Reaction rate

Reaction rate is defined as the change in concentration (or amount) of a reactant or product over time.

A simple formula for finding the rate of a chemical reaction is:

$\text{rate of reaction }=\frac{\text{amount of reactant used up or product formed}}{\text{time}}$

Measuring reaction rates

You can follow the rate of a reaction by measuring either how fast the reactants are used up or how fast the products are formed. Here are three common methods:

1. Measuring a decrease in mass

• For reactions that produce gases, you can measure the rate by observing the decrease in mass over time.
• As gas is released, the mass decreases. This method is straightforward but should be conducted in a fume cupboard for safety.

2.** Measuring the volume of gas given off**

• Alternatively, you can use a gas syringe to measure the volume of gas produced over time.

3.** Timing how long a precipitate takes to form**

• This approach is suitable when the product of the reaction is a precipitate.
• Measure how long it takes for a mark to become obscured by the precipitate.
• For consistent comparison of reaction rates, always use the same observer and reference mark.

Calculating rates from graphs

Plotting the amount of reactant used or product formed against time on a graph, the slope (or gradient) of the curve represents the reaction rate.

To calculate the gradient of the graph:

$\text{Gradient }=\frac{\mathrm{\Delta }y}{\mathrm{\Delta }x}$

Where:

• y = amount of reactant or product
• x = time

Worked example 1 - Calculating average rate of reaction

The graph below shows the mass of a reaction vessel measured at regular intervals during a chemical reaction.

Calculate the average rate of reaction, in cm³ min^-1.

Step 1: Draw a line of best fit

Draw a line of best fit through the data points, starting from the origin (0,0).

Step 2: Choose two distinct points on the line

Identify and choose two points where the line of best fit intersects a gridline distinctly.

Step 3: Create a triangle

Connect these points using vertical and horizontal lines to form a right-angled triangle.

Step 4: Gradient equation

$\text{Gradient }=\frac{\mathrm{\Delta }y}{\mathrm{\Delta }x}$

Step 5: Substitution and correct evaluation

$\text{Gradient }=\frac{4.0-1.0}{4.8-1.2}=\frac{3.0}{3.6}=0.83{\text{ cm}}^{\text{3}}{\text{ min}}^{\text{-1}}$

Worked example 2 - Calculating rate of reaction at a specific time

The graph below shows the mass of a reaction vessel measured at regular intervals during a chemical reaction.

Calculate the rate of reaction, in g min^-1, at 3 minutes.

Step 1: Draw a tangent at 3 minutes

Use a ruler to carefully draw a tangent to the curve at the 3 minute mark on the graph. Extend the tangent across the graph.

Step 2: Choose two distinct points on the tangent

Identify and choose two points where the tangent intersects a gridline distinctly.

Step 3: Create a triangle

Connect these points using vertical and horizontal lines to form a right-angled triangle.

Step 4: Gradient equation

$\text{Gradient }=\frac{\mathrm{\Delta }y}{\mathrm{\Delta }x}$

Step 5: Substitution and correct evaluation

$\text{Gradient }=\frac{0.8-3.2}{5.2-1.2}=\frac{-2.4}{4.0}=-0.60{\text{ g min}}^{\text{-1}}$