Calculating Standard Entropy Changes (HL) (DP IB Chemistry): Revision Note

• This equation is not provided in the data booklet and should be learned

• Standard entropy values (S^⦵) for selected substances are listed in the IB Chemistry data booklet (Section 13)

What is the entropy change when calcium carbonate decomposes?

CaCO₃ (s) → CaO (s) + CO₂ (g)

• S^ꝋ₂₉₈(CaCO₃ (s)) = 93 J K^-1 mol^–1

• S^ꝋ₂₉₈(CaO (s)) = 40 J K^-1 mol^–1

• S^ꝋ₂₉₈(CO₂ (g)) = 214 J K^-1 mol^–1

Answer:

Step 1: Write out the equation to calculate ΔS^ꝋ₂₉₈(reaction)

• ΔS^ꝋ₂₉₈(reaction) = ΣS^ꝋ₂₉₈(products) - ΣS^ꝋ₂₉₈(reactants)

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

• ΔS^ꝋ₂₉₈(reaction) = [S^ꝋ₂₉₈(CaO) + S^ꝋ₂₉₈(CO₂)] - S^ꝋ₂₉₈(CaCO₃)

• ΔS^ꝋ(reaction) = (40 + 214) - 93

• ΔS^ꝋ(reaction) = +161 J K^-1 mol^–1 

What is the entropy change when ammonia is formed from nitrogen and hydrogen?

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• S^ꝋ₂₉₈(N₂ (g)) = 192 J K^-1 mol^–1

• S^ꝋ₂₉₈(H₂ (g)) = 131 J K^-1 mol^–1

• S^ꝋ₂₉₈(NH₃) = 192 J K^-1 mol^–1

Answer:

Step 1: Write out the equation to calculate ΔS^ꝋ₂₉₈(reaction)

• ΔS^ꝋ₂₉₈(reaction) = ΣS^ꝋ₂₉₈(products) - ΣS^ꝋ₂₉₈(reactants)

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

• ΔS^ꝋ₂₉₈(reaction) = [2 x S^ꝋ₂₉₈(NH₃)] - [S^ꝋ₂₉₈(N₂)+ (3 x S^ꝋ₂₉₈(H₂ ))]

• ΔS^ꝋ₂₉₈(reaction) = [2 x 192] - [192 + (3 x 131)]

• ΔS^ꝋ₂₉₈(reaction) = 384 - 584

• ΔS^ꝋ₂₉₈(reaction) = -200 J K^-1 mol^–1

• A negative ΔS^⦵ means the system becomes more ordered