The initial rates method is a way to determine how the speed of a chemical reaction depends on reactant concentrations. It uses data from several experiments that start with different initial concentrations and measures the reaction rate at the very beginning. This matters because the balanced chemical equation usually does not reveal the rate law for a real reaction.
Knowing the rate law helps chemists predict reaction speed and design safer, more efficient reactions.
To use the method, compare two trials where only one reactant concentration changes while the others stay constant. The ratio of the initial rates is matched to the ratio of the changing concentration raised to an unknown power, which gives the reaction order for that reactant. After finding all orders, substitute one trial into the rate law to solve for the rate constant k.
The units of k depend on the overall reaction order, so they must be determined from the final rate law.
Key Facts
- General rate law: rate = k[A]^m[B]^n
- m and n are reaction orders found from experimental data, not from the balanced equation.
- Compare trials where only one reactant concentration changes to find one order at a time.
- Rate ratio equation: rate2/rate1 = ([A]2/[A]1)^m when only [A] changes.
- Overall order = m + n for rate = k[A]^m[B]^n.
- Rate constant: k = rate/([A]^m[B]^n)
Vocabulary
- Initial rate
- The reaction rate measured at the start of a reaction before concentrations change significantly.
- Rate law
- An equation that shows how reaction rate depends on reactant concentrations and a rate constant.
- Reaction order
- The exponent of a reactant concentration in the rate law, showing how that reactant affects rate.
- Rate constant
- The proportionality constant k in a rate law that depends on temperature and the reaction mechanism.
- Overall order
- The sum of all reactant orders in the rate law.
Common Mistakes to Avoid
- Using coefficients from the balanced equation as orders. This is wrong because reaction orders must be determined from experimental rate data unless the reaction is known to be an elementary step.
- Comparing trials where more than one reactant changes. This is wrong because the rate change cannot be assigned to one reactant without extra algebra.
- Forgetting to take ratios of rates and concentrations in the same trial order. This can invert the relationship and give the wrong reaction order.
- Reporting k without units or with generic units. The units of k depend on the overall order and must make the rate units come out correctly.
Practice Questions
- 1 For the reaction rate = k[A]^m[B]^n, trials 1 and 2 keep [B] constant. Trial 1: [A] = 0.10 M, rate = 2.0 x 10^-3 M/s. Trial 2: [A] = 0.20 M, rate = 8.0 x 10^-3 M/s. Find the order m in A.
- 2 Using rate = k[A]^2[B], find k if [A] = 0.30 M, [B] = 0.20 M, and the initial rate is 1.8 x 10^-3 M/s. Include units for k.
- 3 A student compares two trials where [A] doubles and [B] also doubles, then concludes the reaction is second order in A because the rate quadruples. Explain why this conclusion is not justified and what kind of comparison would be better.