Chemistry Form 5 chapter 1.3 Rate of reaction

Factors affecting the rate of Factors affecting the rate of reactionreaction

Experiment 1.1: To investigate the effect of the

surface area of a reactant on the rate of reaction .


Problem statement How does the surface area of a solid reactant

affect rate of reaction?


Hypothesis The smaller the size of the reactant particles, that is, the

larger the total surface area of the reactant particles, the faster the rate of reaction.



Apparatus Conical flask, delivery tube fitted with a rubber stopper,

retort stand and clamp, burette, measuring cylinder and stopwatch.


Materials Marble chips, powdered marble and 0.2 mol dm -3

hydrochloric acid.


Experiment 1 The rate of reaction using large marble chips Procedure 1 A burette is filled with water and inverted over a basin containing

water. The burette is clamped to the retort stand. The water level in the burette is adjusted and the initial burette reading is recorded.


Experiment 1 The rate of reaction using large marble chips Procedure 2. 5.0 g of marble chips are placed in a small conical flask.


Experiment 1 The rate of reaction using large marble chips Procedure 3. 50 cm3 of 0.2 mol dm-3 hydrochloric acid is added to the marble

chips.


Experiment 1 The rate of reaction using large marble chips Procedure 4 The delivery tube with a rubber stopper is inserted into the mouth

of the conical flask (Figure 1.13). The stopwatch is started simultaneously.


Experiment 1 The rate of reaction using large marble chips Procedure 5 The burette readings are recorded at 30-second intervals.


Experiment 1 The rate of reaction using large marble chips Results



Experiment II - The rate of reaction using powdered marble

Procedure 2 The results of the experiment are recorded

in the following table.


Results Based on the results

obtained, a graph of the total volume of carbon dioxide produced against time for each experiment is plotted on the same axes (Figure 1.11).


1 Figure 1.12 shows the graphs that will be obtained if the reactions in Experiments I and II are completed.


2 Figure 1.15 shows that both graphs level off at the same value. This indicates that the maximum volume of carbon dioxide collected at the end of reaction for both Experiments I and II are the same (that is, 120 cm3).


2 Figure 1.15 shows that both graphs level off at the same value. This indicates that the maximum volume of carbon dioxide collected at the end of reaction for both Experiments I and II are the same (that is, 120 cm3). This happens because the masses of the marble and the volumes of the hydrochloric acid used in both the experiments are the same.


3 The gradient of the graphs for Experiments I and II become less steep as the reactions proceed.


3 The gradient of the graphs for Experiments I and II become less steep as the reactions proceed. This shows that the rates of reaction

(a) are very high at the beginning of the reaction,



(a) are very high at the beginning of the reaction, (b) decrease as the reactions proceed,



(a) are very high at the beginning of the reaction, (b) decrease as the reactions proceed, (c) become zero when the reactions have completed.

At this time, the graphs become horizontal.


4 The rate of reaction between the marble and hydrochloric acid decreases because

(a) the mass of the remaining unreacted marble decreases.


4 The rate of reaction between the marble and hydrochloric acid decreases because

(a) the mass of the remaining unreacted marble decreases. (b) the concentration of hydrochloric acid decreases.


5 The reaction in Experiment I stops after t2 minutes while the reaction in Experiment II stops after t1, minutes, where t1 < t2. This shows that the rate of reaction for Experiment II (powdered marble) is faster than the rate of reaction for Experiment I (marble chips).


6 The total volume of carbon dioxide collected in the burette is usually slightly less than the theoretical value.


6 The total volume of carbon dioxide collected in the burette is usually slightly less than the theoretical value. This is because carbon dioxide is slightly soluble in water. To overcome this problem, a gas syringe is used to collect carbon dioxide released during the experiment (Figure 1.13).


Conclusion: Graph (II) is steeper than graph (I). This shows that the rate

of reaction in Experiment II is faster than the rate of reaction in Experiment I. Powdered marble is used in Experiment II. Thus, the rate is faster with powdered marble than with marble chips. Hence, we can conclude that the smaller the particle size, the larger the total surface area exposed for reaction and the faster the rate of reaction.

ConcentrationConcentration


Problem statementHow does the concentration of a reactant affect the rate

of reaction between sodium thiosulphate and dilute sulphuric acid?


