Determining Reaction Rates

Question

Consider the reaction below:

H2(g)+Cl2(g) ----> 2HCl(g)

Why is gas pressure not a good property to monitor in order to determine the reaction rate?

Answer 1

Measuring pressure to monitor reaction rate in this case is not acceptable because you have gas (pressure) as both reactants and products. If say, only the product was a gas, then measuring pressure change would be ok as a means of following reaction rates. Not in this case however. It is much better to measure change in volume.