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Factors Affecting the Rate of Reaction

Mind Map

Total surface area

Big Small

Rate of Reaction

Concentration High Low Rate of Reaction

Temperature High Low Rate of Reaction

Catalyst With catalyst Without catalyst Rate of Reaction

Pressure High Low Rate of Reaction

Factors that influence a Rate of Reaction 1. Rate of reaction is influenced by the following factors:

a) Total surface area of the substance of reaction or the size of the solid substance of reaction.

b) Concentration of the substance of reaction c) Temperature of the reactant d) Catalyst e) Pressure

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Important Tip 1. The definition of effective collision needs to have 3 main concepts:-

• Total energy of the particles that collide must be the same or exceeding the activation energy. • The particles that collide must have the correct orientation of collision • The collision gets to produce a chemical reaction.

2. The definition of activation energy must contain 2 main concepts:- • Minimum energy that must be possessed by particles of the reacting substance that collide. • Energy that must be overcome by the substances of the reaction that collide so that the reaction can

occur to give the products of the reaction. A diagram of energy profile 1. The meaning of energy activation can be pictured by a diagram of energy profile. 2. In the diagram of energy profile, the activation energy is shown by the difference in energy between the peak

of the graph and the level of the energy of the reacting substance.

Collision Theory 1. The particles of the reacting need to touch to enable formation or breaking of the bonds for a reaction to

happen. 2. Collisions of particles of a reacting substance need to achieve a certain minimum energy (Activation

Energy) in order to produce a reaction. 3. Particles that collide also need to have the correct orientation of collision. Activation Energy 1. The activation energy is the minimum energy that the reactants particles must achieve at the time of

collision in order for a chemical reaction to take place. 2. The value of the activation energy is different for different reactions. 3. A reaction with a high activation energy occurs slowly whereas a reaction with a low activation energy

occurs fast. Effective Collision Effective collision is the collision that produces a reaction achieving minimum energy that is sufficient and with a correct orientation of collision.

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The orientation of collision 1. Consider a simple reaction involving a collision

between two molecules - ethene, CH2=CH2, and hydrogen chloride, HCl, for example. These react to give chloroethane.

2. As a result of the collision between the two molecules, the double bond between the two carbons is converted into a single bond. A hydrogen atom gets attached to one of the carbons and a chlorine atom to the other.

3. The reaction can only happen if the hydrogen end of the H-Cl bond approaches the carbon-carbon double bond. Any other collision between the two molecules doesn't work. The two simply bounce off each other.

4. Of the collisions shown in the diagram, only collision 1 may

possibly lead on to a reaction. Frequency of effective collision with rate of reaction and factors that affect the rate of reaction 1. According to the collision theory, the frequency of effective collision influences the rate of reaction. 2. The frequency of effective collision is the number of effective collisions which occur in 1 second. 3. The rate of reaction depends on the frequency of effective collisions that occur. 4. If the frequency of an effective collision for a reaction is high, then the rate of reaction is also high. On the

other hand, if the frequency of an effective collision is low, then the rate of reaction will become low Wrong view 1. The rate of reaction increases when the total number of collisions between particles increases. 2. When the number of effective collisions increases, then the rate of reaction also increases. Actual 1. The rate of reaction increases when the total number of collisions per unit of time between the particles

increases. 2. When the number of effective collisions increases, the rate of reaction does not necessarily have to increase.

The rate of reaction increases only if effective collisions that occur per second increase, that is when the frequency of effective collisions increases. So, the correct statement is

"When the frequency of effective collisions increase, the rate of the reaction will also increase or when the number of effective collisions per second increases, the rate of reaction increases.

3. The factors that influence the rate of reaction can be explained using the collision theory.

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The effect of surface area of a solid on the rate of reaction

a) When the size of the solid substance that reacts is small, the total surface area exposed to the reaction, increases.

b) This means that as the chance for collision increases, that is the frequency of the collisions increase. So, the frequency of effective collisions also increase.

c) When the total surface area that is exposed to the reaction increases, the rate of reaction increases.

