New Way Chemistry for Hong Kong A-Level Book 2 1 Rates of Chemical Reactions 13.1Rates of Chemical Reactions 13.2Expressions of Reaction Rates in Terms

New Way Chemistry for Hong Kong A-Level Book 2

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Rates of Chemical Rates of Chemical ReactionsReactions

13.113.1 Rates of Chemical ReactionsRates of Chemical Reactions

13.213.2 Expressions of Reaction Rates in Terms of RExpressions of Reaction Rates in Terms of R

ates of Changes in Concentrations of Reactaates of Changes in Concentrations of Reacta

nts or Productsnts or Products

13.313.3 Methods of Measuring Reaction RatesMethods of Measuring Reaction Rates

13.413.4 Factors Affecting Reaction RatesFactors Affecting Reaction Rates

1313


2

This highly exothermic reaction does NOT take place at room temperature. Why?


C12H22O11(s) + 12O2(g)

12CO2(g) + 11H2O(l)

ΔHc [C12H22O11(s)] = -5644 kJ mol-1

ø


3

Rates of Chemical Rates of Chemical ReactionsReactions• Impossible to deduce the rate of a reactio

n 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


4

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.

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.



5

13.13.11 Rates of Rates of

Chemical Chemical ReactionsReactions


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13.1 Rates of Chemical Reactions (SB p.3)

DefinitionDefinition

The rate of a chemical reaction is the measure of the extent of the reaction as

it proceeds with time.


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Chemical reaction Reaction rate

1. Reaction between hydrogen and oxygen

2. Reaction between potassium and water

3. Precipitation of insoluble metal hydroxides

4. Displacement reaction of copper and silver nitrate solution

5. Fermentation of sucrose

6. Rusting of iron

7. Weathering of marbles by acid rain

Explosively fast

Vigorous, takes a fraction of a second

Instantaneous

Takes a several minutes

Takes a few weeks

Takes a few months

Slow, take centuries


Rates of Some Chemical Rates of Some Chemical ReactionsReactions


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Filled with the flammable hydrogen gas for buoyancy, the German airship Hindenburg burnt u

p in an accident in May 1937


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Potassium reacts with water vigorously


10



The precipitation of insoluble metal hydroxides is

instantaneous

Cu(OH)2 Fe(OH)2


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The displacement reaction of copper and silver nitrate solution takes several

minutes


12



The fermentation of sucrose takes a few weeks


13



The rusting of iron takes a few months


14



Before corrosion After corrosion

The sandstone statue at the Herten castle in Germany


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How to describe the rate of How to describe the rate of reaction quantitatively? reaction quantitatively?


Change in amount or concentration of a reactant or a product

TimeRate =

• Expressed in units:

mol s-1, mol dm-3 s–1, g s-1, cm3 s-1


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Consider a reaction

A B (reactant) (product)


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• -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

Characteristics of the Characteristics of the Curve Curve


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

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

• Average rate

• Instantaneous rate (rate at a given instant)

• Initial rate


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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.

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.

13.1 Rates of Chemical Reactions (SB p.5)Average Average Rate Rate

Example 13-1AExample 13-1A Example 13-1BExample 13-1B


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Do you think the rate is uniform throughout the reaction?

Do you think the rate is uniform throughout the reaction?

If not, how does it change during the course of the reaction?If not, how does it change during the course of the reaction?

Instantaneous RateInstantaneous Rate



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Consider a reaction A B (reactant) (product)Instantaneous reaction

rate = rate at a given time t1

= slope of tangent line at t= t1



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Initial RateInitial Rate


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


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Consider a reaction A B (reactant) (product)

Initial reaction rate = rate at start= slope of tangent line at t = 0



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Example 13-1CExample 13-1C Check Point 13-1Check Point 13-1


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Expressions of Expressions of Reaction Rates in Reaction Rates in Terms of Rates of Terms of Rates of

Changes in Changes in Concentrations of Concentrations of

Reactants or Reactants or ProductsProducts

13.13.22


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Consider the reaction:

A + B 2C

13.2 Expressions of Reactions Rates in Terms of Rates of Changes in Concentrations of Reactants or Products (SB p.9)

Rate Rate EquationEquation

where –d[A] and –d[B] represent the decreases in concentrations of reactants A and B respectively,

d[C] represents the increase in concentration of product C,

dt represents a very short time interval

Rate = - = - =


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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.

