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CHEMICAL REACTION AND
CHEMICAL EQUATIONS
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TEXTBOOK QUESTIONS AND THEIR ANSWERS
Q.1.
Why should magnesium ribbon be cleaned before
burning in air?
Ans.
Magnesium ribbon is coated with a thin layer of
magnesium oxide. Thus, in order to burn it, it is
scrubbed with sandpaper, so that fresh layer of
magnesium metal is exposed.
Q.2.
What happens when dilute hydrochloric acid is
added to iron filings? Tick the correct choice.
(a) Hydrogen
gas
and
iron
chloride
are
hydroxide
are
produced.
(b) Chlorine
gas
and
iron
produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
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Ans.
(a) is correct choice.
Q.3.
Potassium iodide solution is added to lead nitrate
solution, then a yellow precipitate formed. The
yellow precipitate is :
(a) Potassium nitrate. (b)
(c) Lead nitride
Lead iodide.
(d) None of the above.
Tick the correct choice.
Ans.
(b) is the correct choice as lead iodide is yellow in
colour.
Q.4.
Balance the following chemical equations.
(a) HNO3 + Ca(OH) 2
Ans.
Ca(NO3) 2 + H2O
(b) NaOH + H2SO4
Na2SO4 + H2O
(c) NaCl + AgNO3
AgCl + NaNO3
(d) BaCl2 + H2SO4
BaSO4 + HCl
(a) 2HNO3 + Ca(OH)2
(b) 2NaOH + H2SO4
(c) NaCl + AgNO3
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Ca(NO3)2 + 2H2O
Na2SO4 + 2H2O
AgCl + NaNO3
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(d) BaCl2 + H2SO4
Q.5.
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BaSO4 + 2HCl
Write a balanced chemical equation with state of
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.
Ans.
(i) Barium chloride (Sol) + Sod. sulphate (sol)
Barium Sulphate (ppt) + Sod. Chloride
(sol) ...... Word equation.
BaCl2(aq) + Na2SO4(aq)
BaSO4(s) +
NaCl(aq) ........... Skeletal equation.
BaCl2(aq) + Na2SO4(aq)
BaSO4(s) +
2NaCl(aq) ......... Balanced equation.
(ii) Sodium hydroxide (aq) + Hydrochloric acid (aq)
Sod. chloride (aq) + Water. ....... Word
equation.
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NaOH(aq) + HCl(aq)
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NaCl(aq) + H2O(l)
........ Balanced equation.
Q.6.
What is a balanced chemical equation? Why
should a chemical equation be balanced?
Ans.
A chemical equation in which number of atoms of
each element is same on the side of reactants and
products is called balanced chemical equation.
A chemical equation in which the number of each
element is not same on the side of reactants and
products contradicts law of conservation of mass, as
matter can neither be created nor be destroyed. Thus,
such an equation is not true chemical equation and
hence it should be balanced.
Q.7.
Translate the following statements into chemical
equations and then balance them.
(a) Hydrogen gas combines with nitrogen gas to
form ammonia gas.
(b) Hydrogen sulphide gas burns in air to give
water and sulphur dioxide gas.
(c) Barium chloride reacts with aluminium
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sulphate to give aluminium chloride and a
precipitate of barium sulphate.
(d) Potassium metal reacts with water to give
potassium hydroxide and hydrogen gas.
Ans.
(a) H2(g) + N2(g)
NH3(g) ...... Skeletal
equation
3H2(g) + N2(g)
2NH3(g) ...... Balanced
equation.
(b) H2S(g) + O2(g)
H2O(l) + SO2(g) ......
Skeletal equation
2H2S(s) + 3O2(g)
2H2O(l) + 2SO2(g) ........
Balanced equation
(c) BaCl2(aq) + Al2(SO4)3(aq)
BaSO4(s) +
AlCl3(aq) ...... Skeletal equation
3BaCl2(aq) + Al2(SO4)3(aq)
3BaSO4(s) +
2AlCl3(aq) ..... Balanced equation.
(d) K(s) + H2O(l)
KOH(aq) + H2(g) .....
Skeletal equation
2K(s) + 2H2O(l)
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2KOH(aq) + H2(g) .....
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Balanced equation.
Q.8.
Write balanced chemical equations for the
following reactions.
(a) Calcium hydroxide + Carbon dioxide
Calcium carbonate + Water.
Zinc nitrate +
(b) Zinc + Silver nitrate
Silver.
(c) Aluminium
+
Copper
chloride
Aluminium chloride + Copper.
(d) Barium chloride + Potassium sulphate
Barium sulphate + Potassium chloride.
Ans.
(a) Ca(OH)2(aq) + CO2(g)
CaCO3(s) + H2O
...... Balanced equation
(b) Zn(s) + AgNO3(aq)
Zn(NO3)2(aq) + Ag(s)
...... Skeletal equation
Zn(s) + 2AgNO3(aq)
Zn(NO3)2(aq) +
2Ag(s) ...... Balanced equation
(c) Al(s) + CuCl2
AlCl3 + Cu ...... Skeletal
equation.
