Chapter Text
Elements, Compounds, and Mixtures
Understanding the Building Blocks
Substances are made of atoms
All matter is built from atoms. The periodic table shows the different types of elements and their symbols. Each element is made of just one type of atom.
What Are Compounds?
A compound is a substance that contains two or more different types of atoms chemically bonded together
When atoms bond together chemically, they form compounds. These atoms stay bonded through chemical reactions and form new substances.
Example: Water is a compound with the formula H₂O. Every water molecule has two hydrogen atoms and one oxygen atom bonded together.
Chemical Equations and Balancing
We can show chemical reactions using word equations or chemical equations with symbols.
An important rule: atoms are never created or destroyed in a chemical reaction. This means we must have the same number of each type of atom on both sides of the equation. This is called balancing.
How to balance equations:
Start with atoms that appear in only one compound
Use numbers in front of elements or compounds to multiply them
Never change the small numbers in the formula itself—that would change what the compound is
Mixtures
A mixture is any combination of different types of elements and compounds that aren't chemically bonded together
Mixtures are different from compounds. In a mixture, the parts are not chemically bonded. You can find mixtures all around you—air is a mixture of gases, and salt water is a mixture of salt and water.
Key Difference
The main difference between compounds and mixtures is chemical bonding. Compounds have atoms bonded together. Mixtures do not. In a mixture, you can separate the parts without breaking chemical bonds.
Separating Mixtures
What Are Separation Techniques?
Since mixtures are not chemically bonded, we can separate them into their individual parts. There are several methods to do this, depending on what type of mixture you have.
Filtration
You can separate large insoluble particles from a liquid using filtration like sand from water
Filtration works when you have solid particles that cannot dissolve in a liquid. The filter paper traps the solid particles while the liquid passes through. Sand and water is a classic example—the sand stays behind on the filter while the water flows through.
Crystallization
Crystallization can leave a solute that's the solid dissolved in a liquid behind as you evaporate the solvent
When you have a dissolved solid in a liquid (called a solution), you can use crystallization. Heat the solution gently to evaporate the liquid part. The solid particles left behind form crystals. For example, if you evaporate water from salt water, you'll be left with salt crystals.
Distillation
Distillation is similar to crystallization, but with a key difference. You heat the solution and then cool the gas that forms. The gas condenses back into a liquid. This method collects the liquid that evaporated, not just the solid left behind.
Fractional Distillation
Fractional distillation separates different liquids in a mixture. Since different liquids have different boiling points, they evaporate at different temperatures. By heating to specific temperatures, you can separate each liquid one at a time.
Physical Processes, Not Chemical Reactions
These are all physical processes though and not chemical reactions because no new substances are being made
These separation techniques are physical processes. No new substances form. You're simply dividing the mixture back into its original parts. The atoms and compounds remain unchanged—they're just separated from each other.
States of Matter
The Three Main States
Solid, liquid, and gas are the three main states of matter
Matter exists in three different forms. Each state has its own properties based on how the particles are arranged and how much energy they have.
Solid
In a solid, particles vibrate in fixed positions. They stay in place and cannot move past each other. Ice is a good example of a solid state of water.
Liquid
In a liquid, particles still touch each other but are free to move around. They can slide past one another. Liquid water shows this state well—the molecules move but stay close together.
Gas
In a gas, particles are far apart from each other. They move randomly and quickly because they have the most energy. Water vapor is the gas form of water.
Compressibility
As molecules in a gas are far apart, gases can be compressed while solids and liquids cannot
This is a key difference between states. Gases have lots of empty space between particles, so you can squeeze them into smaller volumes. Solids and liquids have particles already close together, so you cannot compress them.
Phase Changes
To melt or evaporate a substance, you must supply energy, usually in the form of heat
When you want to change a substance from one state to another, you must add heat energy. This energy breaks the forces holding particles in place so they can move more freely. Melting changes a solid to a liquid. Evaporation changes a liquid to a gas.
Physical Changes, Not Chemical Changes
These state changes do not create new substances. Ice, liquid water, and water vapor are all water. The particles stay the same—only their arrangement and movement change. This makes these physical changes, not chemical reactions.
State Symbols
We use special symbols to show what state a substance is in:
S = Solid
L = Liquid
G = Gas
AQ = Aqueous (dissolved in water)
Atomic Structure and Development
How We Discovered Atoms
Our understanding of atoms did not happen all at once. Scientists made discoveries over time that changed how we see atoms.
The Plum Pudding Model
JJ Thompson discovered that atoms are made up of positive and negative charges
JJ Thompson was the first to show that atoms have both positive and negative charges. He suggested the plum pudding model. In this model, a positive charge sits in the middle with lots of tiny negative charges (electrons) scattered around it, like plums in a pudding.
The Nucleus Discovery
Ernest Rutherford found that the positive charge must actually be incredibly small. We now call this the nucleus
Ernest Rutherford tested Thompson's idea with an experiment. He fired tiny alpha particles at thin gold leaf. Most particles went straight through, but some bounced back. This proved atoms are mostly empty space. The positive charge must be packed into a very small center. We call this the nucleus.
