Question
In which of the following reactions, standard reaction entropy changes (ΔSo) is positive and standard Gibb's energy change (ΔGo) decreases sharply with increasing temperature?
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C (graphite) +1/2 O2 (g) → CO (g)
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CO (g) +1/2 (g) → CO2 (g)
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Mg(s) +1/2O2 (g) → MgO (s)
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1/2 C (graphite) +1/2 O2 (g) → 1/2 CO2 (g)
Solution
A.
C (graphite) +1/2 O2 (g) → CO (g)
Among the given reactions only in te case of
C (graphite) +1/2 O2 (g) → CO (g)
entropy increases because randomness (disorder) increases. Thus, standard entropy change (ΔSo) is positive.
Moreover, it is a combustion reaction and all the combustion reactions are generally exothermic, ie. ΔHo=-ve
We know that
ΔGo = ΔHo-TΔSo
ΔGo = -ve-T(+ve)
Thus, as the temperature increases, the value of ΔGo decreases.