The effect of concentration on the rate of reaction

a) The increase in concentration of the reacting substance means there are many more particles per unit of volume.

b) This causes the chance for collisions to increase, that is the collisions increase. So, the frequency of effective collisions also increases.

c) When the concentration of a reacting substance increases, the rate of reaction also increases.

The effect of temperature on the rate of reaction

a) When the temperature of a reaction increases, the particles of the reacting substance have more kinetic energy.

b) The particles of the reacting substance move faster. c) This increases the chance for collision, i.e. the frequency of collisions

increase. So, the frequency of effective collisions also increases. d) When the temperature of a reaction increases, the rate of reaction also

increases. The effect of pressure on the rate of reaction

a) For a reaction that involves a gas, when pressure increases, the particles of gas are compressed to fill the spaces which are small. This makes the number of particles of gas per unit of volume to increase.

b) Chances for collisions to increase, that is the frequency of collisions increase. This causes the frequency of effective collisions to also increase.

c) When pressure of a reaction increases, the rate of reaction also increases. The effect of using a positive catalyst on the rate of reaction

a) When a positive catalyst is used in a reaction, the catalyst prepares an alternative path for the reaction which needs an activation energy which is lower.

b) So, the greater the particles which collide possess energy which exceeds the new activation energy which is lower.

c) So, more effective collisions, means the frequency of effective collisions increases.

d) The use of positive catalysts increases the rate of reaction.

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Example 1

Example 2

Example 3

Example 4

Example 5

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Example 6

Example 7 Experiment Reactant Initial

rate of reaction

1 3.0g magnesium powder with 15cm3 of 2.0 mol dm-3 sulphuric acid

t

2 3.0g magnesium tape with 15cm3 of 1.0 mol dm-3 sulphuric acid

u

3 3.0g magnesium powder with 30cm3 of 1.0 mol dm-3 sulphuric acid

v

Example 8 Which of the following statements best explains why coal dust forms an explosive mixture with air? A Powdering coal breaks chemical

bonds. B Powdered coal catalyses the

explosion. C Powdering coal releases hydrogen

from compounds in coal. D Powdering coal releases methane from

compounds in coal. E Powdering coal produces a large

surface area. Example 9 Two experiments were carried out under the same conditions of temperature and pressure, reacting marble with dilute hydrochloric acid. In Experiment 1, an excess of powdered marble was added to 20cm3 of dilute hydrochloric acid. In Experiment 2, an excess of marble chips was added to 20cm3 of dilute hydrochloric acid of the same concentration. The total volumes of carbon dioxide given off were measured at intervals and plotted against time.

Which of the following pairs of curves would be obtained in the two experiments? Experiment 1

powdered marble Experiment 2 marble chips

A X Y B X Z C Y X D Y Z E Z X Answer 1 D 3 C 5 C 7 D 9 A 2 D 4 B 6 D 8 E 10

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Structure Question 1. Magnesium ribbon reacts with hydrochloric acid to produce hydrogen. A student used an excess of

hydrochloric acid in the apparatus below to investigate this reaction.

His results are shown on the graph

a) What volume of gas was in the syringe at the end of the reaction?

[1 mark]

b) State how the rate of reaction changes during the first four minutes and explain the change.

[2 mark] c) The experiment was repeated using the same quantities of reagents but with the acid at a higher

temperature. Draw on the graph the line that should be obtained at this temperature. d) Some power stations burn coal in the production of electricity. The coal is ground to a fine powder

before being burned in the furnace. Use your knowledge of rates of reaction to suggest why the coal is ground to a fine powder rather than used in large lumps.

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[2 mark] 2. Two experiments are conducted to compare the rate of decomposition of hydrogen peroxide under the

same condition as follows: Experiment I 5 cm3 3-volume hydrogen peroxide solution is poured into a conical cone containing 2.0 g manganese(IV) oxide. Experiment II 5 cm3 6-volume hydrogen peroxide solution is poured into a conical cone containing 2.0 g manganese(IV) oxide. a. Write down the equation to represent the decomposition of hydrogen peroxide.

