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Sodium thiosulphate when merged with a solution of Hydrochloric acid forms an insoluble precipitate of sulphur (S). Sulphur dioxide (SO2) and water (H2O) are also formed as a by-product, but it is the solid sulphur that has the biggest effect here. Sulphur acts as an emulsion in this reaction, staying in suspension and eventually blocking the light from reaching the solution. This modifies the solution from being colourless to milky and then entirely opaque. This mainly occurs due to the precipitates of elemental sulphur that are being formed, which are insoluble and thus cloud the water. The rate of a chemical reaction is directly proportional to the temperature (0C/K). As the temperature rises, the rate of the reaction increases as well. With the rise in temperature the kinetic energy of the molecules increases simultaneously. Commonly, it is observed that for every 100 rises in temperature there is double the reaction rate. Hence, an increase in the reaction rate of hydrochloric acid and sodium thiosulphate mixture also increases the rise in temperature.
| Table of Contents |
Key Takeaways: Acid, Hydrochloric acid, Sodium, Sodium thiosulphate, temperature, molar mass, solution, soluble
Aim
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The aim of the experiment is to study the effect of concentration and temperature difference or variation respectively on the rate of reaction between Sodium sulphate and Hydrochloric acid.
Materials Required
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The apparatus required in the experiment are:
- Conical flask- volume (250 ml)
- Stopwatch
- Tripod stand
- Measuring cylinder
- Burner
- Thermometer
- Wire gauge
- 0.1 M thiosulphate solution
- Concentrated nitric acid
- Water (Distilled)
- 1 M hydrochloric acid
Procedure
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a. The consequences of concentration on rate of reaction:
- Take a conical flask and fill it half with water. This will act as a constant temperature bath, maintained at room temperature.
- Rinse and fill the burette with 1M HCl solution.
- Take a 100 ml beaker and make a mark ‘x’ at the bottom of the vessel. Fill 50ml of 0.1M sodium thiosulphate solution inside it. Place the beaker in the flask.
- Now, the mark ‘x’ will be visible due to the transparency of the system. Make the beaker stand in the flask for a while for few minutes till it attains the temperature of the bath.
- Add 1 ml of 1M HCl solution with the help of a burette. Start the stopwatch when ½ of the HCl solution (0.5ml) has been transferred. Do not forget to swirl while adding HCl.
- Record the time required for the mark ‘x’ to become invisible. This stage indicates the completion of the reaction.
- Perform the experiment again by adding 2, 4, 8, and 16 ml of 1 M hydrochloric acid solution to fresh sodium thiosulphate solution every time till the ‘x’ mark disappears completely in each case.
b. The outcome of temperature on the rate of reaction:
- In a beaker of 100 ml, take 50ml of 0.1M sodium thiosulphate, on the outside surface of the bottom of which a cross ‘x’ has been painted. Keep the beaker in a thermostat maintained at 300 C.
- Add 5ml of hydrochloric acid solution with swirling. Start the stopwatch as soon as ½ (2.5 ml) the amount of hydrochloric acid has been transferred.
- Note the time at which the mark ‘x’ becomes invisible.
- Repeat the experiments at temperatures 400 C, 500 C, 600 C, and 700 C using a fresh solution of sodium thiosulphate each time and then record the time at which the mark ‘x’ disappears.
- After recording the readings. Plot two graphs, one for the volume of HCl added (this determines the concentration of HCl) and the time taken for the mark to disappear. The other graph for temperature and the time taken for the mark to get invisible.
- For plotting the graph, the variation in time is plotted on the x-axis and the variation in volume or temperature is plotted on axis y.
- Sample data and results can be seen as below:
| Beaker | Volume of Na2S2O3 | Volume of H2O | Na2S2O3, M | Reaction time in sec | Reaction rate per sec |
|---|---|---|---|---|---|
| 1 | 50 | 0 | 0.15 | 22.5 | 0.0444 |
| 2 | 40 | 10 | 0.12 | 27.3 | 0.0367 |
| 3 | 30 | 20 | 0.090 | 35.1 | 0.0285 |
| 4 | 20 | 30 | 0.060 | 60 | 0.0167 |
| 5 | 10 | 40 | 0.030 | 159.1 | 0.00629 |
Effects of concentration of reaction time
Effects of concentration of reaction rate
Result
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The result here is apparent that the rate of reaction between sodium thiosulphate solution and hydrochloric acid increases with the rise in temperature.
