7.1 NILAM

7.1 DETERMINE RATE OF REACTION

State the meaning of rate The rate of reaction is a measurement of the change in quantity of reactant or

of reaction. product per unit time.

State the relationship 1 The rate of reaction is high if the reaction occurs fast
between rate of reaction if the reaction occurs slowly
and time. within a short period of time.

Give examples of fast 2 The rate of reaction is low
reactions.
within a long period of time.
Give examples of slow
reactions. 3 The rate of reaction is inversely proportional to time:

How to determine rate of Rate of reaction 1
reaction?
Time taken
How to identify the
change in quantity of – Reaction of marble chip with hydrochloric acid.
reactant/product for – Reaction of magnesium with sulphuric acid.
measuring rate of – Reaction of potassium with water.
reaction? – Burning of fuel.
Give example.
– Rusting of iron in the air.
What are the possible – Photosynthesis.
unit for the rate of – Fermentation of fruit juice to form alcohol.
reaction?
Rate of reaction can be determined by calculating the rate of chemical change or
measured quantity in a chemical change per unit time.

Rate of reaction = Change in quantity of reactant/product
Time taken for the change to occur

The change in amount of reactant or product in any reaction which is chosen for
the purpose of measuring rate of reaction must be observable and measurable.

Example:
(a) Decrease in the mass of reactant.
(b) Increase in the mass of product.
(c) Increase in volume of gas released.
(d) Formation of precipitate as a product.

Units for the rate of reaction depends on the unit for the reactant or product of the
reaction. The possible units are:
(a) g s–1 or g min–1 for increase in mass of product or decrease in mass of reactant
(b) cm3 s–1 or cm3 min–1 for increase of volume gas released
(c) s–1 or min–1

How to measure the Chemical reaction Observable Method of measuring the
observable changes when changes observable changes
a reaction produce gas? Reaction between
magnesium and Decrease in
hydrochloric acid: mass of Hydrochloric Magnesium
magnesium acid

MgCl2(aq) + H2(g) Reading from the balance is recorded
in every 30 seconds.
Increase in
volume of Method I
hydrogen

Hydrochloric Water
acid

Magnesium

Hydrogen gas is collected by water
displacement in a burette. The volume
of hydrogen gas collected is recorded
every 30 seconds.

Method II

Hydrochloric acid

Magnesium

The rate of reaction is measured by the
volume of gas collected in the gas
syringe per unit time.
*This apparatus set-up can also be used

to measure the increase in volume of
other gases that are insoluble for
example oxygen, hydrogen and
carbon dioxide.

How to measure the Chemical reaction Observable Method of measuring the
observable change when changes observable changes
a reaction produces Reaction between
precipitate? sodium thiousulphate Formation of Sodium
and hydrochloric acid: sulphur as a thiosulphate
precipitate solution +
Na2S2O3(aq) + *Volume of sulphur hydrochloric
acid
2NaCl(aq) + H2O(l) + dioxide gas, SO2
SO2(g) + S(s) cannot be Amount of solid sulphur
measured by water formed is measured by the
displacement time taken for the mark ‘X’
because sulphur placed under the conical
dioxide is soluble
in water.

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How to determine the rate of reaction from the graph?
Measurement of Rate of Reaction

Rate of reaction at any given time/temperature/concentration Average rate of reaction

Rate of reaction is measured by Rate of reaction is measured by time taken Average rate of reaction from 0 Average rate of reaction within
volume of gas released in every for the formation of precipitate. second: certain period of time:
30 seconds by water Example Rate of reaction is measured by Rate of reaction is measured by
displacement in a burette. volume of gas released in every 30 volume of gas released in every 30
Example Sodium thiosulphate seconds by water displacement in a seconds by water displacement in a
solution + burette. burette.
Hydrochloric hydrochloric acid Example Example
acid
Water Sketch of graph: Hydrochloric Hydrochloric
Calcium (a) Concentration of sodium acid acid
carbonate Water Water
thiosulphate solution / mol dm–3 Calcium Calcium
carbonate carbonate
Sketch of graph: M1
Volume of carbon dioxide gas / cm3 Sketch of graph:
Sketch of graph: Volume of carbon dioxide gas / cm3
t1 Time / s Volume of carbon dioxide gas / cm3
y Rate of reaction for sodium thiosulphate V V1
V2
x
t1 Time / s solution with concentration M1 mol dm–3
The rate of reaction at t1 second 1 x s–1
= The gradient of tangent to the = t1 s =
curve at t1 s
y cm3 (b) Temperature of sodium 1 2 3 4 5 6 7 8 Time / s 1 2 3 4 5 6 7 8 Time / s
= xs
thiosulphate solution / °C Average rate of reaction in the fourth

minute

T1 minutes = (V2 – V1) cm3 = y cm3 s–1
(4 – 3) s
= (V – 0) cm3 = x cm3 s–1
(4 – 0) s

Rate of reta1 ction for sodium Time / s
thiosulphate

solution at temperature T1 °C
1 y s–1
= t1 s =

Exercise

1 An experiment is carried out to determine the rate of reaction of 20 cm3 of 0.5 mol dm–3 hydrochloric acid with
excess calcium carbonate. The results are shown below.

