1 CHEMICAL REACTIONS AND EQUATIONS SOME BASIC CONCEPTS CHEMICAL REACTION It is process in which the bonds in the reactants break resulting in the formation of new bonds to give products. CHEMICAL EQUATION A chemical equation represents symbolically, the reactants, products, their physical states and conditions under which a reaction occurs. It is balanced in terms of mass and charge. LAW OF CONSERVATION OF MASS In a chemical reaction the total mass of the products must be equal to the total mass of the reactants. BALANCING OF CHEMICAL EQUATIONS Balancing of chemical equations is based upon law of conservation of mass. i.e., number of atoms of each element should be equal on both sides of the equation. BALANCED CHEMICAL EQUATION It is a chemical equation in which number of atoms of each element are equal on both sides of the equation. MOLECULAR EQUATION The balanced chemical equation in which all the substances taking part in the reaction are present in their molecular forms. e.g.
2Na + 2H2O
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COMBINATION REACTION It is a chemical reaction in which two or more substances combine to form a new substance. DECOMPOSITION REACTION It is a chemical reaction in which a single compound breaks down to produce two or more simpler substances. DISPLACEMENT REACTION In this reaction, a more active element displaces or removes other element from a compound in solution. DOUBLE DISPLACEMENT REACTION In this reaction, two different atoms or groups of atoms are displaced by other atoms/groups of atoms. OXIDATION It is a chemical reaction which involves the addition of oxygen or loss of hydrogen. REDUCTION It is a chemical reaction which involves the loss of oxygen or addition of hydrogen. REDOX REACTION It is a chemical reaction in which oxidation and reduction take place simultaneously. OXIDISING AGENT It is a substance which oxidises other substances and itself gets reduced. REDUCING AGENT It is a substance which reduces other substances and itself gets oxidised. NON-REDOX REACTION. It is a reaction in which neither oxidation nor reduction takes place.
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EXOTHERMIC REACTION It is a chemical reaction in which heat energy is given out. ENDOTHERMIC REACTION It is a chemical reaction in which heat energy is absorbed. PRECIPITATION REACTION A chemical reaction in which insoluble salts are produced is called precipitation reaction. NEUTRALISATION The interaction of an acid with a base to form salt and water is called neutralisation. CORROSION It is the slow eating up of metals by the action of air and moisture on their surfaces. RUSTING It is a special case of corrosion of iron. CHEMICALLY RUST Chemically rust is hydrated ferric oxide, Fe2O3, xH2O. RANCIDITY When fats and oils or food containing oils and fats are exposed to air or oxygen, they get oxidised due to which their smells and colours change. This process is called rancidity. RANCIDITY OF FATTY FOODS Rancidity of fatty foods can be prevented by flushing the containers with nitrogen gas by adding antioxidants to them.
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BASIC CONCEPTS IN QUESTION / ANSWER FORM 1.
Why should a magnesium ribbon be cleaned before burning in air?
Solution:Magnesium ribbon should be cleaned before burning to remove the protective layer of basic magnesium carbonate from its surface. 2.
Why is the amount of gas collected in one of the test tubes in double of the amount collected in the other when an electric current is passed through acidulated water? Name this gas. Solution:When electric current is passed through acidulated water, the reaction taking place is 2H 2 O (l) Electric current → 2H 2 (g) + O 2 (g)
Therefore hydrogen and oxygen produced are in the ratio 2 : 1 by volume. Hence volume of gas collected in one test tube is double the volume of gas collected in the other tube. This gas is hydrogen. 3.
Give an example of a double displacement reaction.
Solution:NaCl
4.
(aq)
+ AgNO 3(aq) →NaNO 3(aq) + AgCl (s) ↓
Why is respiration considered as an exothermic reaction? Explain.
Solution:During respiration oxidation of glucose occurs which produces heat energy. 5.
Oil and fat containing food items are flushed with nitrogen. Why?
Solution:This is because food items are prevented from oxidation by oxygen or air.
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6.
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?
Solution:KCl(al) + AgNO 3 (al) →KNO 3 (al) + AgCl(s) ↓ Double displacement reactions
7.
Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.
Solution:∆ 2FeSO 4 → Fe 2 O 3 + SO 2 ↑ + SO 3 ↑
This is a thermal decomposition reaction. 8.
Why do fire flies glow at night?
Solution:Fire, flies ,have a protein which undergoes aerial oxidation in the presence of an enzyme and during this reaction visible light is emitted, therefore, fire flies glow at, night. 9.
