Write a rate equation showing the dependence of rate in reactant concentrations for each of the following elementary reactions.
a. CS2---> CS + S
b. CH3Br + OH- --> CH3OH + Br-
A. k[CS2][S]
B. k[CH3][Br-][OH-]
Neither answer is correct. Forward reaction rates do not involve both reactants and products.
Get rid of the product terms.
A word of warning however: Chemical reactions often proceed by a multistep mechanism, in which case the overall rate cannot simply be written in terms of powers of reactant concentrations. I have misgivings about the way this subject is being taught.
To determine the rate equation for each of the given elementary reactions, we need to consider the stoichiometry of the reaction and the order of each reactant.
a. CS2 ---> CS + S
The stoichiometry of the given reaction indicates that one molecule of CS2 produces one molecule of CS and one molecule of S. Therefore, the rate equation will be:
Rate = k[CS2]
b. CH3Br + OH- ---> CH3OH + Br-
The stoichiometry of the given reaction shows that one molecule of CH3Br reacts with one molecule of OH- to produce one molecule of CH3OH and one molecule of Br-. Thus, the rate equation will be:
Rate = k[CH3Br][OH-]
Therefore, the rate equations for the given elementary reactions are:
a. Rate = k[CS2]
b. Rate = k[CH3Br][OH-]