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2Al(s) + 3CuCl2
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2AlCl3 + 3Cu .....
Balanced equation.
(d) BaCl2(aq) + K2SO4(aq)
BaSO4(s) + KCl
.... Skeletal equation
BaCl2(aq) + K2SO4(aq)
BaSO4(s) + 2KCl
....... Balanced equation
Q.9.
Which of the statements about the reaction below
are incorrect ?
2Pb(s) + CO2(g)
2PbO(s) + C(s)
(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)
Ans.
Q.10.
(iv) all.
(iv) is the answer, as all other choices are incorrect.
Fe2O3 + 2Al
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Al2O3 + 2Fe.
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The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Ans.
Q.11.
(d) displacement reaction is the correct choice.
Why does the colour of copper sulphate solution
change, when iron nail is dipped?
Ans.
The blue colour of copper sulphate is due to Cu2+
ions. These copper ions are discharged and the Fe2+
ions are added in solution, which result in the change
in colour.
Q.12.
In the refining of silver, the recovery of silver
from silver nitrate solution involved displacement
by copper metal. Write down the reaction
involved.
Ans.
Q.13.
2AgNO3(aq) + Cu(s)
Cu(NO3)2(aq) + 2Ag.
Why respiration in living beings is called an
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exothermic process?
Ans.
The food taken by the living beings is ultimately
broken down to glucose by the digestive system. The
glucose so formed is slowly oxidised to carbon
dioxide and water with the release of heat energy.
Thus, respiration is an exothermic process.
Q.14.
Identify the substances that are oxidised and the
substances that are reduced in the following
reactions.
(i) 4Na(s) + O2(g)
(ii) CuO(s) + H2(g)
Ans.
2Na2O(s)
Cu(s) + H2O(l).
(i) Sodium is oxidised and oxygen is reduced.
(ii) Hydrogen is oxidised and copper oxide is
reduced.
Q.15.
What does one mean by exothermic and
endothermic reaction? Give examples.
Ans.
A chemical reaction which proceeds with the
evolution of heat energy is called exothermic
reaction.
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Example : 2Mg(s) + O2(g)
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2MgO(s) + heat.
A chemical reaction which proceeds with the
absorption of heat energy is called endothermic
reaction.
Example : CaCO3(s) ¾ heat
¾ ® CaO(s) + CO2(g).
Q.16.
What is the difference between displacement and
double displacement reactions? Write equations
for these reactions.
Ans.
In a displacement reaction only an active metal
displaces less active metal from its aqueous ionic salt
solution as shown in the following equation.
FeSO4(aq) + Zn(s)
ZnSO4(aq) + Fe(s)
However, in double displacement reaction the
aqueous ionic solutions of two salts react by
exchanging their ions/radicals to form two new
compounds as shown in the following equation.
BaCl2(aq) + Na2SO4(aq)
Q.17.
BaSO4(s) + 2NaCl(aq)
A shiny brown coloured element ‘X’ on heating in
air becomes black in colour. Name the element
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‘X’ and the black compound formed.
Ans.
The shiny brown element is metal copper. The black
compound formed is copper (II) oxide.
Q.18.
Explain the following terms with one example
each.
(a) Corrosion
Ans.
(b) Rancidity
(a) Formation of layers of undesired compounds,
such as metallic oxides and hydroxides on the
surface of metals is called corrosion of metals.
Copper metal when exposed to moist air
containing carbon dioxide gets covered with a
greenish layer of basic copper carbonate.
(b) The oxidation of food materials, such that they
become stale and start smelling is called
rancidity.
Any kind of cooked food gets rancid, if kept at
room temperature (above 25°C) in 24 hours, due
to its oxidation with air.
Q.19.
Why are decomposition reactions called opposite
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of combination reactions? Write equations for
these reactions.
Ans.
During decomposition reaction a single chemical
compound breaks down into two or more different
elements or compounds, whereas during chemical
combination reaction, two or more elements or
compounds react to form a single chemical
compound. Thus, we can say that chemical
decomposition reaction is opposite of chemical
combination reaction. Following examples will
illustrate the point.
2Mg(s) + O2(g)
2MgO(s) ....... Combination
reaction
2CO(g) + O2(g)
2CO2(g) ....... Combination
reaction
¾ ® PbO(s) + CO2(g) .......
PbCO3(s) ¾ heat
Decomposition reaction
¾ ® 2NaNO2(s) + O2(g) .......
2NaNO3(s) ¾ heat
Decomposition reaction
Q.20.
Write balanced chemical equation for the
following and identify the type of reaction in each
case.
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(a) Potassium bromide (aq) + Barium iodide (aq)
Potassium iodide (aq) + Barium
bromide (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).
Ans.
(a) 2KBr(aq) + BaI2(aq)
2KI(aq) + BaBr2(s).
