X-CHP 1-CHEMICAL REACTIONS.pptx

Chemical Reactions And
Equations
• Chemical reactions
• Chemical equations
• Balancing chemical equation
• Types of chemical reactions
• Corrosion and rancidity
COMBINATION DECOMPOSITION
DISPLACEMENT
DOUBLE
DISPLACEMENT
OXIDATION-
REDUCTION

Spoilage of food
Ripening of fruits
Formation of curd
Think what happens when…….

Photosynthesis
Respiration
Digestion of food
Bio- chemical reactions

Exposing iron tawa
to humid atmosphere
Milk is left at room
temperature

Medicines we intake
Burning of fuel

• In all these situations what happened?
SOME CHANGE HAVE HAPPENED FROM THE INITIAL SUBSTANCE.
If a chemical change takes place then we call it as chemical reaction.

PHYSICAL CHANGE
• Can be reversed
• Ex: Melting of wax, evaporation
CHEMICAL CHANGE
• Can not be reversed
• Ex: Digestion ,burning ,rusting

Chemical reaction
A chemical reaction is a change in which one or
more substances or reactant react together to
form a new substance or product with different
properties.

Magnesium ribbon
Bunsen Burner Sand paper
Activity 1.1: Burning of Magnesium ribbon
Apparatus required:
Tongs
Watch glass

Activity 1.2: To study the chemical reaction of lead nitrate with potassium iodide
Materials required: Test tubes, potassium iodide solution, lead nitrate solution
Potassium
nitrate
solution
Lead nitrate
solution
(Colour less)
Potassium iodide
solution
(Colour less)
Lead iodide
(Yellow
precipitate)
Reactants
Products
Pb(NO3)2 +2 KI -----> PbI2 +2 KNO3

Activity 1.3:
To study the chemical reaction of zinc granules and Sulphuric acid/
hydrochloric acid.
Materials required: Conical flask/ test tube, one holed rubber cork, zinc
granules, Dil. Hydrochloric acid or Dil. Sulphuric acid.
Zinc + Hydrochloric acid Zinc chloride + Hydrogen gas
Zinc + Sulphuric acid Zinc sulphate + Hydrogen gas
Reactants Products
Zinc + Nitric acid Zinc nitrate + Water

Observations: Characteristics
Change in odour (smell)
Change in color
Change in physical state
Evolution of gas
Change in temperature
Formation of precipitate

 The process of chemical change is called chemical
reaction.
 During chemical reactions, old bonds break and new
bonds are formed.
 During chemical reaction, the substances which take
part in chemical reaction are called reactants and
the substances which are formed are called products.
 The change in colour, odour, temperature and
physical state, evolution of gas and formation of
precipitate are the characteristic features of different
chemical reactions.

• What is a physical change?
• What is a chemical change?
• Give one example of chemical change.
• What is a chemical reaction?
• What are the products when lead nitrate reacts with potassium iodide?
• What are the changes that can be observed in a chemical reaction?
• Name the gas evolved when metal reacts with acid?
• How will you test the gas evolved ?

RECAP:
 Old bonds are broken and new bonds are formed.
Hydrogen + Oxygen --------> Water
2H₂ (g) + O₂ (g) --------> 2 H₂O (l)

Reactants Product
Magnesium + Oxygen --------> Magnesium oxide (Word equation)
Mg + O₂ --------> MgO ( Skeletal equation)
LHS RHS

WHAT IS A WORD AND SKELETAL
EQUATION?
• WORD EQUATION-A word equation shows the change of reactants to
products through an arrow placed between them in descriptive form.
• The reactants are written on the left hand side with (+) sign in between them
and the products are written on the right hand side with (+) in between them.
• Arrow head points towards the product.
• SKELETAL EQUATION-Unbalanced chemical equation is called as skeletal
chemical equation.
• In unbalanced chemical equation the mass of reactants is not equal to the
mass of products.

BALANCING CHEMICAL EQUATION - RULES
Priority:
 Start balancing with the compound that contains maximum number of atoms.
 Balance Metals.
 Balance Non-metals ( Hydrogen & Oxygen at last)
 Do not alter the subscripts of the formula. (MgCl₂)
 No fraction should exist.
2X + Y = Z
3
 Note: Formula should not be changed.
The method we use for balancing is hit and trial method.

