Aim

Introduction

Procedure

Treatment of Results

Questions

B Varying the concentration of HCl

KINETICS OF THE REACTION BETWEEN SODIUM THIOSULPHATE AND HYDROCHLORIC ACID.

The purpose of this experiment is to determine the rate equation for the reaction between sodium thiosulphate and hydrochloric acid by determining the order of reaction with respect to sodium thiosulphate and hydrochloric acid.

Sodium thiosulphate and hydrochloric acid react according to the stoichiometric equation

Na₂S₂O₃ (aq) + 2HCl (aq) 2NaCl (aq) + H₂O (l) + SO₂ (aq) + S (s)

The rate equation for the reaction is in the form

Rate = k [Na₂S₂O₃]^s [HCl]^t

Where k is the rate constant

s is the order of reaction with respect to sodium thiosulphate and t is the order of reaction with respect to hydrochloric acid.

You will be determining the order of reaction for each reactant by varying the concentration of each species in turn, keeping the others constant.

The reaction can be followed using the precipitated sulphur which causes the solution to become opaque. The rate can be determined by measuring the time taken for a line drawn on a piece of paper and placed under the reaction vessel to become obscured.

Apparatus Chemicals

3 x 50 ml burettes 0.2M sodium thiosulphate solution

2 x 100ml beakers 1M hydrochloric acid

0-100 ^oC thermometer distilled water

stop clock

glass stirring rod

A Varying the concentration of Na₂S₂O₃

1 Set up three burettes in stands for hydrochloric acid, sodium thiosulphate and distilled water.

2 Add 10 cm³ of the hydrochloric acid solution to a 100 cm³ beaker labelled X.

3 In a separate beaker (Y) place 30 cm³ of the sodium thiosulphate solution and 10 cm³ of water.

4 Draw a line on a piece of paper and place beaker Y over it

5 At a convenient time pour the acid from beaker X into beaker Y and start a stop clock.

6 Stir the solution well with a glass rod and watch the line through the solution in beaker Y.

7 Stop the clock when the line on the paper just disappears.

8 Record the time and the temperature of the mixture in the results table.

9 Repeat the procedure for the rest of the mixtures in the table below.

Beaker X	Beaker Y
Volume of HCl /cm³	Volume of Na₂S₂O₃ /cm³	Volume of water /cm³
10	30	10
10	25	15
10	20	20
10	15	25
10	10	30
10	5	35

10 Complete the results table.

1 Follow a similar procedure to that for part A, but keep the volume of thiosulphate solution constant at 10.0 cm³ and vary the volume of hydrochloric acid as shown in the table below.

Beaker X	Beaker Y
Volume of HCl /cm³	Volume of Na₂S₂O₃ /cm³	Volume of water /cm³
30	10	10
25	10	15
20	10	20
15	10	25
10	10	30
5	10	35

2 Complete the results table below.

· For parts A and B, plot separate graphs of 1/Time against volume of reactant under consideration.

1/Time is proportional to the rate of reaction and the volume of reactant is proportional to the concentration, since the total volume is constant.

· Deduce from each graph whether the reaction is first order with respect to the reactant under consideration.

· If you think the reaction is not first order, plot another graph, as explained below.

Suppose that for reactant A

Rate = k₁[A]ⁿ

Under the conditions of the experiment, it follows that

1/Time = k₂ Vⁿ (where V is the initial volume of reactant)

\log (1/Time) = nlog V + log k₂

This is the equation for a straight line of the form y = mx + c

Plotting a graph of log (1/Time) against log V should give a straight line of slope n.

Volume of Na₂S₂O₃/cm³

1/Time s^-1

Temperature /^oC

Average temperature of solutions = ^oC

1. Write the rate equation for the reaction.

2. What is the overall order of reaction?

3. Compare the rate equation with the stoichiometric equation for the reaction. What is the main difference between the two? How do you explain this difference?

4. How would you calculate the rate constant for this reaction?

5. What are the main sources of error in this experiment?