Endothermic & Exothermic 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
The enthalpy changes during an exothermic reaction
Endothermic reactions
- A reaction is endothermic when the products have more energy than the reactants
- Heat energy is absorbed by the reaction from the surroundings
- The temperature of the environment decreases - this can be measured with a thermometer
- The energy of the system increases
- There is an enthalpy increase during the reaction so ΔH is positive