Rate of Reaction

Some reactions take place instantly, but most are much slower and it is possible to measure how long these reactions take to reach a certain stage
As a chemical reaction proceeds, the concentration of the reactants decreases and the concentration of the products increases
- Reversible reactions are an exception to this, where the concentration of reactants and products remains constant once dynamic equilibrium has been achieved
The rate of a reaction is the speed at which a chemical reaction takes place and can be expressed as the change in concentration of a particular reactant or product per unit time
The rate of reaction typically has the units mol dm^-3s^-1

$rate of reaction (mol {dm}^{- 3} s^{- 1}) = \frac{change in concentration of reactants or products (mol {dm}^{- 3})}{time (s)}$

Experimental data from reactions can be shown graphically and used to calculate the rate of reaction

Graphs to show how the concentration of reactants decreases with time and the concentration of products increases with time

As the reaction proceeds, the concentration of reactants and products change with time

The steeper the gradient, the quicker the rate of reaction
The rate of reaction at a particular time can be found by calculating the gradient of the curve at that time
To find the gradient of a curve, draw the tangent to the curve and calculate the gradient of the tangent by using the equation:

$gradient = \frac{Δy}{Δx}$

The rate of reaction is a positive value
The graph of the reactants' concentration over time shows a negative gradient
- So, the sign of the rate should be altered to be a positive value
- E.g. gradient = –20.6 mol dm^-3 s^-1 → rate = 20.6 mol dm^-3 s^-1
The graph of the products' concentration over time shows a positive gradient
- So, the sign of the rate will already be a positive value and not need altering

Iodine and methanoic acid react in aqueous solution.

I₂ (aq) + HCOOH (aq) → 2I⁻ (aq) + 2H⁺ (aq) + CO₂ (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:

Graph to show how the volume of gas collected increases with time Calculate the rate of reaction at 20 seconds.

Answer:

Graph to show how the tangent of the curve can be used to calculate the rate of the reaction

When drawing the tangent to a curve, you should:
- Make the triangle large
- Try to intersect with gridlines if you can
These points should minimise errors of precision and reduce the chance you will accidentally misread the graph values

Rate of Reaction (SL IB Chemistry)