All Free Chemistry MCQs with Answers

321. Ethanoic acid reacts with phosphorus pentachloride to give

• A. ethanoyl chloride, hydrogen chloride and phosphorus oxychloride
• B. chloroethane and water
• C. ethanol and chlorine
• D. ethanal only

Explanation: The hydroxyl group of the acid is replaced by chlorine, giving the far more reactive acyl chloride, and the misty fumes of hydrogen chloride released make this a useful test for a hydroxyl or carboxyl group. Thionyl chloride achieves the same conversion with gaseous by products that are easier to remove. The acyl chloride is then used to prepare esters and amides.

322. Which reagent would distinguish ethanoic acid from phenol?

• A. Sodium hydroxide solution
• B. Sodium metal
• C. Sodium hydrogen carbonate solution
• D. Litmus paper alone

Explanation: Ethanoic acid gives brisk effervescence of carbon dioxide with hydrogen carbonate while phenol, being too weak an acid, gives no reaction at all. Sodium hydroxide is no use, since both compounds are neutralised by it, and both also react with sodium metal because both contain a hydroxyl hydrogen. This is the standard sequence for telling apart an alcohol, a phenol and a carboxylic acid.

323. The IUPAC name of CH3CH2COOH is

• A. ethanoic acid
• B. butanoic acid
• C. propanoic acid
• D. propan-1-ol

Explanation: The molecule has three carbon atoms in total, and the carboxyl carbon is always counted as part of the chain and numbered one, so the name is propanoic acid. Forgetting to count the carboxyl carbon gives ethanoic acid, which is the standard error. The parent acid name is formed by replacing the final e of the alkane with the ending oic acid.

324. An ester is named ethyl ethanoate. This tells us that it was formed from

• A. ethanoic acid and ethanol
• B. ethanol and methanoic acid
• C. ethanoic acid and methanol
• D. ethanal and ethanol

Explanation: The first part of an ester name comes from the alcohol and the second from the acid, so ethyl ethanoate is made from ethanol and ethanoic acid. Methyl ethanoate would come from methanol and the same acid, which is the closest distractor. Reading ester names in this order is essential when working backwards to identify the reactants.

325. A polymer is a large molecule formed by

• A. the joining together of many small repeating units called monomers
• B. the breaking of a large molecule into small units
• C. mixing two liquids without any reaction
• D. heating a metal until it melts

Explanation: Thousands of monomer units link together to form a chain whose properties are quite different from those of the monomer, so gaseous ethene becomes solid polythene. The process is called polymerisation and it is either addition, in which nothing is lost, or condensation, in which a small molecule such as water is eliminated at each link. Breaking a polymer back into monomers is depolymerisation.

326. Polythene is manufactured from ethene by

• A. condensation polymerisation
• B. addition polymerisation
• C. hydrolysis
• D. esterification

Explanation: The double bond of each ethene molecule opens and the units join end to end with no atoms lost, so the empirical formula of the polymer is identical to that of the monomer. Any monomer used in addition polymerisation must therefore contain a carbon to carbon double bond. Condensation polymerisation, by contrast, expels a small molecule at every linkage.

327. Nylon is classified as a condensation polymer because during its formation

• A. no by product is formed
• B. the monomers contain double bonds that open
• C. a small molecule such as water or hydrogen chloride is eliminated at each linkage
• D. the polymer is condensed from a gas

Explanation: A diamine and a dicarboxylic acid react so that an amide linkage forms and water is expelled at each join, which is why the polymer contains fewer atoms than the sum of its monomers. Nylon is a polyamide, and terylene, formed from a diol and a diacid, is the analogous polyester. Every condensation monomer needs two functional groups so the chain can grow at both ends.

328. A thermosetting plastic differs from a thermoplastic in that a thermosetting plastic

• A. softens every time it is heated and can be remoulded
• B. dissolves readily in water
• C. conducts electricity
• D. sets permanently on first heating because cross links form between the chains

Explanation: Covalent cross links lock the chains into a rigid three dimensional network, so the material cannot be softened and remoulded and simply chars if heated strongly, which is why bakelite is used for electrical fittings and pan handles. A thermoplastic such as polythene has separate chains held only by weak forces, so it softens repeatedly on heating and can be recycled. This difference decides how each plastic is processed and whether it can be recycled.

329. The monomer used to make polyvinyl chloride, PVC, is

• A. chloroethene
• B. chloroethane
• C. ethene
• D. phenol

Explanation: Chloroethene, also called vinyl chloride, has the double bond needed for addition polymerisation and carries the chlorine that gives PVC its rigidity and flame resistance. Chloroethane is saturated and so cannot polymerise by addition, which is the distractor to watch for. PVC is used for pipes, cable insulation and flooring, and plasticisers are added when a flexible product is wanted.

330. Which of the following is a naturally occurring polymer?

• A. Polystyrene
• B. Cellulose
• C. Nylon
• D. Teflon

Explanation: Cellulose is a natural condensation polymer of beta glucose and is the main structural material of plant cell walls, as are starch, proteins, natural rubber and DNA among natural polymers. Polystyrene, nylon and teflon are all synthetic. Recognising that natural polymers vastly outnumber synthetic ones in the living world is the point of the question.