1.6.3 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 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 energy of the system increases

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

The enthalpy change during an endothermic reaction

• To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions
• These standard conditions are:
  • A pressure of 100 kPa (you may see some older exam questions that use a figure of 101 kPa; the exact figure is 101 325 Pa, but it has been simplified in the current syllabus for problem-solving purposes)
  • A temperature of 298 K (25 ^oC)
  • Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)

• To show that a reaction has been carried out under standard conditions, the symbol Ꝋ is used
  • ΔH^Ꝋ = the standard enthalpy change

• There are a number of key definitions relating to enthalpy changes that you need to know

Enthalpy Definitions Table

Answer

• • Since two moles of water molecules are formed in the question above, the energy released is simply:

• • ΔH_r^Ꝋ = 2 mol x (-286 kJ mol^-1) = -572 kJ mol^-1

Answer

• • Since two moles of Fe₂O₃ (s) are formed the total change in enthalpy for the reaction above is:
  • ΔH_f^Ꝋ =  2 x ( -824.2 kJ mol^-1) = - 1648 kJ

Answer

Answer 1: ΔH_r^Ꝋ

Answer 2: ΔH_f^Ꝋ as one mole of CO₂ is formed from its elements in standard state and ΔH_c^Ꝋ as one mole of carbon is burnt in oxygen

Answer 3: ΔH_neut^Ꝋ as one mole of water is formed from the reaction between an acid and an alkali