Every time you use a hand warmer on a cold winter day, you are relying on an exothermic reaction.
When asked to describe an exothermic change, you must provide a logical sequence of what happens to the energy:
Step 1: Energy is initially held in the chemical energy store of the reactants.
Step 2: As the reaction takes place, this energy is released and transferred from the chemical system into the thermal energy store of the surroundings.
Step 3: This transfer of thermal energy causes the temperature of the surroundings to rise.
Step 4: You can observe and confirm this exothermic change because a thermometer placed in the reaction mixture will display a positive temperature change ().
Students often confuse the system with the surroundings; remember that the thermometer measures the temperature of the surroundings (like the water), not the chemical molecules themselves.
When asked to 'Describe' an exothermic reaction in an exam, you must explicitly state BOTH the direction of energy flow (from the chemical system to the surroundings) AND the observable effect (a temperature increase).
Remember that energy is conserved: any energy 'lost' by the reactants is 'gained' by the surroundings. This is why the temperature of the surroundings rises.
Exothermic reaction
A chemical reaction in which heat (thermal) energy is given out and transferred to the surroundings.
Chemical system
The specific chemical atoms and molecules (the reactants and products) undergoing a reaction.
Surroundings
Everything outside the chemical system, including the reaction vessel, the solvent, the air, and the thermometer.
Thermal energy
The heat energy that is transferred from the chemical energy store of the reactants to the surroundings during an exothermic reaction.
Chemical energy store
The store of energy within the bonds of the reactant particles.
Thermal energy store
The energy store associated with the temperature of the surroundings.
Put your knowledge into practice — try past paper questions for Chemistry
Exothermic reaction
A chemical reaction in which heat (thermal) energy is given out and transferred to the surroundings.
Chemical system
The specific chemical atoms and molecules (the reactants and products) undergoing a reaction.
Surroundings
Everything outside the chemical system, including the reaction vessel, the solvent, the air, and the thermometer.
Thermal energy
The heat energy that is transferred from the chemical energy store of the reactants to the surroundings during an exothermic reaction.
Chemical energy store
The store of energy within the bonds of the reactant particles.
Thermal energy store
The energy store associated with the temperature of the surroundings.