Download 1 Chemical Reactions and Equations

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Chemical Reactions and Equations
SYLLABUS
Chemical reactions: Chemical equation, balanced chemical equation, implications
of a balanced chemical equation. Types of chemical reactions: combination,
decomposition, displacement, double displacement, precipitation, neutralization,
oxidation and reduction.
BASIC CONCEPTS
1. Chemical reactions taking place are represented by fully balanced chemical equations,
i.e., the number of atoms of each type involved in a reaction are the same on the
reactant and product sides of the chemical equation.
2. A simple equation is balanced by hit and trial method so that same total number of
charges and atoms appear on both sides of the equation. For example when iron
combines with steam to form Fe3O4 and H2, it is balanced in the following steps
(i)
(ii)
(iii)
Fe + H2O → Fe3O4 + H2 (Skeleton equation)
Fe + 4H2O → Fe3O4 + H2 (O is made equal on both sides)
Fe + 4H2O → Fe3O4 + 4H2 (H is made equal on both sides)
(iv) 3Fe + 4H2O → Fe3O4 + 4H2 (Fe is made equal on both sides)
This is balanced equation
Right
Left
Fe
3
3
O
4
4
H
8
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If the reaction is complicated, i.e., it involves large number of reactants and products,
it is written in steps which are balanced as above and then added up to get the final
reactants and products. This is called partial equation method.
3. A complete chemical equation gives the formulae of the reactants and products, their
physical states and brief conditions under which a reaction occurs.
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4. There are several types of reactions. These involve breaking of bonds and formation
of new ones by the effect of heat, light or mechanical shaking.
(i) A decomposition reaction occurs when a compound is broken into smaller parts.
It may be either thermal decomposition or electrolytic decomposition.
(ii) In combination reaction, two or more substances combine to form a new substance.
(iii) The reaction in which an atom or group of atoms in a molecule is replaced by
another atom or group of atoms is called displacement reaction, when two
reacting molecules exchange their partner ions in solution, it will be double
displacement reaction.
(iv) It is said to be a dissociation reaction, when a substance breaks up into its ions.
(v) In precipitation reactions, insoluble salts are formed.
(vi) In redox reactions, a substance gains oxygen or loses hydrogen, i.e., gets oxidised
and simultaneously, another substance loses oxygen or gains hydrogen, i.e., gets
reduced.
5. If the surface of a metal is attacked by air, water, it is said to corrode and the process
is called corrosion. Rancidity is the process in which fats and oils (in food) get
oxidised and the food gets spoiled.
QUESTIONS–ANSWERS
1.1. CHEMICAL EQUATIONS
Q. 1. What is a chemical equation? Illustrate with an example.
Ans. The mode of communicating facts in chemical language about a chemical reaction
is called chemical equation. Thus, a chemical equation is an expression for given chemical
change in terms of symbols or formulae of the reactants and products. For example, the
reaction of zinc with dilute sulphuric acid to produce zinc sulphate and hydrogen is given by
the following chemical equation:
Zn + H2SO4 → ZnSO4 + H2.
Q. 2. How will you know whether a chemical reaction has taken place?
Ans. When a chemical reaction occurs, one or more of the following changes take place :
(i) Change in state
(iii) Evolution of a gas
(ii) Change in colour
(iv) Evolution or absorption of heat (change in
temperature).
Q. 3. How would you justify that a chemical reaction has taken place in the
following cases:
(i) Burning of magnesium ribbon in air.
(ii) Addition of lead nitrate solution to potassium iodide solution.
(iii) Addition of dilute hydrochloric acid to zinc granules.
Ans. (i) Burning of magnesium ribbon in air gives a powder of MgO. So there is a
change of state.
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CHEMICAL REACTIONS AND EQUATIONS
(ii) Addition of colourless lead nitrate solution to potassium iodide solution gives yellow
coloured precipitate (lead iodide). So there is a change of colour.
(iii) Addition of dilute hydrochloric acid to zinc granules gives a gas (H2) with effervescence.
Q. 4. How do you show that respiration is a chemical change?
Ans. During respiration, energy is released.
Q. 5. Why is it necessary to obtain a balanced chemical equation?
Ans. Law of conservation of mass is applied here. Accordingly matter can neither be
created nor destroyed. Thus during a chemical reaction the total mass of the reactants and
products remains the same. Therefore, for a complete chemical equation, the number of
atoms of various elements on both sides are made equal, i.e., the equation is balanced.
Q. 6. How is a chemical equation written? Illustrate with an example.
Ans. Reaction writing should be done in a systematic manner, using the conventions
followed. These are illustrated with the help of an example. Let it be required to write a
chemical equation that describes the reaction between magnesium and sulphuric acid to
produce magnesium sulphate and hydrogen.
