Rates of Chemical Reactions

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Rates of Chemical Reactions
13.1 Rates of Chemical Reactions 13.2 Expressions
of Reaction Rates in Terms of Rates of Changes in
Concentrations of Reactants or Products 13.3 Metho
ds of Measuring Reaction Rates 13.4 Factors
Affecting Reaction Rates
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Rates of Chemical Reactions
This highly exothermic reaction does NOT take
place at room temperature. Why?
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Rates of Chemical Reactions

• Impossible to deduce the rate of a reaction from
  a stoichiometric equation
• Must be determined from experiments
• Reaction rates ? Reaction mechanisms
• Reaction mechanisms
• ? How bonds are broken and how new bonds are
  formed

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Rates of Chemical Reactions
Chemical kinetics is the study of reaction rates,
including factors that affect reaction rates and
the theory of reaction rates in terms of reaction
mechanisms.
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Rates of Chemical Reactions
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13.1 Rates of Chemical Reactions (SB p.3)
Definition
The rate of a chemical reaction is the measure of
the extent of the reaction as it proceeds with
time.
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13.1 Rates of Chemical Reactions (SB p.3)
Rates of Some Chemical Reactions
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13.1 Rates of Chemical Reactions (SB p.3)
Rates of Some Chemical Reactions
Filled with the flammable hydrogen gas for
buoyancy, the German airship Hindenburg burnt up
in an accident in May 1937
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13.1 Rates of Chemical Reactions (SB p.3)
Rates of Some Chemical Reactions
Potassium reacts with water vigorously
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13.1 Rates of Chemical Reactions (SB p.3)
Rates of Some Chemical Reactions
The precipitation of insoluble metal hydroxides
is instantaneous
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13.1 Rates of Chemical Reactions (SB p.3)
Rates of Some Chemical Reactions
The displacement reaction of copper and silver
nitrate solution takes several minutes
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13.1 Rates of Chemical Reactions (SB p.3)
Rates of Some Chemical Reactions
The fermentation of sucrose takes a few weeks
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13.1 Rates of Chemical Reactions (SB p.4)
Rates of Some Chemical Reactions
The rusting of iron takes a few months
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13.1 Rates of Chemical Reactions (SB p.4)
Rates of Some Chemical Reactions
After corrosion
Before corrosion
The sandstone statue at the Herten castle in
Germany
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How to describe the rate of reaction
quantitatively?
13.1 Rates of Chemical Reactions (SB p.4)

• Expressed in units
• mol s-1, mol dm-3 s1, g s-1, cm3 s-1

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13.1 Rates of Chemical Reactions (SB p.4)
Consider a reaction A ? B
(reactant) (product)
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Characteristics of the Curve
13.1 Rates of Chemical Reactions (SB p.4)

• -ve slope of curve of reactant A
• ? Decrease in concentration of reactant A with
  time
• ve slope of curve of product B
• ? Increase in concentration of product B with
  time

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Characteristics of the Curve
13.1 Rates of Chemical Reactions (SB p.4)

• Reaction rate
• Slope of the tangent to the curve at that
  point
• Reaction rate at the beginning is the highest
• ? Concentration of reactant A is the highest

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3 Ways to Express Reaction Rates
13.1 Rates of Chemical Reactions (SB p.5)

• Average rate
• Instantaneous rate (rate at a given instant)
• Initial rate

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Average Rate
13.1 Rates of Chemical Reactions (SB p.5)
Average rate of a chemical reaction is a measure
of the change in the amount or concentration of a
reactant or a product in a given period of time.
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13.1 Rates of Chemical Reactions (SB p.6)
Instantaneous Rate
Do you think the rate is uniform throughout the
reaction?
If not, how does it change during the course of
the reaction?
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13.1 Rates of Chemical Reactions (SB p.6)
Consider a reaction A ? B

(reactant) (product)
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13.1 Rates of Chemical Reactions (SB p.6)
Initial Rate

• Reaction rate at the start (i.e. at time t0)

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13.1 Rates of Chemical Reactions (SB p.6)
Consider a reaction A ? B

