Reaction Order

[Rachna Goli 3E]

Why is the reaction order the sum, rather than the products, of the orders of each reactant?

[Amy Pai]

It might be because of the exponents; meaning, when concentrations are multiplied, you add each part's powers to get the "sum" of the power. I think this also translates to if you have different reactants in your rate law, so the order is just the number of concentrations you have in your rate law, i.e. R=k[A]^3 is third order, and R=k[A][B]^2 is also third order (see question 15.17)

[Nico Medina]

I was wondering this too, thanks for the help!

[Audrey Banzali-Marks 1A]

This is probably because when you multiply variables with exponents attached, you add the exponents instead of multiplying them. For example, a^2 * a^3 would equal a^5, not a^6. Therefore, in a reaction where the rate = k*[x]^2*[y]^1, you add the exponents 2 + 1 to get 3. This rate is a third-order reaction, then.

[Noa Popko 3I]

Follows from exponent rules I think. We add orders together because they are exponents of the concentrations.

[Tammy Shen 2L]

A good way to understand this is that when exponents are multiplied together, the product is a sum of the exponents. We add the orders together because they are the product of the concentrations, which have exponents.

[Jeffrey Vo 2A]

Hi Rachna, the reason why the reaction order is the sum rather than the products ultimately boils down to math. Specifically, exponents. As a person who is weak in mathematics, I googled this and it is because when you multiply exponents, the final answer is the the sum of both exponents. Similarly, the "product" of the concentrations when we have the reaction order, is found by summing them all up because they include exponents. So x^6 * x^7 = x^(6+7). Hope this helps.

[Ashley Presnell 1C]

It kind of follows exponent laws because when you do 3^2 times 3^4 you get 3^6 which is the sum of the two exponents rather than the product of them.

[Adithi Ayyala 2G]

Hello! So essentially the exponents are multiplied together, and the product is the sum of these exponents. Then, we can add the orders together, or the product of the yielded concentrations which have exponents that allow us to do this.

[AbenaaMensahBonsu2F]

Like other said, it's really about the rules of exponents. Since the concentrations of the reactants are multiplied, the total order would be the sum of their exponents.