(i) Write the symbols of all the substances involved in the reaction—first the reactants
and then the products—one after the other ON THE SAME LINE
Reactants
Mg
H2SO4
Products
MgSO4
H2
(ii) Separate the reacting substances from the products by a symbol meaning ‘produce’.
The sign → (an arrow) or sometimes sign (equal) = is used for this purpose
Mg
H2SO4 → MgSO4
H2
(iii) Then put + sign between the reactants indicating that these ‘react’ and a (plus) +
sign between the products which signifies ‘and’,
Mg + H2SO4 → MgSO4 + H2
Q. 7. What do you understand by skeletal and balanced chemical equation?
Ans. If the number of atoms of any element in a chemical equation is not equal on both
sides, then it is a skeleton equation. For example,
Mg + HCl → MgCl2 + H2
Here, the number of chlorine and hydrogen atoms are not equal on both sides.
In a balanced equation, the number of atoms of different elements on both sides of
a chemical equation are equal. For example,
Mg + 2HCl → MgCl2 + H2
Q. 8. Describe the method of balancing a chemical equation taking an example.
Ans.
Example I: Let us consider the formation of water from the combination of oxygen and
hydrogen. The following sequential steps be taken to obtain a balanced chemical equation.
(i) First write the skeleton equation. For example,
H2 + O2 → H2O
Caution: Do not change formula of any constituent while balancing the equation.
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(ii) The same total of charges should appear on the left and right side of the equation.
(iii) Make one of the atoms equal on both sides by multiplying a molecule or compound
with an integral number so that the desired element is balanced. It is convenient to start
with the molecule or compound that contains the maximum number of atoms. Here, H2O
contains maximum number of atoms. It contains one short of oxygen atom and so it is
multiplied by 2 as shown.
H2 + O2 → 2H2O
(iv) Next examine the effect of this multiplication of the molecule on the balance of
other atoms. As is seen, the oxygen balances but now hydrogen on left is 2 less. So, multiply
H2 on the left by 2
2H2 + O2 → 2H2O
(v) Further count the number of atoms of each type on both sides. In the above equation
Right
Left
H
4
4
O
2
2
So, the equation is balanced. If the number of atoms on both sides do not agree,
continue with the above steps till Balanced Chemical Equation is obtained. This is called hit
and trial method.
Example II: Let us take a little more difficult equation, when iron is combined with
steam (H2O).
(i) The skeleton equation for the above reaction is
Fe + H2O → Fe3O4 + H2
(ii) Then, Fe3O4 is selected which contains the maximum of atoms. It contains 4
oxygen atoms whereas there is only one oxygen atom on the other side, i.e., L.H.S, in H2O.
So H2O is multiplied by 4.
Fe + 4H2O → Fe3O4 + H2
(iii) Again examine the effect of step (ii). Oxygen is balanced but Fe and H are not yet
balanced. So to balance H, multiply H2 by 4.
Fe + 4H2O → Fe3O4 + 4H2
(iv) When counting the number of atoms on both sides, it is seen Fe is one on L.H.S.
and is 3 on R.H.S. To equalise Fe on both sides, multiply Fe on L.H.S. by 3.
3Fe + 4H2O → Fe3O4 + 4H2
Now this is a balanced chemical equation.
Example III: Let us consider a reaction in which sodium metal reacts with water to form
sodium hydroxide and hydrogen.
(i) The skeleton equation for the above reaction is:
Na + H2O → NaOH + H2
(ii) When considering number of each type of atoms on both sides, it is seen hydrogen
atoms on both sides are not equal, it is 2 atoms on left side and 3 atoms on right side. So,
we multiply H2O by 2.
Na + 2H2O → NaOH + H2
(iii) This step (ii) makes oxygen atoms deficient on right side and hydrogen atoms
becomes more on left side. So, we multiply NaOH by 2.
CHEMICAL REACTIONS AND EQUATIONS
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Na + 2H2O → 2NaOH + H2
(iv) Now oxygen and hydrogen atoms are equal on both sides but sodium atom is one
less on left side. So, we multiply Na by 2.
2Na + 2H2O → 2NaOH + H2
Now this is a balanced chemical equation.
Example IV: If the reaction is complicated i.e., it involves large number of reactants
and products, it is preferred to write the equation in steps (the actual reaction may or may
not be taking in these steps). Each step should be a balanced chemical equation. For example,
when copper reacts with conc. nitric acid, products are cupric nitrate, nitrogen dioxide and
water.
