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Early Conquests of Ranjit Singh

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Question

Represent the potential energy/enthalpy change in the following processes graphically. In which of the processes potential energy/enthalpy change is a contributing factor to the spontaneity?

$A.$ Throwing a stone from the ground to the roof.

$B. \frac{1}{2} H_{2} (g) + \frac{1}{2} C l_{2} (g) ⇌ H C l (g) Δ_{r} H^{⊖} = - 92.32 k J m o l^{- 1}$

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Solution

$A.$ For the process involved in $(a)$ , Throwing a stone from the ground to roof, the potential energy will be increasing as the stone would cover greater distance from the ground, graphically it can be represented as:

In this process, potential energy change is not a contributing factor to the spontaneity as it will require some external work to toss stone from low potential energy to high potential energy.

The spontaneous reactions are the ones which occur on their own i.e. no external work is required for them to occur.

$B.$ For the reaction: $\frac{1}{2} H_{2} (g) + \frac{1}{2} C l_{2} (g) ⇌ H C l (g)$ $Δ_{r} H^{⊖} = - 92.32 k J m o l^{- 1}$ ,

So, it is an exothermic reaction as heat is evolved and the $Δ H$ value (enthalpy of reaction) is negative. The graphical representation will be:

Amongst those two processes, in the reaction $(b)$ , enthalpy change (i.e. $Δ H$ ) is a contributing factor to the spontaneity, as negative value of enthalpy change favours the negative value of gibb’s free energy change, which led to the spontaneous reaction.

The spontaneous reactions are the ones which occurs on their own, without intervention.

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