This document provides an overview of key topics in chemical reactions covered in Standard 10 Chapter 2, including: 1. Writing and balancing chemical equations, and the main types of chemical reactions - combination, decomposition, displacement, and double displacement. 2. Oxidation and reduction reactions, and examples like corrosion. 3. Neutralization reactions between acids and bases. It defines important concepts like reactants, products, oxidation, reduction, and provides examples for different reaction types. Steps for balancing chemical equations are also summarized.

1. Standard 10 Chapter 2 Chemical Reactions
2.1 Chemical equation
2.1.1 Writing chemical equations
2.1.2 Balancing the chemical equation
2.1.3 Steps involved in balancing
2.2 Types of chemical reactions
2.2.1 Combination (synthesis) reaction
2.2.2 Decomposition reactions
2.2.3 Displacement reactions
2.2.4 Double displacement reactions
2.3 Oxidation and reduction
2.3.1 Oxidation in and around you
2.3.2 Rancidity
2.4 Neutralization

2. Standard 10
Chapter 2
Chemical Reactions
When a chemical change (permanent) occurs a
chemical reaction takes place.
Temporary changes - physical
Permanent changes – chemical

3. 2.1 Chemical equation
Word equation Simple form of representation of
chemical reaction using words
e.g. copper + oxygen → copper oxide
Chemical equation Representation of chemical
reaction using chemical formulae
e.g. Cu + 02 → CuO

4. 2.1.1 Writing chemical equations
Reactants → Products
left hand side (LHS) right hand side (RHS)
The arrow
•points towards the products and tail towards the
reactants
•represents the direction of the reaction
Conditions of the reaction are to be indicated
above and below the arrow

5. A plus (+) sign indicates two or more reactants are
involved, or products are formed
Physical states of the reactants and products make it
more informative. The gaseous, liquid and solid
states are symbolized as (g), (l) and (s)
Mix vegetable oil with nickel powder as a catalyst
and heat it with hydrogen gas, fats are obtained on
cooling at high temperature
Edible oil (l) + Hydrogen (g) nickel Δ Fats (s)

6. 2.1.2 Balancing the chemical equation
The chemical equation
Iron sulphide + Sulphuric acid Ferrous sulphate + Hydrogen sulphide
can be represented as
FeS + H2S04 FeS04 + H2S
Balanced equation The number of atoms is same
on the LHS and RHS of the chemical equation

7. 2.1.3 Steps involved in balancing
Step 1 Rewrite the given equation as it is
SO2 + H2S → S + H2O
Step 2 Compare the number of atoms of each
element in the given equation on both sides of the
equation
Step 3 Choose the reactant or product having
maximum number of atoms. Change the coefficient
SO2 + H2S → S + 2H2O

8. Step 4 'S' and H atoms are not yet balanced. You
may select any one of the two. Select hydrogen
atoms for balancing. Equalise the number of
hydrogen atoms, exactly as mentioned in Step 3.
SO2 + 2H2S → S + 2H2O
Step 5 Select 'S' to be balanced
Step 6 Count the number of atoms of each element
on LHS and RHS
SO2(g) + 2H2S(g) → S(s) + 2H2O(l)

9. 2.2 Types of chemical reactions
Chemical reaction involves breaking and making of
the bonds between the atoms to produce new
substances

10. Reaction Description Example
Combination
(synthesis)
A + B → AB
Two or more
elements combine
to give a compound
2H2 + O2 → 2H2O
Decomposition
AB → A + B
Compound is
broken down into
elements
2H2O → 2H2 + O2
Substitution
(single
replacement)
A + BC → AC + B
OR
A + BC → BA + C
Atom or group of
atoms is replaced by
another atom or
group
Zn + 2HCl →
ZnCl2 + H2
OR
Cl2 + 2NaBr →
2NaCl + Br2
Types of chemical
reactions