Activity 3: The chemical reaction of zinc granules and Sulphuric acid
Zinc + Sulphuric acid ---------> Zinc sulphate + Hydrogen gas
Zn + H₂SO₄ -------------> Zn SO₄ + H₂
All chemical equation must follow law of conservation of mass.
The law states that mass can be neither created nor destroyed. It
means the mass of the reactants and products remains constant.
No. of atoms present in reactant side (LHS) should be equal to the
No. of atoms present in product side (RHS)
Element LHS (No. of atoms in
Reactants)
RHS (No. of. atoms in
Products)
Zn 1 1
S 1 1
H 2 2
O 4 4

Example 1:
Write the chemical equation for reaction of iron with steam to give magnetic oxide (Fe₃O₄)
of iron and hydrogen gas and balance it.
Step 1: write down the word equation.
Iron + steam --------> Magnetic oxide of iron + Hydrogen.
Step 2: Write down the skeletal equation:
Fe + H₂O ---------> Fe₃O₄ + H₂
Step 3: Count the number of reactants and products.
Reactants)
Products)
Fe 1 3
H 2 2
O 1 4

Step 4:
3Fe + H₂O ---------> Fe₃O₄ + H₂
Step 5:
Reactants)
Products)
Fe 3 3
H 2x4=8 2
O 1x4=4 4
3Fe + 4 H₂O ---------> Fe₃O₄ + H₂

3Fe + 4H₂O ---------> Fe₃O₄ + 4 H₂
Reactants)
Products)
Fe 3 3
H 8 2x4 =8
O 4 4
Step 6:
In this equation, the number of different atoms of reactants is equal to the number of different
atoms in products.

Magnesium + Oxygen ---------> Magnesium oxide
Reactants)
Products)
Mg 1 1
O 2 1
Example 2:
Mg + O₂ ---------> MgO
Step 1:
Step 2:
Reactants)
Products)
Mg 1 1x2=2
O 2 1x2=2
Mg + O₂ ---------> 2 MgO

Step 2: Element LHS (No. of atoms in
Reactants)
Products)
Mg 1x2=2 2
O 2 2
2Mg + O₂ ---------> 2 MgO
In this equation, the number of different atoms of reactants is equal to the number of
different atoms in products.

How to make chemical equation more informative?
 Physical state:
(s)- solid (l)- liquid (g)-gas (aq)- aqueous (solution prepared in water)
 Gaseous products –
 Precipitate –
 If reaction is exothermic heat is written along with products (+ Heat) and if the reaction is
endothermic heat is written along with reactants (+ Heat)
 The reaction conditions like pressure, temperature and catalyst are mentioned above and
below the arrow.
N₂(g) + 3 H₂ (g) ----------------------> 2 NH₃ (Haber’s Process)
Fe powder
 ∆ symbol is used to represent heat. It is placed above the arrow.
∆
CaCO₃ (s) -----------------> CaO (s) + CO₂ (g)
500°C, 200 atm
Substance deposited in solid form

EXOTHERMIC REACTION-Reaction in which
heat is liberated along with the products is
called as exothermic chemical reaction.
ENDOTHERMIC REACTION-Reaction in
which energy is absorbed is known as
endothermic reaction.
CATALYST-It is a substance which speeds
up a chemical reaction.

Ch-1 Chemical reactions and equations
I. Intext Questions: (Pg.6)
1) Why should a magnesium ribbon be cleaned before burning in air?
Ans: Magnesium gets covered with a layer of magnesium oxide when kept
in air for a long time. This layer hinders the burning of magnesium. Hence,
it is to be cleaned with sand paper before burning.

2) Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride (Word Equation)
Ans: H2 + Cl2 → HCl (Skeletal Equation)
Element No. of atoms in reactants (L.H.S) No. of atoms in products (R.H.S)
H 2 1
Cl 2 1
2) Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride (Word Equation)
Ans: H2 + Cl2 → HCl (Skeletal Equation)
Ans: H₂ + Cl ₂ →2HCl
H 2 1
Cl 2 1
H 2 1x2=2
Cl 2 1x2=2

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (Word Equation)
Ans: BaCl2 + Al2(SO4)3 → BaSO4 + AlCl3 (Skeletal Equation)
Ba 1 1
Cl 2 3
Al 2 1
S 3 1
O 12 4
Ba 1 1
Cl 2 3x2=6
Al 2 1x2 = 2
S 3 1
O 12 4
BaCl2 + Al2(SO4)3 → BaSO4 + 2AlCl3

Ba 1x3=3 1
Cl 2x3=6 6
Al 2 2
S 3 1
O 12 4
3BaCl2 + Al2(SO4)3 → BaSO4 +2 AlCl3
Ba 3 1x3=3
Cl 6 6
Al 2 2
S 3 1x3=3
O 12 4x3=12
Ans : 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3

