Rate of reaction

1. What is rate of reaction

2. Synthesising factors affecting the rate of reaction

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RATE OF REACTION• The rate of reaction tells us how

quickly a chemical reaction happens.• If a reaction occurs rapidly, the

rate of reaction is high. • The speed of reaction can be

measured by either measuring i. The quantity of a product with time or ii. The quantity of a reactant with time.• The quantity can be number of moles,

mass of solid, volume of gas or concentration of a solution

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e.g. :

• 5 g of magnesium ribbon reacts completely with dilute hydrochloric acid. What is/are the quantity reactant or product that can be measured?

• Hint : the changes must be visible and measurable !

• Answers : mass of magnesium Volume of hydrogen gas

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Experiment to determine rate of reaction between calcium carbonate and hydrochloric acid

• Measure volume of gas released in time

• Using the water displacement method to collect the gas

• Apparatus needed are :1. Conical flask2. Delivery tube3. Burette4. Basin 5. Retort stand

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Set up apparatus

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Results of experiment

Time / s

Burette reading / cm3

Volume of gas liberated / cm3

0

30

60

90

48.90

41.40

37.15

34.40

See example of result

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WAYS OF MEASURING THE RATE

• Two methods can be used to determine the rate of reaction, they are :

1. average rate which is the rate of reaction over an interval of time.

2. instantaneous rate which is the exact rate of reaction at any given time

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AVERAGE RATE

change in amount of reactant or product

Time taken for the change to happen

Average rate of reaction =

Average rate of reaction in the first 2 minute

16.5 cm3 / 120 s=

= 0.14 cm3s-1

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• The instantaneous rate of reaction is determine by :

a. drawing a tangent to the curve at the given time

b. finding the gradient of this tangent

• Therefore,Rate of reaction

at a given time = gradient of the tangent

to the curve at the given time

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DISCUSSION

• The curve at 2nd minutes is steeper than at 4th minutes

• Rate of reaction at 2nd minutes is higher than at 4th minutes

• The maximum volume of gas shows that the reaction has been completed

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FACTORS THAT AFFECT THE RATE OF REACTION

• Five factors are :i. effect of particle size

ii. effect of temperature

iii. effect of concentration

iv. effect of catalyst

v. effect of pressure

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THE COLLISION THEORY• Collision theory states that :a. A chemical reaction occurs when reactant

particles collide with each other.b. Not all collision will result in the

formation of products.c. A collision that lead to a chemical reaction

are known as effective collision.d. Effective collision must satisfy two

conditions:i. the reactant particles have enough

energy to overcome the activation energy, Ea of the reaction

ii. the particles collide in the proper orientation.

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THE COLLISION THEORY

• When the frequency of collision between particles of reactant increase, the frequency of effective collision between particle will also increase

• When the frequency of effective collision increases, the rate of reaction will also increase.

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SIZE OF REACTANT• A smaller size of reactant has a higher

rate of reaction• The smaller pieces of solid have a larger

total surface area exposed for collision• As a result, the frequency of collision

will increase• This causes frequency of effective

collision to increase • Hence the rate of reaction will increase

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Total surface area = 4 x 6

= 24 cm2

Total surface area = 2(4 x 2 )+ 2 (2 x 4)

= 32 cm2

Total surface area = 4(2 x 2 )+ 4 (1 x 4)

= 40 cm2

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EFFECT OF CONCENTRATION

• A reactant with higher concentration has a higher rate of reaction

• When the concentration is increase, the number of particles per unit volume will increase.

• As a result, the frequency of collision will increase

• This causes frequency of effective collision to increase

• Hence the rate of reaction will increase

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EFFECT OF TEMPERATURE• An increase of temperature will

increase the rate of reaction• An increase of temperature will cause

the kinetic energy to increase • As a result, the reactant particle will

move faster and collide more often• This causes frequency of collision to

increase • Hence the frequency of effective

collision rate of reaction will increase• The rate of reaction will increase

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EFFECT OF CATALYST• When a positive catalyst is used, the rate of

reaction will increase• Catalyst will enable reaction to occur

through an alternative path with lower activation energy.

• As a result, more colliding particle can overcome the lower activation energy.

• The frequency of collision will increase• This causes frequency of effective collision

to increase • Hence the rate of reaction will increase

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