[Forrester]

Using the reaction between CaCaO3 and HCl as an example, if I increase the concentration of HCl, the rate of reaction will increase. I understand this is because the number of successful collisions between reactants will increase. However, why does the volume of gas produced in this reaction not also increase? If there are more successful collisions then surely there will also be an increase in products?

[mjc123]

If you have the same amount of CaCO₃ (assuming an excess of HCl), the amount of gas produced will be the same. The rate at which gas is produced will increase with increasing HCl concentration. You say "this is because the number of successful collisions between reactants will increase", which is not really true; it is the rate (the number of successful collisions per unit time) that increases, not the total number of successful collisions. You need to distinguish more clearly between the total amount of something and the rate (amount produced/consumed per unit time).