(reactant) (product)
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13.1 Rates of Chemical Reactions (SB p.6)
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Expressions of Reaction Rates in Terms of Rates
of Changes in Concentrations of Reactants or
Products
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Rate Equation
13.2 Expressions of Reactions Rates in Terms of
Rates of Changes in Concentrations of
Reactants or Products (SB p.9)
Consider the reaction A B
?? 2C
where dA and dB represent the decreases in
concentrations of reactants A and B
respectively, dC represents the increase in
concentration of product C, dt represents a
very short time interval
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13.2 Expressions of Reactions Rates in Terms of
Rates of Changes in Concentrations of
Reactants or Products (SB p.9)
In general aA bB ?? cC dD
where A and B represent reactants in the
reaction, C and D represent the products in the
reaction, a, b, c and d represent the
stoichiometric coefficients in a balanced
chemical equation.
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13.2 Expressions of Reactions Rates in Terms of
Rates of Changes in Concentrations of
Reactants or Products (SB p.9)
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Methods of Measuring Reaction Rates
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13.3 Methods of Measuring Reaction Rates (SB p.10)
Methods of Measuring Reaction Rates

• The rate of a reaction can be determined
  experimentally by measuring either
• The amounts of reactants used up within a
  certain time interval
• The amounts of products formed within a certain
  time interval

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Chemical Method --- Titration
13.3 Methods of Measuring Reaction Rates (SB p.11)

• Reactants are mixed and time is measured
• Reaction carried out in a constant temp.
  environment
• Small samples of reaction mixture are taken out
  for chemical analysis at certain time intervals

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Chemical Method --- Titration
13.3 Methods of Measuring Reaction Rates (SB p.11)

• The reaction of the sample mixture can quenched
  by
• rapid cooling in ice
• removing the catalyst
• removing one of the reactant by another
  chemical reaction
• diluting with a large vol. of cold water

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Chemical Method --- Titration
13.3 Methods of Measuring Reaction Rates (SB p.11)
Conc. of product
Time (min)
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Physical Method --- Determining the Volume of Gas
Formed
13.3 Methods of Measuring Reaction Rates (SB p.12)

• Example
• Zn(s) H2SO4(aq)
  ??ZnSO4(aq) H2(g)
• (b) CaCO3(s) 2HCl(aq)
  ??CaCl2(aq) H2O(l) CO2(g)
• (c) 2H2O2(aq) ?? 2H2O(l) O2(g)

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Physical Method --- Determining the Volume of Gas
Formed
13.3 Methods of Measuring Reaction Rates (SB p.13)
A typical laboratory set-up for measuring the
volume of gas formed in a reaction
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Physical Method --- Colorimetric Measurement
13.3 Methods of Measuring Reaction Rates (SB p.14)

• In some reactions, the reaction mixture may have
  change in colour or change in colour intensity

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Physical Method --- Colorimetric Measurement
13.3 Methods of Measuring Reaction Rates (SB p.14)

• Example
• In the reaction between iodine and propanone, the
  intensity of the dark brown colour of iodine
  decreases as the reaction proceeds
• I2(aq) CH3COOCH3(aq)
• dark brown colourless
• ?? CH3COCH2I(aq) HI(aq)
• colourless colourless

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Physical Method --- Colorimetric Measurement
13.3 Methods of Measuring Reaction Rates (SB p.14)

• Example
• In the oxidation of methanoic acid by bromine,
  the intensity of the orange colour of bromine
  decreases as the reaction proceeds
• Br2(aq) HCOOH(aq)
• orange colourless
• ?? 2Br-(aq) 2H(aq) CO2(g)
• (all products are colourless)

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Physical Method --- Colorimetric Measurement
13.3 Methods of Measuring Reaction Rates (SB p.14)

• Example
• In the oxidation of oxalate ions by
  manganate(VII) ions, the intensity of the purple
  colour of the manganate(VII) ions decreases as
  the reaction proceeds
• 2MnO4-(aq) 5C2O42-(aq) 16H(aq)
• purple colourless
  colourless
• ?? 2Mn2(aq) 10CO2(g) 8H2O(l)
• very pale pink colourless colourless

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Physical Method --- Colorimetric Measurement
13.3 Methods of Measuring Reaction Rates (SB p.14)

• The rates of these reactions can be measured by
  recording the changes in the colour intensity of
  the reaction mixture at different time intervals

