What you'll learn
Energy changes is the unit of AQA GCSE Combined Science: Trilogy that explains where the energy in a chemical reaction comes from and where it goes. The governing principle is conservation of energy: energy is never created or destroyed in a chemical reaction, only transferred between the chemicals and their surroundings. Every reaction is therefore either exothermic, transferring energy out to the surroundings, or endothermic, taking energy in. By the end of this unit you should be able to classify a reaction from a temperature change or a description, give everyday applications of both types, draw and label a reaction profile showing the activation energy and the overall energy change, explain why breaking bonds and making bonds have opposite energy effects, and, at Higher Tier, calculate the overall energy change of a reaction from bond energies. This unit is assessed on Chemistry Paper 1 and includes the required practical on the variables affecting temperature changes in reacting solutions.
Key terms and definitions
Exothermic reaction — a reaction that transfers energy to the surroundings, so the temperature of the surroundings increases
Endothermic reaction — a reaction that takes in energy from the surroundings, so the temperature of the surroundings decreases
Conservation of energy — the principle that the total amount of energy is the same before and after a reaction, because energy is neither created nor destroyed
Reaction profile — a diagram showing the relative energies of reactants and products and the energy barrier between them
Activation energy — the minimum energy that colliding particles must have for a reaction to occur, shown as the height of the barrier on a reaction profile
Bond energy — the energy needed to break one mole of a particular bond, which is also the energy released when that bond forms
Overall energy change — the difference between the energy taken in to break bonds and the energy released when new bonds form
Surroundings — everything outside the reacting chemicals, including the solution, the container and the air, and where the temperature change is measured
Core concepts
Exothermic reactions
An exothermic reaction transfers energy from the chemicals to the surroundings. Because the surroundings gain energy, their temperature rises, and this is what a thermometer in the reaction mixture detects.
The everyday examples you should know are combustion, which is the burning of fuels; oxidation reactions in general; and neutralisation, the reaction of an acid with an alkali. Many everyday uses depend on exothermic reactions, including self-heating cans for food and drinks, and hand warmers, which use a reaction that releases energy slowly and steadily.
An important point of care: the temperature of the surroundings rises, but the chemicals themselves have lost energy from their chemical store. Students often describe an exothermic reaction as one that gains energy because the thermometer reading goes up, which is exactly backwards.
Endothermic reactions
An endothermic reaction takes energy in from the surroundings, so the temperature of the surroundings falls. Examples include thermal decomposition, such as heating a metal carbonate to break it down, and the reaction between citric acid and sodium hydrogencarbonate.
The everyday application is the sports injury pack, a cold pack that becomes cold when the contents are mixed and is applied to reduce swelling.
Reaction profiles
A reaction profile is a diagram with energy on the vertical axis and the progress of the reaction on the horizontal axis. The reactants are drawn on the left and the products on the right, with a curved hump between them.
For an exothermic reaction, the products are drawn lower than the reactants, because energy has been given out. For an endothermic reaction, the products are drawn higher than the reactants, because energy has been taken in. The vertical difference between the two levels is the overall energy change of the reaction.
The hump between reactants and products represents the activation energy: the energy that colliding particles must have before the reaction can occur. It is measured from the reactant level up to the top of the hump, never from the baseline of the diagram and never to the product level. This is one of the most commonly misdrawn features in the whole course.
A catalyst is shown by drawing a second, lower hump. The catalyst provides an alternative pathway with a lower activation energy, but it does not change the energy of the reactants or of the products, so the overall energy change is unaffected. If your diagram shows a catalyst changing the product level, it is wrong.
Bond breaking and bond making
During a chemical reaction, the bonds in the reactants must be broken and new bonds must be made in the products. These two processes have opposite energy effects, and understanding which is which explains everything else in this unit.
Breaking bonds is always endothermic: energy must be supplied to pull bonded atoms apart. Making bonds is always exothermic: energy is released as new bonds form.
Whether the reaction overall is exothermic or endothermic depends on which of the two involves more energy. If more energy is released making bonds than was taken in breaking them, the reaction is exothermic overall. If more energy is taken in breaking bonds than is released making them, the reaction is endothermic overall.
Bond energy calculations at Higher Tier
Bond energies allow the overall energy change to be calculated numerically. The energy needed to break a particular bond is the same as the energy released when that bond forms, so a single value serves for both.
The calculation has three steps. First, add up the bond energies of all the bonds broken in the reactants, remembering to count every bond and to multiply where a molecule contains more than one of the same bond. Second, add up the bond energies of all the bonds made in the products in the same way. Third, subtract: the overall energy change equals the energy taken in to break bonds minus the energy released in making bonds.