Hypothesis The more concentrated the sodium

thiosulphate solution, the higher the rate of reaction.




Apparatus 10 cm3 and 100 cm3 measuring cylinders, 100 cm3

conical flask, white paper marked with a cross 'X', and stopwatch.


Materials 0.2 mol dm -3 sodium thiosulphate solution, 1.0

mol dm-3 sulphuric acid and distilled water.


Procedure 1 50 cm3 of 0.2 mol dm-3 sodium thiosulphate solution is

measured out using a 100 cm3 measuring cylinder. The solution is then poured into a clean, dry conical flask.


Procedure 2 The conical flask is placed on a piece of paper with

across `X' marked on it (Figure 1.14).


Procedure 3 5 cm3 of dilute sulphuric acid is measured out by

using a 10 cm3 measuring cylinder. The acid is then quickly poured into sodium thiosulphate solution. The stopwatch is started immediately.


Procedure 4 The reaction mixture is swirled once and the cross `X'

is viewed from above. A yellow precipitate will appear slowly in the conical flask.


Procedure 5 The stopwatch is stopped as soon as the cross

disappears from view and the time taken is recorded.


Procedure 6 Steps 1 to 5 are repeated with different mixtures of

sodium thiosulphate solution and distilled water as shown in the following table.


Results

50 x M= 40 x 0.2

Experiment 1 2 3 4 5

Volume of Na2S2O3(cm3) 50 40 30 20 10

Volume of water 0 10 20 30 40

Volume of H2SO4(cm3) 5 5 5 5 5

Concentration of Na2S2O3(moldm-3)

0.20 0.16 0.12 0.08 0.04

Time taken(s) 24 30 42 62 111

0.042 0.033 0.024 0.016 0.009)(

1 1sTime

2

112 V

VMM


Discussion 1. Sodium thiosulphate, Na2 S2 O3 , reacts with

dilute sulphuric acid according to the equation: Na2S2O3(aq) + H2 SO4(aq) Na2SO4(aq) + H2O(l) +

SO2(g) + S(s)


Discussion 1. Sodium thiosulphate, Na2 S2 O3 , reacts with

dilute sulphuric acid according to the equation: Na2S2O3(aq) + H2 SO4(aq) Na2SO4(aq) + H2O(l) +

SO2(g) + S(s)

The ionic equation is as follows: S2O3

2- (aq) + 2H+ (aq) S(s) + SO2(g) + H2O(l) The sulphur is precipitated as fine particles and

causes the solution to turn cloudy.


Discussion 2 As the amount of sulphur increases, the cross `X'

becomes more and more difficult to see. Finally, the cross `X' disappears from view when a certain mass of sulphur is precipitated. Hence, the time recorded for the disappearance of the cross `X' is the time taken for the formation of a fixed mass of sulphur.


Discussion 3 Rate of reaction =

takentime

producedsulphur of mass

disappear toX'' cross for the taken time

1reaction of rate Hence,


Discussion 4 The concentration of sodium thiosulphate solution after

mixing with water can be obtained by using the following formula:

Concentration of Na2S2O3

= 3 322

2

11

50

usedNa of volume2.0 moldm

OS

V

VM


Discussion 5 Based on the

experimental results obtained, two graphs can be plotted.

(a) The graph of concentration of sodium thiosulphate against time

(Graph I, Figure 1.15).


(b) The graph of concentration of sodium thiosulphate against

(Graph II, Figure 1.16)

takentime

1


6 The conical flask used for each experiment must have

the same size (for example, 100 cm3 volume). If the conical flask of a larger size (for example, 250 cm3 volume) is used, the time, t, taken for the cross `X' to disappear will increase. When the diameter of the bottom of conical flask increases, a greater amount of sulphur must be formed for the cross `X' to disappear. Conversely, if a smaller conical flask (for example, 50 cm3 volume) is used, the time taken for the cross to disappear will be shorter.


7 If the experiment is repeated with dilute sulphuric acid of different concentrations, but the concentration of sodium thiosulphate is kept constant, the rate of reaction will also be directly proportional to the concentration of the acid used.


Conclusion 1 From graph I, we can conclude that: (a) the higher the concentration of sodium

thiosulphate, the shorter the time taken for a certain mass of sulphur to he precipitated, that is, for the cross `X' to disappear from view.


