RATE OF REACTION

The meaning of rate of reaction

1. The speed of a chemical reaction is called the rate of reaction.2. The rate of reaction can be determined as the rate of disappearance of a reactant. Rate of reaction = Amount of reactant used up Time taken3. The rate of reaction can also be determined as the rate of formation of a product. Rate of reaction = Amount of product formed Time taken4. The amount of reactant or product involved in a reaction can be measured in terms of mass of the substance or the concentration of the substance.5. If a product is a gas, the rate of reaction can be measured as the volume of gas produced per unit time.6. The rate of reaction is inversely proportional to the time taken for the reaction to be completed. Rate of reaction α 1⁄time taken

Measuring the rate of reaction1. There are two ways to express the rate of reaction.a) The average rate over a period of timeb) The rate at any given time (instantaneous rate)

Activity : To find out the reaction rates of the reaction between magnesium and dilute sulphuric acid

Apparatus : Measuring cylinder, conical flask, burette, delivery tube, basin, retort stand/clamp, stopwatch.Procedure :Results :Time/s 0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450Burettereading/cm3

50.0 41.5 35.0 31.0 28.0 25.0 22.5 21.0 19.5 18.0 17.0 16.5 16.0 15.5 15.0 15.0

Vol. of gas/cm3

Graph of volume of hydrogen released against time :

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Discussion :1. The rate of reaction decreases as the reaction progresses with time. 2. The rate of reaction at 90 sec is :

3. The average rate of reaction for the first 100 sec is :

Note : 1. The average rate of reaction can be calculated a) directly from the data givenb) from the graph drawn2. The rate of reaction at a given time (instantaneous rate) can only be calculated from the gradient of the graph at that given time.

Factors that affect the rate of reaction

1. Surface area(particle size) of the solid reactant:1. For the same mass of a solid, the smaller the size of the solid particles, the larger is the total surface area of the solid.2. Hence the smaller the size of the solid particles, the faster(higher) is the rate of reaction.

2. Concentration of reactant (solution)In reactions that involve solutions, the higher the concentration of the reactant, the higher rate of reaction.

3. Effect of TemperatureThe rate of reaction increases when the temperature of the reactants is increased.

4. Presence of a catalyst1. A catalyst is a substance that changes the rate of a reaction but is itself chemically unchanged at the end of the reaction.2. A positive catalyst increases the rate of a reaction while an inhibitor or a negative catalyst decreases the rate of a reaction.3. Only a small amount of catalyst is required to increase the rate of a reaction.4. The rate of reaction is proportional to the amount of catalyst used.5. A catalyst increases the rate of a chemical reaction but it does not change the yield of the reaction.6. In general, catalysts are highly specific and only increase the rate of one type of reaction.

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Experiment : To investigate the effect of the surface area of a reactant on the rate of reactionProblem statement : How does the surface area of a solid reactant affect the 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 higher the rate of reaction.Variables:Manipulated : Size of marble chipsResponding : Volume of gas given off at 30 sec intervals.Constant : Temperature, mass of marble chips, concentration and volume of hydrochloric acid.Apparatus : Burette, conical flask, delivery tube with rubber stopper, retort stand and clamp, measuring cylinder and stopwatch.Materials : Marble chips, 0.2 mol dm-3 hydrochloric acid

Expt. I : Investigating the rate of reaction using large marble chips.1. A burette is filled with water and inverted over a basin of water. The burette is clamped as shown in the diagram and the water level adjusted. The initial burette reading is recorded.2. 5.0 g of marble chips are weighed and placed in a conical flask.3. 50 cm3 of 0.2 mol dm-3 hydrochloric acid is added to the marble chips.4. The delivery tube with the stopper is inserted into the mouth of the conical flask and the stopwatch is started.5. The burette readings are recorded at 30 sec intervals.Results :Time/sBurette reading/cm3

Volume of gas/cm3

Expt. II : Investigating the rate of reaction using smaller marble chips.1.Steps 1 to 4 are repeated using 5.0 g of smaller marble chips.2. All the other conditions are kept constant.

