what is order of a reactionand what r the different ways to calculate it

The sum of concentration terms on which the rate of a reaction actually depends as observed experimentally is called the order of the reaction i.e. the sum of the powers or exponents to which the concentration or pressure terms are raised in order to determine the rate of the reaction.

There are two methods used primarily. 

In the first method, the same reaction is repeated several times, but each time a different concentration of the reactants is used and the initial reaction rate is measured each time. However, it must be done in a logical way for this method to work. The simplest way to do it is to start with one ratio of the reactants. Let's assume we have two reactants, A and B. So we'll start with a concentration of 1 for A and 1 for B. Measure the initial rate. Then we'll double the concentration of A to 2, but keep B the same. If the rate doubles, the order of reactant A is one, but if the rate quadruples, the order is 2. Then we do the same thing for B. Keep A the same, but double B. Again, does it double or quadruple the rate? If you have three reactants you have do do more combinations like this to isolate each variable. Just make sure you change only one thing at a time so you can figure out individual effects. If you change everything at once, it's a mess! 

The other way is to look at the reaction rate as a function of time. Here the reaction is only performed once, but the rate is monitored over time. Based on the predictions for different order reactions, the shape of the curve you plot when you graph the reaction rate versus time will be different for different order reactions. By the shape of the graph you plot, you can tell the reaction order in this way. 

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