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“Facts are not science — as the dictionary is not literature
...
Fischer

CHAPTER

1

Chemical Reactions
and Equations

C

onsider the following situations of daily life and think what happens
when –
milk is left at room temperature during summers
...

grapes get fermented
...

food gets digested in our body
...


In all the above situations, the nature and the identity of the initial
substance have somewhat changed
...
Whenever a chemical
change occurs, we can say that a chemical reaction has taken place
...
How do we come to know that a chemical reaction
has taken place? Let us perform some activities to find the answer to
these questions
...
1
CAUTION: This Activity needs
the teacher’s assistance
...

Clean a magnesium ribbon
about 2 cm long by rubbing
it with sandpaper
...

Burn it using a spirit lamp or
burner and collect the ash so
formed in a watch-glass as
shown in Fig
...
1
...

What do you observe?

Figure 1
...
This powder is
magnesium oxide
...


Activity 1
...

Add potassium
iodide solution
to this
...
3
Take a few zinc granules in a conical flask or a test tube
...
1
...

CAUTION: Handle the acid with care
...
Is there any change in
its temperature?

From the above three activities, we can say that any of
the following observations helps us to determine whether
a chemical reaction has taken place –
change in state
change in colour
evolution of a gas
change in temperature
...
We will study about the various types
of chemical reactions and their symbolic representation
in this Chapter
...
2
Formation of hydrogen
gas by the action of
dilute sulphuric acid on
zinc

CHEMICAL EQUATIONS
1
...
1 can be described as – when a magnesium ribbon is burnt in
oxygen, it gets converted to magnesium oxide
...
It can be written in a
shorter form
...

The word-equation for the above reaction would be –
Magnesium + Oxygen
(Reactants)

→ Magnesium oxide

(1
...
1),
magnesium and oxygen, are the reactants
...

A word-equation shows change of reactants to products through an
arrow placed between them
...
Similarly, products are
written on the right-hand side (RHS) with a plus sign (+) between them
...


2

Science

1
...
1 Writing a Chemical Equation
Is there any other shorter way for representing chemical equations?
Chemical equations can be made more concise and useful if we use
chemical formulae instead of words
...
If you recall formulae of magnesium, oxygen and
magnesium oxide, the above word-equation can be written as –
Mg + O2 → MgO

(1
...
Is the number of atoms of each element the
same on both the sides? If not, then the equation is unbalanced because
the mass is not the same on both sides of the equation
...
Equation (1
...


1
...
2 Balanced Chemical Equations
Recall the law of conservation of mass that you studied in Class IX; mass
can neither be created nor destroyed in a chemical reaction
...

In other words, the number of atoms of each element remains the
same, before and after a chemical reaction
...
Is the chemical Eq
...
2) balanced? Let us
learn about balancing a chemical equation step by step
...
3 may be represented as –
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
The above word-equation may be represented by the following
chemical equation –
Zn + H2SO4 → ZnSO4 + H2

(1
...

Element

Number of atoms in
reactants (LHS)

Number of atoms
in products (RHS)

Zn
H
S
O

1
2
1
4

1
2
1
4

As the number of atoms of each element is the same on both sides of
the arrow, Eq
...
3) is a balanced chemical equation
...
4)

3

Step I: To balance a chemical equation, first draw boxes around each
formula
...

Fe + H2O → Fe3O4 + H2

(1
...
5)
...
It may be a reactant or a
product
...
Using these criteria, we select Fe3O4 and the element
oxygen in it
...

To balance the oxygen atoms –
Atoms of
oxygen
(i) Initial
(ii) To balance

In reactants

1 (in H2O)
1×4

In products

4 (in Fe3O4)
4

To equalise the number of atoms, it must be remembered that we
cannot alter the formulae of the compounds or elements involved in the
reactions
...
Now the partly balanced equation
becomes –
(1
...
Pick any of these elements
to proceed further
...

To equalise the number of H atoms, make the number of molecules
of hydrogen as four on the RHS
...
7)
(partly balanced equation)

Science

Step V: Examine the above equation and pick up the third element which
is not balanced
...

Atoms of
iron

In reactants

In products

1 (in Fe)
1×3

3 (in Fe3O4)
3

(i) Initial
(ii) To balance

To equalise Fe, we take three atoms of Fe on the LHS
...
8)

Step VI: Finally, to check the correctness of the balanced equation, we
count atoms of each element on both sides of the equation
...
9)

3Fe + 4H2O → Fe3O4 + 4H2

(balanced equation)

The numbers of atoms of elements on both sides of Eq
...
9) are
equal
...
This method of balancing chemical
equations is called hit-and-trial method as we make trials to balance
the equation by using the smallest whole number coefficient
...
(1
...
Does this equation tell us anything about
the physical state of each reactant and product? No information has
been given in this equation about their physical states
...
The gaseous, liquid, aqueous and solid states of reactants
and products are represented by the notations (g), (l), (aq) and (s),
respectively
...

The balanced Eq
...
9) becomes
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

(1
...

