Enthalpy Changes (AQA AS Chemistry): Revision Note

Enthalpy Changes

• The total chemical energy contained within a substance is called its enthalpy (or heat content)

• When a chemical reaction occurs, there is a change in chemical energy, and therefore a change in enthalpy

• An enthalpy change is represented by the symbol ΔH, where Δ means change and H represents enthalpy

• An enthalpy change can be either positive or negative

Exothermic reactions

• A reaction is exothermic when the products have less energy than the reactants

• During an exothermic reaction, heat energy is released to the surroundings, causing the temperature of the environment to increase

  • This temperature change can be measured using a thermometer

  • As energy is transferred to the surroundings, the energy of the system decreases

• There is therefore a decrease in enthalpy, and the enthalpy change (ΔH) is negative

• Exothermic reactions are thermodynamically possible because the reactants have a higher enthalpy than the products

• However, if the reaction rate is too slow, the reaction may not occur under the given conditions

  • In this case, the reaction is said to be kinetically controlled

Endothermic reactions

• A reaction is endothermic when the products have more energy than the reactants

• During an endothermic reaction, heat energy is absorbed from the surroundings, causing the temperature of the environment to decrease

  • This temperature change can be measured using a thermometer

  • As energy is absorbed, the energy of the system increases

• There is therefore an increase in enthalpy, and the enthalpy change (ΔH) is positive.

Standard Enthalpy Changes

• To allow meaningful comparison of enthalpy changes between reactions, all thermodynamic measurements are carried out under standard conditions

• These standard conditions are:

  • A pressure of 100 kPa (older exam questions may use 101 kPa; the exact value is 101 325 Pa, but this has been simplified in the current syllabus)

  • A temperature of 298 K (25 °C)

  • All substances in their standard physical states (solid, liquid, or gas)

• To indicate that a reaction has been carried out under standard conditions, the symbol ^Ꝋ is used.

• represents the standard enthalpy change

• There are several important definitions related to enthalpy changes that you need to know

Enthalpy Definitions Table