Orders, rate equations and rate constants

Define rate of reaction

the change in concentration of a reactant or product per unit time

Give the general units for reaction rate

mol dm^-3 s^-1

What does [A] mean?

Concentration of A

Define the rate constant, k

The constant linking the rate of reaction with the concentration of reactants which are raised to the power of their orders in the rate reaction

Define order

The power to which the concentration of a reactant is raised in the rate reaction

Define half-life

The time taken for the concentration of the reactant to reduce by half

Define the rate determining step

The slowest step in the reaction mechanism of a multi-step reaction

Define the initial rate of raection

The change in concentration of a reactant or product per unit time at t=0

Define overall order and how it is calculated

The overall effect of the concentrations of all reactants on the rate of reaction

• Calculated by the sum of all the individual orders of all of the reactants

What does this mean?

The rate of reaction is proportional to the concentration of A raised to a power of n

What is a zero order reactant? How is zero order represented?

When the concentration of a reactant has no effect on the rate

• Zero order: rate ∝ [A]⁰

Give the 2 rules of a zero order reaction

• Any number raised to the power of zero is 1

  • Concentration does not influence rate

What is a first order reaction?

When the rate depends on its concentration raised to the power of 1

• First order: rate ∝ [A]¹

What happens to the reaction rate when [A] is tripled in a first order reaction?

Reaction rate increases by 3

What do you find by calculating the gradient of a first order reactant on a rate-concentration graph?

The rate constant, k

What is a second order reaction?

When the rate depends on a reactant’s concentration raised to the power of 2

• Second order: rate ∝ [A]²

What happens when [A] is tripled in a second order reaction?

The reaction rate increases by 3² = 9

For two reactants, A and B, show the rate equation

Find the overall order and the rate equation

What are the steps for working out the units of the rate constant, k?

1. Rearrange the equation to make k the subject

2. Substitute units into the expression for k

3. Cancel common units and show the final units on a single line

What must be noted about experimental results when determining orders of a reaction?

The rate should always be measured after the same time, ideally as close to the start of the experiment as possible

Step 1: Determine the orders, overall order and rate equation by comparing experimental results