Re: Please explain Rate Law and how to determine the order of achemical reactio

Date: Mon Feb 15 10:14:43 1999
Posted By: Robert L. Judge, Faculty, Chemistry/Science Department, Holy Cross High School
Area of science: Chemistry
ID: 918371690.Ch

Message:

Dear Bill,

An order is a value calculated based on experimental data collected regarding a reaction as [ ] vs. time. It does have a value which must be calculated based on the data collected. It can be determined without plotting the data. Consider the following:

The reaction rate is defined as the *[] of a reactant or product over time t. In the reaction aA + bB ---> cC + dD, the lower case letters indicate the coefficients of the respective reactants and products noted by the upper case letters. From this the rate of this reaction is rate= -1/a *[A]/*t = - 1/b *[B]/*t = 1/c *[C]/*t = 1/d *[D]/*t.

Because as the reaction proceeds, the [] of the reactants decreases and the products increase, the reaction slows, it is important to determine the rate at the start of the reaction if one wnats to determine the rate based on the reactants. The rate law for the above reaction is rate=k[A]^x[B]^x in which k is the proportionality constant and x and y are the powers of the concentrations. These values are determined by calculations based on experimental data. The sum of x and y equals the overall reaction order.

Consider: F2 + 2ClO2 ---> 2FClO2 and [F2](M)   [ClO2](M)       Initial rate
	                           1 0.10      0.010		1.2x10^-3
	                           2 0.10      0.040           4.8x10^-3
	                           3 0.20      0.010           2.4x10^-3

These data tells us that as the [F2] is doubled and the [ClO2] is constant, the reaction is doubled. Also note that as [ClO2] is quadrupled, the reaction is quadrupled. This would tell us that x and y in the equation are both 1 and the overall reaction order is 2. If the [A] is doubled and the reaction is quadrupled, then that would be a second order reaction in A, if the relationship between the [A] and the rate is 1:1 the order is 1 in A. If the relationship between the [A] and the rate is 1:2 the order is 2nd in A etc. You can calculate this relationship by writing the rates of the reactions 1 and 2 as a ratio rate1/rate2. It can also be done by writing the ratio in terms of k[A]^x[B]^y/k'[A]^x'[B]^y'. If then the order is 2 in A and say 1 in B the overall reaction order is 3.

Thanks for the question. You'll need to try some practice problems to get the hang of it.

Happy Mardi Gras
Robert L. Judge

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.