7 Chemical Reactions

The rate of a chemical reaction is a measure of how fast the reactants are being used up to make products.

The rate is usually measured in the amount of reactant used up OR the amount of product made per second.

There are varias ways that we can use to measure the rate that a reaction is taking place. It is easiest to measure the rate of a reaction that produces a gas…

Method 1 - volume of gas given off

The amount of gas evolved can be read from the gas syringe at equal time intervals and plotted on a graph as shown. A measuring cylinder can also be used to collect the gas…

�1

Method 2 - loss of mass

When a gas is lost (usually a gas with a large Mr) we can measure the decrease in mass of the conical flask…

In both of these cases, the speed or rate of the reaction is given by the gradient of the graph at any point.

The reaction rate is greatest at the beginning of the reaction and slowly decreases.

Method 3 - colour and precipitates

If one of the reactants or products is coloured, we can use special devices called colorimeters that can measure how much of the coloured chemical we have at any one moment.

If a precipitate is produced, we can also use this to measure reaction rate.

The faster the cross disappears, the faster the reaction.

The iodine clock reaction is another such case…

p103 Q1-3�2

How the rate changes during a reaction

As we have seen above, the rate of the reaction it any point can be calculated by finding the gradient or slope of the graph at that point…

Calculate the rate of this reaction after 40 seconds:

What would the average reaction rate be here for the whole reaction?

As one of the reactants is used up (the limiting reagent or reactant), the reaction rate decreases until it is finally zero (gradient = 0).

p105 Q1-3

�3

What affects the rate of a chemical reaction?

• Surface area• Catalysts• Concentration (pressure in gases)• Temperature• Light

Surface Area

For particles to react, they have to collide. When a reaction involves a solid the amount of surface which is exposed to collisions is the key factor.

For a given mass of solid reactant, the smaller the particles the larger the surface area and the greater the rate of reaction…

Great surface area means more collisions and therefore more SUCCESSFUL COLLISIONS (those which result in reaction)

This must be taken into account in factories where powder which is potentially flammable can get into the atmosphere (eg. flour mills, wood mills, mines). The reaction rate can be so fast that an explosion occurs…

�4

CatalystsThese are substances that can speed up a chemical reaction but are not used up themselves. They are usually solids (often involving transition metals) but sometimes are in solution or gaseous.

Catalysts that speed up biological processes in organisms are called…

All catalysts lower the activation energy which increases the reaction rate…

This means that more particles have enough energy to overcome the activation energy therefore there are more SUCCESSFUL COLLISIONS.

Eg. The decomposition of hydrogen peroxide using manganese(IV) oxide as a catalyst…

Eqn: After the experiment, the mixture can be filtered, and the catalyst dried and weighed to check that is has not been used up.

p107 Q1-3�5

Concentration and Pressure

Consider the following reaction…

Eqn:

Here we used an EXCESS of acid and the calcium carbonate was the LIMITING REAGENT.

This explains why the graph reaches the same height each time.

Conclusion: (talk about successful collisions)

NB. Increasing the pressure of a gas is the same as increasing its concentration, and therefore increases the collision rate.

In summary, …

�6

Temperature

We all can guess that increasing the temperature of a chemical reaction will increase its rate, but can we explain why?

When we increase the temperature, what happens to the particles in the reaction mixture?

What will this do to the collision frequency?

What will the collisions have much more of and what will they be able to overcome much easier?

Consider the following experiment:

Magnesium is reacted with hydrochloric acid at 2 different temperatures.

Eqn: �7

The results are shown on the following graph…

Explain the shapes of the lines on the graphs (use the term activation energy in your answer)…

p111 Q1-3�8

Photochemical reactions

There are 3 reactions that are affected by light that you must know about:

1. Photosynthesis2. Photography (photoreduction)3. The chlorination of methane

Photosynthesis

Plants take in carbon dioxide and water and convert them into glucose and oxygen.

Energy from the sun drives the reaction.

Green pigments called chlorophylls act as the catalyst for the reaction.

