Model Test – Solved ഉത്തരങ്ങൾ

Alkynes (ആൽക്കൈനുകൾ) have two hydrogen atoms fewer than twice the number of carbons: ethyne C₂H₂, propyne C₃H₄, butyne C₄H₆. (C_nH_2n+2 is for alkanes, C_nH_2n for alkenes.)

Carbon shows catenation more than any other element. With its valency of 4 and its single, double and triple bonds, this is why there are so many carbon compounds.

Both have the word root eth (2 carbons). C₂H₆ has only a single bond – suffix -ane; C₂H₂ (CH≡CH) has a triple bond – suffix -yne: ethyne.

C₂H₆, C₄H₁₀, C₇H₁₆ all fit C_nH_2n+2 (alkanes). In C₅H₁₀, H = 10 = 2 × 5, so it fits C_nH_2n – an alkene (pentene).

Cyclobutane (C₄H₈) is a ring like an alkane – alicyclic. Benzene and naphthalene are aromatic; butene is an open chain.

Bitumen (above C₇₀) collects at the very bottom of the fractionating tower (ഫ്രാക്ഷണേറ്റിങ് ടവർ) and is used for road tarring (Table 8.9).

Naphthalene, C₁₀H₈, has two benzene rings joined side by side. It forms white crystals with a strong smell. It is toxic – keep moth balls away from small children.

Sunlight warms the ground; the warm ground radiates infrared rays. Carbon dioxide, methane, nitrous oxide and other greenhouse gases absorb part of them and keep the Earth warm – the greenhouse effect.

For each formula count the carbons (n) and compare the hydrogens with 2n + 2, 2n and 2n − 2:

Alkanes (ആൽക്കെയ്നുകൾ) C_nH_2n+2: C₈H₁₈, C₁₀H₂₂. Alkenes (ആൽക്കീനുകൾ) C_nH_2n: C₃H₆, C₇H₁₄. Alkynes (ആൽക്കൈനുകൾ) C_nH_2n−2: C₆H₁₀, C₄H₆.

(a) Write each carbon with its hydrogens and keep the bonds between carbons: CH≡C–CH₂–CH₂–CH₂–CH₃.

(b) 6 carbons; hydrogens 1 + 0 + 2 + 2 + 2 + 3 = 10: C₆H₁₀.

(c) 6 carbons → word root hex; a triple bond → suffix -yne: hexyne (go further: hex-1-yne).

(d) It has a triple bond between two carbons – an alkyne; check: 2 × 6 − 2 = 10 ✔.

(a) C₇H₁₆ − C₆H₁₄: carbon 7 − 6 = 1, hydrogen 16 − 14 = 2 → –CH₂–.

(b) Add –CH₂– once more: C₈H₁₈, 8 carbons, an alkane → octane.

(c) Features of a homologous series (ഹോമലോഗസ് ശ്രേണി): all members have one general formula; neighbours differ by –CH₂–; they have similar chemical properties; their physical properties change gradually.

(a) Count the bonds around each carbon. The first carbon has 3 H + 1 = 4 ✔ and the middle carbon has 1 H + 1 + 2 = 4 ✔, but the third carbon has 3 H + a double bond = 5 bonds ✘. Carbon has valency 4, so it can form only four bonds.

(b) Remove one H from that carbon: CH₂=CH–CH₃. Now every carbon has four bonds, and the formula is C₃H₆ (6 H, as the question says). Its name is propene.

(a) Unsaturated hydrocarbons (അപൂരിത ഹൈഡ്രോകാർബണുകൾ) have at least one double or triple bond between carbon atoms; their carbons are not filled with the most hydrogen atoms they could hold. In alkanes every bond between carbons is single – saturated (പൂരിത ഹൈഡ്രോകാർബണുകൾ).

(b) An alkene must have a C=C double bond, and a bond between carbons needs at least two carbon atoms. One carbon can only be CH₄ (an alkane). So the first alkene is ethene, CH₂=CH₂, and the first alkyne is ethyne, CH≡CH.

(a) (i) 7 carbons, all single bonds → heptane (C₇H₁₆). (ii) 8 carbons with a double bond → octene (C₈H₁₆). (iii) 3 carbons with a triple bond → propyne (C₃H₄).

(b) Count the carbons: 8 → word root (പദമൂലം) oct. The bond between carbons includes a double bond → suffix (പ്രത്യയം) -ene. oct + ene = octene (go further: oct-1-ene).

(c) Decane: dec = 10 carbons, -ane → alkane, H = 2 × 10 + 2 = 22 → C₁₀H₂₂. Nonyne: non = 9 carbons, -yne → alkyne, H = 2 × 9 − 2 = 16 → C₉H₁₆.

