Rate of Reaction I
What is rate of reaction?
1. Rate of reaction is defined as the change in the amount of reactants or products per unit time.
2. It means ‘how fast is the reaction’ or 'the speed of the reaction'.
• Fast Reaction High rate
• Slow Reaction Low rate
Quantity change of reactants/products
Rates of reaction =
Total time for the reaction
In a chemical reaction, 2.5g of calcium carbonate
react completely with excess hydrochloric acid to
produce 600cm3 of carbon dioxide gas in 1.5
minutes. Find the rate of reaction in term of
a. decreasing mass of calcium carbonate
b. increasing volume of carbon dioxide gas
[a. -0.0278g/s; b. 6.67cm3/s]
Example of fast reaction
Type of Reaction Example
Combustion Combustion of magnesium in oxygen
Combustion of ethane (C2H6)
Reaction between Reaction between potassium and water
reactive metal and
Reaction between metal Reaction between limestone/ marble and sulphuric acid
carbonate and acid
Ionic precipitation Precipitation of silver(I) chloride
(Double decomposition) AgNO3 + HCl → AgCl + HNO3
Example of slow reaction
Type of Reaction Example
6CO2 + 6 H2O → C6H12O6 + 6O2
4Fe + 3O2 + 2H2O → 2Fe2O3•H2O
C6H12O6 → 2C2H5OH + 2CO2
Measurement and Calculation
Quantity Measure and Average Rate
During a chemical reaction, two things happen 2. The easily measured quantity changes include
The quantities of reactants reduce. a. Mass
The quantities of products increase. b. Concentration (Conductivity)
1. Therefore, the rate of the reaction can be c. Volume of gas
a. measuring the decrease of the amount of
the reactants over time.
b. measuring the increase of the amount of
the products over time.
Example Example 2
Reaction between calcium carbonate and In a reaction, 5 g of calcium carbonate takes 250
hydrochloric acid. seconds to completely react with solution of
hydrochloric acid. Calculate the average rate for
CaCO3 + 2HCl → CaCl2 + CO2 + H2O
this reaction in units
The rate of the reaction can be determined by (a) g s-1 and
a. Measuring the reduces of the mass of the (b) mol s-1
calcium carbonate (reactants) over time [ Relative atomic mass: C 12; 0, 16; Ca, 40]
Mass of CaCO3 reacted
Rate of Reaction =
b. Measuring the increases of the volume of the
carbon dioxide gas produced over time.
Vlolume of CO 2 produced
Rate of Reaction =
[a. 0.02g/s; b. 0.0002mol/s]
Immeasurable quantity and average rate
1. In some reaction, some changes are observable Example 4
but difficult to be measured. For example When the aqueous of ethanadioic is mixed with
a. Change in colour acided potassium manganate(VII) , the reaction
b. Precipitation happen slowly at room temperature. The purple
2. The time taken for the colour of a reactant to colour of the solution is bleached after 40 seconds.
change or certain amount of precipitate to Calculate the average rate of reaction.
form can be used to measure the rate of Answer
Rate of reaction =
If the quantity change is immeasurable
Rates of reaction =
Total time for the reaction [0.025s-1]
Example 3 Cr2O72-(aq) + 14H+ + C2O42- →
6CO2(g) + 7H2O(l) + 2Cr3+ (aq)
2HCl(ak) + Na2S2O3(ak)
2NaCl(ak) + S(p) + SO2(g) + H2O(ce) The chemical equation above represents a reaction
between acidic potassium dichromate(VI) and
In a reaction between dilute hydrochloric acid and ethanedioic. Which of the following can be used
sodium thiosulphate, sulphur precipitate was to determine the rate of reaction?
produced after 2 minute . What is the rate of the I. Volume of carbon dioxide collected over time.
reaction? II. Time for chromium ion to form.
Answer III. Time for the colour of the solution to change
from orange to green.
Rate of reaction = IV. Time for the pH change.
A. I and II
B. I and III
C. I II and III
[0.00833s-1] D. II, III and IV
The graph of quantity of chemical over time
The reaction between dilute hydrochloric acid c) the concentration of hydrochloric acid against
and excess marble will produce calcium chloride time.
and gas of carbon dioxide. Sketch the graph of d) the concentration of calcium chloride against
a) the mass of the marble against time. time.
b) the volume of carbon dioxide against time.
Calculate the Average Rate from the
The rate of reaction is equal to the slope of
the graph of quantity against time.