The sign of the answer tells you the type of reaction. A negative value means more energy was released than taken in, so the reaction is exothermic. A positive value means the reverse, so the reaction is endothermic. Getting the subtraction the right way round is the single most important habit to build here: bonds broken first, bonds made second.
The required practical
The practical investigates how the temperature change of a reaction depends on a chosen variable, for example the volume of an acid or an alkali added, or the mass of a solid added.
Good technique controls the obvious variables: the same starting temperature, the same total volume of solution, the same concentration of the solution not being varied, and the same polystyrene cup with a lid to reduce energy transfer to the surroundings. The polystyrene cup is a genuine exam point, because it insulates and so keeps more of the energy in the mixture where the thermometer can detect it.
Typical results show the temperature rising to a maximum as more of the second reactant is added, then falling once the first reactant is used up, because the extra cold liquid added simply dilutes and cools the mixture. The maximum corresponds to the point at which the two reactants exactly react.
Worked examples
Example 1: Classifying a reaction from data (3 marks)
A student mixes two solutions in a polystyrene cup. The temperature falls from 21 degrees Celsius to 14 degrees Celsius. State whether the reaction is exothermic or endothermic and explain your answer.
The temperature of the surroundings has fallen by 7 degrees Celsius. This means energy has been taken in from the surroundings by the reacting chemicals, so the reaction is endothermic. On a reaction profile the products would be drawn at a higher energy level than the reactants.
Example 2: A bond energy calculation at Higher Tier (4 marks)
A reaction breaks bonds requiring a total of 2,780 kilojoules per mole and forms bonds releasing a total of 3,102 kilojoules per mole. Calculate the overall energy change and state the type of reaction.
The overall energy change equals the energy required to break bonds minus the energy released in making bonds, which is 2,780 minus 3,102. This gives minus 322 kilojoules per mole. Because the value is negative, more energy was released than taken in, so the reaction is exothermic.
Example 3: Explaining a reaction profile (3 marks)
A reaction profile shows the reactants at 400 kilojoules per mole, a peak at 650, and the products at 250. State the activation energy and the overall energy change, and identify the type of reaction.
The activation energy is measured from the reactants to the peak, which is 650 minus 400, giving 250 kilojoules per mole. The overall energy change is measured from the reactants to the products, which is 250 minus 400, giving minus 150 kilojoules per mole. Since the products are lower in energy than the reactants and the change is negative, the reaction is exothermic.
Common mistakes and how to avoid them
The most frequent error in this unit is describing an exothermic reaction as one that takes in energy because the thermometer reading rises. The reading rises because energy has been given out to the surroundings. Reading the definition in terms of the surroundings every time prevents this.
Students regularly state that bond breaking releases energy. It does not; breaking bonds always requires energy in, and making bonds always gives energy out. If this pair is reversed, every subsequent conclusion will be wrong.
On reaction profiles, activation energy is very often drawn or measured from the bottom of the diagram rather than from the reactant level. It is always measured from the reactants up to the peak.
In bond energy calculations, the commonest slip is subtracting in the wrong order, giving an answer with the correct magnitude but the wrong sign. Write the words bonds broken minus bonds made beside the working every time.
Another routine loss is failing to multiply bond energies by the number of that bond present. A molecule of methane contains four carbon–hydrogen bonds, not one.
Exam technique for "Chemistry: Energy Changes"
Whenever a question gives a temperature change, state the direction of energy transfer explicitly before classifying the reaction. Examiners award the mark for the reasoning, not just the label.
When drawing a reaction profile, label four things: reactants, products, activation energy and overall energy change. Unlabelled diagrams score poorly even when the shape is right.
For Higher Tier calculations, set out the bonds broken and the bonds made in two clear lists with their totals before subtracting. Method marks are available and this layout secures them even if the arithmetic goes wrong.
Always give the unit, which is kilojoules per mole, and always keep the negative sign for an exothermic reaction. A missing sign changes the meaning of the answer entirely.
For the required practical, be ready to explain the polystyrene cup and lid in terms of reducing energy transfer to the surroundings, and to identify the maximum temperature change as the point where the reactants are in the exact reacting proportions.
Quick revision summary
Energy is conserved: reactions transfer it between chemicals and surroundings. Exothermic reactions give energy out and the temperature of the surroundings rises — combustion, oxidation and neutralisation, used in self-heating cans and hand warmers. Endothermic reactions take energy in and the temperature falls — thermal decomposition and the citric acid reaction, used in sports injury packs. On a reaction profile, exothermic products sit below the reactants and endothermic products above, with activation energy measured from the reactant level to the peak, and a catalyst lowering only the peak. Breaking bonds is endothermic and making bonds is exothermic, and whichever involves more energy determines the overall type. At Higher Tier, the overall energy change equals bonds broken minus bonds made, with a negative answer meaning exothermic.