Introduction
This experiment investigates the factors which affect the rate of reaction between two chemicals which form an opaque product (often as a suspension). It should fit in with a key stage 4 scheme of work for materials and their properties — rates of reaction.
Equipment required
- DrDAQ data logger connected to a PC
- Home-made light sensor (described below)
- Suitable chemicals (e.g. 1M hydrochloric acid, 40 g/l sodium thiosulphate; or starch solution with iodine indicator with amylase; or milk with trypsin; or neutralisation of alkali using phenolphthalein)
The sensor components
- Block of wood about 10 cm x 5 cm x 5 cm
- LDR (Maplin part number HB10L)
- LED (Maplin part number NP03D)
- 2 resistors (330 ohm and 3.9k ohm)
- 9 volt battery
- Sundries (wire, solder, sticky tape)
Sensor construction
The block of wood is drilled to accommodate the electronics as shown below:
Figure 1: diagram showing the experiment set up
The components are best held in place using the glue from a hot glue gun. The circuit is connected as shown below:
Figure 2: diagram showing the circuit
Carrying out the experiment
The home made sensor is set up and connected to DrDAQ’s voltage terminals. PicoLog is set to measure voltage and display a graph of the results. A time period of 120 seconds is found to be most suited to this type of experiment and the data-logger is allowed to sample results every second. Graph axes are set to automatic.
The test-tube has 5 ml of reactant added (for example hydrochloric acid). Recording is started and 5 ml of the second reactant is added to the test-tube (for example sodium thiosulphate). The reaction is allowed to proceed.
Questions and discussion of results
- Why is the gradient of the graph initially quite steep but becomes less steep as time progresses?
- What shape does the graph take if the chemicals are warmer?
- What shape does the graph take if the chemicals are diluted?
Further study
- What would happen to the shape of the graph if the reactants were warmer (or cooler)?
- What would happen to the shape of the graph if the reactants were diluted (or concentrated)?
- If the reaction employs a catalyst or enzyme, what is the ideal temperature of the chemistry?
- What effect does pH have on enzymes?
