• 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 on a thermometer
  • The?temperature?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, the rate may be too slow to observe any appreciable reaction
  • In this case the reaction is?kinetically?controlled
  • This means the reaction could have a high activation energy which is preventing the reaction from taking place.

The enthalpy change 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?temperature?of the?system increases
• There is an enthalpy increase during the reaction so ΔH?is positive

The enthalpy change during an endothermic reaction