Results :

Discussion :

1. From the graphs, we can conclude that the maximum volume of carbon dioxide collected in both experiments are the same. Why ?2. The reaction in expt. I stops after t2 sec while the reaction in expt. II stops after t1 sec. ( t1 < t2 ). Why ?

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3. In practice, the volume of carbon dioxide collected in the burette is slightly less than the theoretical value. Why ? To overcome the problem, a gas syringe can be used to collect the gas.

Conclusion :1. Graph II is steeper than graph I.2. Hence the rate of reaction in expt. II is higher than the rate of reaction in expt. I.3. The smaller the particle size, the larger the total surface area exposed for reaction, and the faster the rate of reaction.

Experiment : To study the effect of concentration on the rate of reaction between sodium thiosulphate solution and dilute sulphuric acid.

Problem statement : How does the concentration of a reactant affect the rate of reaction between sodium thiosulphate and dilute sulphuric acid?Hypothesis : The higher the concentration of the sodium thiosulphate solution, the higher the rate of reaction.Variables :Manipulated : The concentration of sodium thiosulphate , Na2S2O3, solutionResponding : Time taken for the cross X to disappearConstant : Concentration and volume of sulphuric acid, temperatureApparatus : 10 cm3 and 100 cm3 measuring cylinder, 100 cm3 conical flask, white paper marked with the cross X, stop watchMaterials : 0.2 mol dm-3 sodium thiosulphate solution, 1.0 mol dm-3 sulphuric acid , distilled water.Procedure :1. 50 cm3 of 0.2 mol dm-3 sodium thiosulphate solution is poured into a clean, dry conical flask using a 100 cm3 measuring cylinder.2. The conical flask is placed on a cross X marked on a piece of white paper.3. 5 cm3 of dilute sulphuric acid is measured in a 10 cm3 measuring cylinder and quickly poured into the sodium thiosulphate solution. The stopwatch is started immediately.4. The reaction mixture is swirled once and the cross X is viewed from the top. A yellow precipitate appears slowly in the mixture.5. The stopwatch is stopped as soon as the cross X disappears from view. The time taken is recorded.6. Steps 1 to 5 are repeated with different mixtures of sodium thiosulphate solution and distilled water as shown in the table.

Results :Expt. 1 2 3 4 5Volume of sodium thiosulphate/cm3 50 40 30 20 10Volume of water/cm3 0 10 20 30 40Volume of sulphuric acid/cm3 5 5 5 5 5Concentration of sodium thiosulphate/mol dm-3 0.20 0.16Time taken for cross to disappear/s 24.0 30.0 42.0 62.0 111.01/time(s-1) 0.042 0.033 0.024 0.016 0.009

Discussion:1. Na2S2O3(aq) + H2SO4(aq) → Na2SO4(aq) + H2O(l) + SO2(g) + S(s) Ionic equation :2. A fixed amount of sulphur (fellow solid) is required for the disappearance of the cross X.

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3. Therefore the time recorded for the disappearance of the cross X is the time taken for the formation of a fixed mass of sulphur.4. Rate of reaction = Mass of sulphur produced Time taken

5. Hence rate of reaction

Graphs :

I II

6. The concentration of sodium thiosulphate solution is directly proportional to its volume before dilution because the final volume of the diluted solution is the same at 50 cm3.7. The conical flask used in each expt. must be the same size. Why ?8. The bigger the conical flask, the longer the time taken for the cross X to disappear. Why ?

Conclusion :1. From graph I, the higher the concentration of sodium thiosulphate solution, the faster the rate of reaction.

