Given the enthalpies of reaction: 2p(g) + 3cl2(g) → 2pcl3(g) dh = –574 kj 2p(g) + 5cl2(g) → 2pcl5(g) dh = –887 kj what is the enthalpy change of the following reaction: pcl3(g) + cl2(g) → pcl5(g

Explanation: It's based on Hess's law. We will rearrange the two given equations and make the equation to which it asks to calculate the enthalpy change.

First equation needs to be reversed and divided by 2 so that we could get one on reactant side. When we reversed an equation then the sign of  enthalpy change is also changed.

Second equation also needs to be divided by 2 and its not reversed.

Both the equations are added now:

So, the enthalpy change for the equation is -156.5 kJ.

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