1 rates of chemical reactions 13.1rates of chemical reactions 13.2expressions of reaction rates in...

1

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 Expressions of Reaction Rates in Terms of

Rates of Changes in Concentrations of Rates of Changes in Concentrations of

Reactants or ProductsReactants 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

Chemical KineticsChemical Kinetics

A study of

(1) reaction rates

(2) the factors affecting reaction rates

(3) reaction mechanisms

(the detailed steps involved in reactions)

3

Explosive reactions2H2(g) + O2(g) 2H2O(l)

4

Potassium reacts with water vigorously

Vigorous reactions2K(s) + 2H2O(l) 2KOH(aq) + H2(g)

5

Very rapid reactions

Ag+(aq) + Cl−(aq) AgCl(s)Formation of insoluble salts

6

Fe3+(aq) + 3OH−(aq) Fe(OH)3(s)


Formation of insoluble bases

7


H+(aq) + OH−(aq) H2O(l)

Acid-alkali neutralization reactions

8

Q.1

Ag+(aq) + Cl−(aq) AgCl(s)

Fe3+(aq) + 3OH−(aq) Fe(OH)3(s)H+(aq) + OH−(aq) H2O(l)

All involve oppositely charged ions

9

Rapid or moderate reactions

Zn(s) + 2Ag+(aq) Zn2+(aq) + 2Ag(s)

Displacement reactions of metals : -

10

Rapid or moderate reactions

Zn(s) + 2Ag+(aq) Zn2+(aq) + 2Ag(s)

Cl2(aq) + 2Br(aq) 2Cl(aq) + Br2(aq)

Displacement reactions of metals : -

Displacement reactions of halogens : -

11

Slow reactionsFermentation of glucoseC6H12O6(aq) 2C2H5OH(aq) + 2CO2(g)

12

Slow reactions

2MnO4(aq) + 5C2O4

2(aq) + 16H+(aq)

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

13

Very slow reactionsRusting of iron4Fe(s) + 3O2(g) + 2nH2O(l) 2Fe2O3 · nH2O(s)

14

Extremely slow reactionsCaCO3(s) + 2H+(aq) Ca2+(aq) + CO2(g) + H2O(l)

Before corrosion After corrosion

15

Two Ways to Express Reaction Two Ways to Express Reaction Rates Rates

1. Average rate

2. Instantaneous rate (rate at a given instant)

16

Amount is usually expressed in

Concentration mol dm−3

Mass g

Volume cm3 or dm3

Pressure atm

occur to change the for taken time Totalreactant a or product a of amount in change Total

reaction of rate Average

17

Q.2 0.36 g of magnesium reacted with 50.0 cm3 of 1.0 M hydrochloric acid to give 360 cm3 of hydrogen under room conditions.

The reaction was completely in 90 seconds.

13 s g 104.0s 90g 0.36

rate Average (a)

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

18

133

s cm 4.0s 90

cm 360rate Average (b)

Q.2 0.36 g of magnesium reacted with 50.0 cm3 of 1.0 M hydrochloric acid to give 360 cm3 of hydrogen under room conditions.

The reaction was completely in 90 seconds. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

19

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)2.(c)

mol 0.015mol g 24.3g 0.36

Mg of moles of No. 1

mol 0.0500dm 0.0500dm mol 1.0HCl of moles of No. 33

Mg is the limiting reactant

mol 0.030 mol 0.0152reacted HCl of moles of No.

Decrease in concentration of HCl(aq) in 90 s

33 dm mol 0.60

dm 0.0500mol 0.030

13-3--3

s dm mol 106.7s 90dm mol 0.60

rate Average

20

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)2.(d)

Increase in concentration of MgCl2(aq) in 90 s

3-3 dm mol 0.30 dm mol 0.6021

13-3--3

s dm mol 103.3s 90dm mol 0.30

rate Average

= Rate of reaction w.

r.t. HCl(aq)Rate of reaction w.r.t. MgCl2(aq)2

21

2. Instantaneous rate

The rate at a particular instant of the reaction is called the instantaneous rate.For the chemical reaction

aA + bB cC + dD

)d1

(dt

d[D])

c1

(dt

d[C])

b1

(dtd[B]

)a1

(dtd[A]

rate ousInstantane

[X] = molarity of X

22

2. Instantaneous rate

The rate at a particular instant of the reaction is called the instantaneous rate.For the chemical reaction

aA + bB cC + dD

)d1

(dt

d[D])

c1

(dt

d[C])

b1

(dtd[B]

)a1

(dtd[A]

rate ousInstantane

Units : mol dm3 s1, mol dm3 min1, mol dm3 h1…etc.

