What you'll learn
Energy changes: reaction profiles and activation energy is the topic in AQA GCSE Chemistry that explains where the energy in a chemical reaction comes from and where it goes. The governing principle is conservation of energy: no energy is created or destroyed, only transferred between the chemicals and their surroundings. Every reaction therefore falls into one of two categories, and which one depends entirely on the balance between the energy needed to break bonds and the energy released when new bonds form. By the end of this guide you should be able to classify reactions from temperature data or description, give everyday applications of both types, draw and fully label reaction profiles, explain the effect of a catalyst on a profile, calculate overall energy change from bond energies, describe the required practical, and explain how chemical cells and fuel cells work.
Key terms and definitions
Exothermic reaction — a reaction that transfers energy to the surroundings, raising their temperature
Endothermic reaction — a reaction that takes in energy from the surroundings, lowering their temperature
Conservation of energy — the principle that energy cannot be created or destroyed, only transferred
Reaction profile — a diagram showing the relative energies of reactants and products and the energy barrier between them
Activation energy — the minimum energy colliding particles must have for a reaction to occur
Bond energy — the energy required to break one mole of a particular bond, equal to the energy released when it forms
Overall energy change — the difference between the energy taken in breaking bonds and the energy released making bonds
Chemical cell — a device in which chemical reactions produce a potential difference between two electrodes
Fuel cell — a cell supplied with an external fuel, which is oxidised electrochemically to produce a potential difference
Rechargeable cell — a cell whose reaction can be reversed by supplying an external electric current
Core concepts
Exothermic and endothermic reactions
An exothermic reaction transfers energy from the chemicals to the surroundings, so the temperature of the surroundings rises. Combustion, oxidation reactions and neutralisation are the standard examples. Everyday applications include self-heating cans for food and drink, and hand warmers.
An endothermic reaction takes energy in from the surroundings, so their temperature falls. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are the examples required. The everyday application is the sports injury cold pack.
A precision point worth fixing early: the temperature rise in an exothermic reaction is a rise in the surroundings, while the chemicals themselves have lost energy from their store. Candidates routinely describe an exothermic reaction as gaining energy because the thermometer reading goes up, which inverts the meaning.
Reaction profiles
A reaction profile plots energy on the vertical axis against the progress of the reaction on the horizontal axis. Reactants appear on the left, products on the right, and a curved hump lies between them.
For an exothermic reaction, the products are drawn below the reactants, because energy has been given out. The vertical drop from reactants to products is the overall energy change, and it is negative.
For an endothermic reaction, the products are drawn above the reactants, because energy has been taken in. The overall energy change is positive.
The hump represents the activation energy: the minimum energy that colliding particles must have for the reaction to 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. Misplacing this measurement is the commonest diagram error in the whole course.
A fully labelled profile carries four labels: reactants, products, activation energy and overall energy change. Unlabelled diagrams score poorly even when the shape is correct.
The effect of a catalyst on a profile
A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have sufficient energy to react and the rate increases.
On a reaction profile, a catalyst is shown by drawing a second, lower hump between the same reactant and product levels.
Crucially, the catalyst does not change the energy of the reactants or of the products, so the overall energy change is unaltered. A diagram showing a catalyst changing the product level is wrong, and questions exploit this directly.
Bond breaking and bond making
During a reaction, bonds in the reactants must be broken and new bonds must form in the products, and the two processes have opposite energy effects.
Breaking bonds is endothermic: energy must be supplied to pull bonded atoms apart.
Making bonds is exothermic: energy is released as new bonds form.
Whether a reaction is exothermic or endothermic overall depends on which involves more energy. If more energy is released making bonds than was absorbed breaking them, the reaction is exothermic. If the reverse, it is endothermic.
Getting this pair the right way round is essential, because every conclusion that follows depends on it.
Bond energy calculations
Bond energies allow the overall energy change to be calculated numerically, and the calculation follows three fixed steps.
First, add the bond energies of every bond broken in the reactants, multiplying where a molecule contains several of the same bond.
Second, add the bond energies of every bond made in the products in the same way.
Third, subtract: overall energy change equals energy to break bonds minus energy released making bonds.
The sign of the answer identifies the type. A negative value means more energy was released than absorbed, so the reaction is exothermic. A positive value means the opposite.
The order of subtraction is where most marks are lost. Writing the words bonds broken minus bonds made beside the working prevents the error entirely.
The required practical
The practical investigates how the temperature change of a reaction varies with a chosen variable, such as the volume of acid or alkali added, or the mass of a solid.
The reaction is carried out in a polystyrene cup with a lid. The polystyrene insulates and the lid reduces energy transfer to the surroundings, so more of the energy released stays in the mixture where the thermometer can detect it. This detail is a reliable mark.
Control variables include the starting temperature, the total volume of solution, the concentration of the reagent not being varied, and the same apparatus throughout.
Typical results show the temperature change rising to a maximum as more of the second reactant is added, then falling as further additions merely dilute and cool the mixture. The maximum corresponds to the point at which the reactants are present in exactly reacting proportions.
Chemical cells and batteries
Cells and batteries are assessed on the separate Chemistry course only.
A simple chemical cell consists of two different metals in contact with an electrolyte. Chemical reactions at the electrodes produce a potential difference between them.