Electron Shells
Niels Bohr added to our knowledge. He discovered that electrons do not float randomly. Instead, they exist in shells or orbitals around the nucleus.
The Neutron Discovery
James Chadwick discovered that the nucleus must also contain some neutral charges. We call these neutrons
James Chadwick found that the nucleus contains more than just protons. It also has neutrons, which have no charge.
Subatomic Particles
All atoms are made of three main particles:
Protons
Found in the nucleus
Positive charge (+1)
Relative mass of 1
Neutrons
Found in the nucleus
No charge (0)
Relative mass of 1
Electrons
Orbit in shells around the nucleus
Negative charge (-1)
Protons and electrons have equal and opposite charges
Neutrons have a charge of zero
Very light compared to protons and neutrons
Relative mass of 0 (or nearly 0)
Key Points
Atoms are mostly empty space. The nucleus is incredibly small but holds nearly all the atom's mass. Electrons orbit far away from the nucleus. This structure explains why atoms behave the way they do.
Atomic and Mass Number, Isotopes
Understanding Atomic Number
The bottom number is the atomic number. That's the number of protons in the nucleus
The atomic number tells you how many protons an atom has. This number defines what element the atom is. Every atom is neutral, which means it has the same number of electrons as protons.
Understanding Mass Number
The top number is the mass number or relative atomic mass
The mass number shows the total number of protons and neutrons in the nucleus. For example, carbon-12 has 6 protons and 6 neutrons, giving it a mass number of 12.
Finding the Number of Neutrons
To find how many neutrons an atom has, use this simple formula:
Number of neutrons = Mass number − Atomic number
What Are Isotopes?
These are what we call isotopes. Atoms of the same element, but with different numbers of neutrons
Isotopes are atoms of the same element that have different numbers of neutrons. Carbon-12 and carbon-13 are both carbon atoms, but they have different masses because they have different numbers of neutrons.
Relative Abundance and Average Atomic Mass
This is what we call their relative abundance
Not all isotopes of an element exist in equal amounts. The periodic table sometimes shows the average mass of all isotopes found in nature. For example, chlorine has two main isotopes: 75% of chlorine atoms have a mass of 35, while 25% have a mass of 37.
To calculate the average atomic mass, multiply each isotope's mass by its percentage, then add them together.
Average mass = (35 × 0.75) + (37 × 0.25) = 35.5
This is why you see non-whole numbers on the periodic table.
How the Periodic Table Developed
The periodic table didn't exist from the start. Scientists had to build it over time.
The Early Approach
Before it scientists just put elements in order of their atomic weight
Early scientists arranged elements by their atomic weight. Some elements with similar properties were grouped together, but the atomic weight order still came first.
Dmitri Mendeleev's Breakthrough
Dmitri Mendeleev then came along and grouped elements together based on their properties
Mendeleev changed everything. He organized elements by their properties instead of strictly following atomic weight. This new method worked much better.
Predicting the Unknown
Using this method he found there were gaps in the table. He asserted that these elements were yet to be discovered
Mendeleev noticed empty spaces in his table. He predicted that scientists would discover new elements to fill these gaps. He was right. Over time, the missing elements were found, proving his method worked.
This shows how science improves. A good idea—organizing by properties—can help us understand nature better and even predict what we haven't discovered yet.
Development of the Periodic Table
Electron Configuration
Understanding Electron Shells
Electrons exist in shells around the nucleus
Electrons do not float randomly around atoms. Instead, they arrange themselves in shells or energy levels that surround the nucleus. Each shell can hold a maximum number of electrons.
Maximum Electrons Per Shell
The shells fill up from the inside with a maximum of two electrons on the first shell, eight on the second and eight on the third shell
The shells fill from the inside outward. Here are the limits:
First shell: 2 electrons maximum
Second shell: 8 electrons maximum
Third shell: 8 electrons maximum
Fourth shell: 2 electrons (for GCSE level)
Examples of Electron Configuration
Let's look at some common elements:
Carbon (6 electrons): 2, 4
Magnesium (12 electrons): 2, 8, 2
Calcium (20 electrons): 2, 8, 8, 2
What We Focus On
So we only care about the electron configurations going up to 2882
For GCSE chemistry, you only need to learn electron configurations up to calcium (20 electrons). Transition metals and beyond are covered in A-level chemistry, which is beyond the scope of this course.
The key is remembering that electrons fill shells from the inside out, following the 2, 8, 8, 2 pattern.
Metals and Non-Metals
The Periodic Table Divides Into Two Groups
The periodic table has a clear division. A staircase-like line splits metals from non-metals.
Everything to the left of this staircase is called a metal
Everything to the right of this line is a non-metal. This simple division helps us understand how atoms behave.
How Metals Bond
Metal atoms always donate electrons when they bond to gain an empty outer shell of electrons
Metal atoms give away electrons when they form bonds. They do this to empty their outer shell. Once they lose electrons, they reach a stable state with a full shell underneath.
How Non-Metals Bond
To the right of the staircase, non-metals. They always accept electrons to gain a full outer shell
Non-metals work the opposite way. They accept electrons from other atoms. They do this to fill their outer shell completely. This makes them stable.