Time / s 0 15 30 45 60 75 90 105 120 135 150 165
0.00 10.00 16.00 22.00 27.00 31.50 36.00 39.50 42.00 44.00 44.00 44.00
Volume of CO2 /
cm3

(a) (i) Write a chemical equation for the above reaction.
CaCO3 + 2HCl CaCl2 + H2O + CO2

(ii) State the observable and measurable changes in the experiment.
Increase in volume of carbon dioxide/decrease in mass of calcium carbonate

(iii) State the meaning of the rate of reaction for the above reaction.
Change in volume of carbon dioxide gas in one second/change in mass of calcium carbonate in one
second.

(iv) Draw an apparatus set-up to measure rate of reaction in the given reaction.

Hydrochloric acid Water
Calcium carbonate

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(b) Draw the graph of the volume of carbon dioxide gas collected against time.
Volume of CO2 / cm3

50

40

30

20

10

0 Time / s
30 60 90 120 150 180

(c) From the graph, determine:
(i)

= Time taken for the change to occur

= 27
60

= 0.45 cm3 s–1

(ii) the average rate of reaction in the second minute

= Time taken for the change to occur

= 42 – 27
60

= 0.25 cm3 s–1

(iii) the time when the reaction has completed
135 s

(iv) the average rate of reaction for overall reaction

Total volume of carbon dioxide collected
=

Time taken for the change to occur

= 44
135

= 0.326 cm3 s–1

(v) the rate of reaction at 30 seconds
= the gradient of the graph at 30 seconds

= 0.405 ± 0.1 cm3 s–1

(vi) the rate of reaction at 105 seconds
= the gradient of the graph at 105 seconds

= 0.217 ± 0.1 cm3 s–1

(d) Compare the rate of reaction at 30 seconds and 105 seconds. Explain your answer.
Rate of reaction at 30 seconds is higher than at 105 seconds because the concentration of hydrochloric
acid decreases as time increases.

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2 Excess of zinc powder is added to 50 cm3 of 1 mol dm–3 hydrochloric acid. The volume of hydrogen gas
collected and time taken are recorded. Complete the following table.

Sketch a curve for volume of Tangent on the Volume of hydrogen / cm3
hydrogen gas collected against curve at t1, t2 and V
time for the reaction between t3 respectively
excess of zinc powder with 50 cm3
of 1 mol dm–3 hydrochloric acid. 0 t1 t2 t3 Time / min
The tangents on the curve at t1, t2 2 + H2
and t3 are shown.

Write the balanced equation for
the reaction.

Calculate the volume of hydrogen From the equation,
gas collected in the experiment at 2 mol of HCl : 1 mol H2
room conditions. 0.05 mol HCl : 0.025 mol H2

Volume of H2
= 0.025 mol × 24 dm3 mol–1
= 0.6 dm3
= 600 cm3

Compare the gradient of the curve The gradient of tangent on the curve at t2 is lower than t1. The
at t1 and t2. Explain your answer. rate of reaction at t2 is lower than at t1. The rate of reaction

decreases as the time increases because mass of zinc and
concentration of hydrochloric acid decreases .

What is the gradient at t3? Explain The gradient of tangent on the curve at t3 is zero , the rate of
your answer. reaction at t3 is zero . The reaction is completed at t3. All
hydrochloric acid has reacted because zinc powder used is in

excess . At t3, maximum volume of hydrogen gas is collected.
The maximum volume of hydrogen gas collected is 600 cm3 .

Sketch a curve for mass of zinc Mass of zinc / g
against time.

Time / s

Sketch a curve for concentration Concentration of hydrochloric acid / mol dm–3
of hydrochloric acid against time.

Time / s
3

Calcium carbonate Cotton wool
Hydrochloric acid

Electronic balance

(a) (i) Write a balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid.
CaCO3 + HCl CaCl2 + H2O + CO2

(ii)
The carbon dioxide gas released from the reaction between hydrochloric acid and calcium carbonate

(b)

(c) Describe how this set up of apparatus can be used to measure the rate of reaction.
– The reading of electronic balance are taken at regular intervals of time

(d) The sketch of graph below shows the results of the experiment when the reading is plotted.

A

B
C
80 Time / s

State and describe the rate of reaction in the following points shown in the graph.
(i) Point A

The gradient of the graph is the steepest. The rate of reaction is the highest at the start.

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(ii) Point B
The gradient of the graph becomes less steep. The rate of reaction decreases.

(iii) Point C
The graph is a horizontal straight line. The reaction has stopped.

(e) State one factor that affect the rate of reaction from point A to B.
Concentration of hydrochloric acid decreases as time increases.

(f) Sketch a graph to show the decrease in mass against time on the graph below.

Decrease in mass / g

80 Time / s

(g) The chemical equation below shows a neutralization reaction.

NaOH(aq) + HCl(aq 2O(l)

your answer.

7.2 FACTORS THAT AFFECT THE RATE OF REACTION

1 The rate of reaction is affected by:
(a) Size of solid reactant
(b) Concentration of solution (for the reactant used in the form of solution)
(c) Temperature of solution at which the reaction occurs
(d) Presence of catalyst (for a particular reaction)
(e) Pressure of gas reactant

Rate of
Reaction