Entropy and ionic energy cycles
Introduced| English | Meaning |
|---|---|
| entropy/ˈentrəpi/ | A thermodynamic measure related to energy dispersal and the number of microscopic arrangements consistent with a system's macroscopic state. |
| lattice formation | The exothermic coming-together of gaseous ions into an ionic lattice. |
| hydration/haɪˈdreɪʃn/ | Association or reaction of a substance with water, including water molecules surrounding dissolved ions. |
| covalent character | The degree to which an ionic bond is distorted toward electron sharing by polarisation. |
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Set 80 − 24000/T = 0; T = 24000/80 = 300 K.
A large activation barrier can give kinetic _____ even when reaction is thermodynamically favoured.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Dissolution separates the solid into gaseous ions before hydration in the cycle. Entropy also matters. Second affinity adds to an already negative ion.