• Enzyme rate experiments?are experiments that are carried out to determine the?effect?of?changing?a particular?factor?on the?rate of a reaction?that is catalysed by an enzyme
• Factors that can be?changed?include:
  • Temperature
  • pH
  • Enzyme concentration
  • Substrate concentration

   

• The?key?thing with enzyme rate experiments is to ensure that?only one?of these variables is?changed?during a particular experiment
  • This is known as the?independent?variable

   

• All other variables must be?controlled?(they must stay the?same)
  • These are known as the?control?variables

   

• For example, if investigating the effect of?temperature?on the rate of reaction, the?pH,?enzyme?concentration?and?substrate?concentration?must be exactly the same (kept constant) each time you run the experiment (at each different temperature you are investigating)
• If these control variables are not kept constant, they could?affect the results?of the experiment
  • This would make the results?unreliable

   

Uncertainty

• Uncertainty?is the amount of?error?your measurements might contain
• Results from experiments (including enzyme rate experiments) always contain some error (they are never perfect)
• There will always be a small degree of uncertainty in your readings or measurements
  • This is often because the?sensitivity?of the apparatus being used is?limited

   

• For example, you may want to measure?reaction rate?by measuring how much of a?product?is?made?in a?given time period?(e.g. using a gas syringe to measure the volume of oxygen produced from the breakdown of hydrogen peroxide by catalase)
  • The gas syringe may only give readings to the nearest?1 cm3
  • However, the real volume produced could be up to 0.05 cm3?smaller?or?larger
  • It has an uncertainty value of?± 0.05 cm3
  • A ‘±’ sign tells you the?range?in which the true value lies
  • This range is called the?margin of error

   

• For enzyme rate experiments, you may need to?calculate?the?percentage error?of your measurements
  • As long as you know the?uncertainty value?of your measurements, the percentage error can be calculated using the following formula:

   

percentage error = (uncertainty value ÷ your measurement) x 100

Percentage error = (uncertainty value ÷ your measurement) x 100

Percentage error = (0.05 ÷ 50) x 100

Percentage error = 0.001 x 100

Percentage error = 0.1%

Step 1: Calculate the uncertainty value

The stopwatch can measure to the nearest millisecond (0.001?second)

This means the actual time taken could be up to?0.0005?seconds?shorter?or?longer?than this

This means stopwatch measurements have an uncertainty value of?± 0.0005 s

Step 2: Calculate the percentage error?of the student’s measurement of 5.245 seconds

Percentage error = (uncertainty value ÷ your measurement) x 100

Percentage error = (0.0005 ÷ 5.245) x 100

Percentage error = 0.000095 x 100

Percentage error = 0.0095% or 0.01%