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Physical Method --- Colorimetric Measurement
13.3 Methods of Measuring Reaction Rates (SB p.15)
The main components of a typical colorimeter
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Factors Affecting Reaction Rates
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Collision Theory
13.4 Factors Affecting Reaction Rates (SB p.15)

• The occurrence of a chemical reaction can be
  explained by the collision theory
• The theory states that
• ? reactants must collide for a reaction
  to occur
• ? the collision must occur with enough energy
  to break the bonds in the reactants so that new
  bonds can be formed in the products
• This minimum amount of energy required for the
  reaction to occur is known as the activation
  energy

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Factors Affecting Reaction Rates
13.4 Factors Affecting Reaction Rates (SB p.15)
concentration
particle size
catalyst
pressure
light
temperature
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Concentration of Reactants
13.4 Factors Affecting Reaction Rates (SB p.16)

• e.g. Reaction between Mg and HCl

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13.4 Factors Affecting Reaction Rates (SB p.16)
Concentration of Reactants

• 2.0 M HCl
• (b) 1.0 M HCl
• (c) 0.5 M HCl
• Reaction rate
• (a) gt (b) gt (c)

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13.4 Factors Affecting Reaction Rates (SB p.16)
Concentration of Reactants
? Time for reaction to complete t1 lt t2 lt t3

• The higher the conc. of reactants, the higher
  the reaction rate

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13.4 Factors Affecting Reaction Rates (SB p.17)
Concentration of Reactants

• ? conc. of reactants ? Reactant particles become
  more crowded ? ? effective collisions ? ?
  Reaction rate

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13.4 Factors Affecting Reaction Rates (SB p.17)
Pressure

• For those reactions involving gaseous reactants,
• higher pressure ? higher reaction rate

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13.4 Factors Affecting Reaction Rates (SB p.18)
Temperature

• e.g.
• Na2S2O3(aq) 2HCl(aq)
• ?? 2NaCl(aq) SO2(g) H2O(l) S(s)

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13.4 Factors Affecting Reaction Rates (SB p.18)
Temperature
t time taken for the disappearance of the cross
(b)
(a)
rate ? 1/t
t2 lt t1 (rate b gt a)
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13.4 Factors Affecting Reaction Rates (SB p.19)
Temperature

• ? temperature ? ? kinetic energy of reactant
  particles ? reactant particles move faster ? ?
  collision frequency ? ? reaction rate

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13.4 Factors Affecting Reaction Rates (SB p.19)
Particle size
The smaller the size of each piece of solid
reactant, the larger is the total surface area
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13.4 Factors Affecting Reaction Rates (SB p.19)
Particle size

• e.g.
• CaCO3(aq) 2HCl(aq)
• ?? CaCl2(aq) H2O(l) CO2(g)

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13.4 Factors Affecting Reaction Rates (SB p.20)
Particle size
Rate involvingpowdered solid reactant being
higher
Reason higher chance of contact between reactant
particles
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13.4 Factors Affecting Reaction Rates (SB p.20)
Catalyst
A catalyst is a substance that alters the rate of
a chemical reaction and remains chemically
unchanged at the end of the reaction.
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13.4 Factors Affecting Reaction Rates (SB p.20)
Catalyst

• e.g. H2O2(aq) ?? 2H2O(l) O2(g)

A laboratory set-up for studying the effect of
catalyst on the decomposition of hydrogen
peroxide solution
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13.4 Factors Affecting Reaction Rates (SB p.21)
Catalyst

• A catalyst speeds up a reaction by providing an
  alternative reaction pathway with a lower
  activation energy
• More reactant particles will possess energy equal
  to or greater than the new activation energy, and
  thus the reaction rate increases

