Exothermic & Endothermic Reactions (HL IB Chemistry)

• 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 enthalpy than the reactants
• Heat energy is given off by the system to the surroundings
  • The temperature of the surroundings increases
  • 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, if the rate is too slow, the reaction may not occur.
  • In this case the reaction is kinetically controlled

Energy level diagram for an exothermic reaction

The enthalpy change during an exothermic reaction is negative

Endothermic reactions

• A reaction is endothermic when the products have more enthalpy than the reactants
• Heat energy is absorbed by the system from the surroundings
  • The temperature of the surroundings decreases
  • The temperature of the system increases

• There is an enthalpy increase during the reaction so ΔH is positive

Energy level diagram for an endothermic reaction

The enthalpy change during an endothermic reaction is positive