熵变与离子能量循环
引入| English | 释义 | 拼音 |
|---|---|---|
| entropy/ˈentrəpi/ | 熵 | shāng |
| lattice formation | 晶格形成 | jīng gé xíng chéng |
| hydration/haɪˈdreɪʃn/ | 水合 | shuǐ hé |
| covalent character | 共价性 | gòng jià xìng |
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.
不能仅凭系统熵变判断正反应方向在热力学上是否有利。
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必须将系统熵变与环境熵变相加。
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.
若ΔH = +24,000 J·mol⁻¹，系统熵变ΔS = +80 J·mol⁻¹·K⁻¹，求总熵变为零时的临界温度（单位：K）。
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令80 − ⟨24000 / T⟩ = 0；解得T = 24000 / 80 = 300 K。
即使反应在热力学上有利，较大的活化能垒也会导致动力学 _____。
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动力学稳定性涉及缓慢的反应，而热力学稳定性涉及有利的状态。
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.
下列哪个方程式代表NaCl的晶格形成过程？
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晶格形成涉及气态离子。第三个方程式表示由单质生成化合物。
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.
哪两个陈述是正确的？
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在循环中，溶解过程先将固体分离为气态离子，然后再进行水合。熵也很重要。第二电子亲和势会使一个已经带负电的离子结合电子。