23

Graphical Representation of Reaction Rates – Rate curves

A rate curve is a graph plotting the amount of a reactant or product against time.

24

Consider the reaction

A B + C (reactant) (product)

25

At any time t, the instantaneous rate of the reaction equals the slope of the tangent to the curve at that point.

The greater the slope, the higher the rate of the reaction.

26

-ve slope of curve of reactant A

[A] with time

27

+ve slope of curve of product B

[B] with time

28

The rate at t0 is usually the fastest and is called the initial rate.The curve is the steepest with the greatest slope at time t0.

29

The rate of the reaction gradually as the reaction proceeds.

Flat curve reaction completed

30

Q.3 X + Y 2Z

Time of reaction (min)

Con

cent

ratio

n of

pro

duct

Z

(mol

dm

−3 )

A

B

C

31 Time of reaction (min)

Con

cent

ratio

n of

pro

duct

Z

(mol

dm

−3 )

A

B

C

133

min dm mol 0.39min 7

dm mol 5.421

rate Average

X + Y 2Z


Con

cent

ratio

n of

pro

duct

Z

(mol

dm

−3 )

A

B

C

A at rate ousInstantane

min 0.0)-(1.6dm mol 0.0)-(6.0

21 3

13 min dm mol 1.9 1.6

X + Y 2Z


Con

cent

ratio

n of

pro

duct

Z

(mol

dm

−3 )

A

B

C

B at rate ousInstantane

min 1.0)-(3.0dm mol 2.7)-(5.1

21 3

13 min dm mol 0.6

X + Y 2Z

2.7

5.1


Con

cent

ratio

n of

pro

duct

Z

(mol

dm

−3 )

A

B

C

0 C at rate ousInstantane

X + Y 2Z

35

Methods of Measuring Reaction Methods of Measuring Reaction RatesRates

A.Physical measurements

1. Continuous measurements

2 Initial rate measurements (Clock reactions)

B.Chemical measurements (Titration)

36

1. Continuous measurements1. Continuous measurements

Experiment is done in ONE take.The reaction rates are determined by measuring continuously a convenient property which is directly proportional to the concentration of any one reactant or product of the reaction mixture.Properties to be measured : – Gas volume / Gas pressure / Mass / Color intensity / Electrical conductivity

37

1.1 Measurement of large volume 1.1 Measurement of large volume changeschanges

Examples:(1) CaCO3(s) + 2HCl(aq)

CaCl2(aq) + H2O(l) + CO2

(g)

(2) Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)

(3) 2H2O2(aq) 2H2O(l) + O2(g)

38

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

1.1 Measurement of large volume changes

Temperature is kept constant


Vol

ume

of g

as fo

rmed

(cm

3 )Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)

ratedtdV

slope

40

Q.4

(2) Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)

Volume of CO2

Sigmoid curve

H2(g) is sparingly soluble in water while CO2 is quite soluble in water.

Rate

Rate

41

1.2 Measurement of small volume changes - Dilatometry

Liquid phase reaction mixture

Capillary tube

CH3COOH(l) + CH3CH2OH(l) CH3COOCH2CH3(l) + H2O(l)

42

1.3 Measurement of mass changes

CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2(g)

43

stopwatch

cotton wool plug

limestone piecesof known mass

measured volume of standard hydrochloric acid

electronic balance

The cotton wool plug is to allow the escape of CO2(g) but to prevent loss of acid spray due to spurting.

44


Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)

Which reaction is more suitable to be followed by mass measurement ?

Hydrogen is a very light gas. The change in mass of the reaction mixture may be very small.

The electronic balance used in the school laboratory may not be sensitive enough to detect the small change.

45

time

Loss o

f mass

(m)

mfinal = total mass loss

ratedtdm

slope

dt]d[H

slope

time

mfinal - m

t

mfinal = mfinal – m0

= rate 2

(∵ m0 = 0)

46

1.4 Colorimetry

∵ colour intensity [coloured species]

dtintensity) d(colour

rate

47

H2O2(aq) + 2H+(aq) + 2I(aq) I2(aq) + 2H2O(l)

CH3COCH3(aq) + I2(aq) CH3COCH2I(aq) + H+(aq) + I(aq)

Br2(aq) + HCOOH(aq) 2H+(aq) + 2Br(aq) + CO2(g)

2MnO4(aq) + 16H+(aq) + 5C2O4

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

colour intensity as reaction proceeds

colour intensity as reaction proceeds

49

A colorimeter

cuvettes

50

Blue solution

Yellow filter

Complementary colours

Yellow light

51

Pairs of opposite colours are complementary colours

Red Cyan

52


Red Cyan

Green Magenta

53


Red Cyan

CMYK

Green MagentaBlue Yellow

54

When mixed in the proper proportion, complementary colours produce a neutral color (grey, white, or black).

