Page - 404 Class 10th Chemistry NCERT Book Solution Chapter 1 Chemical equation and reaction
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Chapter 1: Chemical Reactions and Equations
Page 6
Question 1: Why should a magnesium ribbon be cleaned before burning in air?
Answer: Magnesium is very reactive metal like (Na, Ca, etc.). When it expose to air it reacts with oxygen to form a layer magnesium oxide (MgO) on its surface.
Mg + 02 → 2MgO
This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be used for the reaction.
Question 2: Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(i) H2 + Cl2 → 2HCl
(ii) 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
(iii) 2Na + 2H2O → 2 NaOH + H2
Question 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.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Answer:
(i) BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O
Page 10
Question 1: A solution of a substance 'X' is used for white washing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of the substance 'X' named in (i) above with water.
Answer:
(i) The substance 'X' is calcium oxide. Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime)
CaO + H₂O → Ca(OH)2
Question 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.
Answer: During the Electrolysis of water, hydrogen and oxygen is get separated by the electricity. Water (H₂O) contains two parts hydrogen and one part oxygen. Since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.
Page 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 a copper sulphate solution, iron (which is more reactive than copper) displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.
CuSO4 + → FeSO4 + Cu
Blue Colour Green Colour
Therefore, the blue colour of copper sulphate solution fades and green colour appears.
Question 2: Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer: Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.
Na2CO3 + CaCl₂ → CaCO3 + 2NaCl
In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.
Question 3: Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(1)
Answer:
(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H₂O).
Exercises solution
Question
1:Which of the statements about the reaction below are incorrect?
2PbO(s)
+ C(s) → C2Pb(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)
Question 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.
Question 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. The reaction is as follows:
Fe(s) +2HCl(aq) → FeCl2(aq) + H₂↑
Question 4: What
is a balanced chemical equation? Why should chemical equations be balanced?
Answer: A reaction which has an equal number of atoms of all the elements
on both sides of the chemical equation is called a balanced chemical equation.
The law of conservation of mass states that mass can neither be created nor
destroyed. Hence, in a chemical reac-tion, the total mass of reactants should
be equal to the total mass of the products. It means that the total number of
atoms of each element should be equal on both sides of a chemical equation.
Hence, it is for this reason that chemical equations should be balanced.
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) 2H₂S(g) + 302(g) → 2H₂O(l) + 2SO2(g)
(C) 3BaCl2(aq) + Al(SO₂)3(aq) → 2AlCl3(aq) + 3 BaSO4(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 +
HCI
Answer:
(A) 2HNO3 + Ca(OH)2 →
Ca(NO₃)₂ + 2H₂O
(B) 2NaOH + H₂SO₄ → Na₂SO₄ +
2H₂O
(C) NaCl + AgNO3 → AgCl +
NaNO3
(D) BaCl2 + H₂SO₄ → BaSO4 + 2HCI
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 + 2Ag
(C) 2Al + 3CuCl₂ → 2AICI2 + 3Cu
(D) BaCl₂ + K₂SO₄ → BaSO4 + 2KCI
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 de(s)
(B) Zinc carbonate(s) → Zinc
oxide(s) + Carbon dioxide(g)
(C) Hydrogen(g) + Chlorine(g)
→ Hydrogen chloride(g)
(D) Magnesium(s) + Hydrochloric
acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Answer:
(A) 2KBr(aq)
+ Bal(aq) → 2KI(aq) + BaBr(s);
Double displacement reaction
(B) ZnCO3(s) → ZnO(s) + CO2(g);
Decomposition reaction
(C) H2(g) + Cl2(aq) → 2HCl(g);
Combination reaction
(D) Mg(s) + 2HCl(aq) → MgCl2(aq) +H₂(g);
Displacement reaction
Question 9: What does one mean
by exothermic and endothermic reactions? Give examples.
Answer:
Exothermic
reactions:- Chemical reactions
that release energy in the form of heat, light, or sound are called exothermic
reactions.
Example: Mixture of sodium and chlorine to yield table salt
Na(s) + Cl2 → NaCl(s) + 411 kJ of energy
Endothermic reactions:- Reactions that absorb energy or require
energy in order to proceed are called endothermic reactions.
Example: In the process of photosynthesis, plants use the energy from
the sun to convert carbon dioxide and water to glucose and oxygen.
6CO2(s) +6H2O(l) → H6H12O6(aq) + 6O2(g)
Question
10:Why is respiration considered an exothermic reaction? Explain.
Answer: Energy is required to support life.
Energy in our body is obtained from the food we eat. During digestion, large
molecules of food are broken down into simpler substances such as glucose.