It is a double displacement precipitation
reaction.
(b) ZnCO3(s)
ZnO(s) + CO2(g)
It is a chemical decomposition reaction.
(c) H2(g) + Cl2(g)
2HCl(g)
It is a chemical combination reaction.
(d) Mg(s) + 2HCl(aq)
MgCl2(aq) + H2(g).
It is a chemical displacement reaction.
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Q.21.
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Write an equation each for decomposition
reactions, where energy is supplied in the form of
heat, light, or electricity.
Ans.
(a) Energy is supplied in the form of heat, during
the decomposition of ferrous sulphate crystals.
2FeSO4(s) ¾ heat
¾ ® Fe2O3(s) + SO2(g) + SO3(g)
(b) Energy is supplied in the form of light, when
silver iodide decomposes to form silver and
iodine.
¾ ® 2Ag(s) + I2(s)
2AgI(s) ¾ heat
(c) Energy is supplied in the form of electricity,
when molten lead bromide decomposes to lead
and bromine.
PbBr2(l) ¾ elect
¾ ricity
¾¾
® Pb + Br2
Q.22.
What do you mean by precipitation reaction?
Explain by giving examples.
Ans.
Precipitation
reaction
is
basically
double
displacement reaction. It is defined as under :
When the aqueous solutions of two ionic compounds
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react by exchanging their ions/radicals, to form two
new compounds, such that one of the products
formed is insoluble in water, and hence form
precipitate, it is said to be precipitation reaction.
Examples :
1. 2NaOH(aq) + FeSO4(aq)
Na2SO4(aq) +
Fe(OH)2(s)
2. Pb(NO3)2(aq) + Na2SO4(aq)
2NaNO3(aq)
+ PbSO4(s).
3. AgNO3(aq) + HCl(aq)
HNO3(aq) +
AgCl(s)
Q.23.
Explain the following in terms of gain or loss of
oxygen with two examples each :
(a) oxidation, (b) reduction
Ans.
(a) Oxidation of a substance takes place when there
is : (i) addition of oxygen, (ii) removal of
hydrogen from a substance.
Examples :
1. 2Mg(s) + O2(g)
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2MgO(s)
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Oxidation of magnesium takes place as
oxygen is added to it.
2. H2S(g) + Br2(aq)
2HBr(aq) + S(s)
Oxidation of hydrogen sulphide takes place as
hydrogen is removed from it.
(b) Reduction of a substance takes place when there
is : (i) addition of hydrogen, (ii) removal of
oxygen from a substance.
Examples :
¾ ¾® 2HCl(g).
1. H2(g) + Cl2(g) ¾ sunlight
Reduction of chlorine takes place as there is
addition of hydrogen.
heat
2. CuO(g) + H2(aq) ¾ ¾ ® Cu(s) + H2O(s)
Reduction of copper oxide takes place as
oxygen is removed from it.
Q.24.
Write the balanced equation for the following
chemical reactions.
(i) Hydrogen
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Hydrogen
Chlorine
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chloride
(ii) Barium chloride + Aluminium sulphate
Barium sulphate + Aluminium chloride
Sodium hydroxide +
(iii) Sodium + Water
Hydrogen
Ans.
(i) H2 + Cl2
2HCl
(ii) 3BaCl2 + Al2(SO4)3
(iii) 2Na + 2H2O
Q.25
3BaSO4 + 2AlCl3
2NaOH + H2
A solution of 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.
Ans.
(i) The substance X is calcium oxide and its
formula is CaO.
(ii) CaO(s) + H2O(l)
Calcium Water
oxide
Q.26.
Ca(OH)2(aq)
Calcium
hydroxide
Why is the amount of gas collected in one of the
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test-tubes in electrolysis of water is double of the
amount collected in the other? Name this gas.
Ans.
When electric current is passed through acidulated
water, water decomposes to form hydrogen and
oxygen. The reaction takes place is
2H2O(l)
2H2(g) + O2(g)
Thus, hydrogen and oxygen are produced in the ratio
of 2 : 1 by volume. Hence, the amount of gas
collected in one test tube is double the amount
collected in the other.
This gas is hydrogen.
Q.27.
Give an example of a double displacement
reaction other than the one given in Activity 11.0
(of the text book).
Ans.
Barium chloride reacts with aluminium sulphate to
form barium sulphate and aluminium chloride
3BaCl2 + Al2(SO4)3
Q.28.
Ans.
3BaSO4 ↓ + 2AlCl3.
Why do we apply paint on iron articles?
When iron articles are kept in air, then after
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sometime they get corrode due to the formation of
reddish brown powder which is called rust of iron.
So, we paint on iron articles to protect them from
rusting.
Q.29.
Oil and fat containing food items are flushed with
nitrogen. Why?
Ans.
Nitrogen prevents the oil and fat containing food
items from getting oxidised. So, these food items are
flushed with nitrogen.
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