Monitoring the rate of reaction between two liquids

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

1. Why is the gradient of the graph initially quite steep but becomes less steep as time progresses?
2. What shape does the graph take if the chemicals are warmer?
3. 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?