3.4.1 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 changes 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

• An energy level diagram is a diagram that shows:
  • The energy level of the reactants
  • The transition state(s) - an unstable intermediate in the reaction which cannot be isolated and is higher in energy than the reactants and products
  • The energy level of the products
  • The activation energy (E_a)
    • The minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction
  • The enthalpy change for the reaction (ΔH) 
    • This can be describes as the overall energy taken in from / given out to the surroundings OR the energy difference from reactants to products

The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen chloride gas

Answer

Step 1: The chemical equation for the complete combustion of methane is:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

Step 2: Combustion reactions are always exothermic (ΔH is negative) so the reactants should be drawn higher in energy than the products

Step 3: Draw the curve in the energy level diagram

Step 4: Draw arrows to show the E_a and ΔH  including their values

Answer

• • The E_a  is the energy difference from the energy level of the reactants to the top of the ‘hump’
  • E_a (forward reaction) = (+70 kJ mol^-1) + (+ 20 kJ mol^-1 ) = +90 kJ mol^-1

• 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) 
    • ΔH_r^Ꝋ = -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)
    • ΔH_f^Ꝋ = - 1648 kJ mol^-1

Answers

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