Effect of Concentration on Rate of Reaction- Sodium Thiosulphate and Sulphric Acid (Disappearing Cross Lab)

Effect of Concentration on Rate of Reaction- Sodium Thiosulphate and Sulphric Acid (Disappearing Cross Lab)

Aim / Objective:

To investigate the effect of concentration on rate of reaction.

Abstract:

The experiment employed the use of varying concentration of sodium thiosulphate on the rate of reaction. As concentration decreases then the rate of reaction will decreases as well over time.

Introduction:

The rate of a chemical reaction is the time required for a given quantity of reactant(s) to be changed to product(s). The unit of time may be seconds, minutes, hours, days or years.

The rate is affected by several factors, some of which are listed as follows:

(1) Nature of the reactants, i.e., one metal may react vigorously with acid while another does not react.

(2) The particle size of the reactants, i.e., a lump of coal burns slowly but powdered coal may explode.

(3) Temperature increases in general increase the rate of reaction, i.e., a 2O°C rise in temperature doubles the reaction rate.

(4) Catalysts affect the rate by using or allowing a different pathway for the reaction to follow.

(5) Concentration affects the rate of reaction, i.e., if the concentration of one of the reactants is doubled and is an integral part of the reaction then rate increases appropriately.

Some reactions are fast and other reactions are slow. The rate of a specific reaction can be found only by experiment.

Apparatus/Materials:

3 beakers ,16.0 gdm-3 sodium thiosulphate (Na2S2O3), 2 measuring cylinders, 2 retort stands, 2 burettes, 0.5M sulphuric acid (H2SO4),  1 conical flask, distilled water, 1 glass rod, 1 funnel ,filter paper, pencil, stopwatch.

Method / Procedure:

1)    Using a pencil, draw a cross in the middle of the filter paper.

2)    Use a measuring cylinder to pour 50cm3 of 16.0 gdm-3 sodium thiosulphate (Na2S2O3) into an empty beaker.

3)    Center the beaker onto the cross that is drawn on the filter paper.

4)    Add 50cm3 of 0.5M sulphuric acid (H2SO4) to the beaker and start the stopwatch immediately.

5)    View the cross through the beaker from above. Stop the time on the stopwatch immediately as the cross disappears.

6)    Record the time and repeat the procedure 5 times.

7)     Reduce the volume of the Na2S2O3 each time by 5cm and add 5cm of water to the mixture to maintain the 50cm3 in the measuring cylinder at all times.

8)    Record the times and tabulate the results. Plot a graph of volume of sodium thiosulphate against time.

Suggested Results:

Rate of reaction table showing the decomposition of sodium thiosulphate over time.

Rate of reaction table showing the decomposition of sodium thiosulphate over time.

Rate of reaction graph showing the decomposition of sodium thiosulphate over time.

Rate of reaction graph showing the decomposition of sodium thiosulphate over time.

Discussion:

  1. Explain why the cross disappears.

>>>The cross disappeared as a result of the formation of the yellow precipitate of sulphur when the sulphuric acid was added to the sodium thiosulphate.

  1. Write a balanced equation for the reaction

>>>H2SO4 (aq) + Na2S2O3 (aq)     =       S(s) + SO2 (aq) + Na2SO4 (aq) +   H2O (l)

  1. Write the ionic equation for the reaction.

>>> S2O32- (aq) + 2H+ (aq)        =       S(S) + SO2 (aq) + H2O (l)

  1. Identify the variables in the experiment.

>>> The variables were:

  • The manipulated variables were the sodium thiosulphate and the water
  • The responding variable was time
  • The controlled variable was the volume of sulphuric acid.
  1. Based on your experimental data, make a general statement about the effect of concentration of reactants on time and reaction rate.

>>>Concentration affects the rate of reaction. Therefore over time as the concentration of sodium thiosulphate decreased then the rate of the reaction also decreased.

Source of Error/ Limitations/ Assumptions:

  •       Inaccurate timing. Time may be lost during the experiment between the times taken to notice the cross has disappeared to the actual stopping of the watch.
  •       Inaccurate measurement of reactants will affect the overall rate of reaction.
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