Understanding Groups
The column an atom sits in on the periodic table is called its group. The group number tells you how many electrons the atom has in its outer shell. This is very useful for predicting how atoms will bond.
Transition Metals Are Different
Transition metals break the normal rules. They don't follow the simple electron donation pattern of other metals. For now, we don't worry about how their shells work. This topic goes beyond GCSE chemistry.
Groups of the Periodic Table
What Are Groups?
The column an atom is in is called its group. It tells you how many electrons an atom has in its outer shell
Groups are the vertical columns of the periodic table. The group number tells you exactly how many electrons are in an atom's outer shell. This is one of the most useful facts in chemistry.
Group 1: The Alkali Metals
The atoms in group one are called the alkalion metals. They all have one electron in their outer shell
Group 1 elements have just one electron in their outer shell. They easily give away this electron when bonding. All Group 1 metals behave similarly because of this shared feature.
Reactivity Trend in Group 1
As you move down Group 1, the metals get more reactive. Why? The outer electron sits further from the nucleus. The pull between the positive nucleus and the negative electron becomes weaker. This makes the electron easier to lose. So lithium is less reactive than sodium, which is less reactive than potassium.
Group 7: The Halogens
Group seven are what we call the hallogens. They have seven electrons in their outer shell
Group 7 elements have seven outer electrons. They need just one more electron to have a full shell. So they accept electrons from other atoms.
Reactivity Trend in Group 7
Group 7 works the opposite way from Group 1. As you go down the group, halogens become less reactive. An electron further from the nucleus is harder to grab. Boiling points increase as you go down, but reactivity decreases. Fluorine is the most reactive halogen. Iodine is the least reactive.
Group 0: The Noble Gases
Group zero, sometimes referred to as group 8, are called the noble gases. They already have an empty or full outer shell
Group 0 elements have complete outer shells. They don't need to gain or lose electrons. This makes them very unreactive. Under normal conditions, noble gases don't react with other elements. They are stable and content as they are.
How Groups Form Ions
Atoms in the same group form ions in the same way:
Group 1: Lose one electron → ions are 1+
Group 2: Lose two electrons → ions are 2+
Group 6: Gain two electrons → ions are 2−
Group 7: Gain one electron → ions are 1−
Transition metals are different. They can lose different numbers of electrons. Iron can form Fe²⁺ or Fe³⁺ depending on the situation.
Key Takeaway
The periodic table's groups reveal the pattern of electron behavior. Understanding groups helps you predict how atoms will bond and what ions they will form.
Ionic Bonding
Ionic bonding happens when metals bond to non-metals. One atom gives up electrons while the other takes them in.
How Ionic Bonding Works
Metals bond to non-metals through ionic bonding
A group one metal needs to lose one electron. A group seven atom needs to gain one electron. It's a perfect match. For example, lithium donates its outer electron to chlorine.
A group one metal needs to lose an electron while a group seven atom needs to gain one
Drawing Dot and Cross Diagrams
We can show electron transfer using dot and cross diagrams. Only draw the outer shell of each atom. Add brackets around the ions and show their charges.
Balancing Charges
When it comes to ionic bonding, the charges of all ions in an ionic compound must add up to zero
This is the key rule. In lithium chloride (LiCl), lithium is +1 and chloride is -1. They balance to zero.
With beryllium oxide (BeO), beryllium is +2 and oxide is -2. They balance perfectly.
For beryllium chloride (BeCl₂), beryllium is +2. Each chlorine is -1. You need two chlorines to balance the +2 charge.
Chemical Formulas
The charges tell you the formula. When charges balance to zero, you've got the right formula.
Any ionic compound can be called a salt, not only sodium chloride, table salt
Ionic Bonding
Ionic Structures and Properties
Crystal Lattice Structure
Ionic compounds consist of lots of repeating units of these ions in a lattice to form a crystal
Ionic compounds are made up of repeating patterns of ions. These ions arrange themselves in a crystal structure called an ionic lattice. The ions pack together in a regular, repeating pattern.
High Melting and Boiling Points
They have high melting points and boiling points due to the strong electrostatic forces that need to be overcome
Ionic compounds have very high melting and boiling points. This is because the ions are held together by strong electrostatic forces of attraction. A lot of energy is needed to break these forces apart.
Electrical Conductivity
They can conduct electricity but only when they're in liquid form that is molten or dissolved in solution
Ionic compounds can conduct electricity, but only in certain conditions. They conduct when molten (melted) or when dissolved in solution. This is because the ions are free to move in these liquid states and carry electrical charge.
In solid form, ionic compounds cannot conduct electricity. The ions are fixed in place in the crystal lattice and cannot move.
Naming Ionic Compounds
Ionic compounds are also called salts. The name always follows a pattern: the metal ion (cation) comes first, followed by the non-metal ion (anion). For example, sodium chloride (not sodium chlorine).
Some ionic compounds contain molecular ions. These are groups of atoms bonded together that carry a charge. Examples include hydroxide (OH⁻) and sulfate (SO₄²⁻).
Metallic Bonding
How Metallic Bonding Works
Metal atoms bond to each other through metallic bonding
Metal atoms bond together in a special way called metallic bonding. A lattice (or grid) of metal ions forms. Around these ions, there is a "sea" of delocalized electrons. Delocalized means the electrons are free and not attached to any one atom.
