Entropy Change, ΔS (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define entropy.
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Cards in this collection (18)
Define entropy.
Entropy (S) is a measure of the number of possible arrangements of particles and their energy in a system. It is a measure of disorder: the greater the disorder, the higher the entropy.
True or False?
Gases have greater entropy than liquids, which in turn have greater entropy than solids.
True.
The order of entropy is: gas > liquid > solid. Gases are most disordered, solids are most ordered.
Explain why the decomposition of CaCO3 results in an increase in entropy.
CaCO3 (s) → CaO (s) + CO2 (g)
A gaseous CO2 molecule is formed from a solid. Gases are far more disordered than solids, so the number of possible arrangements increases and entropy increases.
In the Haber process: N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Entropy .......... because the number of gas molecules .......... from 4 to 2.
In the Haber process: N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Entropy decreases because the number of gas molecules decreases from 4 to 2.
What happens to the entropy of a substance when it melts, and why?
Entropy increases when a substance melts. The regularly arranged lattice of particles becomes a more randomly arranged liquid in which particles can rotate and slide over each other, increasing disorder.
True or False?
Dissolving a solid in a solvent causes a decrease in entropy.
False.
Dissolving a solid increases entropy. The particles spread out from the ordered solid lattice into a more disordered solution, increasing the number of possible arrangements.
State the order of entropy for the three physical states of matter.
gas > liquid > solid
Gases have the highest entropy (most disordered) and solids have the lowest entropy (most ordered).
When a system becomes more disordered, its .......... increases.
When a system becomes more disordered, its entropy increases.
Why does entropy decrease during crystallisation of a salt from solution?
During crystallisation, particles move from a disordered solution into a regular, ordered solid lattice. The number of possible arrangements decreases, so entropy decreases.
State the equation used to calculate the standard entropy change of a system.
ΔSsystemθ = ΣΔSproductsθ - ΣΔSreactantsθ
True or False?
All elements have standard molar entropy values of zero.
False.
All elements have positive standard molar entropy values. Zero entropy is only reached at absolute zero (0 K).
For the reaction 2Mg (s) + O2 (g) → 2MgO (s):
ΔSsystemθ = (.......... × Sθ[MgO]) - (.......... × Sθ[Mg] + Sθ[O2])
For the reaction 2Mg (s) + O2 (g) → 2MgO (s):
ΔSsystemθ = (2 × Sθ[MgO]) - (2 × Sθ[Mg] + Sθ[O2])
Calculate ΔSsystemθ for: 2Mg (s) + O2 (g) → 2MgO (s)
Sθ[Mg (s)] = 32.60, Sθ[O2 (g)] = 205.0, Sθ[MgO (s)] = 38.20 (all J K-1 mol-1)
ΔSsystemθ = (2 × 38.20) - (2 × 32.60 + 205.0)
= 76.40 - 270.2
= -193.8 J K-1 mol-1
True or False?
The stoichiometry of a balanced equation must be taken into account when calculating entropy changes.
True.
Each entropy value must be multiplied by the corresponding stoichiometric coefficient in the balanced equation before summing.
What are the units of standard entropy and standard entropy change?
J K-1 mol-1
For N2 (g) + 3H2 (g) ⇌ 2NH3 (g):
ΔSsystemθ = (2 × Sθ[NH3]) - (Sθ[N2] + .......... × Sθ[H2])
For N2 (g) + 3H2 (g) ⇌ 2NH3 (g):
ΔSsystemθ = (2 × Sθ[NH3]) - (Sθ[N2] + 3 × Sθ[H2])
Why is a negative value of ΔSsystemθ expected for the reaction 2Mg (s) + O2 (g) → 2MgO (s)?
A gas molecule (O2) is consumed and no gas is produced. The products are all solid, which is more ordered than the reactants. The number of possible arrangements decreases, so ΔSsystemθ is negative.
True or False?
The state of the compounds must be considered when calculating entropy changes.
True.
Entropy values differ significantly between states (gas > liquid > solid), so using the correct state symbol for each compound is essential for an accurate calculation.
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