Consider this reaction:
pcl5(g) + heat pcl3 (g) + cl2 (g)
what will happen when the pressure is decreased?
there will not be a shift in equilibrium.
there will be a shift toward the products.
there will be a shift toward the reactants.
there is not enough information to make a prediction.

The answer is There will be a shift toward the products.

To solve this problem, it is necessary to understand Le Chatelier's principle. According to the principle, if a chemical reaction undergoes changes in conditions, the chemical equilibrium will shift in order to undo the effects. In the reaction PCl5(g) + heat ↔ PCl3 (g) + Cl2 (g), when pressure is decreased, the system will try to increase it. This will result in the increase of gaseous pressure, so the equilibrium will be shifted toward the products. 

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