Electron Movement and Conductivity
Essentially a lattice or grid of ions is formed with a sea of delocalized electrons around them
Because these electrons can move freely throughout the structure, metals are excellent conductors of electricity and heat. The mobile electrons carry electrical charge from one place to another. They also transfer heat energy quickly.
Alloys
When you mix metals together, you create an alloy. The added atoms disrupt the regular lattice structure. This disruption actually makes alloys stronger than pure metals. The irregular arrangement prevents layers from sliding past each other easily.
As these electrons are free to move, metals make good conductors of electricity and heat
Covalent Bonding
What Is Covalent Bonding?
Non-metals bond to each other with covalent bonding to form molecules
Covalent bonding is how non-metals bond together. In this type of bond, atoms share electrons with each other.
Why Atoms Share Electrons
They do this by sharing electrons to gain full outer shells
Atoms want full outer shells. When two non-metal atoms bond, they share electrons to achieve this. Both atoms benefit from the arrangement.
For example, chlorine gas (Cl₂) forms when two chlorine atoms share one electron each. Now both atoms have full outer shells.
Drawing Covalent Bonds
We use two ways to show covalent bonds:
Dot and Cross Diagrams: These show which electrons come from which atom. Only draw the outer shell of each atom.
Structural Formulas: These use lines to represent bonds. Each line is one shared pair of electrons.
Types of Covalent Bonds
The number of electrons an atom needs is the same as the number of covalent bonds it must make
The number of bonds an atom makes depends on how many electrons it needs:
Hydrogen makes 1 bond
Oxygen makes 2 bonds
Carbon makes 4 bonds
Single bonds: One shared pair of electrons (one line)
Double bonds: Two shared pairs of electrons (two lines). Oxygen gas (O₂) has a double bond.
Simple Molecular Structures
Individual molecules that are not bonded to other molecules form simple molecular structures. These molecules can mix together freely.
These structures have low melting and boiling points. Only weak forces hold the molecules together, so little energy is needed to separate them.
Giant Covalent Structures
What Are Giant Covalent Structures?
Giant covalent bonding is similar to the lattice nature of ionic compounds. Atoms form covalent bonds to other atoms, which bond to other atoms, and so on. This creates one giant molecule instead of separate small molecules.
Diamond
Diamond is an example of this. It's a crystal of carbon atoms bonded to each other
Diamond is made of carbon atoms bonded in a continuous network. Each carbon atom forms four covalent bonds. This creates an extremely strong structure.
Diamond is so hard and has such a high melting point because you must break the covalent bonds to change it. These bonds are incredibly strong.
Graphite
Graphite consists of layers of carbons with three covalent bonds each in a hexagonal structure
Graphite is also made of carbon, but it has a different structure. Carbon atoms arrange in layers with three covalent bonds each, forming a hexagonal pattern.
The fourth electron on each carbon atom is delocalized. These spare electrons form weak bonds between the layers. This allows graphite to conduct electricity because electrons can move between the layers.
The layers can also slide over each other easily. This is why graphite is used for pencils.
Graphene
Graphine is just a single layer of graphite
Graphene is one single layer of graphite. It has unique properties that make it useful in many applications.
Fullerenes and Nanotubes
Fullerenes are 3D structures made of carbon atoms. Buckminsterfullerene (C₆₀) is a spherical, football-like structure made of 60 carbon atoms.
Some fullerenes have a tube shape. These are called nanotubes.
Surface-to-Volume Ratio
Surface to volume ratio is just one divided by the other
The surface-to-volume ratio is calculated by dividing the surface area by the volume. When the length of a cube's side doubles, this ratio halves.
Nanoparticles are tiny, so they have a huge surface-to-volume ratio. This large ratio gives them special properties.
Simple Molecular Structures and Properties
What Are Simple Molecular Structures?
Simple molecular structures, also called simple covalent structures, consist of individual molecules that can mix together. Atoms within each molecule are held by strong covalent bonds.
Key Properties
Boiling and Melting Points These structures have relatively low boiling points. This is because only weak intermolecular forces exist between the molecules. Heating breaks these weak forces, not the covalent bonds within molecules.
Electrical Conductivity
Unlike ionic compounds, these can't conduct electricity even as liquids
Simple molecular compounds cannot conduct electricity in any state because they lack free electrons or ions to carry charge.
Important Distinction
When simple molecular compounds melt or boil, the weak intermolecular forces between molecules break. The strong covalent bonds within each molecule remain intact. This is a crucial difference from other bonding types.
Summary
Simple molecular structures are held together by weak forces between molecules, making them easy to melt and boil. They do not conduct electricity in any form.
Moles and Relative Formula Mass
Moles and Relative Formula Mass
Conservation of Mass
Total mass of all substances is conserved in any chemical reaction
In any chemical reaction, the atoms that go in must come out. This is why we balance equations. Nothing is lost or gained—just rearranged.
Relative Formula Mass
We already know about relative atomic mass (RAM). For compounds, we add up the RAM values of all atoms to get the relative formula mass.
For example, CO₂ = 12 + (2 × 16) = 44
What Is a Mole?
