X Chemistry Chemical reaction and equations
1.A compound ‘A’ is used in the manufacture of cement. When dissolved
in water, it evolves a large amount of heat and forms compound ‘B’.
(i) Identify A and B.
(ii) Write chemical equation for the reaction of A with water.
(iii) List two types of reaction in which this reaction may be classified.
(2020)
Answer:
(i) A is calcium oxide, CaO which is used in the manufacturing of cement.
B is calcium hydroxide Ca(OH)3.
(iii) The given reaction is a combination reaction.
Example : NH3(g)(g) + HCl(g) → NH4Cl(s)
2NO(g) + 02(g) → 2NO2(g)
2.Mention with reason the colour changes observe when:
(i) silver chloride is exposed to sunlight.
(ii) copper powder is strongly heated in the presence of oxygen.
(iii) a piece of zinc is dropped in copper sulphate solution.
Answer:
(i) When white silver chloride is left exposed to sunlight, its colour
changes to grey as it decomposes to silver in the presence of sunlight.
This type of reaction is called photodecomposition reaction.
(ii) When copper powder is strongly heated in presence of oxygen, the
reddish brown surface of copper powder becomes coated with a black
substance which is copper oxide.
(iii) When a piece of zinc is dropped in copper sulphate solution, then the
blue colour of copper sulphate fades gradually due to the formation of
colourless zinc sulphate solution and reddish brown copper metal gets
deposited on zinc piece.
3.Lead nitrate solution is added to a test tube containing potassium iodide
solution.
(a) Write the name and colour of the compound precipitated.
(b) Write the balanced chemical equation for the reaction involved.
(c) Name the type of this reaction justifying your answer.
Answer:
(a) When lead nitrate is added to potassium iodide then yellow precipitate
of lead iodide is formed along with potassium nitrate.
(b) Balanced chemical reaction is as follows :
(c) This type of reaction is called precipitation reaction in which one of the
products formed is an insoluble substance or this is also called double
displacement reaction.
4. Identify the type of reactions taking place and write the balanced
chemical equation for the reactions.
(a) Zinc reacts with silver nitrate to produce zinc nitrate and silver..
Answer:
(a) It is a displacement reaction.
5. 2 g of ferrous sulphate crystals are heated in a dry boiling tube.
(a) List any two observations.
(b) Name the type of chemical reaction taking place.
(c) Write balanced chemical equation for the reaction and name the
products formed.
Answer:
(a) Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and
the colour of the crystals changes. It then decomposes to ferric oxide
(Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3) with a smell of
burning sulphur.
(b) This is a thermal decomposition reaction.
6.You might have noted that when copper powder is heated in a China
dish, the reddish brown surface of copper powder becomes coated with a
black substance. (a) Why has this black substance formed?
(b) What is the black substance?
(c) Write the chemical equation of the reaction that takes place.
(d) How can the black coating on the surface be turned reddish brown?
Answer:
(a) The black substance is formed because copper combines with oxygen.
(b) The black substance is copper oxide (CuO).
(d) The black coating on the surface can be turned reddish brown by
passing hydrogen gas over the hot copper oxide.
7.Decomposition reactions require energy either in the form of heat or
light or electricity for breaking down the reactants. Write one equation
each for decomposition reactions where energy is supplied in the form of
heat, light and electricity. (2018)
Answer:
Decomposition reaction involving absorption of heat:
Decomposition reaction involving absorption of light:
Decomposition reaction involving absorption of electrical energy:
8.Take 3 g of barium hydroxide in a test tube, now add about 2 g of
ammonium chloride and mix the contents with the help of a glass rod.
Now touch the test tube from outside.
(i) What do you feel on touching the test tube?
(ii) State the inference about the type of reaction occurred.
(iii) Write the balanced chemical equation of the reaction involved.
Answer:
(i) When barium hydroxide is added into ammonium chloride, the bottom
of test tube is found to be cooler.
(ii) It is an endothermic reaction.
(iii) Ba(OH)2 + 2NH4Cl → BaCl2 + 2NH4OH
9. A solution of potassium chloride when mixed with silver nitrate solution,
an insoluble white substance is formed. Write the chemical reaction
involved and also mention the type of the chemical reaction.
Answer:
It is a double displacement reaction.
10.Name the type of chemical reaction represented by the following
equation:
(i) CaO + H2O → Ca(OH)2
(ii) 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
Answer:
(i) Combination reaction.
(ii) Precipitation reaction or double displacement reaction.
(iii) Thermal decomposition reaction.
11.What is a reduction reaction?
Identify the substances that are oxidised and the substances that are
reduced in the following reactions. (Board Term I, 2015)
(a) Fe2O3 + 2Al → Al2O3 + 2Fe
(b) 2PbO + C → 2Pb + CO2
Answer:
Those reactions in which addition of hydrogen to a substance or removal
of oxygen from a substance take place are called reduction reactions.
Fe2O3 is getting reduced to Fe and Al is getting oxidised to Al 2O3.
PbO is reduced to Pb and C is oxidised to CO2.
12. (a) Can a displacement reaction be a redox reaction? Explain with the
help of an example.
(b) Write the type of chemical reaction in the following:
(i) Reaction between an acid and a base
(ii) Rusting of iron. (Board Term I, 2017)
Answer:
(a) Consider the following displacement reaction:
Zn(s)+ CuSO4(aq) → ZnSO4(aq) + Cu(s)
Here, Zn has changed into ZnSO4 (i.e., Zn2+ ions) by loss of electrons.
Hence, Zn has been oxidised. CuSO4 (i.e., Cu2+) has changed into Cu by
gain of electrons. Hence, CuSO4 has been reduced. Thus, the above
reaction is a displacement reaction as well as a redox reaction.
(b) (i) Neutralisation reaction
(ii) Oxidation reaction.
13.Mention the type of chemical reaction that takes place when:
(i) a magnesium ribbon is burnt in air.
(ii) limestone is heated.
(iii) silver bromide is exposed to sunlight.
(iv) electricity is passed through acidified water.
(v) ammonia and hydrogen chloride are mixed with each other.
Write the chemical equation for each reaction.
Answer:
This is a combination reaction.
This is a thermal decomposition reaction.
This is a photo decomposition reaction.
This is electrolytic decomposition reaction.
This is a combination reaction.
14.What happens when food materials containing fats and oils are left for
a long time? List two observable changes and suggest three ways by
which this phenomenon can be prevented.
Answer:
Food materials containing fats and oils change their taste and smell due
to a process called rancidity. Rancidity is a process in which air reacts with
fats and oils which changes the smell and taste of food.
Methods of prevention:
Vacuum packing,
refrigeration of food materials,
placing of food materials, away from direct sunlight.
14. (i) Why is respiration considered as an exothermic reaction?
(ii) Write chemical name and the formula of the brown gas produced
during thermal decomposition of lead nitrate.
(iii) Why do chips manufactures flush bags of chips with gas such as
nitrogen?
Answer:
(i) The glucose produced in our body during digestion combines with
oxygen in the cells of our body and provides energy. The special name of
this reaction is respiration. Thus respiration is an exothermic process
because energy is produced during this process.
C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l) + Energy
Brown gas evolved is nitrogen dioxide (NO2).
(iii) Chips manufacturers usually flush bags of chips with gas such as
nitrogen because atmospheric oxygen can react with chips which may
cause change in colour, change in taste. So to cut the contact between air
and the chips, nitrogen gas is used which do prevent its oxidation.