Cu + HNO3 → Cu(NO3)2 + NO2 + H2O
We can write this reaction in three steps and balance each step separately.
(i) First, HNO3 is decomposed to give nitrogen dioxide (NO2) and water (H2O) and
atomic oxygen.
2HNO3 → 2NO2 + H2O + O
...(i)
(ii) In the next step, copper is oxidised to copper (II) oxide (CuO)
Cu + O → CuO
...(ii)
(iii) Copper oxide so formed then reacts with nitric acid to form copper nitrate [Cu(NO3)2]
and water.
CuO + 2HNO3 → Cu(NO3)2 + H2O
...(iii)
Next multiply Eq. (i), (ii), (iii) by an integer so that on adding (i), (ii) and (iii), intermediate
products (which do not appear in the final reaction) cancel out. Now in this case, we find that
the integer is one. So add (i), (ii) and (iii) and write the final balanced equation as shown
Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O
This method is known as Partial Equation Method.
Q. 9. Balance the following skeletal equations:
KClO3 → KCl + O2
(i)
Na2CO3 + HCl → NaCl + H2O + CO2
(ii)
CH4 + O2 → CO2 + H2O
(iii)
NaCl + H2SO4 → Na2SO4 + HCl
(iv)
Fe + H2O → Fe3O4 + H2
(v)
H2S + O2 → H2O + S
(vi)
H2 + N2 → NH3.
(vii)
Ans. (i) (a)
(b)
(ii) (a)
(b)
2KClO3 → KCl + 3O2
2KClO3 → 2KCl + 3O2
Na2CO3 + HCl → 2NaCl + H2O + CO2
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
(iii) (a)
CH4 + O2 → CO2 + 2H2O
(b)
CH4 + 2O2 → CO2 + 2H2O
(iv) (a)
2NaCl + H2SO4 → Na2SO4 + HCl
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GOLDEN SCIENCE—X
(b)
2NaCl + H2SO4 → Na2SO4 + 2HCl
(v) (a)
3Fe + H2O → Fe3O4 + H2
(b)
3Fe + 4H2O → Fe3O4 + H2
(c)
3Fe + 4H2O → Fe3O4 + 4H2
(vi) (a)
(b)
H2S + O2 → 2H2O + S
2H2S + O2 → 2H2O + 2S
(vii) (a)
N2 + H2 → 2NH3
(b)
N2 + 3H2 → 2NH3
Q. 10. Balance the following chemical equations:
(i)
NaOH + H2SO4 → Na2SO4 + H2O
(ii)
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(iii)
Ans. (i)
FeCl2 + H2S → HCl + FeS
(A.I. CBSE 2008)
2NaOH + H2SO4 → Na2SO4 + H2O
2NaOH + H2SO4 → Na2SO4 + 2H2O
(ii)
2HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(iii)
FeCl2 + H2S → 2HCl + FeS.
Q. 11. What does a balanced chemical equation convey to a chemist?
Ans. The following information is conveyed by a balanced chemical equation:
(i) Formulae of substances taking part in chemical reaction i.e., reactants.
(ii) Formulae of substances produced in the reaction i.e., products.
(iii) The relative number of molecules of reactants and products.
(iv) The relative masses of reactants and products.
(v) The relative volumes of gaseous substances involved in the reaction.
Q. 12. Enlist the limitations of chemical equation.
Ans. Limitations or defects of a chemical equation:
A chemical equation does not indicate about
(i) the physical states of the reactants and products.
(ii) the concentrations of the reactants and products.
(iii) the heat evolved (exothermic reaction) or absorbed (endothermic reaction) during
the chemical reaction.
(iv) the condition of temperature, pressure, catalyst etc. which are needed for a reaction.
(v) the rate of reaction, whether slow or fast.
(vi) the reversibility of a reaction.
(vii) whether any precipitate is formed or gas is evolved in the reaction.
(viii) the extent upto which reaction takes place.
(ix) time taken by the reaction for completion.
(x ) mechanism of the reaction.
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CHEMICAL REACTIONS AND EQUATIONS
Q. 13. What are the essentials of a chemical equation?
Ans. (i) It must refer to an actual chemical reaction.
(ii) It should be molecular. For example, H, O, N should be written as H2, O2, N2.
(iii) It should be balanced and conform to the law of conservation of matter.
Q. 14. What information is available from the following equation:
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Ans. The following information can be derived from the given equation:
(i) Calcium carbonate and hydrochloric acid are reactants and calcium chloride, water
and carbon dioxide are products.
(ii) 1 molecule of calcium carbonate reacts with 2 molecules of hydrochloric acid to
produce 1 molecule each of calcium chloride, water and carbon dioxide.