Precautions
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The precautions that need to be followed for the given experiment are:
- Start the stopwatch when half the amount of HCl acid solution has been transferred in the reaction flask and stop it when the mark ‘x’ disappears.
- If a bath of constant temperature is not available to maintain the constant temperature, heat the water of the bath in which the beaker is kept from time to time with constant swirling and remove the burner when the temperature is attained.
- To plot a graph, select a suitable scale.
Things to Remember
- Before using the apparatus, rinse it thoroughly with concentrated nitric acid and distilled water.
- Take the precise or exact quantity of solution for the experiment to perform.
- For avoiding the variation in temperature make sure the experiment is done in one go.
- Be aware while starting and stop of the watch.
- View cross mark ‘x’ throughout the reaction from top to bottom from the same height for all observations.
Sample Questions
Ques. Name the solutions used in the above experiment. Give the concentration of each. When should the stopwatch be started? (3 marks)
Ans. The name of solutions used in the experiment are:
- Hydrochloric acid
- Sodium thiosulphate
The concentration of sodium thiosulphate is -0.1 M.
It is important for you to start the stopwatch as soon as you pour half of the HCl in the fresh sodium thiosulphate solution.
Ques. What are the factors on which the rate of reaction depends? Also, give the units of the rate of reaction. (3 marks)
Ans. The factor on which the rate of reaction depends are:
- Nature of the reactants
- Concentration of the reactants
- Temperature
- Presence of catalyst
- Presence of radiation.
The units of the rate of reaction are: Mol L-1 s-1 (Moles per litre per second)
Ques. Explain: (3 marks)
a. What is the mass action law?
b. What is the temperature coefficient of the reaction?
Ans: a. Law of mass action: The law of mass action can be defined as the rate of a reaction being directly proportional to the product of the molar concentrations of the reactants.
- Temperature coefficient: This is described as the ratio of rate constants at two temperatures varying by 100. Its value is generally equal to 2.
Ques. Explain: (5 marks)
a. What are the units of the rate constant for zero-order reactions?
b. What are the units of rate constants for first-order reactions?
c. What is the repercussion of temperature on the rate constant of a reaction?
d. Why certain reactions are so swift in nature?
Ans:
- The unit of rate constant for a zero-order reaction is similar to the rate of the reaction. The unit is Moles/litre/sec.
- The units of the rate constant for a zero-order reaction is per second or /second or s-1.
- The effect is simple, it rises with the rise in temperature.
- Few reactions are too quick to happen because they have very low activation energy.
Ques. Explain: (3 marks)
a. Threshold energy.
b. Why reactions with molecularity of more than three are rare?
c. What is the rate-determining step?
Ans:
- Threshold energy is defined as minimum energy which the colliding molecules must consist of so as to have an effective collision.
- The molecularity of more than three are rare as a simultaneous collision between more than three particles is rare on the basis of probability considerations.
- In some complex reactions, the slowest step determines the overall rate of the reaction. This specific step is known as the rate-determining step.
Ques. Express: (3 marks)
a. Can the order of a reaction be fractional?
b. What is meant complex reaction?
c. What do you understand by 4 volumes of H2O2 solution?
Ans:
- Yes, the order of a reaction can be fractional.
For instance, CH3CHO → CH4 + CO…the order here is 3/2.
- A reaction that consists of more than a single step is known as a complex reaction.
- It is the way of expressing a cone of H2O2 solution. one litre of ‘4 volume’ of H2O2 solution gives four litres of oxygen at N.T.P on decomposition.
Ques. Brief: (3 marks)
(1) Express the cone, of 1 M H2O2 solution in terms of volume strength.
(2) What is the equivalent mass of H2O2 ?
Ans:
- 2 H2O2 → 2 H2O + O2
2 mol 22.4 L
1 litre of 1 M H2O2 consists of 1 mole of H2O2 and thus gives 11.2 L on complete decomposition or with no residue. Hence, 1 M H2O2 solution is 11.2 volume.
- Equivalent mass of H2O2 = (1) x (2) + (16) x (2)
= 34/2 = 17.
Ques. Express: (3 marks)
a. What is the normality of 1 M H2O2 solution?
b. What is the oxidation number of oxygen in H2O2?
c. What is the colour of the starch-iodine complex?
- The normality of 1 M H2O2 solution is 2N.
- The oxidation number of oxygen in H2O2 is -1.
- The colour of the starch-iodine complex is blue.
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