2.2.3Unit 2: Cells, Development, Biodiversity and Conservation
Core Practical 4 – Investigating the Rate of Enzyme Reactions — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 26 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Planning the Investigation
A common version uses amylase and starch, with iodine solution showing when the starch has gone.
Independent variable
- Independent variable The factor you deliberately change, such as temperature, pH or substrate concentration, using at least five values.
Dependent variable
- Dependent variable What you measure, such as time for starch to disappear or volume of oxygen collected from catalase.
Control variables
- Control variables Keep enzyme volume, substrate concentration, pH, temperature and total volume the same except for the variable under test.
Buffers
- Buffers Use buffer solutions to hold pH constant, since the pH of a mixture can drift as the reaction runs.
Water bath
- Water bath Use a thermostatically controlled water bath, and let each solution reach the temperature before mixing them.
Repeats
- Repeats Repeat each condition at least three times and calculate a mean so that anomalous values can be identified.
Section 2 of 5Method for Amylase and Starch
1
Place drops of iodine solution in the wells of a spotting tile, ready for sampling during the reaction.
2
Warm separate tubes of amylase and starch solution in the water bath at the chosen temperature for several minutes.
3
Mix the enzyme and starch, start the stopwatch, and at regular intervals transfer a drop of the mixture to a new iodine well.
4
Record the time at which the iodine stays orange-brown instead of turning blue-black, which means no starch is left.
5
Repeat for each temperature or pH, then calculate the rate as 1 ÷ time and plot rate against the independent variable.
Section 3 of 5Safety, Accuracy and Evaluation
Safety, Accuracy and Evaluation
Safety
- Safety Wear eye protection, and take care with hot water, glassware, and iodine which stains skin and clothing.
Reliability
- Reliability Small volumes measured with syringes or graduated pipettes improve accuracy and reduce error between repeats.
Limitations
- Limitations Judging the end point by eye is subjective, and a colorimeter would give more objective, quantitative data.
Enzyme at extremes
- Enzyme at extremes At high temperature no change is seen because the enzyme is denatured, and rate is then recorded as zero.
Section 4 of 5Worked Example
Question
- In an amylase experiment, starch disappeared after 90 s at 30 °C and after 45 s at 40 °C.
- Calculate the rate at each temperature and the factor by which rate increased.
- Rate = 1 ÷ time. At 30 °C, rate = 1 ÷ 90 = 0.0111 s⁻¹.
- At 40 °C, rate = 1 ÷ 45 = 0.0222 s⁻¹.
- Factor = 0.0222 ÷ 0.0111 = 2.0.
Answer Rates are 0.0111 s⁻¹ and 0.0222 s⁻¹, so the rate doubled for a 10 °C rise.
Section 5 of 5Exam Checklist & Tips
2.2.3 – Key Points for the Exam
- Rate of reaction How quickly a reaction happens, calculated as 1 ÷ time taken or as change ÷ time.
- Buffer A solution that resists changes in pH.
- Control variable A factor kept constant so that it cannot affect the results.
- Anomalous result A value that does not fit the pattern and is excluded from the mean.
Common Mistakes
- Using only two temperatures, which is not enough to find an optimum or show a trend.
- Forgetting to bring the enzyme and substrate to the target temperature before mixing them.
- Writing that time is the rate; rate is calculated as 1 ÷ time.
- Including anomalous results in the mean without identifying and discussing them.
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