Edexcel GCSE Chemistry (1CH0) — Rates of Reaction and Energy Changes
When you crack a disposable cold pack to treat a sports injury, the sudden drop in temperature is caused by a chemical reaction absorbing heat from its surroundings. We can represent energy changes like this visually using a graph.
A reaction profile is a graph showing the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction. To sketch a valid profile, you must set up your axes correctly.
The chemicals are drawn as flat, horizontal lines. Crucially, a curved line must connect the reactants to the products to show the pathway. If a profile lacks this curved "hump", it is merely an energy level diagram and will lose marks.
An exothermic reaction is a reaction in which energy is transferred to the surroundings, resulting in a temperature increase.
Because energy is lost to the surroundings, the reactants start at a higher energy level than the products. The curve rises from the horizontal reactant line to a peak, and then drops significantly below the starting level to meet the horizontal product line.
An endothermic reaction is a reaction that takes in energy from the surroundings, resulting in a temperature decrease.
In this profile, the products finish at a higher energy level than the reactants. The curve rises from the reactant line to a high peak, and then drops slightly to meet the product line. This perfectly demonstrates the conservation of energy; the increase in chemical energy stored in the products is exactly equal to the heat energy absorbed from the surroundings.
The activation energy () is the minimum amount of energy that colliding particles must possess for a reaction to occur, as energy is needed to break existing bonds.
The overall energy change (often called enthalpy change, ) is the difference between the energy of the products and the energy of the reactants.
You can calculate the exact energy change if given numerical values for the energy levels.
Calculate the overall energy change for a reaction where the reactants have a relative energy of 120 kJ and the products have an energy of 350 kJ. State whether the reaction is exothermic or endothermic.
Step 1: Identify the values from the question.
Step 2: Substitute the values into the equation.
Step 3: Calculate the final answer with a sign and units.
In Higher Tier papers, you may be asked to show the effect of a catalyst on a reaction profile. A catalyst provides an alternative reaction pathway with a lower activation energy.
To sketch this, draw a second, lower curved hump beneath the original one. The starting reactant level and ending product level must remain completely unchanged. Because the start and end points do not move, the overall energy change () remains exactly the same.
Students often mistakenly draw the activation energy arrow starting from the product line or the x-axis, but it must always originate from the reactants.
Edexcel examiners frequently penalise 'floating' arrows; ensure your activation energy and overall energy change arrows start and stop exactly on the horizontal lines or the peak.
Avoid labelling the x-axis simply as 'Time'; always use 'Progress of reaction' or 'Reaction pathway' to secure the axis mark.
Do not use double-headed arrows to label energy changes on your profile, as they do not clearly indicate the specific direction of energy transfer.
In calculation questions, always include a positive (+) sign for endothermic overall energy changes (ΔH) to ensure you get full marks.
Reaction profile
A graph showing the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
Exothermic reaction
A chemical reaction in which energy is transferred to the surroundings, usually as heat, causing a temperature increase.
Endothermic reaction
A chemical reaction that takes in energy from the surroundings, resulting in a temperature decrease.
Conservation of energy
The principle that total energy remains constant; energy cannot be created or destroyed, only transferred.
Activation energy
The minimum amount of energy that colliding particles must possess for a reaction to occur.
Overall energy change
The difference between the energy of the products and the energy of the reactants in a chemical reaction.
Put your knowledge into practice — try past paper questions for Chemistry
Reaction profile
A graph showing the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
Exothermic reaction
A chemical reaction in which energy is transferred to the surroundings, usually as heat, causing a temperature increase.
Endothermic reaction
A chemical reaction that takes in energy from the surroundings, resulting in a temperature decrease.
Conservation of energy
The principle that total energy remains constant; energy cannot be created or destroyed, only transferred.
Activation energy
The minimum amount of energy that colliding particles must possess for a reaction to occur.
Overall energy change
The difference between the energy of the products and the energy of the reactants in a chemical reaction.