Why do we store silver chloride in dark coloured bottles?
Solution:This is because it is decomposed to silver by light 2AgCl →2Ag + Cl 2
10. Balance the following chemical equation. MnO 2 + HCl → MnCl 2 + Cl 2 + H 2 O
Solution:MnO 2 + 4HCl → MnCl 2 + Cl 2 + 2H 2 O.
11. Balance the following chemical equation Pb (NO 3 ) 2 (s) Heat → PbO(s) + NO 2 + O 2 (g)
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2Pb(NO 3 ) 2 (s) Heat → 2PbO(s) + 4NO 2 (g) + O 2 (g)
12. Why is respiration considered an exothermic process? Solution:This is because it involves combustion of glucose producing CO2, H2O and a large amount of energy. 13. Predict whether silver can displace copper from copper sulphate solution? Solution:Silver lies below copper in the activity series. This means that silver cannot displace copper from copper sulphate solution. 14. Identify the substance oxidised and reduced in the chemical reaction: MnO 2 + 4HCl → MnCl 2 + Cl 2 + 2H 2 O
Solution:Substance oxidised : HCl Substance reduced: MnO2 15. Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine Hydrogen chloride (ii) Barium chloride + Aluminium Sulphate Barium Sulphate + Aluminium Chloride (iii) Sodium + Water Sodium hydroxide + Hydrogen Solution:(i)
H2
(ii)
BaCl2 +
(iii) 2Na
+ Cl2
2HCl
(NH4)2SO4
+ 2H2O
BaSO4 2NaOH
+ 2NH4Cl
+ H2
16. 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 sulphate and the solution of sodium chloride. (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride and water. EXCELLENT COACHING CENTRE
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Solution:(i) BaCl 2( aq ) + Na 2 SO 4( aq ) →BaSO 4(s) ↓ + 2NaCl (aq) (ii) NaOH ( aq ) + HCl (aq) → NaCl (aq) + H 2 O ( l ) 17. 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. Solution:(i) X is calcium oxide (quick lime) and its formula is CaO. (ii) CaO (s) + H2O (l) Ca(OH)2 (aq). 18. Why does the colour of copper sulphate solution change, when an iron nail is dipped in it? Solution:This is because iron displaces copper from copper sulphate solution. Therefore, the concentration of copper sulphate solution decreases and blue colour of solution gradually fades away. Fe(s)
+ CuSO4 (aq)
(Blue)
FeSO 4(aq) +
Cu(s)
(Green)
The concentration of copper sulphate solution decreases as ferrous sulphate is produced. 19. Identify the substances that are oxidised and the substances that are reduced in the following reactions: (i) 4Na(s) + O2 (g) 2Na2O (s) (ii) CuO(s) + H2(g) Cu(s) + H2O(l). Solution:S. No. (i) (ii)
Substance oxidised Na(s) H2(g)
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20. Translate the following statements into chemical equations and then balance them: (a) Hydrogen gas combines with nitrogen to form ammonia. (b) Hydrogen Hulphide 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. Solution:(a) 3H2 (g)
+ N2 (g)
(b) 2H2S (g)
+
2NH3 (g)
3O2 (g)
(c) 3BaCl2 (aq)
+
2H2O (l)
Al2(SO4)3
+
2SO2 (g)
2 AlCl3 (aq)
+
3BaSO4 (s) ↓
21. Balance the following chemical equations: (a) HNO3 +
Ca(OH)2
(b) NaOH
+ H2SO4
(c) NaCl
+
Ca(NO3)2
Na2SO4
AgNO3
+
AgCl
+
H2O H2O
+
NaNO3
Solution:(a) 2HNO3
+
Ca(OH)2
(b) 2NaOH
+
H2SO4
(c) NaCl
+
AgNO 3
Ca(NO3)2 Na2SO4 AgCl
+
2H2O
+
+
2H2O
NaNO3
22. Write the balanced chemical equations for the following reactions: (a) Calcium hydroxide + Carbon dioxide Calcium carbonate + Water (b) Zinc + Silver nitrate Zinc nitrate + Silver. (c) Aluminium + Copper chloride Aluminium chloride + Copper. Solution:(a) Ca(OH)2 (b) Zn
+
(c) 2Al
+
+
CO2
2 AgNO3 3CuCl2
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CaCO3
+
Zn(NO3)2 2AlCl3
+
H2O +
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23. Why arc decomposition reactions called opposite of combination reactions? Write equations for these reactions. Solution:During decomposition a single substance breaks down into two or more substances which is just the reverse of combination reaction. Examples for decomposition reactions are: ∆ 2Pb(NO 3 ) 2 (s) → 2PbO (s) + 4NO 2 (g) + O 2 (g)
∆ CaCO 3 (s) → CaO (s) + CO 2 ( g )
24. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity. Solution:(i)
CaCO 3 (s) Heat → CaO (s) + CO 2 (g)
Heat energy is supplied to bring about the reaction. → 2Ag (s) + Cl 2 (g) (ii) 2AgCl (s) Sunlight
Here light energy is supplied to bring about the reaction. Spark → 2H 2 (g) + O 2 (g) (iii) 2H 2 O (l) Electric
Here electrical energy is supplied to bring about the reaction. 25. Explain the following in terms of gain or loss of oxygen with two examples each: (a) Oxidation (b) Reduction. Solution:(a) Oxidation. A chemical reaction in which a substance gains oxygen or loses hydrogen is called oxidation. Examples 2Cu + O 2 Heat → 2CuO, Cu is oxidised to CuO. 2Mg + O 2 Burn → 2MgO, Mg is oxidised to MgO.
