AQA A-level Chemistry — Physical Chemistry 2 flashcards
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A-level physical chemistry for AQA 7405: Born–Haber cycles, entropy and free energy, rate equations and Arrhenius, Kp, electrode potentials and commercial cells, pH, Ka, titration curves and buffers.
Learn deck · part of AQA A-level Chemistry · written against: AQA AS and A-level Chemistry (7404/7405) specification, version 1.2, July 2026 (physical, inorganic and organic). Updated 2026-10-10.
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What is the difference between enthalpy of lattice formation and enthalpy of lattice dissociation?Lattice formation makes one mole of a solid ionic compound from its gaseous ions (exothermic); lattice dissociation is the reverse (endothermic).Physical Chemistry › Thermodynamics
Watch out: the two have the same size but opposite signs, so always check which definition a question uses. -
What is meant by the standard enthalpy of atomisation of chlorine, and what equation represents it?The enthalpy change when one mole of gaseous atoms forms from the element in its standard state: ½ Cl₂(g) → Cl(g).Physical Chemistry › Thermodynamics
It equals half the Cl–Cl bond enthalpy, because one mole of bonds breaking gives two moles of atoms. -
What is the first electron affinity of an element, and what is its usual sign?The enthalpy change when one mole of gaseous atoms each gains one electron to form one mole of gaseous 1− ions; it is usually exothermic.Physical Chemistry › Thermodynamics
Example: Cl(g) + e⁻ → Cl⁻(g); the incoming electron is attracted by the nucleus. -
Why is the second electron affinity of oxygen endothermic?The electron is being added to an O⁻ ion that is already negative, so energy is needed to overcome the repulsion.Physical Chemistry › Thermodynamics
Oxygen's first electron affinity is exothermic, as most first electron affinities are, but every second electron affinity is endothermic. -
Use these data (kJ mol⁻¹) to find the lattice formation enthalpy of NaCl: ΔfH −411; atomisation Na +107; first IE Na +496; atomisation Cl +122; electron affinity Cl −349.−787 kJ mol⁻¹Physical Chemistry › Thermodynamics
By Hess's law, ΔLH = ΔfH − (sum of the other steps) = −411 − (107 + 496 + 122 − 349) = −787 kJ mol⁻¹. -
The Born–Haber lattice enthalpy of silver chloride is noticeably larger than the value calculated from a perfect ionic model. What does this show?Silver chloride has some covalent character.Physical Chemistry › Thermodynamics
The Ag⁺ ion polarises the large Cl⁻ ion, distorting its electron cloud towards the cation; the extra attraction is missing from the perfect ionic model. -
Why does magnesium oxide have a much more exothermic lattice formation enthalpy than sodium chloride?Mg²⁺ and O²⁻ have higher charges and are smaller, so the attraction between them is far stronger.Physical Chemistry › Thermodynamics
The ions in MgO are doubly charged and pack closer together than Na⁺ and Cl⁻. -
What is meant by the enthalpy of hydration of an ion?The enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions.Physical Chemistry › Thermodynamics
It is always exothermic, because the ion attracts the polar water molecules: cations attract the δ− oxygen and anions attract the δ+ hydrogens. -
Lattice dissociation enthalpy of NaCl is +787 kJ mol⁻¹; hydration enthalpies are Na⁺ −406 and Cl⁻ −377 kJ mol⁻¹. What is the enthalpy of solution of NaCl?+4 kJ mol⁻¹Physical Chemistry › Thermodynamics
ΔsolH = lattice dissociation + sum of hydration enthalpies = 787 − 406 − 377 = +4 kJ mol⁻¹, which matches the measured value. -
Why is the hydration enthalpy of Mg²⁺ much more exothermic than that of Na⁺?Mg²⁺ is smaller and has a higher charge, so it has a greater charge density.Physical Chemistry › Thermodynamics
It attracts the lone pairs on the oxygen atoms of water molecules more strongly.
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About these cards
Review status: Second-reader reviewed, with corrections applied.
Checked against:
- Topic outline follows the AQA AS and A-level Chemistry (7404/7405) specification, version 1.2 (July 2026) (outline only; no specification text reproduced).
- OpenStax Chemistry 2e and OpenStax Organic Chemistry: A Tenth Edition, consulted as references; no text copied.
- Also checked against NBS/NIST thermochemical tables (reference only; no text copied).
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