(a) Give the formula indicating the relationship... | WAEC SSCE - Chemistry - 1990 (Essay)

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

(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.

Read lesson note on Energy And Energy Changes (WAEC)

Answer Details

(a) Relationship between free energy, enthalpy and entropy changes

The Gibbs-Helmholtz relationship is:

\[\Delta G = \Delta H - T\Delta S\]

(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.

Read lesson note on Energy And Energy Changes (WAEC)

Energy And Energy Changes

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