(b) Reduction. A chemical reaction in which a substance loses oxygen or gains hydrogen is called reduction. ∆ ZnO + C → Zn + CO, ZnO is reduced to Zn.
∆ CuO + H 2 → Cu + H 2O, CuO is reduced to Cu.
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26. A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed. Solution:The element X is copper. 2Cu +
(Brown)
O
2 ( From air)
Heat →
2CuO
Copper (II) oxide (Black)
Therefore the black coloured compound formed is copper (II) oxide (CuO). 27. Why do we apply paint on iron articles? Solution:The iron articles can be protected from rusting by applying paint on them so that the iron surface does not come in contact with air (or oxygen) and moisture which cause rusting. 28. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case: (A) Ethene is burnt in the presence of oxygen to form carbon dioxide water and releases heat and light. (B) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H2SO4. (C) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water. Solution:(A) C2H4 (g) + 3O2 (g) 2CO2 (g) + 2H2O(g) + Heat + Light It is redox or combustion reaction. (B)
+
H C 2 H 5 OH (l) +CH 3COOH (l) →CH 3COOC 2 H 5 (l) + H 2 O(l)
It is a double displacement reaction or esterification reaction (C) NaOH(aq) + CH3COOH (aq) CH3COONa(aq) + H2O(l) It is a double displacement reaction or neutralisation reaction.
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29. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case: (A) Ethanol is burnt in air to form carbon dioxide water and releases heat. (B) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine. (C) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride. (D) Thermit reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide. Solution:(A) C2H5OH (l) + 3O2 (g) 2CO2 (g) + 3H2O (l) - Redox reaction or combustion reaction (B) 2KI (al) + Cl2 (g) 2KCl (al) + I2 (s) displacement .reaction (C) 3Mg(s) + N2 (g) Mg3 N2 (s) - combination reaction (D) Fe2O3(s) + 2Al (s) Al2O3(s) + 2Fe(l) + Heat Displacement search. 30. Complete the missing components/variables given as x and y in the following reactions: x → CaO(s) + CO 2 (g ) (A) CaCO 3 (s )
(B) Zn(s ) + H 2SO 4 (aq) → ZnSO 4 ( x ) + H 2 ( y ) (C) Cu(s ) + 2AgNO 3 (aq ) → Cu(NO 3 ) 2 (aq) + x(s) (D) Pb(NO 3 ) 2 (aq) + 2KI (aq) → PbI 2 ( x ) + 2KNO 3 ( y ) Solution:→ CaO(s) + CO 2 (g ) (A) CaCO 3 (s) Heat
(B) Zn(s) + H 2SO 4 (al) → ZnSO 4 (al) + H 2 (g ) (C) Cu (s) + 2AgNO3 (al) → Cu(NO 3 ) 2 (al) + 2Ag(s) (D) Pb(NO 3 ) 2 (al) + 2KI(al) → PbI 2 (s) + 2KNO 3 (al)
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31. Which among the following changes are exothermic or endothermic in nature? (A) Dissolution of ammonium chloride in water (B) Dissolution of sodium hydroxide in water (C) Dilution of sulphuric acid (D) Decomposition of ferrous sulphate Solution:(A) Endothermic reaction
(B) Exothermic reaction
(C) Exothermic reaction
(D) Endothermic reaction.