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The END
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In a chemical reaction, a total of 0.18 g of
carbon dioxide gas is given out in 1 minute at
room temperature. What is its average rate in mol
s1 for that time interval?
13.1 Rates of Chemical Reactions (SB p.5)
Back
Example 13-1A
Answer
62
In the uncatalyzed decomposition of hydrogen
peroxide solution into water and oxygen at room
conditions, the volume of oxygen given out in 20
hours is 5 cm3. What is its average rate in mol
s1 for that time interval? 2H2O2(l) ??
2H2O(l) O2(g)(Molar volume of gas at room
temperature and pressure 24.0 dm3 mol1)
13.1 Rates of Chemical Reactions (SB p.5)
Example 13-1B
Answer
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13.1 Rates of Chemical Reactions (SB p.5)
Back
Example 13-1B
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The change in concentration of reactant X in a
chemical reaction is illustrated in the graph on
the right.
13.1 Rates of Chemical Reactions (SB p.6)
Example 13-1C
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13.1 Rates of Chemical Reactions (SB p.6)
Example 13-1C
With the use of the graph, calculate(a) the
initial rate of the reaction(b) the average
rate for the time interval from the 1st to the
2nd minute(c) the instantaneous rate at the 3rd
minute. (Give your answers in mol dm3 min1.)
Answer
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13.1 Rates of Chemical Reactions (SB p.6)
Example 13-1C
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13.1 Rates of Chemical Reactions (SB p.6)
Example 13-1C
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13.1 Rates of Chemical Reactions (SB p.6)
Back
Example 13-1C
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13.1 Rates of Chemical Reactions (SB p.8)
Check Point 13-1

• In the hydrolysis of an ester at a constant
  temperature of 398 K, the concentration of the
  ester decreases from 1 mol dm3 to 0.75 mol dm3
  in 4 minutes. What is its average rate in mol
  dm3 s1 for that time interval?

Answer

• Average rate at 398 K
• (1 0.75) mol dm-3 ? (4 ? 60) s
• 0.001 04 mol dm-3 s-1

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13.1 Rates of Chemical Reactions (SB p.8)
Check Point 13-1

• The graph on the right shows the
  change in
  concentration of
  a reactant in a
  chemical reaction.

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13.1 Rates of Chemical Reactions (SB p.8)
Check Point 13-1
With the use of the graph above,
calculate (i) the initial rate of the
reaction (ii) the average rate for the time
interval from the 20th to the 30th
second (iii) the instantaneous rate at the 10th
second.
Answer
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13.1 Rates of Chemical Reactions (SB p.8)
Back
Check Point 13-1
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13.2 Expressions of Reactions Rates in Terms of
Rates of Changes in Concentrations of
Reactants or Products (SB p.10)
Back
Example 13-2
Haemoglobin (Hb) binds with carbon monoxide
according to the following equation 4Hb
3CO ?? Hb4(CO)3 Express the rate of the
reaction in terms of the rate of change in
concentration of any one of the reactants or the
product.
Answer
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13.2 Expressions of Reactions Rates in Terms of
Rates of Changes in Concentrations of
Reactants or Products (SB p.10)
Check Point 13-2
Back
Express the rate of the following reaction in
terms of the rate of change in concentration of
any one of the reactants or the product. 2H2(g)
O2(g) ?? 2H2O(l)
Answer
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13.3 Methods of Measuring Reaction Rates (SB
p.11)
Example 13-3A
Alkaline hydrolysis of ethyl ethanoate (an ester)
using sodium hydroxide solution is represented by
the following equation CH3CO2CH2CH3(l)
NaOH(aq) ?? CH3CO2Na(aq) CH3CH2OH(aq) The
rate of the reaction can be followed by titrating
small volumes of the reaction mixture with
standard dilute hydrochloric acid at successive
five-minute intervals.
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13.3 Methods of Measuring Reaction Rates (SB
p.11)
Example 13-3A
(a) Suggest a method to quench the reaction
mixture so that the concentration of sodium
hydroxide solution can be determined accurately.
Explain briefly why this method can be used.
Answer
(a) The reaction mixture can be quenched by
pipetting a sample of the reaction mixture into a
conical flask containing ice water. The cooling
and dilution of the reaction mixture decrease the
reaction rate sufficiently for chemical analysis.
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13.3 Methods of Measuring Reaction Rates (SB
p.11)
Example 13-3A
(b) Explain why the change in concentration of
sodium hydroxide solution but not that of ethyl
ethanoate is measured in order to determine the
rate of the above reaction.
Answer
(b) Sodium hydroxide is a strong alkali that
reacts with strong mineral acids almost
instantaneously. Therefore, the titration of
sodium hydroxide solution and dilute hydrochloric
acid provides accurate experimental results.
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13.3 Methods of Measuring Reaction Rates (SB
p.11)
Answer
Example 13-3A

• Explain which option, A or B, is a reasonable set
  of experimental results for the above titration.