(iii) Sodium + Water → Sodium hydroxide + Hydrogen (Word Equation)
Na + H2O → NaOH + H2 ↑ ( Skeletal Equation)
Na 1 1
H 2 3
O 1 1
Na + H2O → 2 NaOH + H2 ↑
Na 1x2=2 1x2=2
H 2x2=4 3+1 =4
O 1x2=2 1x2=2
Ans: 2Na + 2H2O → 2 NaOH + H2 ↑

3. Write a balanced chemical equation with state symbols for the following reactions:
i) Solutions of barium chloride and sodium sulphate in water react to give insoluble
barium sulphate and the solution of sodium chloride.
Word equation:
Barium chloride + Sodium sulphate Barium sulphate + Sodium chloride
BaCl₂ + Na₂SO₄ BaSO₄ + NaCl ( Skeletal Equation)
Ba 1 1
Cl 2 1x2=2
Na 2 1x2=2
S 1 1
O 4 4
Ans: BaCl₂(aq) + Na₂SO₄(aq) BaSO₄ (s) + 2 NaCl(aq)

ii). Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce
sodium chloride solution and water.
Ans:
Sodium hydroxide + Hydrochloric acid Sodium Chloride + Water ( word equation)
NaOH + HCl NaCl + H₂O (Skeletal equation)
The number of different atoms in reactants and products are equal, so it is already balanced.
Ans:
NaOH(aq) + HCl (aq) NaCl (aq) + H₂O(l)
Na 1 1
O 1 1
H 2 2
Cl 1 1

TYPES OF CHEMICAL REACTION
 Combination reaction
 Decomposition reaction
 Displacement reaction
 Double Displacement reaction
 Redox reaction

Combination reaction
• Those chemical reactions in which two or more substances
(reactants) combine together to form single substance(product) are
called Combination reaction.
• It is also called Synthesis reaction.
A + B AB
• Most of the combination reactions are exothermic in nature.
i.e. heat is released.

 Hydrogen + Oxygen Water
2H₂ (g) + O₂ (g) 2 H₂O (l)
 Sodium + Chlorine Sodium Chloride
2 Na (s) + Cl₂ (g) 2 NaCl(s)
 Magnesium + Oxygen Magnesium oxide
2Mg (s) + O₂ (g) 2 MgO (s)
 Carbon monoxide + Oxygen Carbon dioxide
2CO (g) + O₂ (g) 2CO₂ (g)
 Sulphur dioxide + Oxygen Sulphur trioxide
2SO₂ (g) + O₂ (g) 2SO₃ (g)

Activity:
• Take a small amount of
calcium oxide or quick lime in a
beaker.
• Slowly add water to this.
• Touch the beaker.
• Do you feel any change in
temperature?
It is an exothermic reaction.

CaO(s) + H ₂ O(l) Ca(OH)₂(aq) + Heat
(Quick lime) (Slaked lime)
Calcium oxide and water combine
together to form a single product,
calcium hydroxide. Such a reaction in
which a single product is formed is
known as a combination reaction.

 A solution of slaked lime is used for white washing walls.
 Calcium hydroxide reacts slowly with the carbon dioxide
in air to form a thin layer of calcium carbonate on the walls.
 Calcium carbonate is formed after two or three days of
white washing and gives a shiny finish to the walls.
Slaked lime + CO₂ Limestone + Water
Ca(OH) ₂ +CO ₂ CaCOꝫ + H₂O

EXOTHERMIC REACTION
Some examples of exothermic reactions are:
1. Burning of natural gas
CH₄ (g) + 2O₂ (g) CO₂ (g) + 2 H₂O(g)
2. Respiration is also an exothermic process.
C₆H₁₂O₆ (aq) + 6O₂(aq) 6CO₂ (aq) + 6 H₂O(l) + Energy
(Glucose).
During digestion, food is broken down into simpler substances.
Carbohydrates are broken down to form glucose. This glucose combines
With oxygen in the cells of our body and provides energy.
3. Decomposition of vegetable matter into compost.