Conclusion 1 From graph I, we can conclude that: (b) This means that the higher the

concentration of sodium thiosulphate, the faster the rate of reaction.


Conclusion 2 From graph II, it can be concluded that the

concentration of sodium thiosulphate is directly proportional to

Concentration of Na2S2O3 ……….(1)

time

1

Higher concentration, shorter time

time

1


3 But the rate of reaction is …………(2)

Hence, combining equations (1) and (2), we have, concentration of Na2S2O3 reaction rate.

That is, rate of reaction concentration of Na2S2O3 solution. The hypothesis is accepted.

time

1

Concentration of Na2S2O3 time

1

time

1

TemperatureTemperature

Experiment 1.3: to study the effect of temperature on the rate of reaction between sodium thiosulphate solution and dilute sulphuric acid


Problem statement How does temperature affect the rate of

reaction between sodium thiosulphate solution and sulphuric acid?


Hypothesis The higher the temperature of the reactant,

the faster the rate of reaction.


Variables (a) Manipulated variable: The temperature

of sodium thiosulphate solution


Variables (b) Responding variable: The time taken for the

cross `X' to disappear (c) Fixed (controlled) variables: The concentrations

and volumes of both sodium thiosulphate solution and dilute sulphuric acid


Apparatus Conical flask, 10 cm3 measuring cylinder,

thermometer, stopwatch, white paper marked with a cross `X', wire gauze, tripod stand, and Bunsen burner.


Apparatus Conical flask, 10 cm3

measuring cylinder, thermometer, stopwatch, white paper marked with a cross `X', wire gauze, tripod stand, and Bunsen burner.


Materials 0.1 mol dm-3 sodium thiosulphate solution and

1.0 mol dm-3 sulphuric acid.


Procedure Experiment I Rate of reaction at room temperature 1 50 cm3 of 0.1 mol dm-3 sodium thiosulphate solution is

measured out using a 100 cm3 measuring cylinder, and poured into a clean, dry conical flask. The temperature of the sodium thiosulphate solution is measured with a thermometer.


Experiment I Rate of reaction at room temperature 2 The conical flask is placed on a white paper marked

with a cross 'X' (Figure 1.17). 3 5 cm3 of l mol dm-3 sulphuric acid is measured out

using a 10 cm3 measuring cylinder. The acid is then quickly poured into the sodium thiosulphate solution.


Experiment I Rate of reaction at room temperature 4 The stopwatch is started immediately and the conical

flask is swirled gently. 5 The cross 'X' is viewed from above. The stopwatch is

stopped as soon as the cross disappears from view and the time taken is recorded.


Experiment II to V 6 The solution in the conical flask is poured out. The

conical flask is washed thoroughly and dried. 50 cm3 of 0.1 mol dm-3 sodium thiosulphate solution is poured into the conical flask. The solution is heated over a wire gauze until the temperature reaches about 45 °C (Figure 1.18).


Experiment II to V Rate of reaction at temperatures above room temperature 7 The hot conical flask is placed over a white paper

marked with a cross X. 8 5 cm3 of 1 mol dm-3 sulphuric acid is measured out

using a 10 cm3 measuring cylinder.


Experiment II to V Rate of reaction at temperatures above room temperature 9 . When the temperature of sodium thiosulphate solution

falls to 40 °C, the sulphuric acid is quickly poured into the thiosulphate solution. The stopwatch is started immediately and the conical flask is swirled gently.


Experiment II to V Rate of reaction at temperatures above room temperature 10 The cross 'X' is viewed from the top and the time

taken for the cross to disappear from view is recorded. 11 Steps 6 to 9 are repeated at higher temperatures as

shown in the following table.


Results


Results Based on the

results of the experiment, a graph temperature of sodium thiosulphate solution against

is plotted (Figure 1.19).

time

1


Conclusion: 1 The graph shows that the temperature of

sodium thiosulphate solution is proportional (but not linearly) to

time

1


Conclusion: 2 Temperature ... (1)

But rate of reaction ... (2)

Combining equations (1) and (2), we have, Rate of reaction temperature

time

1

time

1


Conclusion:3 The higher the temperature of

the experiment, the faster the rate of reaction.

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Chemistry Form 5 chapter 1.3 Rate of reaction