Entropy, $S$, increases most when a substance changes from:
Solid to gas
Gas to liquid
Liquid to solid
Gas to solid
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WhyEntropy measures disorder. A gas is far more disordered than a solid, so a solid-to-gas change gives the largest increase in entropy.
2Multiple choice · Medium
Which equation relates Gibbs free energy to enthalpy and entropy?
$\Delta G = \Delta H - T\Delta S$
$\Delta G = \Delta H + T\Delta S$
$\Delta G = T\Delta S - \Delta H$
$\Delta G = \Delta H \times T\Delta S$
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WhyThe Gibbs free energy change is $\Delta G = \Delta H - T\Delta S$, where $T$ is the absolute temperature.
3Multiple choice · Hard
A reaction has $\Delta H = +50\,\text{kJ mol}^{-1}$ and $\Delta S = +200\,\text{J K}^{-1}\text{mol}^{-1}$. At which temperature does it become spontaneous?
Above $250\,\text{K}$
Below $250\,\text{K}$
At all temperatures
At no temperature
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WhyIt is spontaneous when $\Delta G < 0$, i.e. $T > \dfrac{\Delta H}{\Delta S} = \dfrac{50000}{200} = 250\,\text{K}$. So it is spontaneous above $250\,\text{K}$.
4Fill in the blank · Medium
A chemical reaction is spontaneous (feasible) when the value of $\Delta G$ is (positive or negative).
Answer:
negative / less than zero
WhyA reaction is thermodynamically spontaneous when $\Delta G < 0$, i.e. negative.
5Multiple choice · Medium
Which change is expected to have a positive entropy change (deltaS > 0)?
A reaction that produces gas from solids and liquids
A gas condensing to a liquid
A gas being adsorbed onto a solid
Two gases combining to form one gas
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WhyProducing gas from condensed phases increases disorder, giving a positive entropy change.
6Multiple choice · Medium
For a reaction in which gases are consumed to form a solid, the entropy change of the system is
negative
positive
zero
always greater than deltaH
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WhyLoss of gaseous particles reduces disorder, so deltaS is negative.
7Fill in the blank · Medium
The gaseous state has a entropy than the solid or liquid state.
Answer:
higher / greater
WhyGases are the most disordered, with the highest entropy.
8Fill in the blank · Medium
A positive deltaS corresponds to an in the disorder of the system.
Answer:
increase
WhyPositive entropy change means increased disorder.
9Multiple choice · Medium
Which combination guarantees that a reaction is spontaneous at all temperatures?
deltaH < 0 and deltaS > 0
deltaH > 0 and deltaS < 0
deltaH > 0 and deltaS > 0
deltaH < 0 and deltaS < 0
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WhydeltaG = deltaH - T deltaS is always negative when deltaH is negative and deltaS is positive.
10Multiple choice · Hard
An exothermic reaction has deltaS < 0. The favorability of this reaction is driven by
enthalpy, not entropy
entropy, not enthalpy
both enthalpy and entropy equally
neither term
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WhyWith deltaG = deltaH - T deltaS, the negative deltaH (enthalpy) drives favorability while the negative deltaS opposes it.
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