A mole is just a specific number of atoms or molecules
A dozen equals 12. A score equals 20. A mole equals 6.02 × 10²³. This huge number is called Avogadro's constant.
We use moles to group particles together. We can't count individual atoms or molecules, so moles help us compare amounts of different substances.
Molar Mass
If you have as many grams of a substance as its relative atomic mass or formula mass, you have one mole
This is the key link between mass and moles. One mole of carbon has a mass of 12 grams.
The Moles Equation
Moles equals g over g
Moles = Mass ÷ RAM (or formula mass)
This equation is worth remembering. Use it to find how many moles you have of a substance.
Using Moles in Reactions
In a balanced equation, the numbers in front of each substance show the mole ratio. For methane combustion:
CH₄ + 2O₂ → CO₂ + 2H₂O
This means: 1 mole of methane reacts with 2 moles of oxygen to make 2 moles of water.
Mass-to-Mass Calculations
To find the mass of a product from the mass of a reactant:
Convert mass to moles using: Moles = Mass ÷ RAM
Use the mole ratio from the balanced equation
Convert moles back to mass using: Mass = Moles × RAM
Example: If 64 g of methane reacts completely, how much water is made?
Moles of methane = 64 ÷ 16 = 4 moles
From the equation, 1 mole methane makes 2 moles water
So 4 moles methane makes 8 moles water
Mass of water = 8 × 18 = 144 g
Important Note
Because all these calculations use ratios, you can use any unit (grams, kilograms, or tons). Just keep the same unit throughout your calculation.
Limiting Reactants
Limiting Reactants
What Is a Limiting Reactant?
A limiting reactant is the substance that runs out first in a chemical reaction. Once it's gone, the reaction stops. The other reactants don't fully react because there's nothing left to react with.
Understanding Stoichiometry
The numbers in a balanced equation show the mole ratio. For methane combustion:
CH₄ + 2O₂ → CO₂ + 2H₂O
This means you need 2 moles of oxygen for every 1 mole of methane.
Identifying the Limiting Reactant
If we had that one mole of methane, but only one mole of oxygen, well, that means that not all of the methane would react
In this example, you have 1 mole of methane but only 1 mole of oxygen. You need 2 moles of oxygen to react with 1 mole of methane. Since oxygen runs out first:
We say that the oxygen is the limiting reactant in this case
The methane is left over and doesn't fully react.
Excess Reactant
We could also put it the other way round. We could say that the methane is in excess
An excess reactant is any substance that remains after the reaction finishes. It's the reactant that doesn't run out.
Key Takeaway
Use mole calculations to find which reactant runs out first. This tells you how much product you can actually make.
Solution Concentration
Solution Concentration
What Is Concentration?
Concentration tells you how much of a substance is dissolved in a solution. It measures the amount of solute (the thing being dissolved) in a fixed volume of solvent.
Units of Concentration
Solutions can be measured in two main ways:
Mass Concentration (g/dm³)
The concentration of solutions can be given in g per decime cubed, where a decime cubed is 1,000 cm cubed
This tells you the mass in grams per cubic decimeter.
Molar Concentration (mol/dm³)
If one mole of HCl is dissolved in 1 decime cubed of water, we've made hydrochloric acid that has a concentration of one mole per decime cubed
This measures how many moles of solute are in a cubic decimeter of solution.
Molar Notation
Sometimes you might see this as just one molar, one capital M
Molar concentration is often written as "M" (capital). One molar means one mole per dm³.
Converting Between Units
You can convert between mass and molar concentration. Use the relative formula mass to switch between grams and moles. This helps you work with the units that suit your calculation best.
Key Takeaway
Understanding concentration units helps you solve problems in chemistry. Whether you use grams or moles, you're describing the same thing: how concentrated your solution is.
Yield and Atom Economy (TRIPLE)
Yield and Atom Economy (TRIPLE)
Understanding Percentage Yield
In most reactions, we want to make as much product as possible. However, some reactants usually remain unused at the end.
Percentage yield merely tells us how much product we've actually made compared to how much we could have made in theory
If you start with 20 g of reactants but only make 10 g of ammonia, your percentage yield is 50%. Percentage yield shows what happens in reality. You cannot predict it from the chemical equation alone—it depends on real-world factors like incomplete reactions and loss of material.
Atom Economy: Measuring Efficiency
Atom economy on the other hand tells you how much of a desired product you get out of a reaction compared to the mass of the reactants
Think of atom economy as the efficiency of a reaction. It shows how well a reaction uses its starting materials.
You can think of atom economy as being like efficiency in a reaction
Calculating Atom Economy
Use relative atomic or formula masses to calculate atom economy:
Atom Economy = (RAM of desired product ÷ Total RAM of all products) × 100
Or use this shortcut:
Atom Economy = (RAM of desired product ÷ Total RAM of reactants) × 100
Both methods give the same answer because the total mass of products equals the total mass of reactants.
Example: Making Carbon Dioxide
In greenhouses, methane reacts with oxygen to make carbon dioxide for plants:
CH₄ + 2O₂ → CO₂ + 2H₂O
The desired product is CO₂:
RAM of CO₂ = 44
RAM of all products = 44 + (2 × 18) = 80
Atom Economy = (44 ÷ 80) × 100 = 55%
This means only 55% of the reactant mass becomes the desired product. The remaining 45% becomes water, which is not the target product.
