Question

Explain the method of initial rates to determine rate law and order of reaction.

Answer 1

Method of initial rates:The order of that reactant concentration in the rate law can be determined by observing how the reaction rate varies as the concentration of that one reactant is varied. This method is repeated for each reactant until all the orders are determined.

The mathematical relationship of reaction rate with reactant concentrations is known as the rate law.

For the following hypothetical reaction

$aA+bB\to cC$

the rate law can be expressed as:

$Rate=k\left[A{]}^y\right[B{]}^z$

The sum of the concentration term exponents in a rate law equation is known as its reaction order.

For the following reaction between nitrogen dioxide and carbon monoxide:

$N{O}_2\left(g\right)+CO\left(g\right)\to NO\left(g\right)+C{O}_2\left(g\right)$

The rate law is experimentally determined to be: rate $=k[NO2{]}^2$

Therefore, we would say that the overall reaction order for this reaction is second-order (the sum of all exponents in the rate law is 2), but zero-order for [CO] and second-order for $\left[N{O}_2\right].$

The reaction order is most often a whole number such as 0, 1, or 2; however, there are instances where the reaction order may be a fraction or even a negative value.