5.1.1Physical Chemistry and Transition Elements
Rates and Orders
5 min read 6 parts Lesson 2 of 40
Section 1 of 6Rate of Reaction
Orders and rate constants are found only by experiment, never from the balanced equation.
Definition
- The rate of reaction is the change in concentration of a reactant or product per unit time.
Units
- Rate is measured in mol dm⁻³ s⁻¹ when time is in seconds.
Following a reaction
- Rate can be found from the fall in reactant concentration or the rise in product concentration.
Not constant
- Rate usually decreases as reactants are used up, so the initial rate is the most reliable value for comparison.
Section 2 of 6Orders and the Rate Equation
Orders and the Rate Equation
Rate equation
- For A + B → products, rate = k[A]ᵐ[B]ⁿ, where k is the rate constant.
Order
- The order with respect to a reactant is the power to which its concentration is raised in the rate equation.
Zero order
- If the concentration has no effect on rate, the order is 0 and that reactant does not appear in the equation.
First order
- If doubling the concentration doubles the rate, the order is 1.
Second order
- If doubling the concentration quadruples the rate, the order is 2.
| Concentration doubled | Rate changes by | Order |
|---|---|---|
| No effect | × 1 | 0 |
| Rate doubles | × 2 | 1 |
| Rate quadruples | × 4 | 2 |
Section 3 of 6Rate Constant and Units
1
The overall order is the sum of the individual orders in the rate equation.
2
Rearrange the rate equation to make k the subject, for example k = rate / ([A]²[B]).
3
Substitute the units mol dm⁻³ for each concentration and mol dm⁻³ s⁻¹ for the rate.
4
Cancel the units: overall first order gives s⁻¹, second order gives dm³ mol⁻¹ s⁻¹ and third order gives dm⁶ mol⁻² s⁻¹.
5
The value of k is constant at one temperature and increases when the temperature rises.
Section 4 of 6Finding Orders from Initial Rates
Method
- Vary the concentration of one reactant at a time while keeping the others constant, and measure the initial rate.
Compare
- Look at the factor by which rate changes when the concentration of one reactant changes.
Several reactants
- Each reactant has its own order, and all must be determined separately.
Use the equation
- Once the orders are known, use any experiment to calculate k.
Section 5 of 6Worked Example
Question The initial rate of A + B → products was measured: Experiment 1: [A] = 0.10, [B] = 0.10 mol dm⁻³, rate = 2.0 × 10⁻³ mol dm⁻³ s⁻¹.
- Experiment 2: [A] = 0.20, [B] = 0.10, rate = 8.0 × 10⁻³.
- Experiment 3: [A] = 0.10, [B] = 0.20, rate = 4.0 × 10⁻³.
- Find the orders, the rate equation and k.
- Experiments 1 and 2: [A] doubles with [B] constant and the rate quadruples, so the order with respect to A is 2.
- Experiments 1 and 3: [B] doubles with [A] constant and the rate doubles, so the order with respect to B is 1.
- Rate equationrate = k[A]²[B], overall order 3.What the symbols mean: rate (mol dm−3 s−1) · k = rate constant (units depend on the overall order) · [ ] = concentration (mol dm−3) · each power is the order with respect to that reactant
- k = rate / ([A]²[B]) = 2.0 × 10⁻³ / (0.10² × 0.10) = 2.0, with units dm⁶ mol⁻² s⁻¹.
Answer Rate = k[A]²[B]; k = 2.0 dm⁶ mol⁻² s⁻¹
Section 6 of 6Exam Checklist & Tips
5.1.1 – Key Points for the Exam
- Rate of reaction The change in concentration of a reactant or product per unit time.
- Order of reaction The power to which the concentration of a reactant is raised in the rate equation.
- Rate constant The constant k in the rate equation, which depends on temperature.
- Initial rate The rate at the very start of a reaction, when concentrations are known.
Common Mistakes
- Deducing the order from the coefficients in the balanced equation.
- Forgetting to include units, or giving units that do not match the overall order.
- Treating the overall order as the order for each reactant.
- Using concentration values from the wrong experiment when calculating k.