32. Identify the reducing agent in the following reactions: (A) 2H 2 + O 2 → 2H 2O
(B) Fe 2O 3 + 3CO → 2Fe + 3CO 2
(C) H 2O + F2 → HF + HOF
(D) 4NH 3 + 5O 2 → 4NO + 6H 2O
Solution:(A) H2
(B) CO
(C) H2O
(D) NH3
33. Identify the oxidising agent (oxidant) in the following reactions: (A) CuO + H 2 → Cu + H 2O (B) 3Fe + 4H 2O → Fe 3O 4 + 4H 2 (C) V2O 5 + 5Ca → 2V + 5CaO (D) CuSO 4 + Zn → Cu + ZnSO 4 (E) 2Mg + O 2 → 2MgO (F) Pb 3O 4 + 8HCl → 3PbCl 2 + Cl 2 + 4H 2O Solution:(A) CuO
(B) CuO
(C) H2O
(D) CuSO4
(E) O2
(F) Pb3O4
34. Grapes hanging on the plant do not ferment but after being plucked from the plant get fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change? Solution:EXCELLENT COACHING CENTRE
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Grapes attached to plants are living and hence their immune system prevents fermentation. But in the plucked grapes microbes grow under anaerobic conditions and they undergo fermentation, It is a chemical change. 35. Which among the following are physical or chemical changes? (A) Heating of an iron rod to red hot. (B) Burning of Liquefied Petroleum Gas (LPG) (C) Curdling of milk (D) Sublimation of solid ammonium chloride (E) Evaporation of petrol. Solution:(A) Physical change
(B) Chemical change
(C) Chemical change
(D) Physical change
(E) Physical change.
36. During the reaction of some metals with dilute hydrochloric acid, following observations were made. . (A) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid. (B) The reaction of sodium metal is found to be highly explosive. (C) The temperature of the reaction mixture rises when aluminium (Al) is added. (D) Silver metal does not show any change. Explain these observations giving suitable reactions. Solution:(A) This is because hydrogen gas is produced. Pb + 2HCl → PbCl 2 + H 2 (B) This is because sodium reacts violently with water producing hydrogen which catches fire due to heat produced during the reaction (C) This is because an exthermic reaction takes place (D) This is as Ag lies below hydrogen in activity series and doesn’t react with dil HCl. 37. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved. Solution:EXCELLENT COACHING CENTRE
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Element X is calcium (Ca) Ca
+
Ca(OH)2
2H2O
+
H2
Calcium hydroxide turns red litmus blue. 38. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why? Solution:Zinc lies above hydrogen whereas copper lies below hydrogen in the activity series of metals. Therefore zinc displaces hydrogen from dilute hydrochloric acid, while copper does not. Zn
+
HCl
ZnCl2
Cu
+
HCl
No reaction
+
H2
39. What are non-redox reactions? Explain with the help of examples. Solution:The reaction in which neither oxidation nor reduction takes place are called non-redox reactions e.g. (i) HCl (aq)
+
NaOH (aq)
NaCl (aq)
+
H2O (l)
(ii) BaCl2 (aq)
+
Na2SO4 (aq)
BaSO4 (s)
+
2NaCl (aq)
40. Give two examples from everyday life situations where redox reactions are taking place? Solution:(i) Photosynthesis of carbohydrates (ii) Combustion reactions. 41. Explain why sodium acts as a reducing agent while chlorine acts as an oxidizing agent in the reaction, 2Na
+
Cl2
2NaCl.
Solution:Sodium (Na) loses electrons and forms Na + ions while chlorine (Cl) gains electron and forms Cl- ion. Therefore, Na is oxidising agent and Cl is reducing agent.
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42. Complete and balance the following chemical equations: ………
Na2SO4
+
H2O
(ii) Ca (OH)2 + ………
CaCO3
+
H2O.