55

II0

I0 = intensity before absorption

I = intensity after absorption

56

II0

100%II

ncetransmitta %0

II

logAbsorbance 010

If I = I0 ,

%T = 100%A = log101 = 0zero absorption

If I = 0 ,

%T = 0%

A log10

complete absorption

57

A = bCBeer’s law

58

C

ADeviation at higher concentrations

A calibration curve is first constructed for AC conversion

59

time

[I2]

time

A

ratedt

]d[Islope 2

ratedtdA

slope

Q.5

60

1.5 Measurement of electrical conductivity

Na+OH(aq) + CH3COOH(aq) CH3COONa+(aq) + H2O(l)

∵ conducting mobility : OH > CH3COO

∴ conductivity as the rx proceeds

61

1.5 Measurement of electrical conductivity

2MnO4(aq) + 16H+(aq) + 5C2O4

2(aq)

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

∵ total number of ions

∴ electrical conductivity as the rx proceeds

62

1.6 Measurement of pressure changes

dt)d(P

rate T

PT = total pressure of the reaction mixture

63

(i) 2NO(g) + 2H2(g) N2(g) + 2H2O(g)(ii) 3H2(g) + N2(g) 2NH3(g)

Q.6

At fixed V and T, PT n

In both reactions,

n as the reactions proceed

PT as the reactions proceed

64

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

to data-logger interface and computer

pressure sensor

magnesium ribbon

suction flask

dilute hydrochloric acid

65

A(g) + B(g) products

66

A chemical clock is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes.

In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change in a reproducible time lapse.

67

2. Initial Rate Measurements-Clock Reactions

Experiment

[S2O32(aq)] / M [H+(aq)] / M

1 0.10 1

2 0.08 1

3 0.04 1

4 0.02 1

S2O32–(aq) + 2H+(aq) SO2(aq) + H2O(l) + S(s)

1. A set of experiments is done in which all reaction conditions but one are kept

constant.

68

2. Initial Rate Measurements-Clock Reactions


yellow precipitate

2. The time taken for the reaction to arrive at a particular point at the early stage of the

reaction is measured.

69

The beaker containing the reaction mixture is placed over a cross marked on a white tile.

70

As more sulphur forms, the reaction mixture becomes more cloudy.

71

The cross becomes more and more difficult to see and finally disappears.

72

Average rate in the early stage

Amount of S required to blot out the markTime taken to blot out the mark

=

Since the amount of S required to blot out the mark is a constant,

Averagerate

1

time taken to ‘blot out’ the mark


yellow precipitate

73

Averagerate

1

time taken to ‘blot out’ the mark

The average rate of reaction is inversely proportional to the time taken to ‘blot out’ the mark.

The faster is the reaction, the shorter is the time taken for the mark to disappear.

74

time

amount of S

dtdS

rate initial slope

tS

rate average slope

If S and t are small(early stage)

ΔtΔS

dtdS

75

ΔtΔS

dtdS

Since S is a constant

t1

ΔtΔS

dtdS

76

Initial rate k[S2O32(aq)]x[H+(aq)]y

Initial rate k[S2O32(aq)]x[H+(aq)]y k’[S2O3

2(aq)]x

Since HCl is in large excess,

[H+(aq)]y constant at the early stage

t1

ΔtΔS

rate Initial ∵

x232

'' (aq)]O[Skt1

77

t

1Expt.

[S2O32(aq)

] (M)[H+(aq)]

(M)

Time taken (t)

to mask themark / s

/ s1

1 0.10 1 10

2 0.08 1 13

3 0.04 1 25

4 0.02 1 50

78

Q.7

[S2O32(aq)]

t

1

x232

'' (aq)]O[Sk t

1

Linear x = 1

79

Other Examples of Clock Reactions : -

5I(aq) + IO3(aq) + 6H+(aq) 3I2(aq) +

3H2O(l) Small and fixed amounts of S2O3

2(aq) and starch are added to the reaction mixtures in all runs.

Time taken for the reaction mixture to turn deep blue is measured.