[1 mark] b. Give one method to collect the gas released so that the volume of gas can be read easily every 30

seconds.

[1 mark] c. In the space given below, sketch a graph of the volume of gas collected against time forecast for

Experiment I and II, on the same axis.

d. i) Which experiment gives the highest rate of decomposition of hydrogen peroxide?

[1 mark]

ii) State the reason for your answer in (d)(i).

[1 mark] e. i) State one other substance that can replace manganese(IV) oxide in the experiment.

[1 mark] ii) What is the role of manganese(VI) oxide in the in-crease in rate of decomposition of hydrogen

peroxide? Explain this role in terms of the theory of collision of particles.

[2 marks] f. In addition to the concentration of hydrogen peroxide and catalyst, state one other factor that can

cause the change in the rate of decomposition of hydrogen peroxide.

[1 mark]

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3. Diagram 1 shows a graph for the quantity of product of reaction against time in two experiments.

a. From the above graph (i), calculate the average rate of reaction for the whole reaction.

b. Calculate the average rate of reaction in graph (i) during the first 3 hours.

[3 marks] c. The graphs in Diagram 1 are obtained from reactions that take place at 25°C. Redraw the above graphs

to show changes in the quantity of the product of reaction if the reactions take place at 35°C.

[2 marks] d. How can changes in temperature influence the rates of reaction?

[2 marks]

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Application of Rate of Reaction

Mind Map

Storing Food

Keeping food in a refrigerator 1. Decay and decomposition of food by micro organisms is a chemical reaction which occurs in food

causing the food to get spoilt. 2. If food is kept in the fridge, the food will keep longer because the low temperature will slow down the

rate of the chemical reaction which destroys food. 3. Micro organisms like bacteria and fungus which are active at a temperature of between 10°C until 60°C.

The room temperature is the optimum temperature for bacterial action. So, food easily gets spoilt at room temperature.

Cooking

Cooking food in a pressure cooker 1. In a pressure cooker, the high pressure causes the water in the cooker to boil at a temperature of more

than 100°C. 2. At a higher temperature, the time for the food to get cooked is decreased. 3. Cooking in a pressure cooker saves time and energy.

Cooking Food in Small Pieces 1. Food in the shape of big pieces has a surface area per volume which is small, so the heat takes a longer

time to reach the inside of the food. So, to cook faster, the food needs to be cut into smaller pieces.

Industry The knowledge about factors which influence the rate of reaction are also used to find the optimum state for the industrial process so that energy, time and cost can be saved. The knowledge can be applied in the industrial process

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Haber Process 1. The Haber process is the process of preparing ammonia in industry. 2. In the Haber process, powdered iron is used as the catalyst to raise the rate of reaction. 3. A mixture of nitrogen and hydrogen in the ratio 1:3 is conducted through the powdered iron as catalyst at

a temperature of 450°C to 550°C and a pressure of 200 to 300 atmospheres with molybdenum as a promoter.

N2(g) + 3H2(g)200 300

450 550 ,o

atm

C Iron

−−

→ 2NH3(g)

4. However, the production of ammonia in the process is about 10% only. 5. Ammonia that is produced is cooled and is taken out in liquid form. Hydrogen and nitrogen that does not

react is recycled once again.

Contact Process 1. Sulphuric acid is prepared in industry using Contact process. 2. In the first stage, sulphur is burnt in air to produce sulphur dioxide.

S (s) + O2 (g)→ SO2 (g)

3. In the second stage, sulphur dioxide that is formed is mixed with excess oxygen and is conducted through the catalyst vanadium (V) oxide to raise the rate of reaction. A temperature of 500°C and a pressure of l to 2 atmospheres is used.

2SO2(g) + O2(g)2 5

500o

V O

C→ 2SO3(g)

4. In the third stage, sulphur trioxide that is formed is dissolved in concentrated sulphuric acid to form oleum at stage two.

