<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Dismiss
Skip Navigation
Our Terms of Use (click here to view) and Privacy Policy (click here to view) have changed. By continuing to use this site, you are agreeing to our new Terms of Use and Privacy Policy.

Determining the Rate Law from Experimental Data

Process of using experimental data to determine reaction time

Atoms Practice
Estimated10 minsto complete
%
Progress
Practice Determining the Rate Law from Experimental Data
Practice
Progress
Estimated10 minsto complete
%
Practice Now
Determining the Rate Law from Experimental Data

Measuring time precisely is important in kinetic studies

Credit: Miles Kaufmann
Source: http://commons.wikimedia.org/wiki/File:Timer.jpg
License: CC BY-NC 3.0

How fast?

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.

2NO(g)+2H2(g)N2(g)+2H2O(g)

The following data were collected for this reaction at 1280°C (Table below).

Experiment [NO] [H2] Initial Rate (M/s)
1 0.0050 0.0020 1.25×105
2 0.010 0.0020 5.00×105
3 0.010 0.0040 1.00×104

Notice that the starting concentrations of NO and H2 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 H2 was held constant. The initial rate of the reaction quadrupled, since 5.00×1051.25×105=4. Therefore, the order of the reaction with respect to NO is 2. In other words, rate α [NO]2. Because 22=4, the doubling of [NO] results in a rate that is four times greater.

Comparing experiments 2 and 3: the concentration of H2 was doubled while the concentration of NO was held constant. The initial rate of the reaction doubled, since 1.00×1045.00×105=2. Therefore, the order of the reaction with respect to H2 is 1, or rate α [H2]1. Because 21=2, the doubling of H2 results in a rate that is twice as great.

The overall rate law then includes both of these results.

rate=k[NO]2[H2]

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=rate[NO]2[H2]=1.25×105 M/s(0.0050 M)2(0.0020 M)=250 M2s1

Notice that the rate law for the reaction does not relate to the balanced equation for the overall reaction. The coefficients of NO and H2 are both 2, while the order of the reaction with respect to the H2 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

Questions

  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?

My Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Please to create your own Highlights / notes
Show More

Image Attributions

  1. [1]^ Credit: Miles Kaufmann; Source: http://commons.wikimedia.org/wiki/File:Timer.jpg; License: CC BY-NC 3.0

Explore More

Sign in to explore more, including practice questions and solutions for Determining the Rate Law from Experimental Data.
Please wait...
Please wait...