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-1 or mol L-1 s-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:
- 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
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
- Step 2: Replace the values in the formula and calculate
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
- 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.
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:
Calculate the rate of reaction in mL t-1 at 20 seconds
Answer:
- Step 1: Draw a tangent to the curve at 20 seconds
- 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
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