AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
∆S accounts for the above deficiency, illustrated by physical changes and…
Full specification wording
Gibbs free-energy change, ∆G, and entropy change, ∆S (A-level only): ∆S accounts for the above deficiency, illustrated by physical changes and chemical changes.
Final outcome: Dissolving can have a negative ∆S due to hydration ordering, though physical changes often increase entropy.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Final outcome: Dissolving can have a negative ∆S due to hydration ordering, though physical changes often increase entropy. Initial logic and parameters are validated. Melting and boiling increase particle freedom, giving positive ∆S. Chemical changes follow moles of gas: producing more gas moles gives a positive ∆S. However, dissolving is complex. While dissolving ammonium nitrate gives a positive ∆S (+108 J K⁻¹ mol⁻¹), dissolving ions with high charge density (like CaCl₂) can result in a negative ∆S (e.g., ≈ −45 J K⁻¹ mol⁻¹) because water molecules become highly ordered around the ions, overcoming the entropy increase of the solid lattice breaking down. A positive ∆S can drive an endothermic process.
What earns the marks
- Final result: Dissolving can yield a negative ∆S if the ordering of water molecules during hydration outweighs the disordering of the lattice.
- Established context: ∆S is positive when a solid melts or vaporises due to increased particle freedom.
- Established context: The change in the number of moles of gas is the main influence on ∆S in chemical reactions.
- Final transformation: A positive ∆S can make an endothermic process feasible by offsetting the enthalpy change.
Common mistakes and how to put them right
Final fix: Assuming dissolving always has a positive ∆S.
Put it right by recognising that hydration of highly charged ions can cause a net negative ∆S.
Established context: Ranking entropy increase on melting as larger than boiling.
Put it right by noting gas formation gives the largest increase.
Established context: Ignoring state symbols when judging ∆S.
Put it right by counting moles of gas.
Exam tips
- Final tip: Evaluate both lattice breakdown and hydration ordering when predicting ∆S for dissolving ionic solids.
- Established context: Highlight state symbols in the equation before predicting the sign of ∆S.
Try the lesson
The start of the Gibbs free-energy change, ∆G, and entropy change, ∆S (A-level only): ∆S accounts for the above deficiency, illustrated by physical changes and chemical changes. lesson, made from checked teaching notes for this specification point.
Have a go first
Which physical change has a positive value of ∆S?
Listen
Final outcome: Dissolving can have a negative ∆S due to hydration ordering, though physical changes often increase entropy. Initial logic and parameters are validated. Melting and boiling increase particle freedom, giving positive ∆S. Chemical changes follow moles of gas: producing more gas moles gives a positive ∆S. However, dissolving is complex. While dissolving ammonium nitrate gives a positive ∆S (+108 J K⁻¹ mol⁻¹), dissolving ions with high charge density (like CaCl₂) can result in a negative ∆S (e.g., ≈ −45 J K⁻¹ mol⁻¹) because water molecules become highly ordered around the ions, overcoming the entropy increase of the solid lattice breaking down. A positive ∆S can drive an endothermic process. A strong answer earns marks for these points. Final result: Dissolving can yield a negative ∆S if the ordering of water molecules during hydration outweighs the disordering of the lattice. Established context: ∆S is positive when a solid melts or vaporises due to increased particle freedom. Established context: The change in the number of moles of gas is the main influence on ∆S in chemical reactions. Final transformation: A positive ∆S can make an endothermic process feasible by offsetting the enthalpy change. A common slip: Final fix: Assuming dissolving always has a positive ∆S. Instead, recognising that hydration of highly charged ions can cause a net negative ∆S. Exam tip: Final tip: Evaluate both lattice breakdown and hydration ordering when predicting ∆S for dissolving ionic solids.
See it
Key words
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Hydration ordering
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Lattice disordering
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Moles of gas
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Positive ∆S
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See every definition
- Hydration ordering
- Water molecules become highly ordered around ions with high charge density, which can outweigh the disordering of the solid lattice breaking down.
- Lattice disordering
- The entropy increase caused by the solid lattice breaking down when a substance dissolves.
- Moles of gas
- The change in the number of moles of gas is the main influence on ∆S in chemical reactions.
- Positive ∆S
- A positive ∆S can make an endothermic process feasible by offsetting the enthalpy change.
Label the diagram
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