11. Reaction Description Example
Precipitation
(double
replacement)
AB + CD → AD +
CB
Solutions of two
soluble
compounds mix
to give a solid
compound
AgNO3 + NaCl
→ AgCl +
NaNO3
Neutralisation
(acid + base)
HA + BOH → BA
+ H2O
Acid and base
react to give salt
and water
HCl + NaOH →
H2O + NaCl
Types of chemical reactions

12. Reaction Description Example
Reduction-
Oxidation
(redox)
Red. A + ē → A-
Ox. B → B+
+ ē
Transfer of
electrons
(ionic bond)
Na → Na+
+ ē
ē + Cl-
→ Cl
Combustion
A + O2 → H2O +
CO2
A hydrocarbon
reacts with
oxygen to
produce carbon
dioxide and water
C6H12O6(aq) +
6O2(g) →
6CO2(g) + 6
H2O(l) + heat
exothermic
Types of chemical reactions

13. 2.2.1 Combination (synthesis) reaction
Two or more substances (reactants) combine
(elements or compounds) to form single product
A + B → AB
Formation of iron sulphide by mixing iron and
sulphur
Fe(s) + S(s) → FeS(s)
Increase in formation of C02 in environment leads
to acid rains, when it mixes with water vapour
H2O(g) + CO2(g) → H2CO3(l)

14. At construction sites the wet cement with sand and
gravel sets into concrete which imparts strength to
the building
3Ca0.Al203(s) + 6H2O → 3Ca0.Al203.6H20(s) + Heat
Tricalcium aluminate + Water → Concrete
Plaster of Paris (POP) when mixed with water
sets quickly into hard mass known as gypsum
which is the raw material in manufacturing cement.
2CaS04.H20 + 3H20 → 2CaS04.2H20 + Heat
POP Gypsum

15. Plaster of Paris is used in surgical bandages, casting
and moulding in dentistry, in making statues,
decoration of roofs, crayons manufacturing etc.
During combination of two or more reactants, the
reactants may require or release (liberate) heat
with formation of products

16. Take 1OO ml of distilled water in two polythene
bottles to prevent heat loss. Note temperature of
water in both bottles. Add about 5 gm of potassium
nitrate (KN03) to one bottle. Stir well. Note
temperature of the solution. Add 5 gm of NaOH to
the other bottle. Note the temperature.
The above reactions can be represented as:
KNO3(s) + H20(l) + Heat → 4KN03(aq)
NaOH(s) + H20 (l) → 4NaOH(aq) + Heat

17. In the case of KN03 there is absorption of heat
during the reaction. Hence the temperature of the
solution (product) falls
Endothermic reactions Absorption of heat. The
reactants require (absorb) heat to form products
When NaOH(s) dissolves in water, there is
evolution of heat leading to a rise in temperature
of the product
Exothermic reactions Heat is evolved

18. Carbohydrates such as rice, potato, sago etc. are
major sources of energy in our diet.
During digestion carbohydrates are broken down
into glucose.
Glucose combines with oxygen in our body and
provides energy.
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6 H2O(l) + energy
It is an exothermic reaction

19. 2.2.2 Decomposition reactions (require heat and
light energy)
AB → A + B
A single reactant (sugar) has broken down to give a
simple product (C + H20)
C12H22O11(s) heat (Δ) 12C(s) + 11H2O(g)
Decomposition reactions carried out by heating
(thermal decomposition) During manufacturing of
cement, at a temperature above 1000°C (1273K)
calcium carbonate decomposes
CaCO (s) → CaO(s) + CO (g)

20. The pale yellow silver bromide turns grey when
exposed to sunlight.
2AgBr(s) sunlight 2Ag(s) + Br2(g)
A similar reaction is given by silver chloride
2AgCl(s) sunlight 2Ag(s) + Cl2(g)
Both are used in photography during the process of
developing.

