Entropy, Gibbs energy and feasibility
| English | 中文 | Pinyin |
|---|---|---|
| entropy/ˈentrəpi/ | 熵 | shāng |
| Gibbs energy/ɡɪbz ˈenədʒi/ | 吉布斯能 | jí bù sī néng |
What would explain this observation?
- A reaction can be endothermic and still be thermodynamically favourable. Enthalpy alone does not determine the direction favoured at a given temperature.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Entropy 熵 is associated with energy dispersal and the number of accessible microscopic arrangements. Gibbs energy 吉布斯能 combines enthalpy, entropy and absolute temperature for a process at constant temperature and pressure.
- entropy: A state property related to energy dispersal and accessible arrangements; Gibbs energy: A thermodynamic quantity combining enthalpy and entropy contributions.
What can remain small even when ΔG is negative?
Use ΔG = ΔH - TΔS with consistent energy units. A negative Gibbs energy change indicates thermodynamic favourability for the stated conditions, not a fast rate. An activation barrier can make a favourable process slow.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Use ΔG = ΔH - TΔS with consistent energy units. A negative Gibbs energy change indicates thermodynamic favourability for the stated conditions, not a fast rate. An activation barrier can make a favourable process slow.
- State whether values are standard-state quantities and record temperature in kelvin. Convert entropy from joules per kelvin per mole into kilojoules per kelvin per mole when enthalpy is in kilojoules per mole.
Which two habits make the investigation or model in this case more defensible?
State whether values are standard-state quantities and record temperature in kelvin. Convert entropy from joules per kelvin per mole into kilojoules per kelvin per mole when enthalpy is in kilojoules per mole.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: ΔH = +20 kJ per mole, ΔS = +100 J per kelvin per mole and T = 300 K. Convert ΔS = 0.100 kJ per kelvin per mole. ΔG = ΔH - TΔS = 20 - 300×0.100 = -10 kJ per mole. The process is favourable under the stated approximation.
ΔH=30 kJ per mole, ΔS=0.10 kJ per kelvin per mole and T=400 K. Find ΔG. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
ΔH=30 kJ per mole, ΔS=0.10 kJ per kelvin per mole and T=400 K. Find ΔG.
The result is -10 kJ/mol. Known: ΔH = +20 kJ per mole, ΔS = +100 J per kelvin per mole and T = 300 K. Convert ΔS = 0.100 kJ per kelvin per mole. ΔG = ΔH - TΔS = 20 - 300×0.100 = -10 kJ per mole. The process is favourable under the stated approximation.
Check the conclusion and its limits
- Thermodynamic favourability does not establish reaction rate. Celsius cannot replace kelvin in TΔS, and a unit conversion error can change the result by a factor of 1,000.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A thermodynamically favourable reaction must be fast. This claim is false: Thermodynamic favourability does not establish reaction rate. Celsius cannot replace kelvin in TΔS, and a unit conversion error can change the result by a factor of 1,000.
Entropy, Gibbs energy and feasibility: Use ΔG = ΔH - TΔS with consistent energy units. A negative Gibbs energy change indicates thermodynamic favourability for the stated conditions, not a fast rate. An activation barrier can make a favourable process slow.
A thermodynamically favourable reaction must be fast.
Thermodynamic favourability does not establish reaction rate. Celsius cannot replace kelvin in TΔS, and a unit conversion error can change the result by a factor of 1,000.
A state property related to energy dispersal and accessible arrangements: write the technical term.
entropy means A state property related to energy dispersal and accessible arrangements.