Exothermic reactions
Activity 8.2 follows the temperature of a reaction between a metal and an acid.
Activity 8.2 — magnesium and hydrochloric acid
You will need: a small beaker, a piece of magnesium ribbon, dilute hydrochloric acid, a thermometer
Put about 10 cm3 of dilute hydrochloric acid in the beaker and measure its temperature. Drop in a piece of magnesium ribbon about 2 cm long. When the reaction ends, measure the temperature again.
The observation
The temperature rises: heat is given out as the reaction happens.
Exothermic reaction
A chemical reaction that gives out heat is an exothermic reaction.
reactants → products + heat
Heat is given out because the products have less energy than the reactants. An energy level diagram shows this.
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
heat is given out