Syllabus coverage
- State that an exothermic reaction transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings
- State that an endothermic reaction takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings
- Interpret reaction pathway diagrams showing exothermic and endothermic reactions
- State that the transfer of thermal energy during a reaction is called the enthalpy change, delta H, of the reaction; delta H is negative for exothermic reactions and positive for endothermic reactions
- Define activation energy, Ea, as the minimum energy that colliding particles must have to react
- Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include the reactants, the products, the enthalpy change of the reaction, delta H, and the activation energy, Ea
- State that bond breaking is an endothermic process and bond making is an exothermic process, and explain the enthalpy change of a reaction in terms of bond breaking and bond making
- Calculate the enthalpy change of a reaction using bond energies
Part of the IGCSE Co-ordinated Sciences syllabus 0654 / 0973.
Exothermic reactions transfer thermal energy to the surroundings
In an exothermic reaction, thermal energy is transferred from the reaction mixture to the surroundings. The products end up with less energy than the reactants started with, and the energy that is left over is transferred out of the mixture. Because the surroundings gain that energy, their temperature rises.
The observation you must quote is the temperature of the surroundings increasing. Never write only that 'heat is given out' - link it to the surroundings and say that their temperature rises. In a test tube you would feel the tube get warm, or see the thermometer reading climb.
Combustion is the clearest example. When methane burns in oxygen, the reaction transfers a large amount of thermal energy to the air, which is why a flame is hot. Neutralisation and respiration are also exothermic, and so is the reaction between a reactive metal and an acid.
- Combustion of fuels such as methane, coal and petrol
- Neutralisation of an acid by an alkali
- Respiration in living cells
- Adding water to calcium oxide, which is used in self-heating cans
- Magnesium reacting with dilute hydrochloric acid
Endothermic reactions take in thermal energy from the surroundings
In an endothermic reaction, thermal energy is transferred from the surroundings to the reaction mixture. The products end up with more energy than the reactants, and that extra energy has to come from somewhere - it is taken from the surroundings, so the temperature of the surroundings falls.
This is why an endothermic reaction feels cold. The mixture is not producing cold; it is removing thermal energy from everything around it, including the air, the bench and your hand.
Endothermic reactions are less common than exothermic ones, but they matter. Thermal decomposition always needs energy put in, and photosynthesis takes in the energy of sunlight to build glucose from carbon dioxide and water.
| Reaction | What you observe | Why it is endothermic |
|---|---|---|
| Thermal decomposition of calcium carbonate | The solid only breaks down while it is being heated strongly | Energy must be transferred to the calcium carbonate to break its bonds |
| Dissolving ammonium nitrate in water | The solution becomes noticeably colder | Energy is taken from the water and the surroundings to separate the ions |
| Citric acid and sodium hydrogencarbonate | The mixture fizzes and the temperature falls | The reaction takes in thermal energy from the surroundings as the reactant bonds break |
Enthalpy change and the sign of delta H
The thermal energy transferred during a reaction at constant pressure is called the enthalpy change, delta H. It is quoted per mole and measured in kilojoules per mole, kJ/mol.
The sign of delta H tells you the direction of the energy transfer. For an exothermic reaction energy leaves the mixture, so the products have less energy than the reactants and delta H is negative. For an endothermic reaction energy enters the mixture, so the products have more energy than the reactants and delta H is positive.
Getting the sign right is a mark on its own. A reaction that warms the surroundings always has a negative delta H, and a reaction that cools them always has a positive delta H, however large or small the number is.
| Reaction | Direction of energy transfer | Temperature of surroundings | Sign of delta H |
|---|---|---|---|
| Exothermic | From the reaction mixture to the surroundings | Increases | Negative, delta H < 0 |
| Endothermic | From the surroundings to the reaction mixture | Decreases | Positive, delta H > 0 |
Reaction pathway diagrams and activation energy
A reaction pathway diagram, also called an energy level diagram, is a graph of the energy of the reacting mixture as the reaction proceeds. Energy goes on the vertical axis and the progress of the reaction on the horizontal axis.
Activation energy, Ea, is the minimum energy that colliding particles must have in order to react. Even an exothermic reaction needs energy to get started, because the existing bonds in the reactants must be stretched and broken before new bonds can form. On the diagram the activation energy is the energy difference between the reactants and the highest point of the curve.
The enthalpy change, delta H, is shown as the energy difference between the reactants and the products. On an exothermic diagram the products sit lower than the reactants, so delta H is negative. On an endothermic diagram the products sit higher than the reactants, so delta H is positive.
- The energy of the reactants, marked on the starting level
- The energy of the products, marked on the finishing level
- The highest point of the curve, which the particles must reach to react
- Activation energy, Ea, measured from the reactants up to that highest point
- Enthalpy change, delta H, measured from the reactants across to the products, with its sign
Bond breaking and bond making
Every reaction breaks the bonds in the reactants and makes new bonds in the products. Bond breaking is an endothermic process, because energy must be transferred to the molecule to pull the atoms apart. Bond making is an exothermic process, because energy is transferred to the surroundings when a new bond forms.