SO3 (g) + H2SO4 (aq) → H2S207 (l)

5. Sulphur trioxide does not dissolve directly in the water because this reaction releases too much heat and could even produce an explosion. Then,,, water is mixed with the oleum to produce concentrated sulphuric acid.

H2S207 (1) + H2O (1) →2 H2SO4(aq)

Ostwald Process 1. Nitric acid is prepared in industry through the Ostwald process. 2. At the first stage, ammonia is oxidized in excess in the presence of platinum as catalyst to produce

nitrogen monoxide. The high temperature of 850°C and pressure of 5 atmospheres is used

4NH3(g) + 5O2(g) 5

850o

atm→4NO(g) + 6H2O(1)

3. Nitrogen monoxide that is formed reacts with oxygen to produce nitrogen dioxide at stage two.

2NO (g) + O2 (g) →, 2NO2 (g)

4. At the final stage, nitrogen dioxide together with excess air is dissolved in hot water at a temperature of 80°C to produce concentrated nitric acid.

4NO2(g) + O2(g) + 2H2O(1) → 4HNO3(g)

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Making Margarine 1. Vegetable oil is an organic compound that is not saturated and exists in liquid state at room temperature. 2. Through investigation and continuous development, vegetable oil can be changed to margarine through

the process of hydrogenation using nickel as catalyst at a temperature of 180°C.

Breakdown of petroleum 3. Big molecules of hydrocarbon obtained during fractional distillation of petroleum have been found to be

less useful than small molecules of hydrocarbon. The breakdown process with the use of the catalyst alumina produces smaller hydrocarbons.

Burning of coal 1. Coal contains the element carbon. Burning of coal in air that is in excess will produce carbon dioxide,

water, and heat energy. 2. A big piece of coal takes a long time to burn because the total surface area that is touched by the fire is

small. 3. The rate of burning pieces of coal which are small is higher because the total surface area is bigger. With

this, it provides a lot of heat energy in a short period of time. Example 1 Which of the following pairs of teb catalysts and processes are true? Catalyst Process A Aluminium oxide Production of sulphuric

acid in the Contact process

B Iron Production of ammonia in the Haber process

C Nickel Production of nitric acid in the Ostwald process

D Vanadium(V) oxide

Production of margarine via hydrogenation

Example 2 Which of the following pairs of catalyst and processes are correctly matched? Catalyst Process A Iron Manufacture of ammonia in

the Haber process

B Nickel Manufacture of nitric acid in the Ostwald process

C Vanadium(V) oxide

Manufacture of sulphuric acid in the contact process

D Lead(IV) oxide

Production of oxygen by the decomposition of hydrogen peroxide

A. I and II only B. II and III only C. I, II and IV only D. I, III and IV only

Example 3 Food can be kept longer in the refrigerator. This is because the A. lack of oxygen in the refrigerator slows down

the decomposition of food by bacteria B. lower temperature in the refrigerator slows

down the decay of food C. rate of bacterial reproduction is lowered in the

dark condition inside the refrigerator D. process of food decay releases heat, which is

subsequently absorbed by the refrigerator Example 4 Which of the following statements about the contact process are correct? I. The process is carried out at 450 °C and 250

atmospheres II. The rate of production of sulphur trioxide

becomes faster at high pressures III. Vanadium(V) oxide is used to increase the

yield of sulphur trioxide IV. The rate of reaction between sulphur dioxide

and oxygen is increased at a higher temperature

A. I and II only B. II and IV only C. I, Il and III only D. II I II and IV only

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Example 14 Hydrogen peroxide solution is catalytically decomposed by manganese (IV) oxide. Three experiments were performed using a fixed mass of catalyst. The results are shown below.

The solutions used were : 50 cm3 of 2.0 mol/dm3 hydrogen peroxide; 100 cm3 of 1.0 mol/dm3 hydrogen peroxide; 100 cm3 of 2.0 mol/dm3 hydrogen peroxide; Which curve did each solution pro-duce? (i) (ii) (iii) A Z Y X B X Y Z C X Z Y D Y Z X E Z X Y