Calculating Standard Entropy Changes (HL) (DP IB Chemistry): Revision Note
Calculating Standard Entropy Changes
How to calculate ΔS⦵
The standard molar enthalpy values, Sꝋ, refer to to substances in their standard states at 298.15 K and 100 kPa
The entropy change, ΔSꝋ, can be calculated from thermodynamic data using the following equation:
ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)
Examiner Tips and Tricks
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)
The units of ΔSsystemꝋ are in J K-1 mol–1
Entropy changes depending on the state of the matter
For example, water has different Sꝋ values for the liquid and gaseous phases
Sꝋ298(H2O (l)) = 70 J K-1 mol–1
Sꝋ298(H2O (g)) = 189 J K-1 mol–1
When calculating ΔSꝋ, the coefficients used to balance the equation must be applied when calculating the overall entropy change
For example, when calculating the ΔSꝋ for the reaction below we need to double the value for Sꝋ (NO2 (g))
N2O4 (g) → 2NO2 (g)
ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)
ΔSꝋ = [(2 x Sꝋ298(NO2)] - Sꝋ298(N2O4)
Worked Example
What is the entropy change when calcium carbonate decomposes?
CaCO3 (s) → CaO (s) + CO2 (g)
Sꝋ298(CaCO3 (s)) = 93 J K-1 mol–1
Sꝋ298(CaO (s)) = 40 J K-1 mol–1
Sꝋ298(CO2 (g)) = 214 J K-1 mol–1
Answer:
Step 1: Write out the equation to calculate ΔSꝋ298(reaction)
ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)
Step 2: Substitute in formulas and then values for Sꝋ
ΔSꝋ298(reaction) = [Sꝋ298(CaO) + Sꝋ298(CO2)] - Sꝋ298(CaCO3)
ΔSꝋ(reaction) = (40 + 214) - 93
ΔSꝋ(reaction) = +161 J K-1 mol–1
Worked Example
What is the entropy change when ammonia is formed from nitrogen and hydrogen?
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Sꝋ298(N2 (g)) = 192 J K-1 mol–1
Sꝋ298(H2 (g)) = 131 J K-1 mol–1
Sꝋ298(NH3) = 192 J K-1 mol–1
Answer:
Step 1: Write out the equation to calculate ΔSꝋ298(reaction)
ΔSꝋ298(reaction) = ΣSꝋ298(products) - ΣSꝋ298(reactants)
Step 2: Substitute in formulas and then values for Sꝋ taking into account the coefficients
ΔSꝋ298(reaction) = [2 x Sꝋ298(NH3)] - [Sꝋ298(N2)+ (3 x Sꝋ298(H2 ))]
ΔSꝋ298(reaction) = [2 x 192] - [192 + (3 x 131)]
ΔSꝋ298(reaction) = 384 - 584
ΔSꝋ298(reaction) = -200 J K-1 mol–1
A negative ΔS⦵ means the system becomes more ordered
You've read 0 of your 5 free revision notes this week
Unlock more, it's free!
Did this page help you?