NCERT Class 10 Science Chapter-1
Chemical
Reactions and Equations
In-text questions set
1 Page number – 6
1. Why should a
magnesium ribbon be cleaned before burning in the air?
Answer:
Magnesium ribbon should
be cleaned before burning in air because Magnesium metal reacts with the
atmospheric oxygen and forms Magnesium Oxide (MgO) layer which is a very stable
compound. In order to prevent further reactions with Oxygen, it is therefore
necessary to clean the ribbon by to remove the layer of MgO.
2. Write a balanced
equation for the following chemical reactions.
i) Hydrogen + Chloride
—-> 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 → 2NaOH + H2
3. Write a balanced
chemical equation with state symbols for the following reactions
i) Solutions of Barium
chloride and Sodium sulfate in water react to give insoluble Barium sulfate
and solution of Sodium chloride.
ii) Sodium hydroxide
solution in water reacts with the hydrochloric acid to produce Sodium
chloride solution and water.
Answer:
i) BaCl2 + Na2SO4 → BaSO4 + 2NaCl
ii) NaOH + HCl →
NaCl + H2O
In-text questions set
2 Page number – 10
1. A solution of a
substance, ‘X,’ is used for whitewashing.
(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’
which is used in whitewashing is quick lime or Calcium Oxide and
its formula is CaO.
ii) CaO + H2O → Ca(OH)2
2. Why is the amount
of gas collected in one of the test tubes in Activity 1.7 double the amount
collected in the other? Name this gas.
Answer:
In activity 1.7, gas
collected in one of the test tubes is double of the amount collected in the
other because water gets hydrolysed to release H2 and O2 gas. Here, after
electrolysis two molecules of Hydrogen and one molecule of oxygen gas are
released; hence, the amount of Hydrogen collected would be double than that of
oxygen.
In-text questions set
3 Page number – 13
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 the copper sulphate solution, iron displaces copper from the copper
sulphate because iron is more reactive than copper. Therefore, the colour of
the copper sulphate solution changes. The reaction is
Fe + CuSO4 → FeSO4 + Cu
2. Give an
example of a double displacement reaction other than the one given in Activity
1.10.
Answer:
Reaction Between
silver nitrate (AgNO3) and Sodium chloride (NaCl) is an example of a double
displacement reaction. During the reaction, negative and positive ions trade
positions as a result of the formation of white silver chloride precipitate.
The chemical reaction is given below.
Ag+ + NO3– + Na+ + Cl– → AgCl + Na+ + NO3–
3. Identify the
substances that are oxidised and that are reduced in the following equation.
i) 4Na(s) + O2(g) → 2Na2O(s)
ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
Answer:
The Sodium (Na) in the
first equation is getting oxidized with the addition of Oxygen (O2), and the Copper (Cu)
in the second equation is reduced due to the addition of Hydrogen (H2).
Exercise Questions
Page number – 14-16
1. Which of the
statements about the reaction below are incorrect?
2PbO(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 the above
Answer:
(i) (a) and (b)
Explanation: (a) Because Oxygen is being removed and
(b) Because the
removed oxygen from Lead is added to the elemental Carbon.
3. What happens when
dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
- Hydrogen
gas and Iron chloride are produced.
- Chlorine
gas and Iron hydroxide are produced.
- No
reaction takes place.
- Iron salt and water are
produced.
Answer:
- Hydrogen
gas and Iron chloride are produced.
Explanation: The
Chlorine from Hydrogen chloride is displaced by the Iron fillings to undergo
the following reaction.
2HCl + Fe → FeCl2 + H2
4. What is a balanced
chemical equation? Why should a chemical equation be balanced?
Answer:
A balanced equation is
the one in which number of different atoms on both the reactant and product
sides are equal. Balancing chemical equations is necessary for the reaction
should obey The Law of Conservation of mass. Balancing the chemical equation has
no defined method and is purely a trial-and-error attempt.
5. Translate the
following statements into chemical equations and balance them.
(a) Hydrogen gas
combines with nitrogen to form ammonia.
(b) Hydrogen sulphide
gas burns in the 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) Unbalanced: H2 + N2 → NH3
Balanced: 3H2 + N2 → 2NH3
(b) Unbalanced: H2S + O2 → H2O + SO2
Balanced: 2H2S + 3O2 → 2H2O + 2SO2
(c) Unbalanced:
BaCl2 + Al2(SO4)3 → AlCl3 + BaSO4
Balanced: 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
(d) Unbalanced: K + H2O → KOH + H2
Balanced: 2K +
2H2O → 2KOH + H2
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
7. Write the balanced
chemical equation for the following reactions.
Calcium hydroxide +
Carbon dioxide —-> Calcium carbonate + Water
Zinc + Silver nitrate
—-> Zinc nitrate + Silver
Aluminium + Copper
chloride —-> Aluminium chloride + Copper
Barium chloride +
Potassium sulphate —-> Barium sulphate + Potassium chloride
Answer:
2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
2Al + 3CuCl2 → 2AlCl3 + 3Cu
BaCl2 + K2SO4 → BaSO4 + 2KCl
8. Write a balanced
chemical equation for the following and identify the type of reaction of each
case.
KBr + BaI2 → KI + BaBr2
ZnCO3 → ZnO + CO2
H2 + Cl → HCl
Mg + HCl → MgCl2 + H2
Answer:
2KBr + BaI2 → 2KI + BaBr2 (Double
Displacement Reaction)
ZnCO3 → ZnO + CO2 (Decomposition
Reaction)
H2 + Cl → 2HCl
(Combination Reaction)
Mg + 2HCl → MgCl2 + H2 (Displacement
Reaction)
9. What is meant by
exothermic and endothermic reactions? Give examples.