(iii) 1 g molecule, i.e., 100 g CaCO3 reacts with 2 g molecules, i.e., 73 g HCl and
produces, 1 g molecule, i.e., 111 g CaCl2, 1 g molecule, i.e., 18 g H2O and 1 g molecule, i.e.,
44 g CO2.
(iv) 22.4 litres of CO2 is formed at S.T.P.
Q. 15. What is a thermochemical equation? Give two examples.
Ans. Most chemical reactions are accompanied by either evolution or absorption of heat.
These reactions are known as exothermic and endothermic reactions respectively. Such an
equation in which information about heat change is included is called a thermochemical
equation. It is very important in such a case to indicate the physical state of the various
species involved.
Examples are:
C(s) + O2( g) → CO2( g); ∆H = −393.5 kJ
(i)
(Exothermic reaction)
C(graphite) + 2H2( g) → CH4( g); ∆H = + 74.25 kJ
(ii)
(Endothermic reaction)
Q. 16. Write and balance the following equations presented to you as written
statements:
(i) Magnesium carbonate plus hydrochloric acid produces magnesium chloride plus
water plus carbon dioxide gas.
(ii) Aluminium plus chlorine gas produces aluminium trichloride.
(iii) Nitrogen plus hydrogen produces ammonia.
(iv) Calcium plus sulphur produces calcium sulphide.
(v) Magnesium burns in CO2 to form magnesium oxide and carbon.
Ans. (i)
MgCO3 + HCl → MgCl2 + H2O + CO2
MgCO3 + 2HCl → MgCl2 + H2O + CO2
(ii)
Al + Cl2 ( g) → AlCl3
Al + 3Cl2( g) → 2AlCl3
2Al + 3Cl2( g) → 2AlCl3
(iii)
N2 + H2 → NH3
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GOLDEN SCIENCE—X
N2 + H2 → 2NH3
N2 + 3H2 → 2NH3
(iv)
(v)
Ca + S → CaS
Mg + CO2 → MgO + C
Mg + CO2 → 2MgO + C
2Mg + CO2 → 2MgO + C.
Q. 17. H2 + O → H2O is a balanced chemical equation. It is considered to
be incorrect. Why?
Ans. Oxygen does not occur as oxygen atoms (O) but as oxygen molecules (O2) and so
the above reaction should be written with oxygen molecule and then balanced as.
H2 + O2 → H2O.
2H2 + O2 → 2H2O.
Q. 18. Complete and balance the following equations:
NaOH + ................ → Na2SO4 + H2O
(i)
Ca(OH)2 + ................ → CaCO3 + H2O
(ii)
................ + HCl(aq) → MgCl2(aq) + .................
(iii)
(iv)
Ans. (i)
................ + Na2SO4(aq) → BaSO4(s) + NaCl(aq)
2NaOH + H2SO4 → Na2SO4 + 2H2O
(ii)
Ca(OH)2 + CO2 → CaCO3 + H2O
(iii)
Mg + 2HCl(aq) → MgCl2(aq) + H2( g)
(iv)
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
Q. 19. What are the drawbacks of a chemical equation? Illustrate with an
example.
Ans. Let us consider the following reaction:
2H2S + SO2 → 3S + 2H2O
This is a balanced chemical equation but it does not give the following information:
(i) The physical state of the reactants and products whether these are solids, liquids
or gaseous etc.
(ii) Concentration of reactants and products.
(iii) Condition of reaction such as temperature, pressure, catalyst, light.
(iv) The rate of reaction whether it is slow or fast.
(v) Time taken for completion.
(vi) Heat changes whether heat is evolved or absorbed.
(vii) Whether reaction is reversible or irreversible.
Q. 20. While writing the chemical reaction for the oxidation of calcium in
oxygen, a student wrote the following balanced equation:
Ca + O → CaO
Is it correct? If not, explain and write the correct balanced equation.
Ans. The given equation is not molecular equation. The oxygen should be written in the
molecular form. The correct equation is:
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CHEMICAL REACTIONS AND EQUATIONS
Ca + O2 → CaO
Ca + O2 → 2CaO
and balanced equation is
2Ca + O2 → 2CaO
Q. 21. How can a chemical reaction be made more informative?
Ans. (i) The physical condition of the various species may be indicated by sign (s), (l)
and ( g) for solid, liquid and gas respectively after the formula of a substance, e.g.,
2P(s) + 3Cl2( g) → 2PCl3(l)
(ii) The precipitate formed in the reaction is shown by an arrow pointing downward
(↓) e.g.,
BaCl2 + H2SO4 → BaSO4
↓
+ 2HCl
(iii) (↑) Sign is used for gas evolved.