Option A
Option B
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13.3 Methods of Measuring Reaction Rates (SB
p.11)
Example 13-3A
(c) Sodium hydroxide is a reactant of the
hydrolysis. As the reaction proceeds, the
concentration of sodium hydroxide in the reaction
mixture decreases with time, and hence the amount
of dilute hydrochloric acid used in the
titration. Thus, option A is a reasonable set of
experimental results.
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13.3 Methods of Measuring Reaction Rates (SB
p.11)
Example 13-3A
(d) Name a suitable indicator for the titration.
Answer
(d) Methyl orange / Phenophthalein
Back
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13.3 Methods of Measuring Reaction Rates (SB
p.13)
Example 13-3B
A student recorded the following experimental
results for the reaction of zinc and dilute
hydrochloric acid. Zn(s) 2HCl(aq) ?? ZnCl2(aq)
H2(g)
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13.3 Methods of Measuring Reaction Rates (SB
p.13)
Example 13-3B
(a) Plot a graph of volume of hydrogen gas
produced against time.
Answer
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13.3 Methods of Measuring Reaction Rates (SB
p.13)
Example 13-3B
(b) Describe the change in the rate of the
reaction using your graph in (a).
Answer
(b) As shown in the graph in (a), the volume of
hydrogen gas given out at the beginning of the
reaction (e.g. in the time interval between the
1st and the 2nd minute) is greater than that near
the end of the reaction (e.g. in the time
interval between the 6th and the 7th minute).
Therefore, the rate of the reaction decreases
with time.
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13.3 Methods of Measuring Reaction Rates (SB
p.13)
Example 13-3B
(c) Explain how you can measure the initial rate
of the reaction graphically.
Answer
(c) The initial rate can be found by determining
the slope of the tangent to the curve at time
zero.
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13.3 Methods of Measuring Reaction Rates (SB
p.13)
Back
Example 13-3B
(d) Determine graphically the rate of the
reaction at the 5th minute. State the unit.
Answer
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13.3 Methods of Measuring Reaction Rates (SB
p.15)
Back
Check Point 13-3

• Suggest an experimental method for determining
  the rate of each of the following reactions
• (a) S2O82(aq) 2I(aq) ?? 2SO42(aq) I2( aq)
• CH3COOCH3(aq) I2(aq)
• ?? CH3COOCH2I(aq) HI(aq)
• 2MnO4(aq) 5C2O42(aq) 16H(aq)
• ?? 2Mn2(aq) 10CO2(g) 8H2O(l) H(aq)

Answer

• Colorimetric measurement / titration
• Colorimetric measurement
• Colorimetric mesurement / titration

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13.4 Factors Affecting Reaction Rates (SB p.17)
Let's Think 1
Explain why sawdust burns explosively in pure
oxygen but slowly in air.
Answer
A higher concentration of oxygen increases the
rate of combustion.
Back
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13.4 Factors Affecting Reaction Rates (SB p.21)
Check Point 13-4
(a) List THREE factors that affect the rate of a
chemical reaction.
Answer
(a) Concentration of reactants / pressure /
temperature / surface area / catalyst / light
(any 3)
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13.4 Factors Affecting Reaction Rates (SB p.21)
Check Point 13-4
(b) The figure below shows the laboratory set-up
for measuring the change in mass of the reaction
mixture with time in the course of the
reaction CaCO3(s) 2HCl(aq) ?? CaCl2(aq)
H2O(l) CO2(g)
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13.4 Factors Affecting Reaction Rates (SB p.21)
Check Point 13-4
A certain mass of calcium carbonate was added to
50 cm3 of 2.0 M hydrochloric acid at 20C. Carbon
dioxide was allowed to escape and the mass of the
reaction mixture was measured at regular time
intervals. The results were expressed as the loss
of mass with respect to time. The experiment was
carried out with one change of condition at a
time (i) using 1.0 M hydrochloric acid in place
of 2.0 M hydrochloric acid. (ii) carrying
out the reaction at 30C. (iii) using powdered
calcium carbonate of the same mass.
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13.4 Factors Affecting Reaction Rates (SB p.21)
Back
Check Point 13-4
Match them with the curves A, B and C in the
graph below.
(b) (i) B or C (ii) A (iii) A
Answer