�9

Photography - photo-reduction

Special film coated with a gel containing Silver Bromide reacts when exposed to light to make small particles of millions of black silver atoms.

The more light the more silver is produced and the darker the film turns. This was the basis of photography..

Full Eqn:

Reduction:

Oxidation:

p113 Q1-3End of chapter questions

Chlorination of Methane (see Organic Chemistry, Year11)

Light can break the C-Cl bond which can be useful in some reactions but also caused the depletion of the ozone layer by CFCs (see later)

�10

EXPERIMENTS: RATES OF REACTION1. How does surface area effect the rate of reaction?

Describe and explain the effect of SURFACE AREA on the rate of reactions in terms of collisions between reacting particles

Introduction

In this experiment you will look at the reaction Calcium carbonate with hydrochloric acid.

Calcium carbonate occurs naturally in several forms, limestone, marble, chalk and calcite. All react with dilute hydrochloric acid.

Write a balanced equation for this reaction (with state symbols)

…………………………………………………………………………………………………………………………………………………………………………

The total mass of the flask and reactants will decrease if we allow the carbon dioxide to escape.

We can undertake the experiment on a top-pan balance and measure the mass loss.

Hypothesis

I think the …………………………… marble chips will have the fastest rate of reaction because….

�11

Method

• Use the measuring cylinder to pour 40cm3 of dilute hydrochloric acid into the conical flask.

• Place a loose plug of cotton wool into the neck of the flask. • Weigh out 10g of small marble chips. • Place the conical flask containing the acid and separately the marble on the

balance pan and record the mass • Add the marble chips to the acid and replace the cotton wool plug • Start the stop clock recording the initial mass of the flask and then record

the mass of the flask and its contents every 30s for 5 minutes • Repeat the experiment using larger marble chips

Why do we use a cotton wool plug in the flask?

State 3 variables you will need to control in the experiment

Safety precautions

• Careful handling and pouring of acid (use a funnel) avoid contact with skin and eyes

• Wear goggles • Do not throw away marble chips collect them in the sieves in the sink.

�12

Results

Expt1: Results for small marble chips

Expt 2: Results for larger marble chips

�13

Plot a graph of your results for both experiments on the same axes below…

�

(5)

�14

Conclusion

Did your results agree/disagree with your prediction ?

Explain why

What do you notice about the gradient of the graph as time progresses?

Why does this happen?

How could you improve the reliability of your results ?

What were the major errors in this procedure and how could you improve the experiment?

�15

2. How a catalyst affects reaction rate

Plan an experiment to find out which metal oxide Manganese (IV) oxide or lead (IV)oxide catalyses the decomposition of hydrogen peroxide most effectively.

2H2O2 → 2H2O + O2

Assume you will have

75cm3 of 100vol H2O2

0.5g of powdered Manganese (IV) oxide and lead (IV)oxide

PLAN:

HOW COULD YOU PROVE THAT THE CATALYSTS WERE NOT USED UP IN THE

EXPERIMENT?

�16

How does concentration effect the rate of reaction ?

Aims TO describe and explain the effect of concentration, on the rate of reactions in terms of collisions between reacting particles

Introduction

For reactions in solution the rate of a reaction depends on the concentration of one or more of the reactants.

The concentration is highest at the beginning of a reaction and then decreases as the reaction progresses and reactants are used up.

In this experiment you will look at the reaction of magnesium ribbon with hydrochloric acid.

Write a balanced equation for this reaction (with state symbols)

It is easy to see when the reaction is completed because the Mg ribbon (s) “disappears”.

The surface of the magnesium ribbon can become oxidised and a layer of MgO form, this is easily removed by emery paper.

