Entropy and ionic energy cycles
Введено| English | Значение |
|---|---|
| entropy/ˈentrəpi/ | энтропия |
| lattice formation | образование кристаллической решётки |
| hydration/haɪˈdreɪʃn/ | гидратация |
| covalent character | ковалентный характер |
Why can an endothermic process occur?
- A cold dissolving mixture can change spontaneously even while taking in heat. Enthalpy alone cannot decide feasibility.
- Follow entropy 熵 in both the reacting system and its surroundings.
Add the two entropy terms
- $\Delta S_{total}=\Delta S_{system}+\Delta S_{surroundings}$ and $\Delta S_{surroundings}=-\Delta H/T$.
- Use products minus reactants with coefficients for system entropy. Convert kJ to J before combining with entropy in joules per mole per kelvin.
System entropy alone always decides whether the forward direction is thermodynamically favoured.
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The system and surroundings changes must be added.
Find a feasibility boundary
- For ΔH = +36,000 J per mole and system entropy = +120 J per mole per kelvin, total entropy is 120 − 36000/T.
- The boundary is 300 K; above it total entropy is positive under the constant-data approximation. A favourable direction need not be fast.
At 298 K, ΔH = −40,000 J per mole gives surroundings entropy +134 J per mole per kelvin. Adding system entropy −100 gives total +34.
For ΔH = +24,000 J per mole and system entropy +80 J per mole per kelvin, find the zero-total-entropy boundary temperature in kelvin.
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Set 80 − 24000/T = 0; T = 24000/80 = 300 K.
A large activation barrier can give kinetic _____ even when reaction is thermodynamically favoured.
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Kinetic stability concerns slow reaction, whereas thermodynamic stability concerns the favoured state.
Declare the lattice convention
- Lattice formation 晶格形成 makes one mole of ionic solid from gaseous ions and has a negative enthalpy. Dissociation reverses that route and sign.
- A Born–Haber cycle must count the actual ions: MgCl₂ needs two chlorine atomisations and electron affinities and both magnesium ionisations.
Which is lattice formation for NaCl?
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Lattice formation uses gaseous ions. The third equation is formation from the elements.
Combine lattice and hydration 水合
- Solution enthalpy reverses lattice formation and adds hydration enthalpies. With −850 and −820 kJ per mole, the result is +30 kJ per mole.
- More negative lattice and hydration terms can compete. Solubility also involves entropy, while extra lattice stabilisation beyond a purely ionic model supports covalent character 共价性.
“Negative system entropy makes every reaction impossible” is false. A sufficiently positive surroundings term can outweigh it.
State the reaction direction, units and lattice convention before substituting numbers.
Which two statements are correct?
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Dissolution separates the solid into gaseous ions before hydration in the cycle. Entropy also matters. Second affinity adds to an already negative ion.