Free Reactivity of Carboxylic Acids MCQs with Answers

1. Carboxylic acids are more acidic than alcohols because

• A. they contain more hydrogen atoms
• B. the carboxylate ion formed is stabilised by delocalisation of the negative charge over two oxygen atoms
• C. they are liquids at room temperature
• D. their molecules are larger

Explanation: Losing the acidic hydrogen gives an ion whose charge is shared equally between the two oxygens, so the two carbon to oxygen bonds become identical in length and the ion is far more stable than an alkoxide. That stability makes the loss of the proton much more favourable. This is why an acid liberates carbon dioxide from a carbonate while an alcohol does not.

2. Ethanoic acid reacts with sodium carbonate to give

• A. sodium ethanoate, water and carbon dioxide
• B. sodium ethanoate and hydrogen
• C. ethanol and carbon dioxide
• D. no reaction

Explanation: Being a stronger acid than carbonic acid, ethanoic acid displaces carbon dioxide from a carbonate, and the effervescence produced is the standard test that distinguishes a carboxylic acid from both alcohols and phenols. With sodium metal the acid would give hydrogen instead. Neutralisation with an alkali gives only the salt and water.

3. The presence of chlorine atoms on the carbon next to the carboxyl group, as in chloroethanoic acid, makes the acid stronger because chlorine

• A. donates electrons to the carboxyl group
• B. increases the molar mass
• C. withdraws electron density, spreading and stabilising the negative charge of the anion
• D. makes the molecule non polar

Explanation: The electron withdrawing inductive effect of chlorine pulls charge away from the carboxylate oxygens, so the ion is more stable and the acid ionises more readily, and trichloroethanoic acid is stronger still because three chlorines act together. The effect weakens rapidly with distance along the chain. Electron releasing alkyl groups have the opposite effect, which is why ethanoic acid is weaker than methanoic acid.

4. Carboxylic acids have unusually high boiling points because in the liquid state their molecules

• A. are ionised
• B. form dimers held together by two hydrogen bonds
• C. form covalent bonds with one another
• D. are non polar

Explanation: Two molecules pair up so that the hydroxyl hydrogen of each bonds to the carbonyl oxygen of the other, giving a cyclic dimer with double the effective molar mass, so ethanoic acid boils at 118 degrees Celsius, well above ethanol despite similar size. The same dimerisation occurs in non polar solvents and even in the vapour just above the boiling point. Only in water do the dimers break up, replaced by hydrogen bonds to the solvent.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.