(a) Give the formula indicating the relationship between entropy, free energy and enthalpy changes of a system. (b) For each of the following, state whether...
Question 1 Report
(a) Give the formula indicating the relationship between entropy, free energy and enthalpy changes of a system.
(b) For each of the following, state whether entropy change is positive, negative or zero.
(i) H\(_2\)O\(_{(g)}\) -> H\(_2\)O\(_{(g)}\)
(ii) Cl\(_{2(g)}\) ---> 2CI\(_{(g)}\)
(iii) HCI\(_{(g)}\) -> HCl\(_{(g)}\)
Answer Details
(a) Relationship between free energy, enthalpy and entropy changes
The Gibbs-Helmholtz relationship is:
\[\Delta G = \Delta H - T\Delta S\]where \(\Delta G\) is the free energy change, \(\Delta H\) the enthalpy change, \(\Delta S\) the entropy change and \(T\) the absolute (Kelvin) temperature. A reaction is spontaneous (feasible) when \(\Delta G\) is negative.
(b) Sign of the entropy change
Entropy is a measure of disorder. Disorder increases when the number of gaseous molecules increases; it is unchanged when the state and number of particles are unchanged.
- (i) \(H_2O_{(g)} \to H_2O_{(g)}\): the substance stays in the same physical state with the same number of gaseous molecules, so the entropy change is zero.
- (ii) \(Cl_{2(g)} \to 2Cl_{(g)}\): one mole of gas becomes two moles of gas, so disorder increases and the entropy change is positive.
- (iii) \(HCl_{(g)} \to HCl_{(g)}\): no change in state or number of molecules, so the entropy change is zero.
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