Rate of reaction2

In this chapter, you will learn to:

Analyse the rate of reaction.
Synthesise factors affecting the rate of reaction.
Synthesise ideas on the collison theory
Learn about practising scientific knowledge to enhance quality
of life.

1.2 SYNTHESISE FACTORS
AFFECTING THE RATE OF
REACTION

LEARNING OUTCOMES
Give examples of reactions that are affected by size of
reactant, concentration, temperature, and catalyst.

Explain how each factor affects the rate of reaction.

Describe how factors affecting the rate of reaction are applied
in daily life and in industrial processes.

Solve problems involving factors affecting rate of reaction.

INTRODUCTION

The rate of reaction is affected by the following factors:

 The size of reactants (or surface area)
 Concentration of reactants
 Temperature
 Catalyst
 Pressure

Size of Reactants
For a given quantity of the same reactants, the size of the
particles affects the reaction.
The smaller the sizes of the particles, the larger is the surface
area exposed for reaction.
Example:
 If 2g of marble chips (large particles) and 2 g of calcium carbonate
powder ( small particles) are reacted separately with 50 cm3 of 1
mole dm-3 hydrochloric acid, the following results will be
obtained:
Conditions Time taken to obtain 50 cm3
CO2 (min)
2 g marble chips with 50 cm3 4
HCl, 1 mole dm-3
2 g CaCO3 with 50 cm3 HCl, 1 ½
mole dm-3

Concentration of Reactants
If 2 g of marble chips are reacted separately with dilute
hydrochloric acid of two different strengths, the following
results will be obtained:
CO2 (s)
HCl, 1 mole dm-3
HCl, 2 mole dm-3

The higher the concentration of a reactant, the higher is the
rate of that reaction.

Temperature
The rate of reaction increases when the temperature at which
the reaction is taking place is increased.
Example:
CO2 (min)
HCl, 1 mole dm-3 at 27 oC
HCl, 1 mole dm-3 at 35 oC

Catalyst
Catalyst is a substance which causes a change in the rate of
reaction without itself undergoing a chemical change at the
end of the reaction.
Example:
 Hydrogen peroxide, H2O2, does not decompose at room conditions,
but in the presence of a catalyst, it decomposes into water and
oxygen. The catalyst used is manganese (IV) oxide.
A catalyst can either hasten or slow down a reaction. A
catalyst which quickens (Hastens) a reaction is called a positive
catalyst or promoter, while a catalyst which slow down a
reaction is called a negative catalyst of inhibitor.

Continue…
Characteristics of a catalyst include the following:
 A catalyst does not undergo any chemical change at the end of the
reaction. The quantity and chemical composition of the catalyst
remain the same at the end of the reaction.
 However, the catalyst can undergo a physical change during the
reaction. For example, coarse manganese(IV) oxide change to the
powdery form at end of the decomposition of potassium chlorate(V).
 Only a small quantity of catalyst is needed to change a rate of
reaction but a larger quantity of catalyst can speed up the reaction
and therefore increase the rate of reaction.
 A catalyst only changes the rate of reaction but not the quantity of
products. The product produced in the reaction will not increase by
the addition of a catalyst.
 The efficiency of a catalyst is reduced if impurities are present in it.

Continue…
Some catalysts used in industrial processes are as follows:
Process Reaction Catalyst
Contact process To produce sulphur trioxide in the Vanadium
manufacture of sulphuric acid. pentoxide
V2O5
Haber process To manufacture ammonia Iron
Fe
Ostwald process To produce nitrogen monoxide in Platinum
the manufacture of nitric acid Pt
Hydrogenation of Vegetable oils are converted to Nickel
margarine margarine by reaction with Ni
hydrogen

Pressure
Pressure can affect the rate of reaction only if it involves
gases.
The rate of reaction of solids and liquids are not effected by
changes in pressure.
A higher pressure can increase the rate of reaction.
Example:
 Haber process
 Nitrogen and hydrogen combine to form ammonia when heated, but at
low pressure, the reaction very slow. At a pressure of 250 atm, the rate of
reaction is higher.

Assignment

Group 1
To investigate the effect of the size of the reactant on the rate
of reaction.
 Your presentation should include the following aspect:
 Aim
 All the variables
 Hypothesis
 List of substances and apparatus
 Procedure
 Tabulation of data
 Discussion
 Conclusion

Assignment

Group 2
To investigate the effect of concentration on the rate of
reaction.
 Aim
 Hypothesis
 Procedure
 Discussion
 Conclusion

Assignment

Group 3
To investigate the effect of temperature on the rate of
reaction.
 Aim
 Hypothesis
 Procedure
 Discussion
 Conclusion

Assignment

Group 4
To investigate the effect of catalyst on the rate of reaction.
 Aim
 Hypothesis
 Procedure
 Discussion
 Conclusion

Assignment
Group 5
Solve the following problem:
Volume (cm3)

100

Time (s)

Continue…

1 g of calcium carbonate powder in excess is added to 50 cm3
of 0.1 mole/dm3 hydrochloric acid. The volume of gas
produced is recorded at fixed points of time. A graph, showing
volume against time as given in figure above is obtained. The
experiment is repeated, using the same quantity of calcium
carbonate and 50cm3 of 0.3 mole/dm3 hydrochloric acid.
 Calculate the maximum volume of gas that is produced.
 Draw the graph of volume against time on the same axes of the
earlier experiment.

Rate of reaction2

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