DECOMPOSITION REACTION
• A single reactant breaks down to give simpler products. This is a decomposition
reaction.
This is of three types
• Thermal decomposition=Heat
• Photolytic decomposition=Light energy
• Electrolytic decomposition=Electric current

 FeSO₄.7 H₂O FeSO₄(s) + 7H₂O
Ferrous sulphate Ferrous sulphate ( anhydrous)
Heptahydrate (Green colour)
2FeSO₄(s) Fe₂O₃(s) + SO₂(g) + SO₃(g)
Ferrous Sulphate Ferric oxide Sulphur dioxide Sulphur trioxide
(Green colour)
Heat
Heat

 Ferrous sulphate crystals (FeSO₄.7 H₂O)
lose water when heated and the colour
of the crystal changes. It then
decomposes to ferric oxide
(Fe₂O₃),sulphur dioxide (SO₂) and sulphur
trioxide (SO₃).
 Ferric oxide is a solid, while SO₂ and SO₃
are gases.

THERMAL DECOMPOSITION REACTION
 Those reactions in which decomposition is carried out by heating are called thermal
decomposition reaction.
 For example, decomposition of limestone.
CaCO₃(s) CaO(s) + CO₂ (g)
Calcium carbonate Calcium oxide Carbon dioxide
 CaO is called lime or quick lime which is used in the manufacture of cement.
 Decomposition of ferrous sulphate into ferric oxide, Sulphur dioxide and Sulphur trioxide
on heating is another example of thermal decomposition.
Heat

Decomposition of lead nitrate takes place when it is heated.
Lead nitrate is a white solid. Nitrogen dioxide gas is evolved on heating lead
nitrate and a yellow solid of lead oxide is left behind in the boiling tube.
2Pb(NO₃)₂(s) 2PbO(s) + 4 NO₂(g) + O₂(g)
Lead Nitrate Lead oxide Nitrogen dioxide Oxygen
(Colourless) (yellow) (reddish brown fumes)
Heat

IDENTIFY----
HOW PHOTOLYTIC DECOMPOSITION IS
USED IN PHOTOGRAPHY?
In this reaction decomposition of AgBr is used in photography technique in camera roll
.The film contain silver bromide crystals on surface when light fall on roll, object
reflection on film takes place ,then AgBr gets decomposed making an impression and
undecomposed silver bromide gives differentiation.

In text questions: ( Pg.10)
1. A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
Ans) The substance “X’ that is used for white washing is calcium oxide. Its
chemical formula is CaO.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water
Ans) Calcium oxide (CaO), commonly known as quicklime. Quick lime
reacts vigorously with water to produce calcium hydroxide (slaked lime)
releasing a large amount of heat. So the reaction is highly exothermic.
Calcium oxide and water react to form only a single product, calcium
hydroxide. So the reaction is a combination reaction.

2) Why is the amount of gas collected in one of the test tubes in
activity 1.7 double of the amount collected in the other? Name
this gas.
Ans) Activity 1.7 is an experiment on electrolysis of water.
Electrolysis of water is its decomposition to give hydrogen and
oxygen gases due to the passage of an electric current. Water
(H2O) is formed when hydrogen and oxygen combine in the ratio
of 2:1 by volume. Therefore, the gas with double volume is
hydrogen as two atoms of hydrogen combine with one atom of
oxygen to form water hence, volume of hydrogen is double.

DISPLACEMENT REACTION
 Those chemical reactions in which more reactive metal or non-metal
displaces a less reactive metal or non-metal from its aqueous salt
solution are called displacement reactions.

Displacement chemical reaction: Activity
 Take 2 iron nails & two test tubes marked as A and B. in each test tube, take about 10
ml copper sulphate solution.
 Immerse one nail in the copper sulphate solution in test tube for about 20 minutes.
 After 20 minutes, take out the iron nails from the copper sulphate solution.
 Compare the intensity of the blue colour of copper sulphate solutions in test tubes.
 Also, compare the iron nail dipped in the copper sulphate solution with the one kept
aside.

Hydrochloric acid + Magnesium Magnesium chloride + Hydrogen
2 HCl(aq) + Mg (s) MgCl₂ (aq) + H₂ (g)
Sulphuric acid + Iron Iron Sulphate + Hydrogen
H₂SO₄(aq) + Fe(s) FeSO₄(aq) + H₂ (g)
Nitric acid + Magnesium Magnesium nitrate + Hydrogen
2HNO₃(aq) + Mg(s) Mg(NO₃)₂(aq) + H₂ (g)
Zinc + Hydrochloric acid Zinc chloride + Hydrogen
Zn(s) + 2 HCl (aq) ZnCl₂(aq) + H₂ (g)
Lead + Sulphuric acid Lead Sulphate + Hydrogen
Pb(s) + H₂SO₄(aq) Pb(SO₄)₂(aq) + H₂ (g)
Lead + Copper chloride Lead Chloride + Copper
Pb(s) (bluish grey) + CuCl₂(aq)(green) PbCl₂(aq)( Colourless) + Cu(s) red brown

Sodium sulphate + Barium chloride -------------- Barium sulphate + Sodium Chloride
Na₂SO₄ (aq) + BaCl₂ (aq) -------------- BaSO₄(s) + 2 NaCl (aq)

 The white precipitate of
BaSO₄ is formed by the
reaction of sulphate and
barium ions.
 The other product formed
is sodium chloride which
remains in the solution.
 Such reactions in which
there is an exchange of ions
between the reactants are
called double displacement
reactions.