Key Takeaway
Percentage yield and atom economy measure different things. Percentage yield shows what actually happens in practice. Atom economy shows how efficient the reaction is at converting reactants into the desired product. Both matter in real chemistry.
Gas Volume (TRIPLE)
Molar Volume of Gases at RTP
One mole of any gas takes up a volume of 24 dm³. This is true regardless of what the gas is or its relative mass.
Room Temperature and Pressure (RTP)
RTP stands for room temperature and pressure. These conditions are:
Temperature: 20°C
Pressure: 1 atmosphere
One mole of any gas takes up a volume of 24 decime cubed regardless of its relative mass
This is true for RTP, room temperature and pressure. That's 20° C and a pressure of 1 atmosphere
Converting Between Moles and Gas Volume
You can easily convert between moles and volume by multiplying or dividing by 24.
To find volume: Moles × 24 = Volume (dm³)
To find moles: Volume (dm³) ÷ 24 = Moles
This simple relationship makes gas calculations straightforward at RTP.
Gas Volume (TRIPLE)
Reactivity Series and Displacement Reactions
Understanding Metal Reactivity
Metals vary in how reactive they are. Some metals give up their electrons more easily than others. This difference in reactivity is shown in the reactivity series.
The reactivity series lists common metals in order, from most reactive to least reactive. Hydrogen and carbon are also included in this series. We compare metals to these elements to predict what will happen in reactions.
We know that metals vary in their reactivity as some donate their electrons more readily than others
Displacement Reactions
A more reactive metal can push out a less reactive metal from a compound. This is called a displacement reaction.
Example: When zinc is placed in copper sulfate solution, copper forms on the zinc. The zinc displaces the copper because zinc is more reactive. The zinc takes the place of copper in the compound, forming zinc sulfate.
A more reactive metal will displace a less reactive metal from a compound
Alkali Metals and Water
Alkali metals react with water because they are more reactive than hydrogen. They displace the hydrogen from water molecules, leaving behind an alkali (a hydroxide compound) and releasing hydrogen gas.
For example, potassium reacts with water to form potassium hydroxide and hydrogen gas.
Metal Extraction: Smelting
Metals less reactive than carbon can be extracted from their ores using carbon. Carbon displaces these metals from their compounds in the ore.
Any metal less reactive than carbon can be displaced by it
This process is called smelting. Iron oxide, for example, can be reduced to iron using carbon or coke.
This is called smelting
Oxidation and Reduction
What Are Oxidation and Reduction?
Oxidation and reduction are two types of chemical processes that happen together.
Oxidation: Loss of electrons
Reduction: Gain of electrons
These reactions don't always involve oxygen, even though the words might suggest they do.
Even if oxygen is not involved in a reaction though we can still say that reduction and oxidation are happening
The OIL RIG Mnemonic
Remember this simple phrase to keep oxidation and reduction straight:
The pneummonic is oil rig oxidation is loss reduction is gain of electrons
Oxidation Is Loss of electrons
Reduction Is Gain of electrons
Half Equations and Ionic Equations
When writing half equations, electrons are shown as gained or lost. An important rule to remember:
We never really have a minus in any half equation
This means you write electrons as positive values in your equations. When an atom gains electrons, it becomes negatively charged. When it loses electrons, it becomes positively charged.
Identifying Oxidation and Reduction
Look at what happens to electrons in a reaction. If a substance loses electrons, it has been oxidized. If a substance gains electrons, it has been reduced. This helps you understand which reactant is the oxidizing agent and which is the reducing agent.
Neutralisation and Acid-Base Reactions
Neutralisation and Acid-Base Reactions
What Are Alkalis?
Alkalies have a pH greater than 7. These can react with acids
Alkalis are substances with a pH above 7. When an alkali meets an acid, they can react together.
Acid-Alkali Reactions
When an acid and an alkali react, they produce two things:
This produces a salt and water
For example, sodium hydroxide and hydrochloric acid make sodium chloride (salt) and water. The resulting solution has a neutral pH of 7.
Neutralisation
When the right amounts of acid and alkali are mixed, they neutralize each other completely. This means no acid or alkali is left over. The reaction uses up all the reactants.
Metals and Acids
Metals more reactive than hydrogen can displace it from an acid too. So most metals react with acid. This always produces a salt
Metals that are more reactive than hydrogen can push hydrogen out of acids. This creates a salt.
Different acids make different salts:
Sulfuric acid produces sulfates
Nitric acid produces nitrates
Salts in Solution
When salts dissolve in water, their ions separate. The water also breaks apart into H⁺ and OH⁻ ions. These dissolved ions are what we find in salt solutions.
pH Scale and Acid Strength
Understanding the pH Scale
The pH scale measures how acidic or basic a substance is. It uses a special system called a logarithmic scale base 10. This means the scale is not linear—each step represents a big change.
