What is Reaction Rate?
Definition
Reaction rate
The reaction rate is the speed at which substrates are converted into products.
In enzyme-catalysed reactions, this is often measured as the change in the amount of substrate or product over time.Note
The units for reaction rate are typically expressed as the change in concentration (e.g., moles per liter) per unit of time (e.g., seconds).
Measuring Reaction Rates: The Two Main Approaches
- Fixed Time Method: Allow the reaction to occur for a set period and measure the amount of substrate used or product formed.
- Completion Time Method: Measure the time taken for all the substrate to be converted into products.
The fixed time method is often preferred for enzyme experiments because it provides a clearer picture of the reaction rate when substrate concentration is high.
Experimental Setup: Measuring Catalase Activity
- Catalase is an enzyme that breaks down hydrogen peroxide (H₂O₂) into water and oxygen.
- To measure its activity, we can use a simple apparatus:
- Reaction Vessel: Contains hydrogen peroxide and yeast (a source of catalase).
- Gas Collection System: Collects the oxygen produced, which can be measured using a graduated cylinder or gas syringe.
- Imagine you want to measure the effect of substrate concentration on catalase activity.
- You could prepare solutions of hydrogen peroxide at different concentrations (e.g., 0.2 M, 0.4 M, 0.6 M) and measure the volume of oxygen produced in a fixed time (e.g., 1 minute).
Calculating Reaction Rates
- To calculate the reaction rate, divide the change in the amount of substrate or product by the time taken.
- For example, if 10 cm³ of oxygen is produced in 1 minute, the reaction rate is:
- $$
\text{Reaction Rate} = \frac{\text{Volume of Oxygen}}{\text{Time}} = \frac{10 , \text{cm}^3}{1 , \text{min}} = 10 , \text{cm}^3/\text{min}
$$
- $$
Be consistent with units when calculating reaction rates. If you measure time in seconds, convert the rate to cm³/s.
Variables in Enzyme Experiments
- Independent Variable: The factor you change (e.g., substrate concentration, temperature, pH).
- Dependent Variable: The factor you measure (e.g., volume of oxygen produced).
- Control Variables: Factors kept constant to ensure a fair test (e.g., enzyme concentration, reaction time).
- Always repeat experiments multiple times to ensure reliability.
- Calculate the average reaction rate from your trials to reduce the impact of anomalies.
Using Graphs to Determine Reaction Rates
Graphs are powerful tools for visualizing how reaction rates change under different conditions.
Example: Substrate Concentration and Reaction Rate
- When plotting the effect of substrate concentration on enzyme activity, the graph typically shows:
- Initial Increase
- As substrate concentration increases, the reaction rate rises because more substrate molecules are available for collisions with the enzyme.
- Plateau
- Initial Increase