(i) NaOH
+
Solution:(i) 2NaOH
+
(ii) Ca(OH)2 +
H2SO4
Na2SO4
+
2H2O
CO2
CaCO3
+
H2O
43. What happens chemically when quick lime is added to water? Solution:Slaked line is produced and a large amount of heat energy is given out. CaO
+
H2O
Quick lime
Ca(OH)2
+
Heat energy
Slaked lime
44. Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating. Solution:Take a small amount of ferrous sulphate crystals in a dry test tube. Heat it. On heating ferrous sulphate decomposes producing reddish brown ferric oxide, SO2 (g) and SO3(g). 2FeSO 4 . 7H 2 O ( s )
Heat → Fe 2 O 3( s ) + SO 2(g) ↑ + SO 3(g) ↑ + 7H 2 O
( Green )
(Reddish brown)
45. What is an oxidation reaction? Identify in the following reaction: (i) the substance oxidised and (ii) the substance reduced: ZnO + C Zn + CO Solution:Oxidation reaction. It is a reaction in which gain of oxygen or removal of hydrogen takes place. (i) C
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46. Write the balanced chemical equation for the following chemical reactions: (i) Aqueous solution of sulphuric acid and sodium hydroxide react to form aqueous sodium sulphate and water. (ii) Phosphorus burns in chlorine gas to form phosphorus pentachloride. Solution:(i)
H2SO4 (aq)
(ii) P4
+
+
2NaOH (aq)
10Cl2(g)
Na2SO4(aq)
+
2H2O (l)
4 PCl5
47. What is the difference between displacement and double displacement reactions? Write equations for these reactions. Solution:In a displacement reaction, a more active element displaces another element from a compound. These reactions mostly occur in solution form. e.g. Zn(s) + CuSO4(aq) ZnSO4 (aq) + Cu(s) In a double displacement reaction, two different atoms or groups of atoms are displaced by other atoms/group of atoms. These reactions usually occur in ionic compounds. e.g. BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl (aq) 48. Write a chemical equation for each of the following reactions: (i) Zinc metal reacts with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas. (ii) When solid mercury (II) oxide is heated, liquid mercury and oxygen gas are produced. Solution:(i) Zn(s)
+
2HCl(aq)
ZnCl2 (aq)
+
H2(g)
∆ → 2Hg(l) + O 2 (g) (ii) 2HgO (s)
49. Give four points of similarities between rusting and burning. Solution:(i) Both require oxygen. (ii) Both produce oxides. (iii) Both produce heat energy, (iv) Both involve a chemical change. EXCELLENT COACHING CENTRE
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50. Give important points of distinction between rusting and burning. Solution:Rusting 1. It is a slow process
Burning 1. It is a fast process
2. It occurs only in case of iron.
2. It occurs in a variety of substances.
3. Heat is evolved at every very slow rate.
3. Heat is evolved rapidly.
4. Air (or O2) and moisture are required.
4. Only oxygen is required.
5. There is a slow rise in temperature.
5. There is a sudden rise in temperature.
51. Mention five ways by which rusting can be prevented. Solution:(1) By painting metal surfaces with enamel paints. (2) By applying oils and greases on surfaces. (3) By plastic coating on the surfaces. (4) By galvanisation. (5) By forming alloys. 52. Mention two ways by which rancidity of fatty foods can be prevented. Solution:(a) By adding antioxidants to the fatty foods. (b) By flushing the food containers with nitrogen to prevent food from oxidation. 53. Can we keep an aqueous solution of ferrous sulphate in a vessel made of zinc? Solution:Zinc lies above iron in the activity series. Therefore, it is more reactive than iron. This means zinc will displace iron from ferrous sulphate solution. We can also say that a displacement reaction take place between zinc and ferrous sulphate. Zn + Zinc
FeSO
4 Ferrous sulphate
→ ZnSO 4
Zinc sulphate
+ Fe Iron
Because of chemical reaction, the zinc vessel will be slowly eaten up and FeSO 4 solution is destroyed. Thus, a ferrous sulphate solution cannot be stored in zinc vessel.
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54. Write the following equations in the balanced form: (i)
Fe
+
(ii) Al(OH)3 (iii) NH3
Cl2
+
FeCl3
Al2Oa
CuO
+ Cu
H2O +
N2
+
H2O.
Solution:(i)
2Fe
+
(ii) 2Al(OH)3 (iii) 2NH3
+
3Cl2
Al2O3
3CuO
2FeCl3 +
3H2O
3Cu
+
N2
+
3H2O
55. Oxidation can never occur without reduction and vice-versa. Comment. Solution:In any reaction, the electrons lost during the oxidation half-reaction must be gained during the reduction half-reaction. Therefore, oxidation and reduction always take place simultaneously i.e. oxidation can never occur without reduction and vice-versa. 56. From the figure given below, indicate the type of reaction and chemical equation for it. Also indicate ratio of volumes of hydrogen and oxygen collected. Solution:The figure indicates electrolytic decomposition reaction. is 2H 2 (l) Electrolys → 2H 2 (g) + O 2 (g) 2 Vol.
