Enduring Understanding 4.A: Reaction Rates
- Chemical reactions vary widely in the speed at which they occur.
- The reaction rate is defined as the change in concentration of a substance divided by the time interval:
- Reactions can be examined experimentally to determine their rate. If a reactant or product is colored, it can be followed by spectrometry using Beers Law.
- Factors that influence a rate of reaction include:
- Concentration of reactants (except in zero order processes)
- Pressure (for reactions involving gases)
- Surface area (for a solid)
- Temperature (increased temperature = faster reaction)
- Presence of a catalyst.
- Example: A reaction in a closed container, A→2B, is monitored for 60 seconds, giving the following results. What concentrations of A and B are present at 90 seconds?
- Every 30 seconds, the concentration of A drops by half and the concentration of B increases by twice the drop in A.
- Therefore, after 90 seconds, [A] will be 0.25 mol/L and [B] will be 3.50 mol/L.
- The rate of a chemical reaction can be expressed mathematically by a rate law. For the reaction:
- The rate law for this reaction would take the form rate = k[A]x[B]y
- k is the specific rate constant, the rate when [A] and [B] are 1.
- The exponents, x and y, are the order of the reaction with respect to that reactant. The sum of the exponents, x + y, is the overall order of the reaction.
- If x = 0, doubling [A] has no effect on the rate (zero order)
- If x = 1, doubling [A] doubles the rate (first order)
- If x = 2, doubling [A] increases the rate by 2^{2} or four times (second order).
- Example: For the reaction: O_{2} + 2NO → NO_{2}
- The rate equation is k[O_{2}][NO]^{2}
- If the concentration of O_{2} doubles, the rate doubles.
- If the concentration of NO doubles, the rate increases by 2^{2}, or quadruples.
- The overall order of the reaction is 3.
- Rate laws cannot be determined from the chemical equation; they must be determined experimentally.
- Sample Question: Give the following data, what is the rate law for the reaction:
- Doubling [A] quadruples the rate, so the order of A is 2.
- Doubling [B] doubles the rate, so the order of B is 1.
- Doubling [C] has no effect on the rate, so the order of C is zero.
- The rate law must therefore be k[A]^{2}[B]
- Radioactive decay is an example of a first-order process. The time taken for half of a sample of a radioactive isotope to decay is called the half-life.
- The amount of a sample remaining after n half-lives is given by the equation:
Time, seconds | [A], mol/L | [B], mol/L |
---|---|---|
0 | 2.00 | 0 |
30 | 1.00 | 2.00 |
60 | 0.50 | 3.00 |
90 | ? | ? |
[A] | [B] | [C] | Initial Rate (mol/s) |
---|---|---|---|
0.10 | 0.10 | 0.10 | 5.0 |
0.20 | 0.10 | 0.10 | 20.0 |
0.20 | 0.20 | 0.10 | 40.0 |
0.20 | 0.20 | 0.20 | 40.0 |
Related Links: Chemistry Chemistry Quizzes AP Chemistry Notes Redox Reactions |
To link to this Reaction Rates page, copy the following code to your site: