# Where does the rate equation come from? (r = k [A]^x [B]^y)

Equations are great, but they're much better when you can understand them. I don't quite get why this works. Could someone explain it to me?r = k [A]^x [B]^y

## 1 Answer

A to the power of x or B to the power of y are telling you how the concentration of that reactant will affect the rate of reaction. For example, if you find that doubling the concentration of reactant A quadrupled the reaction rate than A could be raised to the power of 2. If doubling the concentration of B doubled the rate of reaction than you could raise B to the power of 1. K is the Arrhenius equation and its value is taking into account the frequency that the particles are colliding, the amount that are colliding with the right orientation and the energy required for the reaction to take place. K = Ae^(-Ea/RT) where A is the product of the frequency of the colliding particles and the amount that have the correct orientation. e is 2.718281828 and is raised to the power of negative the activation energy divided by RT where R is the ideal gas constant with a value of 8.314 and T is the temperature. You could use algebra to solve for A if you have the other values.

• Agreed. Rate constants are found experimentally, and only indirectly reflect the mechanisms occurring at the molecular level. In fact: many reaction mechanisms are much more complex than you might imagine! – colnegn Mar 4 '16 at 5:34