Usually physical states are not included in a chemical equation unless
it is necessary to specify them
...
, for the reaction are indicated above and/or below the arrow
in the equation
...
11)

Sunlight

6CO2 (aq) + 6H2 O(l)  C6 H12 O6 (aq) + 6O2 (aq)
Chlorophyll →

(1
...
(1
...


Why should a magnesium ribbon be cleaned before burning in air?

2
...

(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate +
Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

3
...

(i) Solutions of barium chloride and sodium sulphate in water react
to give insoluble barium sulphate and the solution of sodium
chloride
...


REACTIONS
1
...
Nor do atoms
disappear from the mixture or appear from elsewhere
...
You will study about types of bonds formed
between atoms in Chapters 3 and 4
...
2
...
4
Take a small amount of calcium oxide
or quick lime in a beaker
...

Touch the beaker as shown in Fig
...
3
...
3
Formation of slaked
lime by the reaction of
calcium oxide with
water

Calcium oxide reacts vigorously with water to produce slaked lime
(calcium hydroxide) releasing a large amount of heat
...
13)

(Slaked lime)

In this reaction, calcium oxide and water combine to form a single
product, calcium hydroxide
...


6

Science

Do You Know?

A solution of slaked lime produced by the reaction 1
...
Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin
layer of calcium carbonate on the walls
...
It is interesting to
note that the chemical formula for marble is also CaCO3
...
14)

(Calcium
carbonate)

Let us discuss some more examples of combination reactions
...
15)

Formation of water from H2(g) and O2(g)
2H2(g) + O2(g) → 2H2O(l)

(1
...

In Activity 1
...

This makes the reaction mixture warm
...

Other examples of exothermic reactions are –
(i) Burning of natural gas
CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)
(ii)

(1
...
We get this energy
from the food we eat
...
For example, rice, potatoes and bread contain
carbohydrates
...

This glucose combines with oxygen in the cells of our body and provides
energy
...

C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy

(1
...

Identify the type of the reaction taking place in Activity 1
...


Chemical Reactions and Equations

7

1
...
2 Decomposition Reaction
Activity 1
...

Note the colour of the ferrous sulphate
crystals
...
1
...

Observe the colour of the crystals after
heating
...
4
Correct way of heating
the boiling tube
containing crystals
of ferrous sulphate
and of smelling the
odour

Have you noticed that the green colour of the ferrous sulphate crystals
has changed? You can also smell the characteristic odour of burning
sulphur
...
19)

(Ferric oxide)

In this reaction you can observe that a single reactant breaks down
to give simpler products
...
Ferrous
sulphate crystals (FeSO4, 7H2O) lose water when heated and the colour
of the crystals changes
...
Ferric oxide is a solid,
while SO2 and SO3 are gases
...
Calcium oxide is called lime or quick lime
...
When a decomposition
reaction is carried out by heating, it is called thermal decomposition
...
20)

(Quick lime)

Another example of a thermal decomposition reaction is given
in Activity 1
...


Activity 1
...

Hold the boiling tube with a pair of tongs and
heat it over a flame, as shown in Fig
...
5
...

Figure 1
...

These fumes are of nitrogen dioxide (NO2)
...
21)

(Oxygen)

Let us perform some more decomposition reactions as given in
Activities 1
...
8
...
7
Take a plastic mug
...

Insert carbon electrodes in these rubber
stoppers as shown in Fig
...
6
...

Fill the mug with water such that the
electrodes are immersed
...

Take two test tubes filled with water and
invert them over the two carbon electrodes
...

You will observe the formation of bubbles
at both the electrodes
...

Is the volume of the gas collected the same in both the test tubes?
Once the test tubes are filled with the respective gases, remove
them carefully
...

CAUTION: This step must be performed carefully by the teacher
...
6
Electrolysis of water

Activity 1
...

What is its colour?
Place this china dish in sunlight for some time
(Fig
...
7)
...


Figure 1
...
This is Silver chloride turns grey
due to the decomposition of silver chloride into silver and chlorine by in sunlight to form silver
metal
light
...
22)

9

Silver bromide also behaves in the same way
...
23)

The above reactions are used in black and white photography
...

Reactions in which energy is absorbed are known as endothermic
reactions
...
Add 1 g of ammonium chloride and mix
with the help of a glass rod
...
What do you
feel? Is this an exothermic or endothermic reaction?

Q
1
...

(i) Name the substance ‘X’ and write its formula
...


2
...
7 double of the amount collected in the other? Name this gas
...
2
...
9

Figure 1
...

Take two test tubes marked as (A) and
(B)
...

Tie two iron nails with a thread and
immerse them carefully in the copper
sulphate solution in test tube B for
about 20 minutes [Fig
...
8 (a)]
...

After 20 minutes, take out the iron nails
from the copper sulphate solution
...
1
...

Also, compare the colour of the iron nails
dipped in the copper sulphate solution
with the one kept aside [Fig
...
8 (b)]
...
8 (b) Iron nails and copper sulphate solutions compared before and after the experiment

Why does the iron nail become brownish in colour and the blue colour
of copper sulphate solution fade?
The following chemical reaction takes place in this Activity–
Fe(s) + CuSO4(aq) →
(Copper sulphate)

FeSO4(aq)

+ Cu(s)

(1
...
This reaction is known as
displacement reaction
...
25)
(Copper sulphate)

Pb(s) + CuCl2(aq)

→

(Copper chloride)

(Zinc sulphate)

PbCl2(aq)

+

Cu(s)

(1
...
They displace
copper from its compounds
...
2
...
10
Take about 3 mL of sodium sulphate
solution in a test tube
...