Answer:
An endothermic
reaction occurs when energy is absorbed from the surroundings in the form of
heat (For example, Photosynthesis, melting of ice, evaporation). Conversely, an
exothermic reaction is one in which energy is released from the system into the
surroundings (Examples: Explosions, concrete setting, nuclear fission and
fusion).
10. Why is respiration
considered to be an exothermic reaction?
Answer:
For the survival of
life, we require energy. We obtain this energy from the food we eat. The food
molecules, through the process of digestion, is broken down into simpler
molecule like glucose. These substances come in contact with the Oxygen present
in our body cells to form Carbon dioxide and water along with a certain amount
of energy (Respiration process). Since the energy is in the form of heat (that
maintains our body temperature), respiration is considered to be an exothermic
reaction. The reaction taking place is:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
11. Why are
decomposition reactions called the opposite of Combination reactions? Write
equations for decomposition reactions.
Answer:
Combination reaction
is said to be the reaction between two or more molecules to form a larger
molecule; whereas the decomposition reaction is defined as the splitting of
larger molecules into two or more smaller molecules. This essentially explains
that the decomposition reaction is the opposite of the combination reaction.
In most cases, the
decomposition reaction is endothermic since the heat from the surrounding or
induced heat is used to break the bonds of the larger molecule. A few examples
of decomposition reactions are
ZnCO3 → ZnO + CO2
CaCO3 + Energy → CaO +
CO2
2HgO → 2Hg + O2
12. Write one equation
each for decomposition reactions in which energy is supplied in the form of
heat, light or electricity.
Answer:
(a) Thermal
decomposition reaction (Thermolysis)
Decomposition of potassium
chlorate: When heated strongly, potassium chlorate decomposes into potassium
chloride and oxygen. This reaction is used for the preparation of oxygen.
2KClO3 + Heat → 2KCl + 3O2
(b) Electrolytic
decomposition reaction (Electrolysis)
Decomposition of
sodium chloride: On passing electricity through molten sodium chloride, it
decomposes into sodium and chlorine.
Electricity
(c) Photodecomposition
reaction (Photolysis)
Decomposition of hydrogen
peroxide: In the presence of light, hydrogen peroxide decomposes into water and
oxygen.
Light
13. What is the
difference between displacement and double displacement reactions? Write
relevant equations for the above.
Answer:
A displacement
reaction is one when a more reactive substance displaces a less reactive one
from its salt solution, whereas a double displacement reaction is one where a
mutual exchange of ions happens between two compounds.
In a displacement
reaction, only a single displacement takes place, whereas in the double
displacement reaction, as the name suggests, two displacement takes place
between the molecules.
Example:
Displacement reaction
Mg + 2HCl → MgCl2 + H2
Double displacement
reaction
2KBr + BaI2 → 2KI + BaBr2
14. In the refining of
Silver, the recovery of silver from Silver nitrate solution involves
displacement reaction by Copper metal. Write down the reaction involved.
Answer:
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
15. What do you mean
by a precipitation reaction? Explain by giving examples.
Answer:
When two solutions
containing soluble salts are combined, a double displacement reaction takes
place in which the ions are exchanged between the compounds. When one of such
compounds formed is in solid form (that is insoluble in aqua), then it settles
down at the bottom of the container. This solid is known as the precipitate and
the respective reaction is termed as the precipitation reaction. A few examples
of precipitation reactions are
CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)
2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) +
Mg(OH)2(s)
16. Explain the
following in terms of the gain of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Answer:
(a) In a chemical
reaction, when the oxygen is added to the element to form its respective oxide
it is the element being oxidised. Example:
4Na(s) + O2(g) → 2Na2O(s)
H2S + O2 → H2O + SO2
(b) In a chemical reaction, when the oxygen is
removed from the compound, then it is said to be reduced. Example:
CuO(s) + H2(g) → Cu(s) + H2O(l)
2HgO → 2Hg + O2
17. A shiny brown
coloured element ‘X’ on heating in the air becomes black in colour. Name the
element ‘X’ and the black-coloured compound formed.
Answer:
The shiny brown-coloured
element is Copper metal (Cu). When the metal is heated in air, it reacts with
atmospheric oxygen to form copper oxide. Hence, the black-coloured compound is
copper oxide.
2Cu(s) + O2(g) → 2CuO(s)
18) Why do we apply
paint on iron articles?
Answer:
Iron articles are
painted to prevent them from rusting. When left unpainted, the metal surface
comes in contact with the atmospheric oxygen and in the presence of moisture it
from Iron(III) oxide. But if painted, the surface does not come in contact with
moisture and air, thus prevents Rusting.
19) Oil and Fat
containing food items are flushed with Nitrogen. Why?
Answer:
The main purpose of
flushing Nitrogen into food packets that contain oil and fat items is to
prevent Rancidity which occurs when the oil or fat reacts with the oxygen
letting out an unpleasant smell and taste. Therefore, by flushing Nitrogen, an
unreactive surrounding is created, thus preventing rancidity.
20) Explain the
following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer:
(a) Corrosion is a
process where a refined metal is oxidised by atmospheric oxygen to form a more
stable compound, such as oxides. The metal gradually degrades during the
corrosion process. Rusting of iron is a good example of corrosion where the
iron is converted to Iron oxide. Millions of dollars are spent annually in
preventing rusting from bridges and other monuments.
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