Summary AQA AS Chemistry Revision Notes 2022 updated

Enthalpy Changes The total chemical energy inside a substance is called the enthalpy (or heat content) When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy) An enthalpy change can be positive or negative Exothermic reactions A reaction is exothermic when the products have less energy than the reactants Heat energy is given off by the reaction to the surroundings The temperature of the environment increases – this can be measured with a thermometer The energy of the system decreases There is an enthalpy decrease during the reaction so ΔH is negative Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products) However, if the rate is too slow, the reaction may not occur In this case the reaction is kinetically controlled