AQA GCSE Chemistry (8462) — Key ideas
Every time you snap a sports injury cold pack, it instantly turns freezing cold. Why does this happen if energy cannot simply disappear?
The principle of the states that energy cannot be created or destroyed, only transferred. In chemistry, energy is transferred between the chemical (the reactants and products) and the (the reaction vessel, solvent, thermometer, and the air).
In an , energy is transferred from the chemical to the . Because the gain thermal energy, their temperature increases, while the products end up with less chemical energy than the reactants.
Conversely, in an , energy is taken in from the into the chemical . The temperature of the decreases, and the products end up with more chemical energy than the reactants. Reversible reactions are always exothermic in one direction and endothermic in the opposite direction, with the exact same magnitude of energy transferred.
You can think of a chemical reaction like a journey over a hill; the starting and ending heights reveal the overall energy shift.
A is a graph showing the progress of a chemical reaction against the relative energy of the substances. It visually represents the energy transfer that has occurred between the and .
For , the product energy level is lower than the reactant level, creating a downward step. For , the product energy level is higher than the reactant level, creating an upward step.
Why doesn't a match spontaneously burst into flames while just sitting in its box?
According to , chemical reactions can only occur when reactant particles collide with each other. However, they must collide with sufficient energy to break the initial chemical bonds.
This minimum energy required for particles to react when they collide is called the activation energy (). On a , this is represented by an upward "hump" or energy barrier that starts from the reactant energy level.
The height of this barrier directly influences the . A higher activation energy hump means a slower reaction rate, as fewer particles possess the required energy to overcome the barrier.
Understanding how to speed up reactions explains everything from why food lasts longer in the fridge to how chemical plants maximise yield.
Increasing the temperature makes particles move faster, leading to more frequent collisions. More importantly, a higher proportion of particles will now have energy greater than or equal to the activation energy, making these collisions successful.
A speeds up reactions by providing an alternative reaction pathway with a lower activation energy. Because the energy barrier is reduced, a higher proportion of particles meet the new energy requirement, resulting in an increased frequency of successful collisions.
In chemistry, simply knowing that a reaction gets hotter isn't always enough; scientists need to calculate exactly how much energy will be transferred.
For Higher Tier students, the overall energy change is calculated by comparing bond energies:
Worked Example:
Step 1: Identify the bond energies.
Step 2: Substitute into the formula.
Step 3: Calculate the final value.
A negative result means the reaction transfers energy to the and is therefore exothermic.
The overall speed of a reaction is calculated using the rate equation:
Students often confuse the temperature of the surroundings with the energy of the system; in an exothermic reaction, the temperature of the surroundings goes up, but the chemical energy of the system goes down.
AQA mark schemes strictly penalise saying energy is "made", "produced", or "created" — always use the words "released" or "transferred".
When explaining the effect of temperature on rate, examiners expect you to make two distinct points: particles collide more frequently, AND a higher proportion of particles have energy greater than or equal to the activation energy.
When describing collision theory, examiners require the word "frequent"; simply saying "there are more collisions" is often insufficient to score the mark.
When drawing a reaction profile, ensure the activation energy arrow starts exactly from the reactant energy level line and goes to the peak of the curve, never starting from the x-axis or the products.
Conservation of energy
A principle stating that energy cannot be created or destroyed; it can only be transferred from one store to another.
System
The chemical reactants and products involved in a reaction.
Surroundings
Everything outside the chemical system, including the reaction vessel, solvent, thermometer, and the air.
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.
Reaction profile
A graph showing the progress of a chemical reaction against the relative energy of the substances involved.
Collision theory
The theory that chemical reactions happen only when reacting particles collide with sufficient energy.
Activation energy (Ea)
The minimum amount of energy that particles must have to react when they collide.
Rate of reaction
The speed at which the amount of reactant is used up or the amount of product is formed over time.
Catalyst
A substance that increases the rate of reaction by providing an alternative pathway with lower activation energy, while remaining chemically unchanged at the end.
Put your knowledge into practice — try past paper questions for Chemistry
Conservation of energy
A principle stating that energy cannot be created or destroyed; it can only be transferred from one store to another.
System
The chemical reactants and products involved in a reaction.
Surroundings
Everything outside the chemical system, including the reaction vessel, solvent, thermometer, and the air.
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.
Reaction profile
A graph showing the progress of a chemical reaction against the relative energy of the substances involved.
Collision theory
The theory that chemical reactions happen only when reacting particles collide with sufficient energy.
Activation energy (Ea)
The minimum amount of energy that particles must have to react when they collide.
Rate of reaction
The speed at which the amount of reactant is used up or the amount of product is formed over time.
Catalyst
A substance that increases the rate of reaction by providing an alternative pathway with lower activation energy, while remaining chemically unchanged at the end.
