1.2.2 - Laboratory Activity: The Effect of Concentration of the Reactant on the Rate of Reaction

Laboratory Activity 1.2.2: The Effect of Concentration of the Reactant on the Rate of Reaction

Aim: To understand the relationship between concentration of reactants and the rate of reaction.
Problem statement: How the concentration of reactants affect the rate of reaction?
Hypothesis: The higher the concentration of the reactants, the faster the rate of reaction.
Variable:

	»	Fixed variable : Volume and temperature of reactants
	»	Manipulated variable : Concentration of reactants
	»	Responding variable : Rate of reaction

Material: » Sodium thiosulphate 0.2mol dm-3 » Hydrochloric acid 0.2 mol dm-3 » Distilled water

Apparatus: » Conical flask » Measuring cylinder » Stopwatch » Filter paper

Procedure:

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	►	1. 50cm3 of sodium thiosulphate, Na2S2O3 0.2mol dm-3, is measured with a measuring cylinder and poured it into a conical flask. 2. A cross 'X' is drawn in the middle of the filter paper. A conical flask containing the sodium thiosulphate solution is placed on top of the marked filter paper. 3. The cross 'X' can be easily seen through the conical flask containing the sodium thiosulphate solution. 4. 5cm3 of hydrochloric acid 0.2mol dm-3 is measured by measuring cylinder. 5. The hydrochloric acid is added quickly into the conical flask containing sodium thiosulphate solution. 6. The conical flask solution is swirled slowly. 7. The stopwatch is stopped when the cross can no longer be seen from the above. 8. The time taken for cross 'X' disappear are recorded. 9. The experiment is repeated by using different volumes of sodium thiosulphate, hydrochloric acid and distilled water. 10. This video shows the observation on the effect of concentration on the rate of reaction.

Results:

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Volume of sodium thiosulphate, V (cm3) Volume of water added (cm3) Concentration of sodium thiosulphate solution after dilution (mol dm-3) Time to produce the precipitation (s) 50 0 0.2 19 40 10 0.16 23 30 20 0.12 33 20 30 0.08 48 10 40 0.04 115

Discussion:

	►	Sodium thiosulphate solution reacts with hydrochloric acid to form sulphur precipitate slowly as shown in the following equation: ○ Na2S2O3(aq) + H4SO4(aq) → Na2SO4(aq) + H2O(l) + SO2(g) + S2 ○ The time taken for the precipitation of the sulphur is used to to determine the rate of reaction.
	►	Graph of concentration against time ○ Based on the graph of concentration against time, we can be observed that when the concentration of sodium thiosulphate decreases, the time it takes for cross 'X' to disappear increase. ○ This means that when the sodium thiosulphate having lower concentration, the rate of reaction decreases as well.
	►	Graph of concentration against $\frac{1}{\mathrm{time}}$. ○ The graph of concentration against $\frac{1}{\mathrm{time}}$ is a straight line. This means that the concentration of sodium thiosulphate directly proportional to $\frac{1}{\mathrm{time}}.$ ○ The higher the concentration of the reactants, the higher is the rate of reaction.

Conclusion:

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A reaction becomes faster when the concentration of a reactant increases.

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