The size of the potential difference depends on several factors: the two metals chosen, and specifically how far apart they lie in the reactivity series, with a greater difference producing a larger voltage; the electrolyte used; and the conditions.
In a non-rechargeable cell such as an alkaline battery, the reaction proceeds until one of the reactants is used up, after which no more electricity is produced. The reaction cannot be reversed, so the cell cannot be recharged.
In a rechargeable cell, the chemical reactions can be reversed by supplying an external electric current, which regenerates the original reactants.
Fuel cells
A fuel cell is supplied with a fuel and oxygen from outside, and produces a potential difference for as long as the fuel is supplied.
The hydrogen fuel cell is the example required. Hydrogen is oxidised electrochemically and the overall reaction is simply hydrogen combining with oxygen to produce water. Hydrogen is oxidised at the negative electrode and oxygen is reduced at the positive electrode.
Compared with rechargeable cells, fuel cells have advantages: they do not need to be recharged, only refuelled; they produce only water, so there are no polluting products at the point of use; and they can be made in a wide range of sizes.
The disadvantages are also examinable: hydrogen is a gas and is difficult and expensive to store and transport, it is highly flammable, and producing the hydrogen in the first place usually requires energy from fossil fuels, so carbon dioxide may be released elsewhere in the process.
Worked examples
Example 1: Classifying from temperature data (3 marks)
Two solutions at 22 degrees Celsius are mixed in a polystyrene cup. The temperature rises to 31 degrees Celsius. Classify the reaction and explain.
The temperature of the surroundings has risen by 9 degrees Celsius, which means energy has been transferred from the reacting chemicals to the surroundings. The reaction is therefore exothermic. On a reaction profile the products would be drawn at a lower energy level than the reactants, and the overall energy change would be negative.
Example 2: A bond energy calculation (4 marks)
In a reaction, breaking the bonds in the reactants requires 3,450 kilojoules per mole and forming the bonds in the products releases 3,180 kilojoules per mole. Calculate the overall energy change and classify the reaction.
The overall energy change equals the energy required to break bonds minus the energy released in making bonds, which is 3,450 minus 3,180. This gives a positive 270 kilojoules per mole.
Because the value is positive, more energy was taken in to break bonds than was released in forming them, so the reaction is endothermic. On a reaction profile the products would lie above the reactants.
Example 3: Interpreting a profile with a catalyst (4 marks)
A reaction profile shows reactants at 250 kilojoules per mole, a peak at 480, and products at 90. A second curve for the catalysed reaction peaks at 370. State the activation energy for each route and the overall energy change, and explain why the overall change is the same for both.
For the uncatalysed route, the activation energy is measured from the reactants to the peak, which is 480 minus 250, giving 230 kilojoules per mole. For the catalysed route it is 370 minus 250, giving 120 kilojoules per mole.
The overall energy change is measured from reactants to products, which is 90 minus 250, giving minus 160 kilojoules per mole, and the reaction is exothermic.
The overall change is identical for both routes because the catalyst provides an alternative pathway with a lower activation energy but does not alter the energy of the reactants or of the products. Only the height of the barrier changes.
Common mistakes and how to avoid them
The most frequent error is reversing the definitions because of the thermometer reading. Exothermic means energy out, and the surroundings warm as a result.
Students regularly state that breaking bonds releases energy. Breaking bonds always requires energy in; making bonds always releases it.
On profiles, activation energy is often measured from the bottom of the diagram rather than from the reactant level, or drawn to the product level instead of the peak.
In bond energy calculations, subtracting in the wrong order produces a correct magnitude with the wrong sign, which changes the classification entirely.
Another routine loss is forgetting to multiply a bond energy by the number of that bond present, such as counting one carbon-to-hydrogen bond in methane instead of four.
Finally, candidates often show a catalyst lowering the product level on a profile. It lowers only the peak.
Exam technique for "Energy changes: reaction profiles and activation energy"
State the direction of energy transfer explicitly before classifying a reaction. The mark is usually for the reasoning rather than the label.
Label all four features on any profile you draw, and take an extra second to check that activation energy runs from the reactant level to the peak.
For bond energy questions, set out two clear lists — bonds broken with their total, bonds made with their total — before subtracting. Method marks survive an arithmetic slip only if the working is visible.
Always give the unit, kilojoules per mole, and retain the negative sign for an exothermic reaction.
For cells and fuel cells, remember these are separate Chemistry content and that evaluation questions expect both the point-of-use advantages and the production and storage drawbacks.
Quick revision summary
Energy is conserved and is transferred between chemicals and surroundings. Exothermic reactions release energy so the surroundings warm — combustion, oxidation and neutralisation, used in self-heating cans and hand warmers. Endothermic reactions absorb energy so the surroundings cool — 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 adding a lower peak without changing either level or the overall energy change. Breaking bonds is endothermic and making bonds exothermic, and whichever is greater determines the overall type; the overall energy change equals bonds broken minus bonds made, with a negative answer meaning exothermic. The required practical uses an insulated polystyrene cup with a lid and finds a maximum temperature change at the exact reacting proportions. Chemical cells use two different metals in an electrolyte, with voltage depending on their separation in the reactivity series; rechargeable cells reverse their reaction with an external current; hydrogen fuel cells produce only water but face storage, flammability and hydrogen production problems.