Glucose combines with oxygen in the cells and provides energy. The special name
of this combustion reaction is respiration. Since energy is released in the
whole process, it is an exothermic process.
C6H12O6(aq)
+ 602(g) → 6CO2(g)
+
6H2O + Energy
Glucose Oxygen Carbon dioxide Water
Question 11: Why are
decomposition reactions called the opposite of combination reactions? Write
equations for these reactions.
Answer:
Decomposition reactions are
those in which a compound breaks down to form two or more substances. These
reactions require a source of energy to proceed. Thus, they are the exact
opposite of combination reactions in which two or more substances combine to give
a new substance with the release of energy.
Decomposition reaction: AB + Energy
→ A + B
2H₂O →(Electrolysis) 2H2 + O2(g)
Combination reaction: A+B → AB+
Energy
2H2(g) +
O2(g) → 2H₂O(l) + Energy
Question 12:
Write one equation each for decomposition reactions where energy is supplied in
the form of heat, light or electricity.
Answer:
(a) Thermal decomposition:
2 FeSO4 → Fe₂O + SO2(g)
+ SO3
Ferrous sulphate Ferric oxide Sulphur dioxide Sulphur trioxide
(b)
Decomposition by light:
2 AgCl →(Light) 2Ag(s)
+ Cl2
Silver chloride Silver Chlorine
(c)
Decomposition by electricity:
2A1203 →(Electricity) 4 Al + 302
Aluminium oxide Aluminium Oxygen
Question 13:
What is the difference between displacement and double displacement reactions?
Write equations for these reactions.
Answer: In a displacement reaction, a more
reactive element replaces a less reactive element from a compound.
A + BX → AX + B; where A is more reactive than B
In a
double displacement reaction, two atoms or a group of atoms switch places to
form new compounds.
AB + CD → AD + CB
For example:
Displacement reaction: CuSO4(aq) + Zn(s)
→ ZnSO4(aq) + Cu(s)
Double displacement reaction:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) +
2NaCl(aq)
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: 2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)
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 is called a precipitation reaction.
For example:
Na₂CO3(aq) + CaCl2(aq) → CaCO3(s) + 2NaCl(aq)
In this reaction, calcium
carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.
Another example of
precipitation reaction is:
Na₂SO(aq)
+ BaCl(aq) → BaSO(s) + 2NaCl(aq)
In this reaction, barium sulphate is obtained as a precipitate.
Question 16:
Explain the following in terms of gain or loss of oxygen with two examples
each.
(a) Oxidation
(b) Reduction
Answer:
(a) Oxidation is the gain of
oxygen.
For example:
(i) CO₂ + H₂ → CO + H₂O
(ii) 2Cu + O2 → CuO
In equation (i), H₂ is
oxidized to H₂O and in equation (ii), Cu is oxidised to CuO.
(b)
Reduction is the loss of oxygen.
For example:
(i) CO₂ + H₂ → CO + H₂O
(ii) CuO + H2 → Cu + H₂O
In equation (i), CO₂ is reduced to CO and in equation (ii), CuO is reduced to
Cu.
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: 'X' is copper (Cu) and the black-coloured
compound formed is copper oxide (CuO). The equation of the reaction involved on
heating copper is given below.
2Cu + O2 →
2CuO
Shiny brown in colour
(Black in colour)
Question 18:
Why do we apply paint on iron articles?
Answer: Iron articles are painted because it
prevents them from rusting. When painted, the contact of iron articles from
moisture and air is cut off. Hence, rusting is prevent-ed their presence is
essential for rusting to take place.
Question 19:
Oil and fat containing food items are flushed with nitrogen. Why?
Answer:
Nitrogen
is an inert gas and does not easily react with these substances. On the other
hand, oxygen reacts with food substances and makes them rancid. Thus, bags used
in packing food items are flushed with nitrogen gas to remove oxygen inside the
pack. When oxygen is not present inside the pack, rancidity of oil and fat
contain-ing food items is avoided.
Question 20:
Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer:
(a) Corrosion:
Corrosion is defined as a
process where materials, usually metals, deteriorate as a result of a chemical
reaction with air, moisture, chemicals, etc. For example, iron, in the
pres-ence of moisture, reacts with oxygen to form hydrated iron oxide.
4Fe + 302 + nНО → 2FeO3nH2O
Hydrated
iron oxide
This hydrated iron oxide is rust.
(b)
Rancidity:
The process of oxidation of fats and oils that can be easily noticed by the
change in taste and smell is known as rancidity.
For example: the taste and smell of butter changes when kept for long.
Rancidity can be avoided by:
1. Storing food in air tight containers
2. Storing food in refrigerators
3. Adding antioxidants
4. Storing food in an environment of nitrogen
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