I2(aq) + starch deep blue complex

(excess) (fixed)

I2(aq) + 2S2O32(aq) 2I(aq) + S4O6

2(aq)

(fixed) (fixed)

80


5I(aq) + IO3(aq) + 6H+(aq) 3I2(aq) +

3H2O(l) I2(aq) + 2S2O3

2(aq) 2I(aq) + S4O62(aq)

(fixed) (fixed)

I2(aq) + starch deep blue complex

(excess) (fixed)

By changing the concentration of any one of the reactants, deep blue colour will appear in different time lapses a chemical clock !

Halloween clock

81


5Br(aq) + BrO3(aq) + 6H+(aq) 3Br2(aq) +

3H2O(l) OH

+

OH

BrBr

Br

3Br2

(fixed) (fixed)

Br2 + methyl red colourless(excess) (fixed)

82

Advantages of physical measurements

1. Suitable for fast reactions.

2. Small sample size

3. More accurate than chemical method (titration)

4. No interruption continuous measurements

5. Can be automated.

83

Disadvantages of physical measurements

1. More sophisticated

2. More expensive

3. More specific – only suit a limited number of reactions.

84

B.Chemical Measurements (Titration Methods)

1. Start a reaction with all reaction conditions but one fixed.

2. Withdraw and quench fixed amounts of the reaction mixture at different times.

85

• Diluting the reaction mixture with a sufficient amount of cold water or an appropriate solvent.

• Cooling the reaction mixture rapidly in ice.

Quenching methods:

Temperature

Concentration

• Removing one of the reactants or the catalyst (if any) by adding another reagent.

86

B.Chemical Measurements (Titration Methods)

1. Start a reaction with all reaction conditions but one fixed.

2. Withdraw and quench fixed amounts of the reaction mixture at different times.

3. Titrate the quenched samples to determine the concentration of one of the reactants or products.

87

H+ as catalystCH3COCH3 + I2 CH3COCH2I + HI

Q.8

The reaction is quenched by adding to it NaHCO3(aq) that removes the catalyst.

HCO3(aq) + H+(aq) H2O(l) + CO2(g)

88


Q.9

Titrated with standard solution of Na2S2O3(aq) using starch as indicator (added when the end point is near)

2S2O32(aq) + I2(aq) S4O6

2(aq) + 2I(aq)

Colour change at the end point : deep blue to colourless

89


Q.10

The excess S2O32(aq) would react with H+ to

give a cloudy mixture with a pungent smell.

S2O32(aq) + 2H+(aq) S(s) + SO2(g) + H2O(l)

90

Advantages of titrimetric method

1. Only simple apparatus are required.

2. Can be applied to a great variety of slow reactions.

91

Disadvantages of physical measurements

1. Not suitable for fast reactions.

It takes time to withdraw samples and perform titration.

2. Reactions are disturbed – NOT continuous

3. Time consuming – NOT automated

92

Factors Factors Affecting Affecting Reaction Reaction

RatesRates

93

Collision TheoryCollision Theory

No reaction

Sufficient K.E.

Incorrect orientation

94


Correct orientation Insufficient K.E.

No reaction

95


Sufficient K.E.

Correct orientation

Effective collision

96


Activation energy

Bond breaking and bond forming occur at the same time

Ea < B.E.(s) of the bond(s) to be broken

97


Activation energy

Higher Ea

more K.E. required for effective collision

slower reaction

98


Activation energy

Lower Ea

less K.E. required for effective collision

faster reaction

99


Activation energy

Rate of reaction depends on Ea which in turn depends on the nature of reactants.

E.g. K is more reactive than Mg

100

Factors Affecting Reaction RatesFactors Affecting Reaction Rates

concentration

concentration

temperature

temperature

pressurepressure

particle sizeparticle size

catalystcatalyst

lightlight

101

• e.g. Reaction between Mg and HCl

Effect of concentrationEffect of concentration

102

(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)


103

Time for reaction to complete: t1 < t2 < t3

Time for reaction to complete: t1 < t2 < t3

Higher [HCl(aq)]

Faster reactionHigher [HCl(aq)]

Faster reaction


104

[X]

Reactant particles are more crowded

Collision frequency

Number of effective collisions Reaction rate

105

For the reaction aA + bB cC + dD

Rate k[A]x[B]y

where x and y are the orders of reaction with respect to A and B

k is the rate constant units mol dm3 s1/(mol dm3)x+y

106

For the reaction aA + bB cC + dD

Rate k[A]x[B]y

x and y can be integers or fractional x y is the overall order of reaction.

x, y can ONLY be determined experimentally.