The overall enthalpy change of a reaction is the balance between these two. If more energy is needed to break the bonds in the reactants than is released when the bonds in the products form, the reaction is endothermic and delta H is positive. If more energy is released in bond making than is absorbed in bond breaking, the reaction is exothermic and delta H is negative.
The strength of a bond is measured by its bond energy, the energy needed to break one mole of that bond in the gaseous state, in kJ/mol. A large bond energy means a strong bond, so more energy is released when that bond forms.
- Bond breaking: endothermic, energy is taken in
- Bond making: exothermic, energy is released
- Overall energy change = energy absorbed breaking bonds - energy released making bonds
- A negative answer means the reaction is exothermic overall
Calculating the enthalpy change from bond energies
To calculate delta H from bond energies, work through the reaction in three steps. First add up the bond energies of every bond that is broken in the reactants. Then add up the bond energies of every bond that is made in the products. Finally subtract the second total from the first.
So delta H = energy absorbed to break bonds - energy released when bonds are made. A negative answer means more energy was released than absorbed, so the reaction is exothermic. A positive answer means the reaction is endothermic.
Count the bonds carefully, including the coefficients in the equation. In H2 + Cl2 -> 2HCl one H-H bond and one Cl-Cl bond are broken, but two H-Cl bonds are made, because the equation produces two molecules of hydrogen chloride. Forgetting that factor of two is the most common error in this calculation.
| Step | Working | Value |
|---|---|---|
| Energy absorbed to break bonds | H-H + Cl-Cl = 436 + 242 | 678 kJ/mol |
| Energy released when bonds are made | 2 x H-Cl = 2 x 431 | 862 kJ/mol |
| Enthalpy change | 678 - 862 | -184 kJ/mol, so exothermic |
Key terms you must be able to define
- Exothermic reaction
- A reaction that transfers thermal energy to the surroundings, so the temperature of the surroundings increases and delta H is negative.
- Endothermic reaction
- A reaction that takes in thermal energy from the surroundings, so the temperature of the surroundings decreases and delta H is positive.
- Enthalpy change, delta H
- The thermal energy transferred during a reaction at constant pressure, measured in kJ/mol; negative for exothermic reactions and positive for endothermic reactions.
- Activation energy, Ea
- The minimum energy that colliding particles must have in order to react.
- Reaction pathway diagram
- A diagram with energy on the vertical axis and the progress of the reaction on the horizontal axis, labelled with the reactants, the products, the activation energy and the enthalpy change.
- Bond energy
- The energy needed to break one mole of a particular bond in the gaseous state, in kJ/mol.
Applied to the real world
Cambridge sets Co-ordinated Sciences in unfamiliar contexts - a hospital, a power station, a farm, a laboratory. Use these as models for how to apply the science, then write your own.
Burning coal, oil or natural gas is exothermic. The thermal energy released heats water into steam, and the steam drives turbines that turn generators to make electricity. The energy originally came from the bonds in the fuel, which release more energy when they are broken and remade as carbon dioxide and water than is needed to get the reaction started.
Instant cold packs are used to reduce swelling after a sprain. The pack contains water in one compartment and ammonium nitrate in another. Squeezing the pack breaks the seal and the ammonium nitrate dissolves. This endothermic change takes thermal energy from the pack and then from the injured tissue, so the pack becomes cold.
A self-heating can of coffee uses an exothermic reaction. Pressing a button lets water mix with calcium oxide, and the reaction transfers thermal energy to the drink, warming it without a stove. Hand warmers work on the same principle, using the exothermic reaction of iron with oxygen in the air.
To find the enthalpy change of neutralisation, a student mixes a known volume of dilute acid with a known volume of alkali in an insulated cup and records the temperature rise. The temperature rise shows the reaction is exothermic, and the size of the rise allows the enthalpy change in kJ/mol to be calculated.
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Exam technique for this topic
- Learn the direction of the energy transfer, not just the two words. Exothermic transfers thermal energy to the surroundings so their temperature rises; endothermic takes thermal energy from the surroundings so their temperature falls.
- Always give the sign of delta H with a value: negative for exothermic, positive for endothermic. A number on its own can lose the mark.
- On reaction pathway diagrams label all four required features: reactants, products, delta H and activation energy Ea. Draw Ea from the reactants up to the peak, and delta H from the reactants across to the products.
- In bond energy calculations, write out the bonds broken and the bonds made before you subtract, and count every bond including the coefficients in the equation.
- Finish a calculation with the value, its sign and the unit kJ/mol, and say whether the reaction is exothermic or endothermic.
- Use the syllabus wording: 'thermal energy' rather than 'heat', and 'transferred' rather than 'produced' or 'destroyed'. Energy is never created or lost.
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Where this fits in IGCSE Co-ordinated Sciences
This page is one topic of Cambridge IGCSE Co-ordinated Sciences 0654 / 0973. IGCSE Co-ordinated Sciences is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE Co-ordinated Sciences syllabus page.