1 Vol.
Hydrogen and oxygen produced are in the ratio 2 : 1 by volume. 57. For the figure given ahead: (a) Indicate the type of reaction (b) Write down the chemical equation for the reaction. Solution:(a) Thermal decomposition reaction. ∆ → 2PbO + 4NO 2 ↑ + O 2 ↑ (b) 2Pb(NO 3 ) 2
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58. What happens when zinc granules are treated with dilute solution of H 2SO4, HCl, HNO3, NaCl and NaOH, also write the chemical equations if reaction occurs. Solution:The reaction Zn granules with (a) Dilute H2SO4 Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g) (b) Dilute HCl Zn(s) + 2HCl (aq) ZnCl2 (aq) + H2(g) (c) Dilute HNO3 Reaction with dilute HNO3 is different as compared to other acids because nitric acid is also an oxidising agent and it oxidises H2 gas evolved to H2O. 4 Zn(s) + 10HNO3(aq) 4Zn(NO3)2 (aq) + 5H2O(l) + N2O(g) (d) NaCl solution Zn (s) + NaCl (aq) No reaction (Zn lies between Na in the activity series) (e) NaOH solution Zn(s) + 2NaOH (aq) Na2ZnO2(aq) + H2(g) Sodium zincate 59. On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained. (A) Write a balanced chemical equation of the reaction involved (B) What other name can be given to this precipitation reaction? (C) On adding dilute hydrochloric acid to the reaction mixture, white preipitate disappears. Why? Solution:2 SO 3 (aq ) + (A) Na Sodium Sulphite
BaCl
2 Barium chloride
(aq) → BaSO 3 (s) + 2NaCl(aq) Barium sulphite
Sodium chloride
(B) This reaction is also known as double displacement reaction (C) BaSO3 is a salt of a weak acid (H 2SO3), therefore dilute acid such as HCl decomposes barium sulphite to produce sulphur dioxide gas smell of burning sulphur. BaSO3(s) +
2HCl (aq) BaCl2 + H2O + SO2(g)
White ppt. EXCELLENT COACHING CENTRE
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CLASS - X 1. 26
CHEMICAL REACTION
BaCl2 is soluble in water, hence white precipitate disappears 60. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl. Dilute HNO3, ZnCl2 and H2O. In which of the above containers these solutions can be kept? Solution:(A) When solutions are kept1n copper container is (a) Dilute HCl Copper does not react with dilute HCl. Therefore, it can be kept. (b) Dilute HNO3 Nitric acid acts as a strong oxidising agent and reacts with copper vessel, therefore it cannot be kept. (c) ZnCl2 Zinc is more reactive than copper (Cu) therefore, no displacement reaction occurs and hence it can be kept. (d) H2O Copper does not react with water. Therefore, it can be kept. (B) When solutions are kept in aluminum container (a) Dilute HCl Aluminum reacts with dilute HCI to form its salt and hydrogen is evolved. Therefore, it cannot be kept. 2Al + CHCl ďƒ 2AlCl3 + 3H2 (b) Dilute HNO3 Aluminum gets oxidised by dilute HNO3 to form a this protecture layer of Al2O3 and it can be kept. (c) ZnCl2 Aluminium being more reactive than zinc can displace zinc ion from the solution. Therefore, the solution cannot be kept 2Al + 3ZnCl2
ďƒ 2AlCl3 + 3Zn
(d) H2O Aluminium does not react with cold or hot water. Therefore, water can be kept. However, aluminium is attacked by steam to form aluminium oxide and hydrogen EXCELLENT COACHING CENTRE
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CLASS - X 1. 26
CHEMICAL REACTION
2Al(s) + 3H2O (g) Al2O3 (s) + 3H2 (g). 61. Indicate the oxidising and reducing agent in the reactions: (a) H2S
+
2HCl
Cl2
+
S
2HBr
(b) H2
+
Br2
(c) Zn
+
CuSO4
ZnSO4
(d) 2Na
+
Cl2
2NaCl
(e) 2Mg
+
O2
2MgO
+
Cu
Solution:Reaction (a) (b) (c) (d) (e)
Oxidising agent Cl2 Br2 CuSO4 Cl2 O2
Reducing agent H2S H2 Zn Na Mg
62. You are given the following materials: (i)
Iron nails
(ii) Copper sulphate solution (iii) Barium chloride solution (iv) Copper powder (v) Ferrous sulphate crystals (vi) Quick Lime Identify the type of chemical reaction taking place when (a) Barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed. (b) On heating copper powder in air in a China dish, the surface of copper powder turns black. (c) On heating green coloured ferrous sulphate crystals, reddish brown solid is left and smell of a gas having odour of burning sulphur is experienced. (d) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away. (e) Quick lime reacts vigorously with water releasing a large amount of heat. Solution:EXCELLENT COACHING CENTRE
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CLASS - X 1. 26
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(a) Precipitation reaction.
