Determining the Rate Law from Experimental Data

Learning Objectives

  • Use experimental data to determine the rate law for a reaction.

How fast?

Measuring time precisely is important in kinetic studies

Timer. From Miles Kaufmann/Wikimedia.

Determining the amount of time a process requires calls for a timer. These devices can be simple kitchen timers (not very precise) or complex systems that can measure to a fraction of a second. Accurate time measurement is essential in kinetics studies for assessing rates of chemical reactions.

Determining the Rate Law from Experimental Data

In order to experimentally determine a rate law, a series of experiments must be performed with various starting concentrations of reactants. The initial rate law is then measured for each of the reactions. Consider the reaction between nitrogen monoxide gas and hydrogen gas to form nitrogen gas and water vapor.

2text{NO}(g)+2text{H}_ 2(g) rightarrow text{N}_ 2(g)+ 2text{H}_ 2text{O}(g)

The following data were collected for this reaction at 1280°C.

Experiment [NO] [H 2 ] Initial Rate (M/s)
1 0.0050 0.0020 1.25 times 10^{-5}
2 0.010 0.0020 5.00 times 10^{-5}
3 0.010 0.0040 1.00 times 10^{-4}

Notice that the starting concentrations of NO and H 2 were varied in a specific way. In order to compare the rates of reaction and determine the order with respect to each reactant, the initial concentration of each reactant must be changed while the other is held constant.

Comparing experiments 1 and 2 : the concentration of NO was doubled, while the concentration of H 2 was held constant. The initial rate of the reaction quadrupled, since frac{5.00 times 10^{-5}}{1.25 times 10^{-5}}=4 . Therefore, the order of the reaction with respect to NO is 2. In other words, rate α [NO] 2 . Because 2^2 = 4 , the doubling of [NO] results in a rate that is four times greater.

Comparing experiments 2 and 3 : the concentration of H 2 was doubled while the concentration of NO was held constant. The initial rate of the reaction doubled, since frac{1.00 times 10^{-4}}{5.00 times 10^{-5}}=2 . Therefore, the order of the reaction with respect to H 2 is 1, or rate α [H 2 ] 1 . Because 2^1 = 2 , the doubling of H 2 results in a rate that is twice as great.

The overall rate law then includes both of these results.

text{rate}=k[text{NO}]^ 2 [text{H}_ 2]

The sum of the exponents is 2 + 1 = 3 , making the reaction third-order overall. Once the rate law for a reaction is determined, the specific rate constant can be found by substituting the data for any of the experiments into the rate law and solving for k .

k=frac{text{rate}}{[text{NO}]^2 [text{H}_2]}=frac{1.25 times 10^{-5} text{ M/s}}{(0.0050 text{ M})^2(0.0020 text{ M})}=250 text{ M}^{-2} text{s}^{-1}

Notice that the rate law for the reaction does not relate to the balanced equation for the overall reaction. The coefficients of NO and H 2 are both 2, while the order of the reaction with respect to the H 2 is only one. The units for the specific rate constant vary with the order of the reaction. So far, we have seen reactions that are first or second order with respect to a given reactant. Occasionally, the rate of a reaction may not depend on the concentration of one of the reactants at all. In this case, the reaction is said to be zero-order with respect to that reactant.

Summary

  • The process of using experimental data to determine a rate law is described.

Practice

Use the site below to practice determination of rate constant with experimental data.

http://ibchem.com/IB/ibnotes/full/kin_htm/order_calculation.htm

Review

  1. How do you carry out experiments for determining rate constants?
  2. Why is the reaction order with regard to NO a value of 2?
  3. Why is the reaction order with regard to hydrogen value of 1?