Exothermic & Endothermic Reactions (Oxford AQA IGCSE Chemistry)

Exothermic & Endothermic Reactions

• During chemical reactions, energy is either transferred to or taken in from the surroundings, usually in the form of heat

• The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, of the reaction

• ΔH is either a positive or a negative value depending on whether the reaction is exothermic or endothermic 

Exothermic Reactions

• In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases

• Combustion, oxidation, and neutralisation reactions are typical exothermic reactions

  • Hand warmers used in the wintertime are based on the release of heat from an exothermic reaction

  • Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers

• The enthalpy change for an exothermic reaction is negative due to energy being lost to the surroundings

  Exothermic reaction diagram

Endothermic Reactions

• In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases

• These types of reactions are much less common than exothermic reactions

  • Thermal decomposition, whereby a substance is broken down using heat, is a common example of an endothermic reaction

  • Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling

• The enthalpy change for an endothermic reaction is positive due to energy being gained from the surroundings

  Endothermic Reaction Diagram

  

Energy Changes in Reversible Reactions

• A reversible reaction is one in which the products of the reaction can react to produce the original reactants

• The symbol for a reversible reaction is ⇌

• Reversible reactions can be represented as:

  A + B ⇌ C + D

  • The reaction to produce the products is known as the forward reaction

  • The reaction to reform the reactants is known as the reverse reaction

• If the forward reaction is exothermic, then the reverse reaction will be endothermic 

  • The same amount of heat is transferred in both directions

• A good example of this is the hydration of anhydrous copper(II) sulfate:

  • When anhydrous copper(II) sulfate crystals are added to water, they turn blue 

    • The forward reaction is exothermic 

  • If the hydrated copper(II) sulfate crystals are then heated, the blue crystals form a white powder 

    • The reverse reaction is endothermic

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

Diagram to show the reversible reaction of copper(II) sulfate