Rate =



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Example 13-2Example 13-2 Check Point 13-2Check Point 13-2


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Methods of Methods of Measuring Measuring Reaction Reaction

RatesRates

13.13.33


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The rate of a reaction can be determined experimentally by measuring either:

1. The amounts of reactants used up within a certain time interval

2. The amounts of products formed within a certain time interval

13.3 Methods of Measuring Reaction Rates (SB p.10)

Methods of Measuring Reaction Methods of Measuring Reaction RatesRates


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Chemical Method --- TitrationChemical Method --- Titration

• 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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The reaction of the sample mixture can quenched by:

1. rapid cooling in ice

2. removing the catalyst

3. removing one of the reactant by another chemical reaction

4. diluting with a large vol. of cold water


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Time (min)

Conc. of product


Example 13-3AExample 13-3A


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Physical Method --- Determining Physical Method --- Determining the Volume of Gas Formedthe Volume of Gas Formed


Example:

(a) 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 Physical Method --- Determining the Volume of Gas Formedthe Volume of Gas Formed


A typical laboratory set-up for measuring the volume of gas formed in a reaction

Example 13-3BExample 13-3B


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Physical Method --- Physical Method --- Colorimetric MeasurementColorimetric Measurement

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



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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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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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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) + 5C2O4

2-(aq) + 16H+(aq)

purple colourless colourless

2Mn2+(aq) + 10CO2(g) + 8H2O(l)

very pale pink colourless colourless




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The change in colour intensity at different time can be followed by using a colorimeter


Physical Method --- Physical Method --- Colorimetric MeasurementColorimetric Measurement• The rates of these reactions can be me

asured by recording the changes in the colour intensity of the reaction mixture at different time intervals


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The main components of a typical colorimeter

Check Point 13-3Check Point 13-3


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Factors Factors Affecting Affecting Reaction Reaction

RatesRates

13.13.44


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Collision TheoryCollision 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 RatesFactors Affecting Reaction Rates

concentrationconcentration

temperaturetemperature

pressurepressure

particle sizeparticle size

catalystcatalyst

lightlight



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Concentration of Concentration of ReactantsReactants


• e.g. Reaction between Mg and HCl


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(a) 2.0 M HCl

(b) 1.0 M HCl

(c) 0.5 M HCl

Reaction rate:

(a) > (b) > (c)

(a) 2.0 M HCl

(b) 1.0 M HCl

(c) 0.5 M HCl

Reaction rate:

(a) > (b) > (c)

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


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Time for reaction to complete: t1 < t2 < t3

Time for reaction to complete: t1 < t2 < t3

Concentration of Concentration of ReactantsReactants

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

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



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

ConcentrationConcentration of Reactantsof Reactants



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PressurePressure


• For those reactions involving gaseous reactants,

higher pressure higher reaction rate


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TemperatureTemperature


• e.g.

Na2S2O3(aq) + 2HCl(aq)

2NaCl(aq) + SO2(g) + H2O(l) + S(s)


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TemperatureTemperaturet = time taken for the disappearance of the cross

t = time taken for the disappearance of the cross

rate 1/trate 1/t

t2 < t1 (rate: b > a)t2 < t1 (rate: b > a)


(a) (b)


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TemperatureTemperature


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


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Particle sizeParticle size


The smaller the size of each piece of solid reactant, the larger is the total surface

area


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• e.g.

CaCO3(aq) + 2HCl(aq)

CaCl2(aq) + H2O(l) + CO2(g)


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Rate involvingpowdered solid reactant being higher

Rate involvingpowdered solid reactant being higher

Reason: higher chance of contact between reactant particles

Reason: higher chance of contact between reactant particles




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A catalyst is a substance that alters the rate of a chemical reaction and

remains chemically unchanged at the end of the reaction.

A catalyst is a substance that alters the rate of a chemical reaction and

remains chemically unchanged at the end of the reaction.

CatalystCatalyst



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CatalystCatalyst

• 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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CatalystCatalyst

• 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


Check Point 13-4Check Point 13-4


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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 s–1 for that time interval?


Number of moles of CO2 =

= 0.0041 mol

Average rate =

= 6.83 × 10–5 mol s–1

1-mol g 2) 16.0(12.0

g 0.18

s 60mol 0.0041

Answer

Back


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 s–1 for that time interval? 2H2O2(l) 2H2O(l) + O2(g)(Molar volume of gas at room temperature and pressure= 24.0 dm3 mol–1)


Answer


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Number of moles of O2 =

= 2.08 × 10–4 mol

Average rate =

= 2.89 × 10–9 mol s–1

13

3

mol cm 000 24cm 5

s 60)60(20mol 10 2.08 -4

Back


64

The change in concentration of reactant X in a chemical reaction is illustrated in the graph on the right.



