All Free Chemistry MCQs with Answers

221. Cracking of long chain alkanes in the petroleum industry is carried out mainly to

• A. produce shorter, more useful alkanes and alkenes for fuel and plastics
• B. remove sulphur from crude oil
• C. convert alkenes into alkanes
• D. increase the molar mass of the products

Explanation: Fractional distillation yields more of the heavy fractions than the market wants and too little petrol, so cracking breaks the long chains into shorter alkanes plus alkenes such as ethene, which are the feedstock for polymers. Catalytic cracking uses zeolites at about 500 degrees Celsius. Removing sulphur is a separate process called desulphurisation.

222. When ethene is passed into cold concentrated sulphuric acid and the product is then diluted with water and warmed, the compound obtained is

• A. ethane
• B. ethanoic acid
• C. ethanol
• D. ethanal

Explanation: Ethene adds sulphuric acid to give ethyl hydrogen sulphate, and hydrolysing that with water releases ethanol and regenerates the acid, which is the industrial route to ethanol from petroleum. The modern direct route passes ethene and steam over a phosphoric acid catalyst. Fermentation of sugar is the alternative, renewable route to the same product.

223. The addition of hydrogen to an alkene in the presence of a nickel catalyst is called

• A. dehydration
• B. hydrogenation
• C. hydrolysis
• D. halogenation

Explanation: Hydrogen adds across the double bond to give the corresponding alkane, and this is the process used to convert liquid vegetable oils into solid fats such as margarine. Nickel at about 150 degrees Celsius is the usual catalyst, though platinum works at lower temperatures. Excessive hydrogenation in food manufacture produces trans fats, which is why the extent of the reaction is controlled.

224. Benzene is described as aromatic because it

• A. has a pleasant smell
• B. contains only carbon and hydrogen
• C. reacts readily by addition
• D. is planar, cyclic and has a delocalised system of six pi electrons obeying Huckel's rule

Explanation: Aromaticity in the modern sense requires a planar cyclic conjugated system containing 4n plus 2 pi electrons, which for benzene means six, and this confers exceptional stability. The word originally referred to the smell of such compounds, which is why the first option is the historical but chemically wrong answer. Aromatic compounds characteristically undergo substitution rather than addition.

225. Organic compounds are built around carbon because a carbon atom

• A. forms four covalent bonds and can catenate into long chains and rings
• B. readily forms ions of charge four
• C. has a very large atomic radius
• D. always forms only single bonds

Explanation: With four valence electrons and a small radius, carbon forms four strong covalent bonds and bonds to itself almost as strongly as to other elements, so chains, branches and rings of unlimited length are possible. It also forms stable double and triple bonds, which silicon below it cannot do. Carbon rarely forms simple ions because removing or adding four electrons would cost far too much energy.

226. A homologous series is a family of compounds in which each member

• A. has the same molecular formula
• B. differs from the next by a CH2 group and shares the same general formula and functional group
• C. has the same physical properties
• D. contains the same number of carbon atoms

Explanation: Members share a general formula and a functional group, so their chemical properties are similar, while physical properties such as boiling point change gradually as the chain lengthens. The alkanes, with the general formula CnH2n+2, are the simplest example. Compounds with the same molecular formula but different structures are isomers, not members of a series.

227. Which of the following is a heterocyclic compound?

• A. Benzene
• B. Cyclohexane
• C. Pyridine
• D. Naphthalene

Explanation: A heterocyclic compound has a ring containing at least one atom other than carbon, and in pyridine one of the six ring positions is occupied by nitrogen. Benzene, cyclohexane and naphthalene are all carbocyclic, since their rings are made only of carbon. Furan, thiophene and pyrrole are the other common five membered heterocycles.

228. The functional group present in an aldehyde is

• A. a hydroxyl group
• B. a carbonyl group at the end of the chain, written CHO
• C. a carboxyl group
• D. a carbonyl group between two carbon atoms

Explanation: In an aldehyde the carbonyl carbon carries at least one hydrogen and so must be terminal, whereas in a ketone the same carbonyl group lies between two carbon atoms. This structural difference is why aldehydes are readily oxidised to carboxylic acids and give a positive test with Tollens reagent while ketones do not. The carboxyl group of an acid combines a carbonyl and a hydroxyl on the same carbon.

229. The general formula CnH2n could represent

• A. an alkane
• B. an alkyne
• C. an alcohol
• D. an alkene or a cycloalkane

Explanation: Both an alkene, with one double bond, and a cycloalkane, with one ring, have two hydrogens fewer than the corresponding alkane, so they share this general formula and are functional isomers of each other. Alkanes are CnH2n+2 and alkynes CnH2n-2. This is why a general formula alone can never identify a compound with certainty.

230. The functional group of a carboxylic acid is

• A. COOH
• B. CHO
• C. OH
• D. CO

Explanation: The carboxyl group combines a carbonyl and a hydroxyl on the same carbon atom, and this arrangement makes the hydrogen far more acidic than in an alcohol, because the resulting carboxylate ion is stabilised by delocalisation. CHO is the aldehyde group and CO between two carbons is a ketone. Acids therefore liberate carbon dioxide from carbonates, which alcohols do not.