• The standard molar enthalpy values,?S^?, relate to standard conditions of temperature and pressure
• The entropy change, ΔS^?, can be calculated from thermodynamic data using the following equation:

ΔS^?₂₉₈(reaction) = ΣS^?₂₉₈(products) - ΣS^?₂₉₈(reactants)

• • This equation is provided in the data booklet
• The units of ΔS_system^??are in?J K^-1?mol^–1
• Entropy will change depending on the state of the matter
• Taking water as an example the values for?S^??will be different for the liquid and gaseous phases
  • S^?₂₉₈(H₂O (l)) = 70.0 J K^-1?mol^–1
  • S^?₂₉₈(H₂O (g)) = 188.8 J K^-1?mol^–1
• When calculating ΔS^?, the coefficients used to balance the equation must be applied when calculating the overall entropy change
• For example, when calculating the ΔS^??for the reaction below we need to double the value for?S^??(NO (g))
  • N₂O₄?(g) →?2NO₂?(g)
  • ΔS^?₂₉₈(reaction) = ΣS^?₂₉₈(products) - ΣS^?₂₉₈(reactants)
  • ΔS^??= [(2?x?S^?₂₉₈(NO₂)] -?S^?₂₉₈(N₂O₄)

Answer:

Step 1:?Write out equation to calculate ΔS^?₂₉₈(reaction)

• • • ΔS^?₂₉₈(reaction) = ΣS^?₂₉₈(products) - ΣS^?₂₉₈(reactants)

Step 2:?Substitute in formulas and then values for?S^?

• • • ΔS^?₂₉₈(reaction) = [S^?₂₉₈(CaO) +?S^?₂₉₈(CO₂)] -?S^?₂₉₈(CaCO₃)
    • ΔS^?(reaction) = (39.7 + 213.6) - 92.9
    • ΔS^?(reaction) =?+160.4 J K^-1?mol^–1

Answer:

Step 1:?Write out equation to calculate ΔS^?₂₉₈(reaction)

• • • ΔS^?₂₉₈(reaction) = ΣS^?₂₉₈(products) - ΣS^?₂₉₈(reactants)

Step 2:?Substitute in formulas and then values for?S^??taking into account the coefficients

• • • ΔS^?₂₉₈(reaction) = [2 x?S^?₂₉₈(NH₃)]?- [S^?₂₉₈(N₂)+ (3 x?S^?₂₉₈(H_2?))]
    • ΔS^?₂₉₈(reaction) = [2 x 192.3] - [191.6 + (3 x 131)]
    • ΔS^?₂₉₈(reaction) = 384.6 - 584.6
    • ΔS^?₂₉₈(reaction) =?-200 J K^-1?mol^–1