65

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 dm–3 min–1.)


Answer


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(a) Initial rate

= Slope of the tangent to the curve at t0

=

= -0.05 mol dm-3 min-1

min )0 .21(dm mol 0.160) (0.100 3


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(b) Average rate

=

= -0.03 mol dm-3 min-1

min )1 (2dm mol 0.110) (0.080 3


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(c) Instantaneous rate at the 3rd minute

= Slope of the tangent to the curve at the 3rd minute

=

= -0.021 mol dm-3 min-1

min )2 (3.5dm mol 0.077) (0.046 3

Back


69

(a) In the hydrolysis of an ester at a constant temperature of 398 K, the concentration of the ester decreases from 1 mol dm–3 to 0.75 mol dm–3 in 4 minutes. What is its average rate in mol dm–3 s–1 for that time interval?


Answer(a) 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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(b) The graph on the right shows the change in concentration of a reactant in a chemical reaction.


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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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(i) Initial rate =

= -1 10-3 mol dm-3 s-1

(ii) Average rate =

= -3 10-4 mol dm-3 s-1

(iii) Instantaneous rate =

= -5 10-4 mol dm-3 s-1

s 10)(0dm mol 0.01) - 0.02( -3

s 30)(20dm mol 0.006) - 0.009( -3

s 10)(0dm mol 0.013) - 0.018( -3

Back


73

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.


The rate of the reaction is expressed as:

dtd

dtd

dt

d ]CO[31]Hb[

41](CO)[Hb

Rate 34

Answer

Back


74

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

Back

Rate = dt

d

dt

d

dt

d )]g(O[)]g(H[

21O(l)][H

21 222


75

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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(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 sam

ple of the reaction mixture into a conical flask containing i

ce water. The cooling and dilution of the reaction mixture

decrease the reaction rate sufficiently for chemical analysi

s.


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


Answer

Time after mixing (min)

Volume of HCl added at the end point (cm3)

5 1010 8

Time after mixing (min)

Volume of HCl added at the end point (cm3)

5 8

10 10

Option A

Option B


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(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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(d) Name a suitable indicator for the titration.

Answer(d) Methyl orange / Phenophthalein

Back


81


A student recorded the following experimental results for the reaction of zinc and dilute hydrochloric acid.

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)Time (min)

0.0 1.0 2.0

3.0 4.0 5.0 6.0 7.0

8.0 9.0

Volume of H2(g) produced (cm3)

0 15 26 33 38 40 41 42 42 42


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(a) Plot a graph of volume of hydrogen gas produced against time.

Answer(a)


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(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.


84


(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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(d) Determine graphically the rate of the reaction at the 5th minute. State the unit.

Answer

(d) From the graph in (a),

rate of reaction

= slope of the tangent to the curve at the 5 minute

=

= 2 cm3 min-1

min )28(cm 34)(46 3

Back


86

Suggest an experimental method for determining the rate of each of the following reactions:

(a) S2O82–(aq) + 2I–(aq) 2SO4

2–(aq) + I2( aq)

(b) CH3COOCH3(aq) + I2(aq)

CH3COOCH2I(aq) + HI(aq)

(c) 2MnO4–(aq) + 5C2O4

2–(aq) + 16H+(aq)

2Mn2+(aq) + 10CO2(g) + 8H2O(l) + H+(aq)


Answer(a) Colorimetric measurement / titration

(b) Colorimetric measurement

(c) Colorimetric mesurement / titration

Back


87


Explain why sawdust burns explosively in pure oxygen but slowly in air.

Back

AnswerA higher concentration of oxygen increases

the rate of combustion.


88


(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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(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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A certain mass of calcium carbonate was added to 50 cm3 of 2.0 M hydrochloric acid at 20°C. 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 30°C.

(iii) using powdered calcium carbonate of the same mass.


91


Match them with the curves A, B and C in the graph below.

Answer

(b) (i) B or C

(ii) A

(iii) A

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New Way Chemistry for Hong Kong A-Level Book 2 1 Rates of Chemical Reactions 13.1Rates of Chemical Reactions 13.2Expressions of Reaction Rates in Terms