(b) Combination reaction.
(c) Decomposition reaction.
(d) Simple displacement reaction
(e) Combination (and exothermic) reaction. 63. From the figures given below, explain why: (a) Iron nails become brownish in colour after some time (b) Blue colour of copper sulphate solution gradually fades away (c) Type of the reaction (d) Balanced chemical equation for the reaction. Solution:(a) Iron displaces copper from copper sulphate solution and this copper gets deposited over iron nails giving a brownish colour. (b) Blue colour of the copper sulphate solution gradually fades away because concentration of copper ions (Cu2+) in solution goes on decreasing as copper ions (Cu2+) are oxidised to copper metal by iron. (c) Displacement reaction. (d) Fe(s) + CuSO4(aq) ďƒ FeSO4(aq) + Cu(s)
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ASSIGNMENT Very short answer type questions 1.
Write the balanced equation for the following reactions. (a) hydrogen (g) and chlorine (g) react to form hydrogen chloride (g) (b) barium chloride + aluminium sulphate barium sulphate + aluminium chloride (c) potassium + water potassium hydroxide + hydrogen (d) sodium hydroxide solution in water reacts with hydrochloric acid solution in water to produce sodium chloride solution and water (e) zinc carbonate(s) zinc oxide(s) + carbon dioxide(g) (f) potassium bromide(aq) + barium iodide(aq) potassium iodide(eq) + barium bromide(aq) (g) zinc metal reacts with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas.
2.
Balance the following equations. (a) H 2 SO 4 (aq) + NaOH(aq) → Na 2 SO 4 (aq) + H 2 O(l) (b) Al(s) + HCl(aq) → AlCl 3 (aq) + H 2 (g ) (c) CH 4 (g ) + O 2 (g ) → CO 2 (g ) + H 2 O(g ) (d) KCN(ag ) + H 2 SO 4 (aq ) → K 2 SO 4 (aq ) + HCN(g) (e) Pb(NO 3 ) 2 (aq ) + Fe 2 (SO 4 ) 3 (aq ) → Fe(NO 3 ) 3 (aq ) + PbSO 4 (s) (f) MnO 2 (s) + Al(s) → Mn(s) + Al 2 O 3 (s) (g) Fe 2 O 3 (s) + CO(g) → Fe(s) + CO 2 (g ) (h) Zn (s) + AgNO 3 (aq ) → Zn(NO 3 ) 2 (aq) + Ag(s) (i) KNO 3 (s) → KNO 2 (s) + O 2 (g ) (j) C 6 H 12 O 6 (aq) + O 2 (aq) → CO 2 (g ) + H 2 O(l) + energy
3.
Classify the following reactions as combination, decomposition, displacement and double displacement reactions: → 2Ag(s) + Cl 2 (g ) (a) 2AgCl(s) sunlight → Fe 2 O 3 (s) + SO 2 (g ) + SO 3 (g ) (b) 2FeSO 4 (s) heat
(c) CaO(s) + H 2 O(l) → Ca(OH) 2 (aq ) (d) Pb(s) + CuCl 2 (aq) → PbCl 2 (aq) + Cu(s) EXCELLENT COACHING CENTRE
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(e) H 2 (g ) + O 2 (g ) → 2H 2 O(l) (f) Zn (s) + dil H 2SO 4 (aq ) → ZnSO 4 (aq ) + H 2 (g ) (g) Cl 2 (g ) + 2KBr(aq) → 2KCl(aq) + Br2 (g ) (h) AlCl3 (aq) + 3NH 4 OH(aq) → Al(OH)3 ↓ + 3NH 4 Cl(aq) → 2PbO(s) + 4NO 2 (g ) + O 2 (g ) (i) 2Pb( NO 3 ) 2 (s) heat
4.
