Question

Why do we multiply molar concentrations and not sum them, and why there's no squared sign here?

From Wikipedia's page Antigen-Antibody Interaction

Why do we multiply [Ab] and [Ag] and not, say, sum them up?

Why the final term has no squared signs? We've basically multiplied two similar terms.

Answer 1

By the law of mass action rate of reaction in certain direction is proportional to the active mass or the molar concentration of the reactants.So following the law of joint variation the molar concentrations (of reactants) ralsed to proper powers are multiplied to get the rate of reaction in certain direction.

Here the reacion is represented as follows

`A_b+A_g rightleftharpoons A_bAg`

where `A_b and A_g` are two different species interacting in 1:1 mole ratio. So the multiplication of their concentration terms raised with unit power does not result the squaring.

Again `(A_bA_g)` being associated single species with unit stoichiometric coefficient squaring does not arise.