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1.	The compound with the highest boiling point is:A. (a) n-PentaneB. (b) n-HexaneC. (c) 2-Methyl butaneD. (d) 2,2-Dimethyl propane
Answer» Correct Answer - B With the increase in C atoms and decrease in branching (or straight chain), the boiling point of alka-nes increase. So, n-bexane has the highest boiling point.

2.	The highest boiling point is expected for:A. (a) IsooctaneB. (b) n-OctaneC. (c) 2,2,3,3-Tetramethyl butaneD. (d) n-Butane
Answer» Correct Answer - B All have the same number of C atoms. n-Ocatane is a straight-chain compound that has a large surface area. So, there are more van der Waals forces of attraction resulting in a boiling point.

3.	Classify cycloalkanes by size and ring strain.
Answer» i. Small rings `(C_(3) - C_(4))` have large strain. ii. Common rings `(C_(5) - C_(6))` have little or no strain. iii. Medium rings `(C_(7) - C_(12))` have little strain. iv. Large rings `(gt C_(12))` are strain free.

4.	Give the decreasing order of the octane rating of the following: i.
Answer» The tendency of knocking in the decreasing order is: stright-chain alkane gt branched-chain alkane gt alkenes gt cyclo alkane gt aromatic hydrocarbons. The ocatane rating is reversed, therefore, the decreasing order of ocatane rating is (vi) gt (v) gt (iv) gt (iii) gt (ii) gt (i)

5.	Fifteen milliliters of gaseous hydrocarbo (A) was required for complete combustion 357 ml of air (21 % oxygen by volume) and gaseous products occupied 327 ml (all volumes being measured at STP). The molecular formula of the hydrocarbon (A) is:A. `C_(2)H_(6)`B. `C_(2)H_(4)`C. `C_(3)H_(6)`D. `C_(3)H_(8)`
Answer» Correct Answer - D Let the formula of hydrocarbon A is CxHy. `C_(x)H_(y)(g) +(x+(y)/(4))O_(2) (g) rarr xCO_(2)(g) + (y)/(2)H_(2)O(I)` 1 ml `(x + (y)/(4))ml` x ml ---- 15 ml `(x + (y)/(4))ml` 15x ml ---- Volulme of `O_(2) =(357xx21)/(100) =75 ml` , volume of `N_(2)` `=357 - 75 =282 ml` Volume of `CO_(2) =327 - 282 =45 ml` `:. 15x=45, x=3` `:. ({:(15(x+y/4)=75),(x+y/4=5):})` `3 + (y)/(4) =5` , On solving, we get y = 8. Formula of (A) `= C_(3)H_(8)`

6.	Twenty millilitres of a gaseous hydrocarbon (A) was exploded with excess of oxygen in eudiometer tube. On cooling, the volume was reduced by 50ml. On further treatement with KOH solution, there was a further contraction of 40 ml. The molecular formula of the hydrocarbon (A) is:A. `C_(2)H_(6)`B. `C_(2)H_(4)`C. `C_(3)H_(6)`D. `C_(3)H_(8)`
Answer» Correct Answer - A `{:(C_(x)H_(y)(g),+ (x + (y)/(4)) O_(2) rarr x, CO_(2)(g) + (y)/(2)H_(2)O(I)),(1ml,(x+(y)/(4))ml,xml-),(20ml,20(x+(y)/(4))ml,20x" ml Contraction"(50 ml)):}` `CO_(2)` is absorbed in KOH. `:. 20x = 40 implies x = 2` `20 ml + 20 (2 + (y)/(4)) =40 + 50` `20 + 40 + (20 y)/(4) = 90` `5y = 90 - 60 = 30` `y =6` Formula of the compound `= C_(2)H_(6)`

7.	Write the IUPAC name and condensed formula of the following compounds whose bond line structures are given as folllows: (a)
Answer» (a) IUPAC name: 3,3-Diethyl pentane Condensed formula: `C(C_(2)H_(5))_(4)` or `Cet_(4)` (b) IUPAC name: 4,4-Dipropyl heptane Condensed formula: `C(C_(3)H_(7))_(4)` or `CPr_(4)` (c) IUPAC name: 3-Ethyl-4-methyl hexane Condensed formula: `(C_(2)H_(5))_(2)CHCH(CH_(3))C_(2)H_(5)` or `Et_(2)CHCH(Me)Et`

8.	Explain the difference in the melting point, boiling point, and densities of cycloalkane with those of the corresponding normal alkanes.
Answer» a. Cycloalknes have higher melting point than normal alkanes with the same number of C atoms because they have more compact shapes than n-alkanes and are packed more closely in the crystal lattice or solid state. They behave like compact branched-chain alkanes. b. Cycloalknes have lower boiling point than normal alkanes with the same number of C atoms because they are more compact or have lesser surface area and, thus, have weaker intermolecular van der Waals forces of attraction. c. Densities of cycloalknes are more than normal alknes with the same number of C atoms because they can be closely packed in teh liquid state also.