You will be given:

• conical flasks, • stop clocks • measuring cyinders, • emergy paper • one 18cm legnth of Mg ribbon,

• 60cm3 of HCl in the following concentrations:

-2.0 moldm-3 -1.5 moldm-3 - 1.0 moldm-3 - 0.5 moldm-3

�17

Hypothesis

I predict the rate of the reaction will be greatest with ………………………..moldm-3 HCl because

Method

1. Dependent variable: ………………………………………………………………………………………………………

2. Independent variable: ……………………………………………………………………………………

3. Control variables: ……………………………………………………………..……………………………………………

4. To improve reliability we will: ……………………………………………………………………………….………

5. Write a method for your investigation (including a diagram)

�18

Results

Complete the results table (including headers and units).

Conclusion 11. Did your results agree/disagree with your prediction?

12. Explain your results

How could you show the gas produced was hydrogen ?

In this experiment which reactant is used in excess ?

0.5

1.0

1.5

2.0

�19

How does temperature effect the rate of reaction ?

Aims-to Describe and explain the effect of temperature, on the rate of reactions in terms of collisions between reacting particles

Safety precautions

• Tell your teacher if you are asthmatic before the experiment • Careful handling and pouring of solutions (use a funnel) avoid contact with

skin and eyes • Wear goggles • Do not overheat solutions • Clear desk • Try not to inhale SO2 when looking through flask at cross.

Introduction In this experiment you will look at the reaction of sodium thiosulfate with hydrochloric acid. Solid sulphur is formed (a precipitate) making the reaction mixture becomes increasingly cloudy.

Na2S2O2 (aq)+ 2HCl(aq) → 2NaCl(aq) +H2O(l) + SO2 (g) + S (s)

We can time the length of time it takes for the mixture to become so cloudy that we can no longer see through it.

Hypothesis

1. I think the …………………………………….. temperature will have the fastest rate of

reaction because

�20

Method

- Use a 100cm3 measuring cylinder to pour 40cm3 of thiosulphate solution into the conical flask.

- Measure out 5 cm3 of dilute hydrochloric acid in a smaller measuring cylinder - Place the conical flask with the sodium thiosulphate solution in a large beaker

containing hot water (a water bath). - Wait until the temperature of the solution reaches 30ºC. Remove the beaker

and allow the temperature to stabilise. - Place the flask on a piece of paper marked with a cross. - Add the hydrochloric acid and start the stop clock - Look through the solution. - Stop the timer when the cross disappears.

Rinse all equipment thoroughly and repeat heating the sodium thiosulphate solution to 40, 50, 60 and then try it at 20ºC

Explain why we use the paper marked with a cross………………………………………………..

State 3 variables you will control in the experiment

Results

5. Complete the results table

Temperature of Thiosulphate at start (ºC)

Time for cross to disappear (s) 1/Time (s-1)

�21

6. Draw a graph of your results

Start temperature against 1/time

!

�22

Conclusion

Did your results agree/disagree with your prediction?

Explain why

Evaluation

Why must the volumes of sodium thiosulphate and hydrochloric acid remain constant?

What is the advantage of plotting the graph of 1/time?

If the sodium thiosulfate started to go cloudy before you added the acid why

might this be?

How might this effect your results ?

�23

�24

Appendix

35Cambridge IGCSE Chemistry 0620. Syllabus for examination in 2016, 2017 and 2018.

8.

