7.9–7.11Energy transfer during exothermic and endothermic reactions
Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred.
An exothermic reaction is one that transfers energy to the surroundings, so the temperature of the surroundings increases.
An endothermic reaction is one that takes in energy from the surroundings, so the temperature of the surroundings decreases.
You should be able to distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings, and evaluate uses and applications of exothermic and endothermic reactions given appropriate information.
An evaluation needs more than "it gets hot, so it is useful". Ask how much the temperature changes, how long the effect lasts, whether the pack can be reused, how much it costs, and whether it is safe to hold against skin — then reach a judgement using the figures you were given.
Required practical 4
Investigate the variables that affect temperature changes in reacting solutions
- Stand a polystyrene cup in a beaker for support and add a measured volume of the first solution, for example 30 cm³ of dilute hydrochloric acid.
- Record the starting temperature with a thermometer.
- Add a measured amount of the second reactant, stir, and put a lid on the cup to reduce energy transfer to the air.
- Record the highest — or, for an endothermic reaction, the lowest — temperature reached.
- Repeat with different values of the variable being investigated, keeping everything else the same, and repeat each value to check for anomalies.
Common mistake
In an exothermic reaction the temperature of the surroundings rises but the products end up with less energy than the reactants. The two are not a contradiction — the energy has left the chemicals and gone into the surroundings.
- 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
7.15–7.16Reaction profiles
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.
Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
You should be able to draw simple reaction profiles — energy level diagrams — for exothermic and endothermic reactions, showing the relative energies of reactants and products, the activation energy and the overall energy change, with a curved line to show the energy as the reaction proceeds.
Four features must appear on a diagram that scores full marks:
• energy on the vertical axis and progress of reaction on the horizontal axis • a level for the reactants and a level for the products, at the right heights relative to each other • a curved line rising from the reactants to a peak and falling to the products • two labelled arrows — activation energy from the reactants level up to the peak, and overall energy change between the two levels.
You should also be able to use reaction profiles to identify reactions as exothermic or endothermic, and explain that the activation energy is the energy needed for a reaction to occur.
Reading a profile: if the products are lower than the reactants, energy has been given out and the reaction is exothermic. If the products are higher, energy has been taken in and the reaction is endothermic. The height of the peak has nothing to do with which it is — a very exothermic reaction can still have a large activation energy, which is why petrol does not ignite until it is given a spark.
Common mistake
The activation energy is measured from the reactants up to the peak, not from the bottom of the axis and not from the products. Drawing that arrow in the wrong place is the most common error on these diagrams.
- Activation energy
- the minimum amount of energy that particles must have in order to react
- Reaction profile
- an energy level diagram showing the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
7.12–7.14The energy change of reactionsHigher tier
During a chemical reaction:
• energy must be supplied to break bonds in the reactants • energy is released when bonds in the products are formed.
The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies. The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
You should be able to calculate the energy transferred in chemical reactions using bond energies supplied.
energy change = energy in to break bonds − energy out from making bonds
A negative answer means energy was given out overall, so the reaction is exothermic. A positive answer means energy was taken in, so it is endothermic.
Worked example
Calculate the energy change for H₂ + Cl₂ → 2HCl. Bond energies in kJ/mol: H–H 436, Cl–Cl 242, H–Cl 431.
- List the bonds broken: one H–H and one Cl–Cl.
- Energy in = 436 + 242 = 678 kJ/mol.
- List the bonds made: 2HCl means two H–Cl bonds.
- Energy out = 2 × 431 = 862 kJ/mol.
- Energy change = 678 − 862 = −184 kJ/mol.
- The answer is negative, so more energy came out than went in.
Answer: −184 kJ/mol — exothermic, because the energy released forming the new bonds is greater than the energy needed to break the existing ones.
Common mistake
Count every bond, including the ones inside a molecule that does not appear to change, and multiply by the balancing numbers. In 2HCl there are two H–Cl bonds, not one; forgetting the 2 halves the energy released and flips the answer's sign.
- Bond energy
- the energy needed to break, or released when forming, a particular bond; the overall energy change is the difference between the energy to break bonds in the reactants and the energy released forming bonds in the products
Before you practise
- Exothermic gives heat energy out and the surroundings warm up; endothermic takes it in and they cool down.
- Combustion, neutralisation and most oxidations are exothermic; thermal decomposition and citric acid with sodium hydrogencarbonate are endothermic.
- On a reaction profile, the products are lower than the reactants for an exothermic reaction and higher for an endothermic one.
- The activation energy is the hump: from the reactants up to the peak, not from zero.
- Breaking bonds takes energy in; making bonds gives energy out.
- energy change = energy to break all the bonds − energy released making all the bonds.
- A negative answer means exothermic and a positive answer means endothermic — always say which.