107

Only applicable to reactions involving gaseous reactants.

Effect of pressureEffect of pressure

108

Pressure

Reactant particles are more crowded

Collision frequency

No. of effective collisions

Rate of reaction

109

Effect of temperatureEffect of temperature

Applicable to ALL reactions

110

T K.E. of particles Collision frequency (minor effect) and

No. of particles with K.E. > Ea (major effect) No. of effective collisions Rate of reaction

111

Rate

T / C

Rate of reaction exponentially with temperature

In general, a 10oC in T doubles the rate.

RTEa

eRate

112

Effect of particle sizeEffect of particle size

For a fixed volume of solid,

Smaller particle size greater surface area

113

Rate involvingpowdered solid reactant is higher

Rate involvingpowdered solid reactant is higher

Reason: higher chance of contact between reactant particles

Reason: higher chance of contact between reactant particles

CaCO3(aq) + 2H+(excess) CaCl2(aq) + H2O(l) + CO2(g)

114

0.5 g powder

0.5 g granule

Q.11

115

Effect of CatalystEffect of Catalyst

A catalyst is a substance that alters the rate of a chemical reaction by providing analternative reaction pathway with a different activation energy.

A negative catalyst slows down a reaction by providing an alternative reaction pathway with a higher Ea.

A positive catalyst speeds up a reaction by providing an alternative reaction pathway with a lower Ea.

116

Catalysts remain chemically unchanged at the end of reactions.

Effect of CatalystEffect of Catalyst

117

MnO2 as catalystH2O2(aq) 2H2O(l) +

O2(g)Physical measurement


Vol

ume

of g

as fo

rmed

(cm

3 )MnO2 as catalyst

H2O2(aq) 2H2O(l) + O2(g)

119

Titrimetric method (Q.12)

MnO2 as catalystH2O2(aq) 2H2O(l) +

O2(g)Pipette samples at different times

Remove MnO2(s) by filtration

Titrate with MnO4(aq)/H+(aq)

5H2O2(aq) + 2MnO4(aq) + 6H+(aq)

2Mn2+(aq) + 8H2O(l) + 5O2(g)

120

Q.13

time

[H2O2

]

Without MnO2

With MnO2

121

Effect of lightEffect of light

Light with specific frequency (E h) can provide sufficient energy to break a particular chemical bond in a reactant leading to a photochemical reaction.

Br – Br hBr + Br C6H14 + Br C6H13 + HBr

C6H13Br…

122

AutocatalysisAutocatalysis

Catalysis in which the product acts as the catalyst of the reaction

2MnO4(aq) + 16H+(aq) + 5C2O4

2(aq)

2Mn2+(aq) + 10CO2(g) + 8H2O(l) CH3COCH3(aq) + I2(aq)

CH3COCH2I(aq) + H+(aq) + I(aq)

123 time

[MnO4]

Q.14

Rate

Rate

Sigmoid curve

124

The END

125

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?

13.1 Rates of Chemical Reactions (SB p.5)

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

126

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

127


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

128

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


129

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

130


(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

131


(b) Average rate

=

= -0.03 mol dm-3 min-1

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

132


(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

133

(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

134


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

135

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

136


(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

137

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.

13.2 Expressions of Reactions Rates in Terms of Rates of Changes in Concentrations of Reactants or Products (SB p.10)

The rate of the reaction is expressed as:

dtd

dtd

dt

d ]CO[31]Hb[

41](CO)[Hb

Rate 34

Answer

Back

138

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)

13.2 Expressions of Reactions Rates in Terms of Rates of Changes in Concentrations of Reactants or Products (SB p.10)

Answer

Back

Rate = dt

d

dt

d

dt

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

21O(l)][H

21 222

139

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.

13.3 Methods of Measuring Reaction Rates (SB p.11)

140

(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.

141

(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.

142

(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 10

10 8

Time after mixing (min)

Volume of HCl added at the end point (cm3)

5 8

10 10

Option A

Option B

143


(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.

144


(d) Name a suitable indicator for the titration.

Answer(d) Methyl orange / Phenophthalein

Back

145


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

146


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

Answer(a)

147


(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.

148


(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.

149


(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

150

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

151

13.4 Factors Affecting Reaction Rates (SB p.17)

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

Back

AnswerA higher concentration of oxygen increases

the rate of combustion.

152


(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)

153


(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:


154


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.

1 rates of chemical reactions 13.1rates of chemical reactions 13.2expressions of reaction rates in...

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