Zn + 2HCl → ZnCl2 + H2
↑
(iv) If a substance dissolved in water is used or produced, sign (aq) is used.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O
(v) Heat changes occurring during a chemical reaction may also be indicated such as
2H2( g) + O2( g) → 2H2O(l ) ; ∆H = + 571.6 kJ
(Endothermic reaction)
H2( g) + Cl2( g) → 2HCl( g) ; ∆H = − 184.7 kJ
(Exothermic reaction)
Q. 22. Which of the following statement is true for a balanced chemical
equation?
(i) The number of atoms of each kind on each side should be equal.
(ii) The sum of coefficients on both sides is the same.
(iii) Atoms and ions are conserved.
(iv) Atoms and mass are conserved.
Ans.
(i) The number of atoms of each kind on each side should be the same.
(ii) Atoms and mass are conserved.
Q. 23. (i) Rewrite the following equation in a balanced form:
Fe + O2 → Fe2O3
(ii) Rewrite the following equation indicating that a solution of lime in water
has been used and that a precipitate is formed in the reaction.
Ca(OH)2 + CO2 → CaCO3 + H2O
Ans. (i)
(ii)
4Fe + 3O2 → 2Fe2O3
Ca(OH)2(aq) + CO2 → CaCO3 ↓ + H2O
Q. 24. Calcium carbonate on heating forms calcium oxide and carbon dioxide. Write
the equation for the reaction and indicate the evolution of gas.
Ans.
CaCO3 → CaO + CO2 ↑
Q. 25. What happens when silver chloride is placed in sunlight in a china dish.
Write the balanced chemical equation.
Ans. Silver chloride (white) on exposure to sunlight for sometime becomes grey. This
is due to decomposition of silver chloride into silver and chlorine by sunlight.
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GOLDEN SCIENCE—X
2AgCl(s) → 2Ag(s) + Cl2( g)
White
Grey
Q. 26. A student strongly heated green ferrous sulphate (FeSO4.7H2O) solid in
a dry test tube. He found that after sometime reddish brown residue was left and
some pungent smelling gas evolved. Explain the chemical reaction and write
balanced chemical equation.
Ans. On heating ferrous sulphate, SO2 and SO3 are produced leaving behind reddish
brown ferric oxide.
2FeSO4.7H2O(s) → Fe2O3(s) + SO2( g) + SO3( g) + 14H2O(l)
Green
Reddish brown
Q. 27. Convey the following information in the form of a balanced chemical
equation:
On adding aqueous solution of sodium hydroxide to an aqueous solution of copper
sulphate, copper hydroxide is precipitated and sodium sulphate remains in solution.
Ans.
2NaOH(aq) + CuSO4(aq) → Cu(OH)2 ↓ + Na2SO4(aq)
Q. 28. Rewrite the following equation in a balanced form and show that in it,
Al(OH)3 is an insoluble product
Al2(SO4)3 + NaOH → Al(OH)3 + Na2SO4
Al2(SO4)3 + 6NaOH → 2Al(OH)3 ↓ + 3Na2SO4
Ans.
Q. 29. How are the following information indicated in a chemical equation?
(i) Formation of a precipitate
(ii) Evolution of a gas
(iii) A solution made in water
(iv) Evolution of heat
(v) Absorption of heat.
Ans. (i) ↓ or (s)
(iii) (aq)
(ii) ↑ or ( g)
(iv) + heat (∆H = − ve)
(v) − heat (∆H = +ve).
Q. 30. Write a balanced equation with state symbols for the following reactions:
(i) Solutions of sulphuric acid and sodium hydroxide in water react to give sodium
sulphate solution and water.
(ii) When solid mercury oxide is heated, liquid mercury and oxygen gas are produced.
(iii) When aluminium metal is dipped in a solution of hydrochloric acid in water,
hydrogen gas is evolved with the formation of aluminium chloride solution.
(iv) Solutions of silver nitrate and sodium chloride react to give insoluble silver chloride
and the solution of sodium nitrate.
Ans. (i)
H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)
(ii)
2HgO(s) → 2Hg(l) + O2( g)
(iii)
2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
(iv)
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Q. 31. Write balanced chemical equations to represent the following statements:
(i) Carbon (coke) burns in air to form carbon dioxide gas.
(ii) A piece of sodium metal on putting in water forms caustic soda and hydrogen gas.
Ans. (i)
C(s) + O2 ↑ → CO2 ↑
(ii)
2Na(s) + 2H2O → 2NaOH(aq) + H2 ↑.