21. Some decomposition reactions are brought
about by acids
CaS(s) + 2HCl(l) → CaCl2(s) + H2S(g)
Calcium sulphide + Hydrochloric acid → Calcium chloride + Hydrogen sulphide

22. 2.2.3 Displacement reactions (substitution/ single
replacement)
A + BC → AC + B
A more reactive element removes the element
having less reactivity from its compound
Zinc is more reactive than copper. It removes
copper from copper sulphate.
CuS04(aq) + Zn(s) → ZnS04(aq) + Cu(s)

23. 2.2.4 Double displacement reactions
(precipitation)
AB + CD → AD + CB
A brown insoluble substance, known as precipitate
(ppt), is formed.
CuCl2 + 2KI → Cul2↓ + 2KCl
copper chloride + potassium iodide → cupric iodide (ppt) + potassium chloride

24. Precipitation reactions Precipitates are formed
AgNO3 + NaCl → AgCl(s) ↓ + NaNO3
silver nitrate + sodium chloride → silver chloride (ppt) + sodium nitrate
White precipitate of AgCl is
formed by exchange of ions Ag+
and Cl-
between the reactants

26. 2.3 Oxidation and Reduction (redox)
Reduction A + ē → A-
Oxidation B → B+
+ ē
Gain of ē Loss of ē
When aluminium burns in presence of oxygen,
oxide of aluminium (known as alumina) is
formed.
4Al + 302(g) → 2Al203
aluminium + oxygen → alumina
Reaction of metallic sodium with alcohol
2C2H5OH(l) + 2Na → 2C2H5ONa + H2↑
ethyl alcohol + sodium → sodium ethoxide + hydrogen

30. Na atomic number 11 (2,8,1), 1s2
2s2
2p6
3s1
Na (atom; loss of ē) → Na+
(ion; 2,8) + ē
Cl atomic number 17 (2,8,7), 1s2
2s2
2p6
3s2
3p5
ē + Cl (atom) → Cl-
(chlorine ion; 2,8,8; gain of ē)

33. Oxidation reaction
•Reactants gain oxygen to form corresponding
oxide
•Reactants lose hydrogen to form product
C(s) + 2H2(g) → CH4(g)
carbon + hydrogen → methane

34. 03 light 02 + [O]
ozone → oxygen + nascent oxygen
Oxygen is freshly liberated. This oxygen is often
called freshly born or "Nascent" oxygen.
In a chemical equation, nascent oxygen is always
denoted by showing symbol of oxygen (O) in square
brackets such as [0].

35. Reduction reaction
•Reactants gain hydrogen
•Reactants lose oxygen to form product
Redox reaction Oxidation and reduction take place
simultaneously in a given chemical reaction

36. Oxidation Reduction
Reactants gain oxygen gain
hydrogen
lose hydrogen lose oxygen
Comparison

37. 2.3.1 Oxidation in and around you
Due to the effect of moisture a layer of
reddish brown colour is deposited over the
surface of iron, called rust. Its chemical formula is
Fe203.H20.

38. Corrosion
•Is a slow process of decay or destruction of metal
due to the effect of air, moisture and acids
•Can be prevented by using antirust solution,
coating surface by paint, galvanising and
electroplating with other metals

39. 2.3.2 Rancidity
Spoilage of food in such a way that it becomes
undesirable (and usually unsafe) for consumption
When oil and fats are oxidized or even allowed to
stand for a long time, they become 'rancid'
Antioxidants are used to prevent oxidation of food
containing fats and oils. Storage of food in air tight
containers also retards oxidation

41. 2.4 Neutralization
HA + BOH → BA + H2O
Acid + Alkali (Base) → Salt + Water

42. • When used plates of food are cleaned with
soap/ detergent, we observe changes in color
• The yellow oily left over stains turn red/ orange
because of neutralization
• Compounds in edible oil are neutralized by
alkaline soap/ detergent
• Edible oils are organic compounds of alcohols
and organic acids (carboxylic acids)
• The compounds formed are known as esters of
carboxylic acids
• This neutralization reaction is indicated by
turmeric (yellow) which turns red
• Acid + Alkali (Base) → Salt + Water

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