Rate Laws

Rate laws essentially use concentration and relate it to the reaction rate.

The general formula is shown above, with several variables included.

• k - This is a constant which depends on temperature, it is large for reactions with a fast rate. The units themselves vary depending on the reaction order.

• n, m - These 2 variables represent reaction orders, they have no correlation to the actual coefficients of the original equation. The overall reaction order is defined by n + m.

• a, b, c, d - These represent the actual coefficients of the original equation.

• A, B, C, D - These variables represent the elements or molecules that are used in the equation.

Reaction Order

There are several orders of reaction, varying from 0^th order up until the i^th order. The most common you'll find are 0^th order, 1^st order, and 2^nd order.

Each of these different ordered reactions relies on the value of n. The higher the order goes, the quicker the reaction takes place.

• 0^th order reactions are linear in nature, they are constant and have a straight slope throughout.

Example

In the example above, you're asked to find the rate law of the equation. There are 2 parts to this because you need both n and m in order to find the overall reaction order.

So to do this you first have to find the ratio of 2 different rates, along with the ratio of 2 different molarities of 2 respective molecules/elements. In doing this you cancel out the k constant, and factor out the n, leaving you with a simplified equation. To find n value, you then solve by plugging in the given values and finding that n = 2.

In the 2nd part of the question, you're asked to find the order of O₂. To find this, you need to again go through the same process as above. The resulting answer is m = 1, which when combined with n gives the overall reaction order which is 3!

Note: The answer is B) First order, because it asks what the order of O₂ was and not the overall reaction order.