All Free Chemistry MCQs with Answers

311. The reaction between a carboxylic acid and an alcohol in the presence of concentrated sulphuric acid is called

• A. saponification
• B. esterification
• C. hydrolysis
• D. decarboxylation

Explanation: Esterification joins the two molecules with the elimination of water and is reversible, so the sulphuric acid serves both as catalyst and as a dehydrating agent that pulls the equilibrium towards the ester. Saponification is the reverse process carried out with alkali, which produces a soap from a fat. Decarboxylation removes carbon dioxide and shortens the chain.

312. The hydrolysis of an ester with hot sodium hydroxide solution is called saponification and gives

• A. the carboxylic acid and an alkene
• B. an aldehyde and water
• C. the sodium salt of the acid and an alcohol
• D. an amide and hydrogen

Explanation: Alkaline hydrolysis produces the carboxylate salt rather than the free acid, and because that salt cannot react back with the alcohol the reaction goes to completion, unlike acidic hydrolysis which is reversible. When the ester is a natural fat the sodium salt produced is soap, which is the origin of the name. Glycerol is the alcohol released in that case.

313. The most reactive of the carboxylic acid derivatives towards nucleophilic attack is

• A. the amide
• B. the ester
• C. the carboxylate salt
• D. the acyl chloride

Explanation: Chlorine is a good leaving group and withdraws electron density strongly, so the carbonyl carbon of an acyl chloride is highly electron deficient and it reacts violently even with cold water. Reactivity falls in the order acyl chloride, anhydride, ester, amide, since each successive group donates electron density more effectively. This is why acyl chlorides are the reagents of choice for making esters of unreactive alcohols and phenols.

314. Ethanoyl chloride reacts with ethanol to give

• A. ethyl ethanoate and hydrogen chloride
• B. ethanoic acid and chlorine
• C. ethane and water
• D. ethanal and hydrogen chloride

Explanation: The alcohol attacks the carbonyl carbon and chloride is expelled, giving the ester rapidly and irreversibly along with fumes of hydrogen chloride. This route is preferred over direct esterification when a high yield is needed, since no equilibrium limits it. The vigorous reaction with water is the reason acyl chlorides must be kept dry.

315. Reduction of a carboxylic acid with lithium aluminium hydride gives

• A. an aldehyde only
• B. a primary alcohol
• C. a ketone
• D. an alkane

Explanation: Lithium aluminium hydride is powerful enough to reduce the carboxyl group all the way to a primary alcohol, passing through the aldehyde without stopping there. Milder reducing agents such as sodium borohydride will not touch a carboxylic acid at all, which is why the choice of reagent matters. The reaction must be carried out in dry ether, since the reagent reacts violently with water.

316. Which acid is the strongest?

• A. Ethanoic acid
• B. Propanoic acid
• C. Trichloroethanoic acid
• D. Butanoic acid

Explanation: Three electronegative chlorine atoms withdraw electron density powerfully and stabilise the carboxylate ion, so trichloroethanoic acid is a strong acid, comparable in strength to some mineral acids. Among the unsubstituted acids, strength falls slightly as the alkyl chain lengthens because alkyl groups release electron density. This is why methanoic acid is the strongest of the simple aliphatic acids.

317. Soap is manufactured by the alkaline hydrolysis of

• A. ethanol
• B. benzene
• C. starch
• D. natural fats and oils, which are esters of glycerol

Explanation: Boiling a fat with sodium hydroxide splits the three ester linkages and gives sodium salts of long chain fatty acids, the soap, together with glycerol as a valuable by product. Soap cleans because the hydrocarbon tail dissolves in grease while the ionic head remains in the water. Using potassium hydroxide instead gives a softer, more soluble soap.

318. The functional group of an amide is

• A. CONH2, in which nitrogen is bonded directly to the carbonyl carbon
• B. COOH
• C. COOR
• D. COCl

Explanation: In an amide the nitrogen lone pair is delocalised into the carbonyl group, which is why amides are almost neutral rather than basic like amines, and why they are the least reactive of the acid derivatives. The peptide link joining amino acids in a protein is exactly this group. COOR is the ester group and COCl the acyl chloride.

319. Methanoic acid differs from other carboxylic acids in that it also

• A. fails to react with alkalis
• B. acts as a reducing agent, giving a silver mirror with Tollens reagent
• C. contains no carboxyl group
• D. is insoluble in water

Explanation: Its carboxyl carbon also carries a hydrogen, so the molecule contains an aldehyde group as well and can be oxidised further to carbon dioxide, which no other carboxylic acid can do. This is why methanoic acid reduces Tollens and Fehling's reagents. It is otherwise a typical, and in fact the strongest, of the simple aliphatic acids.

320. Carboxylic acids up to four carbon atoms are completely miscible with water because

• A. they ionise completely
• B. they are non polar
• C. the carboxyl group forms hydrogen bonds with water molecules
• D. they react with water chemically

Explanation: Both the carbonyl oxygen and the hydroxyl hydrogen can take part in hydrogen bonding with water, so the smaller acids dissolve freely, and solubility then falls as the hydrocarbon chain grows and dominates the molecule. Complete ionisation is not the reason, since these are weak acids only slightly dissociated in solution. Dissolving is a physical process, not a chemical reaction.