1. Why should a magnesium ribbon be cleaned before
burning in air?
Solution:
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
Solution:
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 sulphate in water react to give insoluble Barium
sulphate and solution of Sodium chloride.
ii) Sodium
hydroxide solution in water reacts with hydrochloric acid solution to produce
Sodium chloride solution and water.
Solution:
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.
Solution:
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 of the
amount collected in the other? Name this gas
Solution:
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 is released, hence the amount of Hydrogen collected would be double than that of oxygen.
1. Why does the colour of copper sulphate solution
change when an iron nail is dipped in it?
Solution:
When an iron nail 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.
Solution:
Reaction Between silver nitrate (AgNO3)
and Sodium chloride (NaCl) is an example of double displacement reaction.
During the reaction negative and positive ions trade positions as a result in
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 oxidized and that are reduced in the following equation.
i) 4Na(s)
+ O2(g) → 2Na2O(s)
ii) CuO(s)
+ H2(g) → Cu(s) + H2O(l)
Solution:
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
Solution:
(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
2.
Fe2O3 + 2Al → Al2O3 + 2Fe
The
above reaction is an example of a
Combination
reaction.
Double
displacement reaction.
Decomposition
reaction.
Displacement
reaction.
Solution:
Answer is 4. Displacement reaction.
Explanation: The Oxygen from the
Ferrous oxide is getting displaced to the Aluminium metal to form Aluminium
Oxide. In this reaction Aluminum is more reactive metal than Fe. Therefore Al
will displace Fe from its oxide. This type of chemical reactions in which one
of the elements displace another is called displacement reaction. Here less
reactive metal is displaced by more reactive metal. Since one-time displacement
is occurring, therefore, it is called a single displacement reaction.
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.
Solution:
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?
Solution:
A balanced equation is the one in
which number of different atoms on both the reactant and product sides are
equal. Balancing chemical equation is necessary for the reaction should obey
The Law of Conservation of energy. 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 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.
Solution:
(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
Solution:
(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
Solution:
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
Solution:
2KBr + BaI2 →
2KI + BaBr2 (Double
Displacement Reaction)
1. ZnCO3 →
ZnO + CO2 (Decomposition
Reaction)
2. H2 +
Cl → 2HCl (Combination Reaction)
3. Mg +
2HCl → MgCl2 +
H2 (Displacement
Reaction)
9. What is meant by
exothermic and endothermic reactions? Give examples.
Solution:
An endothermic reaction occurs when
energy is absorbed from the surroundings in the form of heat.(Example:
Photosynthesis, melting of ice, evaporation). Conversely, an exothermic
reaction is one in which energy is released from the system into the
surroundings. (Example: Explosions, concrete setting, nuclear fission and
fusion)
10. Why is respiration
considered to be an exothermic reaction?
Solution:
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 a 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) the 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.
Solution:
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 of the cases the
decomposition reaction is endothermic since heat from the surrounding or
induced heat is used to break the bonds of the larger molecule. 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.
Solution:
(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.
(c) Photodecomposition reaction
(Photolysis)
Decomposition of hydrogen peroxide:
In the presence of light, hydrogen peroxide decomposes into water and oxygen.
13. What is the
difference between displacement and double displacement reactions? Write
relevant equations for the above.
Solution:
A displacement reaction is the one
when a more reactive substance displaces a less reactive one from its salt
solution whereas a double displacement reaction is the 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.
Solution:
Cu(s) + 2AgNO3(aq)
→ Cu(NO3)2(aq)
+ 2Ag(s)
15. What do you mean by a
precipitation reaction? Explain by giving examples.
Solution:
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. 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 gain of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Solution:
(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
being 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.
Solution:
The shiny brown coloured element is
the 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
the copper oxide.
2Cu(s) + O2(g)
→ 2CuO(s)
18)
Why do we apply paint on iron articles?
Solution:
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
preventing Rusting.
19) Oil and Fat containing
food items are flushed with Nitrogen. Why?
Solution:
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
Solution:
(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.
(b) The condition produced by the aerial oxidation of the oil and fat present in the food material that produces an unpleasant taste and smell. The rancidity is retarded when the food is kept inside the refrigerator since the low temperature does not promote the oxidation reaction.
