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CBSE-Chemistry Question Paper

Question 1
Give the formula of westron

Ans
Westron is C2 H2 Cl4 (1, 1, 2, 2 tetra chloro ethane).

Question 2
State the components of the fuel. which was used in the Apollo rocket.

Ans.
The Apollo rocket which took man to the moon, had fuel made of a mixture of methyl hydrazine out of the oxidizer dinilrogen tetra oxide.

Question 3
Which compound of radioactive isotope is injected in making a brains scan?

Ans.
Technisium (Tc O-4).

Question 4
Name the monomers of Buna -Srubber.

Ans.
1,3 – butadiene and styrene (C6 H5 CH = CH2).

Question 5
What happens when
(a) Bleaching powder is treated with small quantities of dilute sulphuric acid and with excess of dilute H2SO4.
(b) Bleaching powder contains a little cobalt chloride?

Ans.
(a) 2 Ca OCl2 + H2SO4 – Ca SO4 +Ca Cl2 + HOCl

(Bleaching powder) (Dilute) (Hypochlrous acid)

.When bleaching powder is treated with excess of dilute H2SO4, then Cl2 gas is liberated.

Ca O Cl2+ H2SO4 ——-> Ca SO4+ H2O+Cl2

(Dilute)
(Excess)

(b) In the presence of little cobalt chloride, it decomposes to liberate O2 gas.
COCl3

2 Ca OCl2 ——-2 Ca Cl2+O2

Question 6
How is olefinic bond detected in a hydrocarbon?

Ans.
Olefinic bond is double bond and it can be detected by bromine in carbon tetrachloride. A compound containing olefinic bond decolorizes bromine in CCl4.

Question 7
How would you prepare ethanol from molasses by fermentation?

Ans.
Ethyl alcohol is prepared by fermentation of molasses i.e. sugar, by the enzymes (invertase and zymase) present in the yeast plants. Fermentation is a process by which organic compounds are broken up by microorganisms into simple products with the formation of CO2. It is exothermic process carried out at a temperature of 20-30 degree C.

invertase

C12+H22+O11+H2O———–> C6H12O6+ C6H12 O6

(cane sugar sucrose in molasses)(Glucose)(Fructose)
Zymase

C6H12 O6 ———-> 2 C2 H5 OH + 2CO2

(Glucose) (Ethyl alcohol )

Question 8
Explain what happens when
(a) White phosphorus is boiled with alkali solution.
(b) White phosphorus is heated with nitric acid.
(c) Calcium dihydrogen phosphate is heated with baking soda.

Ans.
(a) When white phosphorus is boiled with alkali solution, phosphorus liberates phosphine gas.

4P + 3 Na OH + 3H2O -(Phosphine)(Sodium hypophosphite)- PH3 + 3 Na H2PO2

(b) When white phosphorus is heated with nitric acid, phosphorus gets oxidized to phosphoric acid.

4P+2OHNO3 ———-> 4H3PO4+20 NO2+ 4H2O
(nitricAcid) (Phosphoric Acid)

(c) When calcium dihydrogen phosphate is heated with baking soda, then it evolves CO2 gas.

Ca (H2 PO4) 2 + 2 NaHCO3 – 2CO2 + 2H2O+ (Ca HPO4)(5) + Na2HPO4)

Question 9
How would you remove the chlorides and sulphates of magnesium and calcium from hard water?

Ans.
(a) By adding soda ash – It converts the sulphates of Ca or Mg into insoluble carbonates :-

Ca SO4 + Na2 CO3 – Ca CO3 +Na2 SO4
(soda Ash )

(b) By Permutit Process – It is an artificial zeolite with the formula Na2 Al2 Sl2 O8 which reacts with Ca or Mg chloride and forms insoluble calcium or magnesium zeolite and soluble sodium chloride.

(c)Na2 Al2 Sl2 O8 + Mg Cl2 – Mg (Al2 Si2 O ) + Na Cl

(in hard water) (Sodium)

Question 10
Explain the effect of heat on sulphur .

Ans.
On heating in a test tube, sulphur melts to a pale yellow straw coloured liquid at 115 degree C. On further heating it becomes thicker and darker and at 250 degree C, it is almost immobile. Further heating makes it mobile again and it boils at 445 degree C releasing brown vapors, which condense. On the cooler part of the test tube in the form of fine crystals, known as flowers of sulphur.

Question 11
Explain the various steps involved in the extraction of impure copper from chloropyrites. Give all the chemical equations involved.

Ans.Chlorophriles
Steps involved in the extraction of impure copper from chloropyrites are :

(a) Crushing and Grinding :- The ore is crushed in big jaw crushes and then powdered in ball mills.

(b) Concentration of Ore :- The finely powdered ore is concentrated by the froth floatation process.

(c) Roasting :- The concentrated ore is roasted in a current of air in a reverberatory furnace. Arsenic and antimony are removed as volatile oxides. A part of the sulphur is also removed as sulphur dioxide and a mixture mainly consisting of cuprous and ferrous sulphides is left behind.

2 CuF e S2+ O2 – Cu2 S + 2 FeS + SO2

(d) Smelting – The roasted ore is mixed with smoke, coke and sand and smelled in a water jacketed blast furnace. Ferrous sulphide is oxidised to ferrous, which forms a fusible slag with sand and is removed through a slag hole.

2FeS + 3O2 – 2 FeO + 2 SO2
FeO + SiO2 – Fe SiO3

Any cuprous oxide formed by the oxidation of cuprous sulphide reacts with ferrous sulphide to form cuprous sulphide. This is because iron has more affinity for oxygen than for copper,

Cu2O+FeS – FeO +Cu2 S
Feo +SiO2 – Fe SiO3

The slag is formed in the upper layer while the lower layer called matte consists mainly of cuprous sulphide with some ferrous sulphide.

(e) Bessemerisation – The molten matter is poured into a pear – shaped converter. It is made of steel plates lined with line or magnesium oxide. It is fixed on pivots. A blast of sand and air is blown into the mattle. Iron is removed as slag. Some of the cuprous sulphide is changed into the oxide which reacts with more cuprous sulphide to give copper.

2Cu2S + 3O2 – 2 CU2 O + 2SO2
Cu2 S+2CU2 O ———–>6 Cu+ SO2

The matter copper thus obtained is 99 percent pure and is used for many purposes.