Answer the following questions:
1. Name the three types of decomposition reaction.
Ans: Thermal, Electrolytic and photolytic reactions.
2. What is the colour of silver chloride after it is exposed to sunlight?
Ans: Grey
3. State the main difference between endothermic reaction and an exothermic reaction.
Ans: In endothermic reaction, heat is absorbed. In exothermic reaction, heat is evolved.
4. Write the essential condition for the following reaction to take place:
2AgBr --------->2Ag + Br2 Write application of this reaction.
Ans: The reaction will take place in presence of sunlight. This reaction is used in black
and white photography.
5. In the electrolysis of water:
i. Name the gas collected at the cathode and anode respectively.
ii. Why is volume of gas collected at one electrode double than that at the other?
iii. How will you test this gas?
Ans: i. Hydrogen is collected at the cathode, oxygen is collected at the anode.
ii. It is because H2O contains hydrogen and oxygen in the ratio 2 : 1.
iii. Bring a burning matchstick near the gas, if the gas burns with ‘pop’ sound, the gas is H2.

BHA- Beta
Hydroxy
Anisole
BHT- Beta
Hydroxy
Toluene

Page Number: 13
Question 1
Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?
Answer:
When an iron nail is dipped in copper sulphate solution, the displacement reaction takes place. The colour of
copper sulphate solution fades due to the formation of light green solution of iron sulphate.
Question 2
Give an example of a double displacement reaction other than the one given in Activity 1.10 (NCERT
Text Book).
Answer:
Sodium hydroxide and hydrochloric acid react to form sodium chloride and water.

Question 3
Identify the substances that are oxidised and the substances which are reduced in the following
reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO (s) + H2(g) → Cu (s) + H2O(l)
Answer:
(i) Substances oxidised is Na as it gains oxygen and oxygen is reduced.
(ii) Substances reduced is Cu as hydrogen is oxidised as it gains oxygen.

Book exercise: page 14
1. Which of the statements about the reaction below are incorrect ?
2 PbO(s) + C(s) → 2Pb (s) + CO2(g)
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b) (ii) (a) and (c) (iii) (a), (b) and (c) (iv) All
Answer:
(i) (a) and (b)
2. Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a _________.
(a) combination reaction (b) double displacement reaction
(c) decomposition reaction (d) displacement reaction
Answer:
(d) Displacement reaction.
3. What happens when dilute hydrochloric acid is added to iron filings ? Tick the correct answer :
(a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place. (d) Iron salt and water are produced.
Answer:
(a) Hydrogen gas and iron chloride are produced.

Question 4
What is a balanced chemical equation ? Why should chemical equations be balanced ?
Answer:
A balanced chemical equation has an equal number of atoms of different elements in the reactants
and products.
The chemical equations should be balanced to satisfy the law of conservation of mass.
Question 5
Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of
barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) 3H2 (g) + N2 (g) → 2NH3 (g)
(b) H2S (g) + 3O2 (g) → SO2 (g) + 2H2O(l)
(c) 3BaCl2 (aq) + Al2(SO4)3 (aq) → 2AlCl3 (aq) + 3BaSO4 ↓(s)
(d) 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

Question 6
Balance the following chemical equations :
(a) HNO3 + Ca (OH)2 → Ca (NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer:
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + 2HCl
Question 7
Write the balanced chemical equations for the following reactions :
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer:
(a) Ca (OH)2 + CO2 → CaCO3 + H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2 Ag
(c) 2Al + 3 CuCl2 → 2AlCl3 + 3 Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl

Question 8
Write the balanced chemical equation for the following and identify the type of reaction in each case :
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium
(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)
(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)
Answer:
(a) 2KBr (aq) + Bal2(aq) → 2Kl(aq) + BaBr2(s) Type : Double displacement reaction
(b) ZnCO3 (s) → ZnO (s) + CO2 (g) Type : Decomposition reaction
(c) H2 (g) + Cl2 (g) → 2HCl(g) Type : Combination reaction
(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) Type : Displacement reaction
Question 9
What does one mean by exothermic and endothermic reactions ? Give examples.
Answer:
Exothermic reactions : Those reactions in which heat is evolved are known as exothermic reactions. An exothermic
reaction is indicated by writing “+ Heat”on the products side of an equation.
Example :
(i) C (s) + O2 (g) → CO2 (g) + Heat
(ii) N2 (g) + 3H2 (g) → 2NH3 (g) + Heat
Endothermic reactions : Those reactions in which heat is absorbed are known as endothermic reactions. An
endothermic reaction is usually indicated by writing “Heat” on the product side of a chemical equation.
Examples :
(i) C (s) + 2S (s) → CS2 (l) – Heat
(ii) N2 (g) + O2 (g) → 2NO(g) – Heat

Question 10
Why is respiration considered an exothermic reaction ? Explain.
Answer:
Respiration is an exothermic process because during respiration glucose combines with oxygen in the cells of our
body to form carbon dioxide and water along with the production of energy.
Question 11
Why are decomposition reactions called the opposite of combination reactions? Write equations for these
reactions.
Answer:
In a decomposition reaction, a single compound breaks down to produce two or more simpler substances.
For example:
While, in a combination reaction, two or more substances simply combine to form a new substance.
For example:

Question 12
Write one equation each for the decomposition reactions where energy is supplied in the form of heat,
light or electricity.
Question 13
What is the difference between displacement and double displacement reactions? Write equations for
these reactions.
Answer:
In displacement reactions, a more reactive metal displaces a less reactive metal from its solution. For example,
Fe(s) + CuSO4(aq) → Cu(s) + FeSO4(aq)
This is a displacement reaction where iron displaces copper from its solution.
In double displacement reactions, two reactants in solution exchange their ions. For example,
AgNO3(aq) + NaCl (aq) → AgCl(s) + NaNO3 (aq)
This is a double displacement reaction where silver nitrate and sodium chloride exchange Cl– and NO3
– ions
between them.

Question 14
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by
copper metal. Write down the reaction involved.
Answer:
Question 15
What do you mean by a precipitation reaction ? Explain by giving examples.
Answer:
A reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a
precipitation reaction.
Example : When a solution of iron (III) chloride and ammonium hydroxide are mixed, a brown precipitate of iron
(III) hydroxide is formed.

Question 16
Explain the following in terms of gain or loss of oxygen with two examples each:
(a) Oxidation and
(b) Reduction.
Answer:
(a) Oxidation : The addition of oxygen to a substance is called oxidation.
Example :
(i) S(s) + O2(g) → SO2(g) (Addition of oxygen to sulphur)
(ii) 2Mg(s) + O2 (g) → 2MgO(s) (Addition of oxygen to magnesium)
(b) Reduction : The removal of oxygen from a substance is called reduction.
Example: (i) CuO + H2 Heat−→−− Cu + H2O
Here, copper oxide is being reduced to copper because oxygen gets removed from copper oxide.
(ii) ZnO + C → Zn + CO
Here, zinc oxide is being reduced to zinc because oxygen gets removed from zinc oxide.
Question 17
A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the
black coloured compound formed.
Answer:
Element ‘X’ is copper (Cu).
The black coloured compound is copper oxide (CuO). The reaction involved is

Question 18
Why do we apply paint on iron articles ?
Answer:
Paint does not allow iron articles to come in contact with air, water and saves iron articles from damage due to
rusting.
Question 19
Oil and fat containing food items are flushed with nitrogen. Why ?
Answer:
To keep food items fresh and save from getting oxidised, food items are flushed with nitrogen.
Question 20
Explain the following terms with one example each (a) Corrosion, (b) Rancidity.
Answer:
(a) Corrosion : It is the process in which metals are eaten up gradually by the action of air, moisture or a
chemical (such as an acid) on their surface.
Example : When iron is exposed to moist air for a long period of time, its surface acquires a coating of a brown,
flaky substance called rust. Rust is mainly hydrated iron (III) oxide [Fe2O3.xH20].
(b) Rancidity : The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell
and taste is called rancidity.
Rancidity spoils the food materials prepared in fats and oils which have been kept for a considerable time and
makes them unfit for eating.
Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils. It can also be prevented by
flushing fat and oil containing foods with nitrogen before sealing.

X-CHP 1-CHEMICAL REACTIONS.pptx

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