9.	What is the effect of branching on melting and boiling points of alkanes ?
Answer» For the same number of C atoms, increased branching leads to a more compact molecule that can pack more clocely into a crtstal lattice. The intermolecular van der Waals forces of attraction are stronger and have higher melting poins. But the point decreases with the increase in branching for the same number of C atom due to more compact molecules with less surface area and, thus, having weaker intermolecular van dar Waals forces of attraction.

10.	Which of the following statements is wrong ?A. (a) The decreasing order of the numerical value of heat of combustion is: B. (b) Cycloalkanes are planar.C. (c) Cyclopropane has higher heat of combustion per methylene `(--CH_(2)--)` group than that of cyclobutane.D. (d) With the exception of cyclopropane, cycloalkanes are non-planar.
Answer» Correct Answer - B i. More the number of C atoms, more is the heat of combustion. So, the correct order is (a). ii. Cyclopropane has the highest heat of combustion per `(CH_(2))` group. iii. Except cyclopropane, cycloalkanes are non-planar. So, the answer is (b).

11.	Reactivity of hydrogen atoms attached to different carbon atoms in alkane has the order:A. (a) `Tertiary gt Primary gt Secondary`B. (b) `Tertiary gt Secondary gt Primary`C. (c) `Secondary gt Tertiary gt Primary`D. (d) Both (a) and (c).
Answer» Correct Answer - B `3^(@) gt 2^(@) gt 1^(@)`

12.	The decreasing order of melting points of the following compounds is: (I) A. (a) `(I) gt (II) gt (III) gt (IV)`B. (b) `(IV) gt (III) gt (II) gt (I)`C. (c) `(II) gt (I) gt (IV) gt (III)`D. (d) `(III) gt (IV) gt (I) gt (II)`
Answer» Correct Answer - A Cycloalkanes, due to their compact nature, have high boiling and melting points than the corresponding alkanes. (III) gt (IV) gt (I) gt (II)

13.	Why dose a fuel with high ocatane number has less tendency to knock, whereas fuel with high centane number has more tendency to knock in an automobile engine ?
Answer» Octane number is the percentage of isooctane in the mixture of isooctane and n-heptane. Octane number is for gasoline (petrol) fuel. A gasoline with octane number of 80 means `80%` of isooctane and `20%` of n-heptane, i.e., branched-chain alkane is more, so kncking is less. Branched-chain hydrocarbons burn more smoothyl to form more stable, less reactive `3^(@)` radicals, wheres straigth-chain hydrocarbons (e.g., n-heptane) form less stable, more reactive `1^(@)` and `2^(@)` radicals. In contrast, centane number is used for diesel fuel and is defined as the percentage of cetane in a mixture of cetane number 80 means `80%` of cetane `(C_(16)H_(34)` (a straight-chain hydrocarbon) and `20%` of `alpha`-methyl napthalene (aromatic compound). So, ity will not not burn smoothly and produce high knocking. But straight-chain hydrocarbons, e.g., cetane, ignite spontaneously in comparison to aromatic compounds. So, centane number determines the ignition of fuel. Higher cetane number means fuel will ignite faster but will prouduce higher knocking. Lower centane number means fuel will ignite slowly and will produce lesser knocking (rattling sound).

14.	Out of 2-methylhexane and 2,2-dimethyl butane, which one has higher melting point and which one has higher boiling point ?
Answer» As we known that more the branching, higher is the melting point and lower is the boiling point. So, melting point of 2,2-dimethyl butane (more branching) is higher than that of 2-methyl hexane (less branching), but boiling point of 2-mrthyl hexane is higher than that of 2,2-dimethyl butane.

15.	Arrange the following compounds in the increasing order of homolytic (C--C) bond dissociation energy. I. Propane II. Ethane III. 2,2-Dimethyl propane IV. 2-Methyl propaneA. (a) `(III) lt (IV) lt (II) lt (I)`B. (b) `(II) lt (I) lt (IV) lt (III)`C. (c) `(III) lt (IV) lt (I) lt (II)`D. (d) `(I) lt (III) lt (II) lt (IV)`
Answer» Correct Answer - C

16.	Marsh gas mainly contains:A. (a) COB. (b) `H_(2)S`C. (c) `C_(2)H_(2)`D. (d) `CH_(4)`
Answer» Correct Answer - D By the decay of plants or animals present in wet or spongy land (called swamps or marsh) and by the action of bacteria on them, methane gas is produced. Because of this method of formation, methane gas is also called marsh gas.

17.	Give the decreasing order of the stability of the following or increasing order of heat of combustion. (I) A. (a) `(I) gt (II) gt (III)`B. (b) `(III) gt (II) gt (I)`C. (c) `(II) gt (III) gt (I)`D. (d) `(I) gt (III) gt (II)`
Answer» Correct Answer - B Lower is the heat of combustion, mmore stable is the compound. Lesser is the angle strain, more stable is the compound. So, the answer is (a) (I) gt (II) gt (III).

18.	i. Compound (A) ii. Compound (B) iii. Alkene (C) and alkane (D) Compound (A) is:A. B. C. D.
Answer» Correct Answer - D Isomeric hexyl chlorides obtaines obtained from (B) are (VI) and (VII).