The pH scale is a logarithmic scale base 10. It's not linear
How the pH Scale Works
The scale runs from 0 to 14:
pH 7 is neutral
Below 7 is acidic
Above 7 is alkaline (basic)
Each acid contains H⁺ ions. The key thing to know is that a small change in pH means a huge change in H⁺ concentration.
An acid that has a pH of three will have 10 times the concentration compared to the same acid that has a pH of 4
For example, an acid at pH 3 has 100 times more H⁺ ions than the same acid at pH 5. Alkalis work the same way, but with OH⁻ ions instead. The higher the pH above 7, the greater the concentration of OH⁻ ions.
Strong Acids vs. Weak Acids
Strong acids completely break apart in water. They release all their H⁺ ions.
A strong acid is one that dissociates or ionizes completely when in solution
Examples include hydrochloric acid, nitric acid, and sulfuric acid.
Weak acids only partly break apart in water. They release some H⁺ ions but not all.
Weak acids, on the other hand, only partially dissociate
Examples include ethanoic acid, citric acid, and carbonic acid.
What Affects pH?
The pH of an acid depends on two things: its strength and its concentration. A strong acid can have a high or low pH depending on how concentrated it is. A weak acid might have a similar pH to a strong acid if it is more concentrated.
Titration (TRIPLE)
What is Titration?
Titration is a method used to find the concentration of an acid or an alkali. It involves a controlled reaction between two solutions to determine an unknown concentration.
The Procedure
Setting up:
Use a glass pipette to measure a known volume of alkali (or acid) with a known concentration
Place this solution in a conical flask
Add a few drops of an indicator (methyl orange or phenolphthalein)
We use a glass pipette to measure out a volume of alkali of known concentration
Running the titration:
Place the acid (or alkali) of unknown concentration in a burette
Open the tap and let the liquid drip slowly into the flask
Swirl the flask continuously as you add the liquid
Detecting the Endpoint
The indicator shows when neutralization occurs:
Methyl orange: turns pink when the endpoint is reached
Phenolphthalein: turns colorless when the endpoint is reached
When it turns pink, if we're using methyl orange or goes colorless, if it's phenylthalen, we close the tap
Once the color change occurs and remains after swirling, neutralization is complete. Close the tap immediately.
Improving Accuracy
First, perform a rough titration to get an approximate volume. Then carry out a precise titration, adding the liquid drop by drop near the endpoint for a more accurate result.
Calculating Concentration
Using the balanced equation and stoichiometry:
Convert the volume to decimetres cubed (dm³)
Calculate the number of moles using: moles = concentration × volume
Use the stoichiometry ratio to find moles of the unknown solution
Calculate the unknown concentration using the formula: concentration = moles ÷ volume
Electrolysis of Molten Compounds
What is Electrolysis?
Electrolysis is a process that uses electricity to break down ionic compounds. When you melt an ionic compound, the ions can move freely. By passing an electric current through the molten compound using inert electrodes, you can separate it into its elements.
If you melt an ionic compound, let's say aluminium oxide, it can conduct electricity
How It Works
In electrolysis, two things happen at the same time:
At the cathode (negative electrode):
Positive ions (cations) move here
They receive electrons and turn into atoms
Cations are always reduced at the cathode
The positive metal ions or cations move to the negatively charged electrode. We call that the cathode
At the anode (positive electrode):
Negative ions (anions) move here
They lose electrons
Anions are always oxidized at the anode
The negative ions or annions move to the positive electrode the anode where they lose electrons
Inert Electrodes
Inert electrodes are used because they don't react with the compounds being electrolyzed. Carbon is commonly used. However, in some reactions like aluminium oxide electrolysis, the oxygen produced at the anode reacts with the carbon electrode itself. This means the electrodes need replacing over time.
Practical Applications
Electrolysis is used to extract metals from their compounds when other methods won't work. For example, aluminium is extracted from aluminium oxide through electrolysis. This process is essential when a metal's reactivity makes displacement reactions impossible.
Electrolysis of Solutions
What Happens in Solution?
When you electrolyze an ionic solution, multiple ions are present at once. For example, sodium chloride solution contains Na+, Cl−, H+, and O²− ions. This creates a competition: which ions move to which electrode?
How Reactivity Decides
The key is reactivity. The more reactive ion stays in solution. The less reactive ion moves to the cathode and gets reduced.
The more reactive ion stays in solution while the less reactive one moves to the cathode
At the Cathode
What forms at the cathode depends on the metal's reactivity compared to hydrogen:
If the metal is more reactive than hydrogen: H+ ions are reduced instead, producing hydrogen gas
If the metal is less reactive than hydrogen: The metal deposits on the cathode (for example, copper forms on the cathode in copper sulfate solution)
If the metal in solution is less reactive than hydrogen, say copper in copper sulfate solution, it forms on the cathode
At the Anode
At the anode, non-metal ions are oxidized. If the ion is a halide like Cl−, it gets oxidized at the anode.
If the non-metal ion is a halid, like the Cl minus here, it's oxidized at the anode
Key Difference from Molten Compounds
Electrolysis of solutions is different from molten compounds because competing ions are present. In solutions, reactivity determines which species gets discharged at each electrode.
