Measuring Reaction Rate

In chemistry, it is important to determine how quick products are formed during a chemical reaction
The rate of reaction is the speed at which a chemical reaction takes place
The rate of reaction can be expressed as the change in concentration of a reactant or product per unit of time
- The most common units for rate of reactions are M s^-1or mol L^-1s^-1

How to calculate the rate of reaction

The rate of reaction is always a positive value
The formula for the rate of reaction in terms of the concentration of product is shown below:

$rate of reaction (M s^{- 1}) = \frac{change in concentration of products (M)}{time (s)}$

If the rate of reaction is calculated using the change in concentration of reactants, the formula must have a negative sign
- This occurs because the change in concentration of reactants is negative

$rate of reaction (M s^{- 1}) = \frac{change in concentration of reactants (M)}{time (s)}$

Worked example

Calculate the rate for the reaction

A → B

If the concentration of A has decreased from 2.5 M to 0.6 M in 20 seconds

Answer:

Step 1: Identify the formula that must be used and replace the values

Since A is a reactant, the equation that must be include the concentration of reactants

$rate of reaction (M s^{- 1}) = \frac{change in concentration of reactants (M)}{time (s)}$

Step 2: Replace the values in the formula and calculate

$rate of reaction (M s^{- 1}) = \frac{Final concentration - Initial concentration}{time}$

$rate of reaction (M s^{- 1}) = \frac{0.6 M - 2.5 M}{20 s}$

$rate of reaction (M s^{- 1}) = - 0.095 M s^{- 1}$

Using graphs to calculate instantaneous rate

Experimental data from reactions can be used to determine the rate of reaction graphically
Two different graphs can be constructed: concentration of reactants vs time, and concentration of products vs time
The steeper the gradient, the quicker the rate of reaction
To find the instantaneous rate of reaction at any point in the graph, draw a tangent to the curve, and calculate the gradient of the tangent
- The gradient of of a line can be calculated using the equation below

$gradient = \frac{∆ y}{∆ x}$

If the data used corresponds to the graph: concentration of reactants vs time, the sign must be changed
- This mathematical manipulation must be done because the reaction rate is always a positive value
If the data corresponds to the graph: concentration of products vs time, there is no need for a mathematical manipulation

Worked example

Iodine and methanoic acid react in aqueous solution.

I_{2 (aq)}+ HCOOH _(aq) → 2I⁻ _(aq) + 2H⁺ _(aq) + CO_{2 (g)}

The rate of reaction can be found by measuring the volume of carbon dioxide produced per unit time and plotting a graph as shown:

rate-of-reaction-worked-example-calculating-rate

Calculate the rate of reaction in mL t^-1 at 20 seconds

Answer:

Step 1: Draw a tangent to the curve at 20 seconds

rate-of-reaction-worked-example-calculating-rate-answer

Step 2: Complete the triangle and use values of x and y to calculate the gradient. The result of the gradient calculation is the rate of reaction

$gradient = \frac{∆ y}{∆ x}$

$gradient = \frac{27 mL - 3 mL}{40 s - 0 s}$

$gradient = \frac{24 mL}{40 s} = 0.60 mL s^{- 1}$

Exam Tip

When drawing the tangent to a curve, you must:

Make the triangle as large as possible
Intersect with grid lines if they are given

Measuring Reaction Rate (College Board AP Chemistry)

Revision Note