App

endi

x

8.1

The

Perio

dic

Tabl

e

Group

I

II

III

IV

V

V

I

VII

VIII

Ke

y

1 H

hydrogen

1

2

He

helium

4

3

Li

lithiu

m

7

4

Be

berylliu

m

9

a

to

mic n

um

be

r

atom

ic sym

bol

nam

e

re

la

tive

a

to

mic m

ass

5

B

boron

11

6

C

carbon

12

7

N

nitrogen

14

8

O

oxygen

16

9

F

flu

orin

e

19

10

Ne

neon

20

11

Na

sodiu

m

23

12

Mg

magnesiu

m

24

13

Al

alu

min

ium

27

14

Si

silic

on

28

15

P

phosphorus

31

16

S

sulfur

32

17

Cl

chlo

rin

e

35

.5

18

Ar

argon

40

19

K

potassiu

m

39

20

Ca

calc

ium

40

21

Sc

scandiu

m

45

22

Ti

titaniu

m

48

23

V

vanadiu

m

51

24

Cr

chrom

ium

52

25

Mn

manganese

55

26

Fe

iron

56

27

Co

cobalt

59

28

Ni

nic

kel

59

29

Cu

copper

64

30

Zn

zin

c

65

31

Ga

galliu

m

70

32

Ge

germ

aniu

m

73

33

As

arsenic

75

34

Se

sele

niu

m

79

35

Br

brom

ine

80

36

Kr

krypton

84

37

Rb

rubid

ium

85

38

Sr

strontiu

m

88

39

Y

yttriu

m

89

40

Zr

zirconiu

m

91

41

Nb

nio

biu

m

93

42

Mo

moly

bdenum

96

43

Tc

technetiu

m

–

44

Ru

rutheniu

m

10

1

45

Rh

rhodiu

m

10

3

46

Pd

palladiu

m

10

6

47

Ag

silver

10

8

48

Cd

cadm

ium

11

2

49

In

indiu

m

11

5

50

Sn

tin

11

9

51

Sb

antim

ony

12

2

52

Te

telluriu

m

12

8

53

I

iodin

e

12

7

54

Xe

xenon

13

1

55

Cs

caesiu

m

13

3

56

Ba

bariu

m

13

7

57

–7

1

lanthanoid

s

72

Hf

hafniu

m

17

8

73

Ta

tantalu

m

18

1

74

W

tungsten

18

4

75

Re

rheniu

m

18

6

76

Os

osm

ium

19

0

77

Ir

irid

ium

19

2

78

Pt

pla

tin

um

19

5

79

Au

gold

19

7

80

Hg

mercury

20

1

81

Tl

thalliu

m

20

4

82

Pb

lead

20

7

83

Bi

bis

muth

20

9

84

Po

polo

niu

m

–

85

At

astatin

e

–

86

Rn

radon

–

87

Fr

franciu

m

–

88

Ra

radiu

m

–

89

–1

03

actin

oid

s

10

4

Rf

rutherfordiu

m

–

10

5

Db

dubniu

m

–

10

6

Sg

seaborgiu

m

–

10

7

Bh

bohriu

m

–

10

8

Hs

hassiu

m

–

10

9

Mt

meitneriu

m

–

11

0

Ds

darm

stadtiu

m

–

11

1

Rg

roentgeniu

m

–

11

2

Cn

copernic

ium

–

1

14

Fl

fle

roviu

m

–

1

16

Lv

liverm

oriu

m

–

lan

th

an

oid

s

57

La

lanthanum

13

9

58

Ce

ceriu

m

14

0

59

Pr

praseodym

ium

14

1

60

Nd

neodym

ium

14

4

61

Pm

prom

ethiu

m

–

62

Sm

sam

ariu

m

15

0

63

Eu

europiu

m

15

2

64

Gd

gadoliniu

m

15

7

65

Tb

terbiu

m

15

9

66

Dy

dysprosiu

m

16

3

67

Ho

holm

ium

16

5

68

Er

erbiu

m

16

7

69

Tm

thulium

16

9

70

Yb

ytterbiu

m

17

3

71

Lu

lutetiu

m

17

5

actin

oid

s

89

Ac

actin

ium

–

90

Th

thoriu

m

23

2

91

Pa

protactin

ium

23

1

92

U

uraniu

m

23

8

93

Np

neptuniu

m

–

94

Pu

plu

toniu

m

–

95

Am

am

eric

ium

–

96

Cm

curiu

m

–

97

Bk

berkelium

–

98

Cf

californiu

m

–

99

Es

ein

stein

ium

–

10

0

Fm

ferm

ium

–

10

1

Md

mendele

viu

m

–

10

2

No

nobelium

–

10

3

Lr

law

renciu

m

–

Th

e vo

lum

e o

f o

ne

m

ole

o

f a

ny g

as is

2

4 dm

3

a

t ro

om

te

mp

era

tu

re

a

nd

p

re

ssu

re

(r.t.p

.)