In a reaction between calcium carbonate and liquid
hydrochloric acid, carbon dioxide gas that is released
is collected in a burette. The graph below shows the
volume of carbon dioxide collected over time. Find
the average rate of reaction in the first 60s.
1. The rate of reaction changes from time to time as the reaction happens.
2. The rate of reaction at a particular time is called the instantaneous rate.
3. The instantaneous rate of a reaction is equal to the gradient of tangent at a particular time.
Rate of reaction =
δ (product) ∆(product)
Rate of reaction = Rate of reaction =
δ (Time) ∆(Time)
δ(Product) = Small change of the amount of product ∆ (Product) = Change of the amount of product
δ(Time) = Small change of the time ∆ (Time) = Change of the time
This method is not practical because δ(Product) and
δ(Time) is too small to be measured.
Simple Exercise 1
Find the reaction rate at t = 40s.
a. Find the rate of reaction at 50 second.
b. Find the rate of reaction at 200 second.
Example 9 Time/s Mass lost/g
In a reaction between sodium and water, the mass 30 1.10
lost of sodium is recorded every 30s, as shown in 60 1.75
the table below. By using a graph, 90 2.20
Find the rate of reaction at 1 minute. 120 2.50
[0.019 g s-1] 150 2.58
Find the average rate of reaction at the third 180 2.60
minute. 210 2.60
[0.0016g s-1] 240 2.60
Sketching a Graf
1. To sketch a graph, we need to determine
• the total quantity of the product that formed
• relative reaction rate ( higher or lower ).
Mole of reactant/product
For solution For gas (When volume is given)
MV Volume of gas
n= Molar volume of gas (22.4dm3 at stp / 24dm3 at rtp)
For solid, liquid or gas ( When mass is given )
Molar mass (RAM/RMM)
Example 12 a. Calculate the number of moles of
In an experiment, a quantity of excess zinc hydrogen that is produced when the
powder is added to 25 cm3 of liquid reaction is completed. [Molar volume of
hydrochloric acid. Figure below shows the gas: 24 dm3 mol-1 at room temperature].
curve obtained when hydrogen that is
released is plotted against time.
b. Calculate the concentration of
hydrochloric acid that is used in the
1. The rate of reaction between calcium carbonate with liquid hydrochloric acid is determined by the
6 g chips of calcium carbonate are mixed with 100 cm3 of solution of hydrochloric acid 1.0 mol dm-3 in a
conical flask. The flask is place on to a balance and the decrease in mass of its contents is recorded at
certain time intervals. A graph of the decrease in mass against time is drawn and the results are as shown
a. Write an equation for the reaction between calcium carbonate and liquid hydrochloric acid.
b. State the change in rate of reaction in the first 240 seconds. Why?
c. Why is it that the decrease in mass of the contents of the conical flask does not change after 120
d. The experiment is repeated using 25 cm3 of hydrochloric acid 2.0 mol dm-3, instead of 100 cm3
hydrochloric acid 1.0 mol dm-3 and other factors are maintained. Sketch the change of the shape of
the curve that will be obtained on the graph above.
e. Calculate the volume of carbon dioxide that is released in the experiment that uses 100 cm3
hydrochloric acid 1.0 mol dm-3 [Molar volume of gas:24 dm3mol-1 at room temperature. The relative
atomic mass: C, 12; O, 16]
f. Sketch the shape of the graph that will be obtained if the readings of the balance against time are
2. A conical flask that contains 50 cm3 of solution of sodium thiosulphate is left on top of a sheet of white
paper which has been marked with an "X". The stopwatch is started as soon as 5 cm3 of hydrochloric
acid 2 mol dm-3 is mixed into the conical flask. The contents of the conical flask, is stirred. The reaction
is left to occur until the mark "X" is not visible and then the stopwatch is stopped. This experiment is
repeated four times, each time using a different concentration of sodium thiosulphate while all other
conditions are kept constant.
a. State the variable that is
b. Write one equation for the reaction that occurs in the experiment
c. What causes the mark "X" to become not visible?
d. Sketch one graph for the concentration of sodium thiosulphate solution against time.
e. If a bigger conical flask is used in this experiment, sketch on the same axes, the expected graph to be
obtained using a dashed line.
f. What conclusion can be drawn about the graph of the concentration of sodium thiosulphate solution
g. Sketch one graph of concentration of sodium thiosulphate solution against 1 /time.
h. What is the unit for the rate of reaction calculated from this graph?