Enthalpy

KEY POINTS:

• Every reaction involves a change in the amount of energy that is present. The reactants moving to products is sometimes referred to as the system.
• The internal energy of a substance is referred to as the enthalpy (symbol H), therefore reactions have enthalpy changes (ΔH).
• We cannot measure the internal energy of substance directly. We can only measure enthalpy changes as indicated by the change in temperature of the surroundings of a reaction.
• For many reactions, the surroundings consist of the air immediately around the reactants. However for reactions occurring in aqueous solutions, the surroundings are the water of the solutions.
• If the reaction is exothermic – heat energy is given out to the surroundings, and the temperature of the surroundings rises. The enthalpy change is negative i.e. ΔH is negative. The products are at a lower energy level than the reactants.
• If the reaction is endothermic – heat energy is taken in from the surroundings, and the temperature of the surroundings decreases. The enthalpy change is positive i.e. ΔH is positive. The products are at a higher energy level than the reactants.