2. From graph II, is directly proportional to the concentration of sodium thiosulphate.

3. That is the rate of reaction is directly proportional to the concentration of sodium thiosulphate solution.

The Collision Theory of particles1. The Kinetic Theory of matter states that matter is made up of tiny, discrete particles that are continually moving.2. These particles collide with each other. According to the collision theory, some of these collisions will result in a chemical reaction while some will not.3. The collision that are successful in producing a chemical reaction are called effective collisions.4. Effective collisions are collisions that achieved the minimum energy and collide in the right orientation.5. The minimum energy that the reactant particles must possess at the time of collision in order for a chemical to take place is called the activation energy, Ea.6. The collision theory can be used to explain the factors affecting the rate of chemical reactions.7. The rate of a reaction depends on the frequency of effective collisions. The higher the frequency of effective collisions, the higher the rate of reaction.

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8. Small solids reactants have a larger total surface area exposed for collisions. This enables more collisions between the reacting particles. Therefore the frequency of effective collisions increases and the rate of reaction increases.9. When the concentration of solution increases, the number of particles per unit volume increases. Hence the frequency collision increases. Therefore the frequency of effective collisions increases and the rate of reaction increases.10. When the temperature is increased, the reacting particles move faster and collide with a higher energy. Hence the frequency of collisions increases and the frequency of effective collisions increases. Therefore the rate of reaction increases.11. A catalyst provides an alternative reaction route. A positive catalyst provides an alternative route that has a lower activation energy. As a result, more reacting particles possess sufficient energy to overcome the lower activation energy required for effective collisions. Therefore, the frequency of effective collisions increases and the rate of reaction increases.

Structure Question

1. Two experiments were carried out in which hydrogen peroxide solution was added to manganese(IV) oxide powder. The table below shows the description of the reactants used in the experiments.

Experiment Reactants

I 10 cm3 of 2-volume hydrogen peroxide solution + 3.0 g manganese(IV) oxide powder

II 10 cm3 of 4-volume hydrogen peroxide solution + 3.0 g manganese(IV) oxide powder

The volume of gas liberated was recorded at regular time intervals.

(a) Draw a figure of the set-up of apparatus to carry out this experiment. [2 marks]

(b) Write the chemical equation for the decomposition of hydrogen peroxide. [1 mark]

(c) Sketch a graph of volume of gas liberated against time for experiments I and II on the same axes[2 marks]

(d) (i) Among the two reactions, which occurs at a higher rate? [1 mark]

(ii) State the reason for your answer in (d)(i) [1 mark]

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(e) Manganese(IV) oxide acts as a catalyst in experiments I and II. Using the collision theory, explain the role of manganese(IV) oxide in increasing the rate of liberation of gas in the experiments. [3 marks]

2. (a) Food cooks faster in a pressure cooker. Explain why. [3 marks]

(b) A student carries out three experiments to investigate factors that affect the rate of reaction. The table below shows the description of each experiment. In each experiment, gas was collected until the reaction stopped.

Experiment

Reactants Temperature (°C)

Total volume of gas

collected at 2 minutes (cm3)

IExcess zinc powder + 20 cm3 of 0.2 mol dm-3 sulphuric acid

30 20.0

IIExcess zinc chips + 20 cm3 of 0.2 mol dm-3 sulphuric acid

30 16.0

IIIExcess zinc powder + 20 cm3 of 0.2 mol dm-3 sulphuric acid + copper(II) sulphate solution (catalyst)

30 32.0

(i) Calculate the average rate of reaction at the first two minutes for experiment I, II, and III in cm3s-1 [3 marks]

(ii) Write the chemical equation for the reaction between zinc and sulphuric acid. Calculate the maximum volume of gas produced in experiment I. [4 marks]

(iii) Deduce the maximum volume of gas produced in experiment III [1 mark]

(iv) Sketch a graph of the volume of gas collected against time for experiments I, II, and III on the same axes. [3 marks]

(v) Compare the rate of reaction between the following:

(i) Experiment I and experiment II

(ii) Experiment I and experiment III

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In each case, explain why there is a difference in the rate of reaction based on the collision theory. [6 marks]

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