(a) Count C and H: P – 5 C, 8 H: C₅H₈; Q – 6 C, 6 H: C₆H₆; R – 6 C, 12 H: C₆H₁₂.

(b) A ring compound takes the prefix (മുൻപ്രത്യയം) cyclo before the IUPAC name of the open-chain hydrocarbon with the same carbons and bonds: P – 5 C with one double bond → cyclopentene; R – 6 C, single bonds only → cyclohexane.

(c) Q is benzene (ബെൻസീൻ): an aromatic hydrocarbon (അരോമാറ്റിക് ഹൈഡ്രോകാർബണുകൾ) with three alternating double bonds (unsaturated), one H on each carbon, a characteristic smell. R is cyclohexane: alicyclic, only single bonds (saturated), two H on each carbon.

(a) Fractional distillation (അംശികസ്വേദനം). Crude oil (ക്രൂഡ് ഓയിൽ) is heated and its vapour rises in the fractionating tower. The tower is hottest at the bottom (about 350 °C) and coolest at the top (about 50 °C). Small molecules (fewest carbons) boil at low temperatures, so they stay as gases and rise to the top; heavier fractions condense lower down.

(b) (i) gases C₁–C₄: domestic and industrial fuel; (ii) petrol: C₅–C₁₀; (iii) kerosene C₁₀–C₁₆; (iv) paraffin wax: making candles, boot polish, wax paper and tarpaulin sheets.

Key box: the number of carbon atoms and the structure of the carbon chain decide the properties of a fuel.

(a) Match the percentages of carbon: W 83 % → bituminous coal; X 57 % → peat; Y 94 % → anthracite; Z 67 % → lignite. Anthracite (ആന്ത്രസൈറ്റ്) has the most carbon and peat (പീറ്റ്) the least.

(b) Carbonisation (കാർബണൈസേഷൻ): the process in which plant remains buried in the soil turn into carbon at high temperature and pressure in the absence of air. Coal is formed this way – it is a fossil fuel.

(c) Aromatic compounds from coal: benzene, naphthalene, phenol (any two). Other uses: household and industrial fuel; making coke, coal tar and coal gas; synthetic petrol; some fertilisers, medicines and perfumes (any one).

(a) Butane: CH₃–CH₂–CH₂–CH₃, C₄H₁₀. It is an alkane, C_nH_2n+2: n = 4 → H = 2 × 4 + 2 = 10 ✔.

(b) Methane, CH₄: the carbon atom in the middle with four H atoms around it, each joined by a single bond – 4 bonds (valency of carbon = 4).

(c) CNG – compressed natural gas (സി.എൻ.ജി): natural gas (mainly methane) filled at high pressure. LNG – liquefied natural gas (എൽ.എൻ.ജി): natural gas changed into a liquid at very high pressure, so a large amount can be carried by ship.

(d) Natural gas (പ്രകൃതിവാതകം) is mainly methane, a small molecule with only one carbon atom. It burns more completely, giving less smoke and less carbon pollution than petrol and diesel – so it is eco-friendly.

(e) Methane CH₄ → ethane C₂H₆ → propane C₃H₈ → butane C₄H₁₀. Difference: C 4 − 1 = 3, H 10 − 4 = 6 → C₃H₆, that is three –CH₂– groups.

(a) Sunlight warms the Earth’s surface. The warm surface gives out heat as infrared rays (ഇൻഫ്രാറെഡ് രശ്മികൾ). Greenhouse gases (ഹരിതഗൃഹവാതകങ്ങൾ) such as CO₂, CH₄ and N₂O absorb part of these rays and send heat back, so the atmosphere stays warm – the greenhouse effect (ഹരിതഗൃഹപ്രഭാവം).

(b) Largest share: carbon dioxide, 55 %. Fastest increase: chlorofluorocarbons, 4 % every year (methane, 0.9 %, is next).

(c) No. Without the greenhouse effect the Earth would be too cold for life. The problem is the extra greenhouse gases from burning fossil fuels, cutting forests and CFCs: they trap more heat and raise the average temperature – global warming (ആഗോളതാപനം).

(d) Any two: melting of glaciers and polar ice; rise in sea level (danger to Kerala’s coast); droughts and heat waves; loss of crops; spread of diseases; bleaching of coral reefs.

(e) Any two: plant and protect trees (they take in CO₂); save electricity and fuel; walk, cycle or use buses and CNG vehicles; use solar energy; avoid burning plastic and waste; use CFC-free appliances.