Why don't aluminium sheets corrode easily?
5.
Can a displacement reaction be a redox reaction?
6.
What will happen if silver nitrate is placed in an iron pot?
7.
Why are gold, silver and platinum called noble metals?
8.
Give an example of a precipitation reaction.
9.
An iron rod dipped in copper sulphate develops a brown coating on it and the colour of the solution starts fading. Explain this observation scientifically.
10.
Give one example of thermal decomposition reaction.
11.
Give one example of photolytic decomposition reaction.
12.
Give one example of electrolytic decomposition reaction.
13.
Give one example of a reaction involving combination of an element with a compound.
14.
Give an example of a reaction involving combination of an element with another element.
15.
Given an example of a reaction involving combination of a compound with another compound. .
16.
What type of reaction are the following: (a) digestion of food (b) respiration (c) rusting (d) electric current is passed through sodium chloride (e) slaked lime mixes with carbon dioxide to form limestone (f) limestone is heated (g) sulphuric acid is added to barium chloride solution
17.
Why should an equation be balanced?
18.
How do we come to know that butter has turned rancid?
19.
Write the formula of rust.
20.
What will happen if a silver coin is exposed to air?
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21.
Why does a copper vessel develop a greenish coating on exposure to moist air?
22.
Name the substance oxidised and reduced in the following reactions: (a) 2PbO(s) + C(s) → 2Pb(s) + CO 2 (g ) (b) Pe 2 O 3 + 2Al → Al2 O 3 + 2Fe (c) MnO 2 + 4HCl → MnCl 2 + 2H 2 O + Cl 2 (d) Fe 2 O 3 + 3CO → 2Fe + 3CO 2 (e) ZnO + C → Zn + CO (f) CuO + H 2 → Cu + H 2 O (g) 4 Na + O 2 → 2Na 2 O
23.
What do the following symbols represent in a chemical equation (a) Δ
(b) - heat
(c) + heat
(d) (aq)
(e) (l)
(f) (s)
(g) (g)
Short answer type questions 1.
Differentiate between displacement and double displacement reactions.
2.
Differentiate between oxidation, reduction, oxidising agent and reducing agent in a redox reaction.
3.
Can a displacement reaction also be a redox reaction? Give an example and justify your answer.
4.
How is thermal decomposition different from electrolytic decomposition? Explain giving an example for each.
5.
What type of reactions form the basis for black and white photography? Explain this type of reaction with an example.
6.
A student performed an experiment using iron nails and copper sulphate solution as shown in the diagram. (a) What happens to the colour of iron nails? (b) What happens to the blue colour of copper sulphate? (c) What type of reaction takes place? (d) Write the equation for the chemical reaction that takes place.
7.
(a) Write the formula of ferrous sulphate crystals
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1. 26
(b) When ferrous sulphate crystals are heated, the colour changes from green to red and a strong smell is felt. Name the products formed in the reaction which bring about this change. (c) Write the equation for the thermal decomposition of ferrous sulphate. (d) What is the correct way of heating the boiling tube containing the crystals of ferrous sulphate? 8.
(a) Describe how electrolysis of water is performed. (b) Why is dilute sulphuric acid added to water before electric current is passed through it? (c) How can you find out which gas is collected in which test tube?
9.
(a) What would happen if a silver spoon is placed in a concentrated solution of copper sulphate? (b) What would happen if a copper spoon is placed in a concentrated silver nitrate solution? (c) Write equations of the reaction(s) that take(s) place.
10.
Why is respiration considered an endothermic reaction?
11.
(a) Write an equation to represent photosynthesis. (b) What type of a chemical reaction is it? Why do you say so?
12.
Why decomposition reactions called opposite of combination reaction? Give equation for these reactions and explain.
13.
Give two examples of double displacement reactions and explain them.
14.
A shiny brown coloured element X on heating in air becomes black in colour. (a) Name the element and the black substance formed (b) Write an equation to show the reaction that is taking place.
15.
(a) When do we say that a metal has been corroded? (b) What will be the change in colour in silver metal when it gets corroded? (c) Write an equation to show the reaction between silver and hydrogen sulphide gas present in air which corrodes it.
16.
What is meant by rancidity? Suggest three ways of preventing rancidity of food.
17.
What is meant by rusting of iron? What are the essential conditions for rusting to take place? Suggest two ways of preventing rusting of iron.
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