How is it possible to have a reaction equation where one reactant does not effect the rate of the reaction?

Question

THanks! If you could help me that'd be great, I have a chem test very soon and need this answered :)

Answer 1

When a reaction takes place, it's not as simple as a reaction equation makes it out to be. Two (or one) do the shimmie and come out as something else. There is a series of steps which occur before the end product is evolved, this is known as the reaction mechanism. One simple reaction, which involves only two steps is the reaction:

NO2 + CO → NO + CO2

the two steps are:

1. NO2 + NO2 → NO + NO3
2. NO3 + CO → NO2 + CO2

These two steps each have their own reaction speed and the slowest is called the "rate-determining step." This means that as the slowest is reacted, the products are immediately reacted in turn. This is much the same as an assembly line. The slowest person dictates the speed.

The rate determining step doesn't have to include both the reactants. In the above reaction, the first step is the slowest and doesn't use CO. So the reaction rate has nothing to do with CO.

Answer 2

Are you confusing a limiting reactant with the rate of reaction? Usually chemical reactions continue until one reactant is "used up." Rates of reaction can be sped up or slowed down by a variety of things other than the reactants so it is quite common for none of the reactants to affect the rate of reaction. For instance, temperature can have a profound effect on reaction rates, but it is not a reactant. Likewise, chemicals can be added as catalysts or retardants and they don't "blend" with the reactants in the experiment, (the products don't include them) they simply change the rate of reaction.

Answer 3

Because for example, if there are two reactants, you would need a certain amount of each. You have, for example, 1 gram of A and 2 grams of B. If you need 1 gram of each for a reaction, you will have 1 gram of B left over. So adding more B wouldn't do anything because there's not enough A. It would actually slow down the reaction very slightly.