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5.2.10Additional lessons

The Rate Constant (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 105 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5The Rate Equation

5.2.10 The Rate Constant (HL)

Orders cannot be read directly from the balanced equation.

Form
  • For A + B → products, rate = k[A]ᵐ[B]ⁿ, where m and n are the orders with respect to each reactant.
Rate constant
  • k is the proportionality constant, which is constant at a given temperature and independent of concentration.
Order of reaction
  • The power to which a reactant concentration is raised in the rate equation, found only by experiment.
Overall order
  • The sum of the individual orders, which determines the units of the rate constant.

Part 2 of 5Determining Orders

Rate-concentration graphs show a horizontal line for zero order, a straight line for first order and a curve for second order.

1
Compare experiments in which only one concentration changes while all other concentrations stay constant.
2
If doubling the concentration has no effect on rate, the reaction is zero order with respect to that reactant.
3
If doubling the concentration doubles the rate, it is first order; if rate quadruples, it is second order.
4
Write the rate equation using the orders found and substitute any data set to solve for k.
5
Work out the units of k by substituting units into the rate equation and cancelling.

Part 3 of 5Units and Temperature

Units and Temperature
Zero order overall
  • k has units of mol dm⁻³ s⁻¹.
First order overall
  • k has units of s⁻¹, because rate in mol dm⁻³ s⁻¹ divides by one concentration.
Second order overall
  • k has units of mol⁻¹ dm³ s⁻¹.
Temperature
  • k increases with temperature, which is why reactions speed up when heated.

Part 4 of 5Worked Example

Question

  • For a reaction with rate = k[A][B], the initial rate is 4.0 × 10⁻³ mol dm⁻³ s⁻¹ when [A] = 0.10 mol dm⁻³ and [B] = 0.20 mol dm⁻³.
  • Calculate k with units.

  1. Rearrangek = rate ÷ ([A][B]).
  2. k = 4.0 × 10⁻³ ÷ (0.10 × 0.20) = 4.0 × 10⁻³ ÷ 0.020.
  3. Unitsmol dm⁻³ s⁻¹ ÷ (mol dm⁻³)² = mol⁻¹ dm³ s⁻¹.

Answer k = 0.20 mol⁻¹ dm³ s⁻¹

Part 5 of 5Exam Checklist & Tips

5.2.10 – Key Points for the Exam

  • Rate constant The proportionality constant k in the rate equation.
  • Order of reaction The power of a reactant concentration in the experimentally determined rate equation.
  • Overall order The sum of the orders for all reactants.
  • Rate equation An expression linking rate to the concentrations of reactants raised to their orders.

Common Mistakes

  • Taking the order from the coefficients in the balanced equation.
  • Leaving the units of k out or giving the wrong units for the overall order.
  • Changing two concentrations at once when determining an order.
  • Believing that k does not depend on temperature.

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