Mix the two solutions (Fig
...
9)
...
This insoluble substance
formed is known as a precipitate
...

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
(Sodium
sulphate)

(Barium
chloride)

Chemical Reactions and Equations

(Barium
sulphate)

(1
...
9
Formation of barium
sulphate and sodium
chloride

(Sodium
chloride)

11

What causes this? The white precipitate of BaSO4 is formed by the
reaction of SO2– and Ba2+
...
Such reactions in which there is an
exchange of ions between the reactants are called double displacement
reactions
...
2, where you have mixed the solutions of lead(II) nitrate
and potassium iodide
...

(iii) Is this also a double displacement reaction?

1
...
5 Oxidation and Reduction
Activity 1
...
1
...

What do you observe?

Figure 1
...
Why has this black
substance formed?
This is because oxygen is added to copper and
copper oxide is formed
...
28)
→
If hydrogen gas is passed over this heated material (CuO), the black
coating on the surface turns brown as the reverse reaction takes place
and copper is obtained
...
29)
If a substance gains oxygen during a reaction, it is said to be oxidised
...

During this reaction (1
...
The hydrogen is gaining oxygen and is being oxidised
...
Such reactions are called oxidation-reduction reactions
or redox reactions
...
30)
Some other examples of redox reactions are:
ZnO + C → Zn + CO
MnO2 + 4HCl → MnCl 2 + 2H2 O + Cl 2

12

(1
...
32)

Science

In reaction (1
...

In reaction (1
...

From the above examples we can say that if a substance gains oxygen
or loses hydrogen during a reaction, it is oxidised
...

Recall Activity 1
...
Is magnesium being oxidised or
reduced in this reaction?

HAVE
OXID
XIDA
1
...
3
...
This process
is commonly known as rusting of iron
...
Have you noticed the colour of the coating
formed on copper and silver? When a metal is attacked by substances
around it such as moisture, acids, etc
...
The black coating on silver and the green
coating on copper are other examples of corrosion
...
Corrosion of
iron is a serious problem
...
You will learn more about corrosion in
Chapter 3
...
3
...
Usually substances which prevent oxidation
(antioxidants) are added to foods containing fats and oil
...
Do you know that
chips manufacturers usually flush bags of chips with gas such as
nitrogen to prevent the chips from getting oxidised ?

Q

U

E

S

T

I

O

N

1
...


Give an example of a double displacement reaction other than
the one given in Activity 1
...


3
...

(i) 4Na(s) + O2(g) → 2Na2O(s)

(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Chemical Reactions and Equations

S

?
13

What you have learnt
A complete chemical equation represents the reactants, products and their physical
states symbolically
...
Equations must always be balanced
...

Decomposition reactions are opposite to combination reactions
...

Reactions in which heat is given out along with the products are called exothermic
reactions
...

When an element displaces another element from its compound, a displacement
reaction occurs
...

Precipitation reactions produce insoluble salts
...

Oxidation is the gain of oxygen or loss of hydrogen
...


E X E R C I S E S
1
...

(b)
Carbon dioxide is getting oxidised
...

(d)
Lead oxide is getting reduced
...
Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction
...


14

Science

(c)

decomposition reaction
...


3
...

(a)

Hydrogen gas and iron chloride are produced
...


(c)

No reaction takes place
...


4
...
Translate the following statements into chemical equations and then balance them
...


(b)

Hydrogen sulphide gas burns in air to give water and sulpur dioxide
...


(d)

Potassium metal reacts with water to give potassium hydroxide and hydrogen
gas
...
Balance the following chemical equations
...
Write the balanced chemical equations for the following reactions
...
Write the balanced chemical equation for the following and identify the type of
reaction in each case
...
What does one mean by exothermic and endothermic reactions? Give examples
...
Why is respiration considered an exothermic reaction? Explain
...
Why are decomposition reactions called the opposite of combination reactions?
Write equations for these reactions
...
Write one equation each for decomposition reactions where energy is supplied in
the form of heat, light or electricity
...
What is the difference between displacement and double displacement reactions?
Write equations for these reactions
...
In the refining of silver, the recovery of silver from silver nitrate solution involved
displacement by copper metal
...

15
...

16
...

(a) Oxidation
(b) Reduction
17
...

Name the element ‘X’ and the black coloured compound formed
...
Why do we apply paint on iron articles?
19
...
Why?
20
...

(a) Corrosion
(b) Rancidity

Group Activity
Perform the following activity
...

Put 25 mL of water in A, B and C beakers and copper sulphate solution in beaker D
...

Add two spatulas of potassium sulphate, ammonium nitrate, anhydrous copper
sulphate and fine iron fillings to beakers A, B, C and D respectively and stir
...

Find out which reactions are exothermic and which ones are endothermic in nature

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