For each of the two reactions below, use the thermodynamic data given in the tab

Question

For each of the two reactions below, use the thermodynamic data given in the table on the next page to answer the following:

i. Is the reaction exothermic or endothermic?

ii. Is it spontaneous at 298 K?

iii. Will raising the temperature make the reaction more or less thermodynamically favourable?

iv. In each case, explain why the reaction is spontaneous or not spontaneous by explaining why the enthalpy and entropy changes are large, small, positive or negative.

Reaction 1: 2As(g) + 3H2(g) 2AsH3 (g)

Reaction 2: CuSO4•5H2O(s) CuSO4(s) + 5H2O(g)

As(g) H20g) AsH3(g) CuSO4 5H20(s) CuSO4(s) H20 (g) AH (kJ mol t) 302.5 0.0 66.4 2279.7 771.36 241.82 (J mol K 174.21 130.684 222.78 300.4 109 188.83

Explanation / Answer

I. Heat of reaction is given by sum of formation enthalpy of products-sum of formation enthalpy of reactants.

For reaction 1:- reaction enthalpy=2(66.4)-302.5=-169.7

So reaction is exothermic

For reaction 2:- reaction enthalpy=(771.36)+(-5×241.82)-(-2279.7)=299.24

So reaction is endothermic.

II. Reaction 1:-spontaneous at 298K as G=H-TS is negative.

Reaction 2:- non spontaneous at 298K as G is positive.

III. Reaction 1:- increase temperature would make reaction less favourable.

Reaction 2:- increase in temperature would make reaction more favourable.

IV. Reaction 1 is spontaneous due to negative enthalpy which makes G negative

Reaction 2 is nonspontaneous due to positive enthalpy change and negative entropy change which makes G positive.

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