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:
- $$
\text{Reaction Rate} = \frac{\text{Volume of Oxygen}}{\text{Time}} = \frac{10 , \text{cm}^3}{1 , \text{min}} = 10 , \text{cm}^3/\text{min}
$$
- $$
Be consistent with units when calculating reaction rates. If you measure time in seconds, convert the rate to cm³/s.
Variables in Enzyme Experiments
- Independent Variable: The factor you change (e.g., substrate concentration, temperature, pH).
- Dependent Variable: The factor you measure (e.g., volume of oxygen produced).
- Control Variables: Factors kept constant to ensure a fair test (e.g., enzyme concentration, reaction time).
- Always repeat experiments multiple times to ensure reliability.
- Calculate the average reaction rate from your trials to reduce the impact of anomalies.
Using Graphs to Determine Reaction Rates
Graphs are powerful tools for visualizing how reaction rates change under different conditions.
Example: Substrate Concentration and Reaction Rate
- When plotting the effect of substrate concentration on enzyme activity, the graph typically shows:
- Initial Increase
- As substrate concentration increases, the reaction rate rises because more substrate molecules are available for collisions with the enzyme.
- Plateau
- At high substrate concentrations, the reaction rate levels off because all enzyme active sites are occupied.
- This is known as saturation.
- Initial Increase
- Suppose you measure the reaction rate of catalase at different hydrogen peroxide concentrations and plot the data.
- The graph shows a steep increase in rate at low concentrations, but the curve flattens at higher concentrations, indicating saturation.
Determining Reaction Rates from Graphs
To find the reaction rate from a graph, calculate the slope of the curve.
Example: Calculating the Initial Rate of Reaction
- Identify the Linear Region: Focus on the initial part of the curve, where the reaction rate is highest and substrate concentration is not limiting.
- Calculate the Slope: Use the formula:
- $$
\text{Slope} = \frac{\text{Change in Product}}{\text{Change in Time}}
$$
- $$
If the graph shows that 5 cm³ of oxygen is produced in the first 30 seconds, the initial rate of reaction is:$$
\text{Initial Rate} = \frac{5 , \text{cm}^3}{30 , \text{s}} = 0.17 , \text{cm}^3/\text{s}
$$
The initial rate of reaction is the most accurate measure of enzyme activity because it reflects conditions before substrate depletion or product accumulation affects the reaction.
Factors Affecting the Reaction Rate (During Experimentation)
- You'll eed to control these variables to ensure accurate results.
- Temperature: As temperature increases, the reaction rate usually Collision theory due to Collision theorymolecular motion, until the enzyme denatures at high temperatures.
- pH: Each enzyme has an optimal pH at which it performs best. Outside this range, the enzyme may denature or lose activity.
- Substrate Concentration: As substrate concentration increases, the reaction rate increases, but it will Collision theorywhen the enzyme becomes saturated.
- Enzyme Concentration: More enzyme molecules usually lead to a faster reaction, as more active sites are available for substrate binding.
Applications of Measuring Reaction Rates
Understanding enzyme activity has practical applications in fields like medicine, industry, and environmental science.Example
Firefly Bioluminescence
- Fireflies produce light through an enzyme-catalysed reaction involving luciferase.
- By measuring the rate of light production, scientists can study how fireflies regulate this process for communication and mating.
Challenges and Considerations
- Ensuring Accurate Measurements
- Failing to control variables like temperature or pH can lead to inconsistent results.
- Always monitor these factors closely.
- Dealing with Non-Linear Data
- The reaction rate may decrease over time as substrate is depleted or products accumulate.
- This is why the initial rate is often used for analysis.
- Choosing the Right Method
- Select a measurement method that suits your enzyme and reaction.
- For example, use a colorimeter to measure changes in light absorbance for reactions involving coloured products.
- What is the difference between the fixed time method and the completion time method for measuring reaction rates?
- How does substrate concentration affect the shape of a reaction rate graph?
- Why is the initial rate of reaction often used to assess enzyme activity?