Exothermic and Endothermic Reactions
Energy Transfer in Chemical Reactions
Any chemical reaction involves energy transfers as energy is needed to break chemical bonds while energy is released when chemical bonds form.
Every reaction requires two things to happen at the same time. First, energy is needed to break the bonds in the reactants. Second, energy is released when new bonds form in the products.
Exothermic Reactions
If there's more energy released from bonds being made than energy needed to break bonds, we say this is a net energy released.
When a reaction releases more energy than it uses, the extra energy escapes. This makes the surroundings warmer. You will see the temperature increase.
This is an exothermic reaction.
Think of explosions. They are exothermic. The "exo" part means "out" – energy goes out into the surroundings.
Endothermic Reactions
If it's the other way round and there's a net energy input into the reaction, the reaction should get colder. This is an endothermic reaction.
When a reaction needs more energy to break bonds than it releases from making new ones, it takes heat from the surroundings. The temperature drops. The reaction feels cold.
Temperature Changes Show What's Happening
You can spot exothermic and endothermic reactions by watching the temperature:
Exothermic: Temperature rises
Endothermic: Temperature falls
Practical Neutralization Experiment
You can test this with a simple experiment:
Mix an acid and an alkali in a well-insulated cup
Place a thermometer through the lid
Measure the highest temperature reached
Repeat with different amounts of alkali
Plot the results on a graph
The temperature will rise to a peak, then fall as the solution cools. Where two lines meet on your graph shows how much alkali was needed to neutralize the acid.
Energy Profile Diagrams
Energy profile diagrams show the energy difference between reactants and products. The y-axis shows potential energy.
In an exothermic reaction, the products have lower potential energy than the reactants. This lost potential energy becomes kinetic energy, making the temperature higher.
In an endothermic reaction, the products have higher potential energy than the reactants. This extra energy must come from the surroundings, making the temperature lower.
Activation Energy
Fuel needs a spark to start burning. This spark provides the activation energy – the energy needed to get the reaction going. Once started, an exothermic reaction releases enough energy to keep itself going.
Bond Energies and Energy Calculations
Bond Energy Basics
Every bond needs a very specific amount of energy to break.
Each chemical bond has its own specific energy value. This means different bonds require different amounts of energy to break apart. For example, 413 kJ of energy is needed to break one mole of carbon-hydrogen bonds.
If a mole of these bonds are made though, it's the same amount of energy that's released instead.
When bonds form, the same amount of energy is released. This is important: breaking bonds requires energy input, and making bonds releases energy.
Calculating Energy Changes
To find the total energy change in a reaction, you need to:
Calculate the energy needed to break all bonds in the reactants
Calculate the energy released when all bonds form in the products
Subtract: products energy minus reactants energy
The total bond energies of the products minus that of the reactants gives us the overall energy change.
Example: Combustion of Methane
Let's look at how methane burns:
Breaking bonds in reactants:
Four carbon-hydrogen bonds: 4 × 413 = 1,652 kJ
Two oxygen double bonds: 2 × 495 = 990 kJ
Total energy needed: 2,642 kJ
Making bonds in products:
Two carbon-oxygen double bonds: 2 × 799 = 1,598 kJ
Four oxygen-hydrogen bonds (in water): 4 × 467 = 1,868 kJ
Total energy released: 3,466 kJ
Overall energy change: 3,466 kJ − 2,642 kJ = 824 kJ released
Determining Exothermic or Endothermic
The calculation tells you what type of reaction it is:
If products release more energy than reactants need: The reaction is exothermic (negative value)
If reactants need more energy than products release: The reaction is endothermic (positive value)
In our methane example, more energy is released than goes in, so it's exothermic. This makes sense because combustion reactions produce heat.
Key Points
Bond energies are always given to you – you don't need to memorize them
The units (usually kJ per mole) show energy amounts
The calculation reveals whether a reaction releases or takes in heat
Energy changes help predict how reactions behave in the real world
Cells and Batteries
What Are Cells and Batteries?
Cells and batteries contain chemicals that produce a potential difference (or voltage) to power electrical devices. They are essential components in modern technology, from smartphones to remote controls.
Basic Composition
The fundamental structure of a cell consists of:
Two different metals in contact with each other
An electrolyte (a chemical substance that conducts electricity)
This simple arrangement allows the chemical energy to be converted into electrical energy.
Non-Rechargeable Batteries
Non-rechargeable batteries or dry batteries stop working when the reactants are used up.
Once the chemical reactants inside are completely consumed, these batteries cannot produce any more electrical energy. They must be replaced with new ones.
Rechargeable Batteries
Rechargeable batteries can be recharged when a supplied current causes the reverse reaction to occur.
Unlike non-rechargeable batteries, rechargeable batteries can have their chemical reactions reversed. When you supply an electric current, the used-up reactants are restored, allowing the battery to produce electricity again.
Hydrogen Fuel Cells
Hydrogen fuel cells operate on a similar principle. Water is split into hydrogen and oxygen through electrolysis. When these gases recombine, they produce a potential difference that generates electrical power. This makes fuel cells a promising clean energy source.
Key Takeaway
Understanding how cells and batteries work helps explain how we power the devices we use every day. The chemistry inside these devices directly converts chemical energy into the electricity that powers our world.