Answers8 Carboxylic acids and their derivatives

Answers to Topic 8 Test yourself questions

1 Hexanoic acid, CH3CH2CH2CH2CH2COOH;octanoic acid

CH3CH2CH2CH2CH2CH2CH2COOH;decanoic acidCH3CH2CH2CH2CH2CH2CH2CH2CH2COO

H.2 C3H6O2, CH3CH2COOH, propanoic acid

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4 a) In aqueous solution there is a large excess of water molecules. This means that hydrogen bonding by ethanoic acid molecules is largely with water molecules. In a non-polar solvent the acid molecules can only form hydrogen bonds with each other.

b)

5 a) Pentanoic acid, CH3CH2CH2CH2COOH

b) Butanoic acid, CH3CH2CH2COOHc) Propanoic acid, CH3CH2COOH

6 a) CH3CN(l) + 2H2O(l) + HCl(aq) CH3COOH(aq) + NH4Cl(aq)

b) CH3CN(l) + H2O(l) + NaOH(aq) CH3COONa(aq) + NH3(aq)

7 Hydrolysis of a nitrile with alkali produces the salt of the acid. This is ionic and not volatile. Carboxylic acids are weak acids, and so acidifying the solution of the salt with a strong acid converts the anion of the acid in the salt to the molecular acid.

8 a) CH3COOH(aq) + KOH(aq) CH3COOK(aq) + H2O(l)

The product is potassium ethanoate.

b) 2CH3CH2COOH(aq) + Na2CO3(aq) 2CH3CH2COONa(aq) + CO2(g) + H2O(l)

The product is sodium propanoate.c) CH3CH2CH2COOH(aq) + NH3(aq)

CH3CH2CH2COONH4(aq)The product is ammonium butanoate.

9 Sodium ethanoate is a salt. There is ionic bonding between sodium ions and ethanoate ions. Ionic bonding is strong bonding. Ethanoic acid is molecular. The bonds between molecules are intermolecular forces: hydrogen bonds and London forces. These are much weaker than ionic bonds.

10 a) H3C6H5O7(aq) + 3NaOH(aq) Na3C6H5O7(aq) + 3H2O(aq)

b) A measured sample of a fruit juice could be titrated with a standard solution of a strong alkali such as sodium hydroxide. Phenolphthalein could be used for this titration of weak acids with a strong base. Since fruit juices are not clear and colourless, it might be better to follow the titration with a pH meter rather than an acid–base indicator. The result could be used to calculate the total acidity of the fruit juice but it could not measure only the citric acid concentrations. Grape juice, for example, contains tartaric acid and malic acid as well as citric acid. Apple juice also contains a significant amount of malic acid.

11 a) CH3CH2CH2CH2OH, butan-1-olb) CH3CH2COCl, propanoyl chloride

12 a) The reaction is rapid at room temperature and gives off hydrogen chloride which fumes in moist air. The test indicates the presence of OH groups, which is the reason that alcohols also give a positive result.

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© G. Hill and A. Hunt 2009 Edexcel Chemistry for A2

Answers8 Carboxylic acids and their derivatives

b) Aqueous solution of alcohols are neutral. Aqueous solutions of carboxylic acids have a pH around 3–4 and react with a solution of sodium carbonate to give carbon dioxide gas with effervescence.

13 ● Using an excess of one reactant: adding more of one reactant forces the equilibrium to the right, increasing the proportion of the other (more valuable) reactant converted to the ester.

● Using more concentrated sulfuric acid than is needed for its catalytic effect: sulfuric acid is a dehydrating agent so it removes water (one of the products) from the reaction mixture; this shifts the equilibrium to the right.

● Distilling off the product at formed: this removes the ester (the other product) from the reaction mixture, also shifting the equilibrium to the right.

14 CH3COCl(l) + H2O(l) CH3COOH(l) + HCl(g)The carbon–chlorine bond is broken by reaction with water.

15 CH3COCl(l) + 2NH3(aq) CH3CONH2(aq) + NH4Cl(aq)

The second mole of ammonia reacts with hydrogen chloride formed in the reaction to produce ammonium chloride.

16 CH3CH2CONHCH2CH2CH2CH3,N-butylpropanamide

17 a)

propyl butanoateb)

methyl ethanoatec)

butyl ethanoate18 Ethyl ethanoate is a larger molecule

than either ethanol or ethanoic acid; it has a permanent dipole but no –OH groups so molecules of the ester cannot form hydrogen bonds with each other. Hydrogen bonding is stronger than the attraction between permanent dipoles. Ethanol and ethanoic acid molecules are both held together by hydrogen bonding. So ethanol has a higher boiling temperature than might be expected from its molecular size. An ethanoic acid molecule can form more hydrogen bonds than an ethanol molecule.

19 a) Propan-1-ol and butanoic acidb) Ethanol and sodium methanoate

20 a) Excess of dilute acid so that there is plenty of water.

b) To make the ester no water is added and the acid is concentrated.

21 Saturated: palmitic and stearic; Unsaturated: oleic and linoleic

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23 Alkaline hydrolysis is not reversible and goes to completion. Also, using alkali produces the salt of the fatty acid which is the soap.

24 Stearic acid, octadecanoic acid25 a)

trans-octadec-9-enoic acid (elaidic acid)

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© G. Hill and A. Hunt 2009 Edexcel Chemistry for A2

Answers8 Carboxylic acids and their derivatives

cis-octadec-9-enoic acid (oleic acid)

b) Normally the isomers are distinct because the molecules cannot rotate around a double bond. Rotation must somehow become possible in the presence of the catalyst for hydrogenation.

c) The trans isomer is a linear molecule like a saturated fatty acid. This means that triglycerides with elaidic acid can pack more closely together with stronger intermolecular forces.

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27 a) Sodium methoxide is made using sodium (a very reactive metal) and methanol (a highly flammable liquid). The compound is a very strong base.

b) Sodium methoxide is rapidly hydrolysed by water. The methoxide ion is a strong base and reacts with water to form methanol and hydroxide ions.

28 The relatively large molecules in vegetable oil make it more viscous than diesel fuel. The reaction with methanol produces separate smaller molecules so that the biodiesel is more free-flowing and so easier to use as a fuel without having to modify engines.

29 If there is moisture present with the alkali the result is hydrolysis to form the sodium salts of fatty acids instead of methyl esters.

30 Making biodiesel: the triglyceride reacts with an alcohol such as methanol to produce methyl esters of fatty acids and glycerol.Raising melting temperature of oils: triglycerides react with each other leading to an exchange of fatty acids between the molecules.

31 a) When biodiesel burns it gives out about the same amount of carbon dioxide as the carbon dioxide taken in as the vegetable oil formed when the plants grew.

b) Carbon dioxide is given out during the manufacture of fertilisers, plant cultivation, harvesting, extraction of oil and processing of the oil into fuel.

32 Vegetable oils and corn starch are used to make food. Using them to make biodiesel leads to food shortages and an increase in food prices. Using waste plant material, such as straw, means that the manufacture of biodiesel does not compete with the production of food.

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© G. Hill and A. Hunt 2009 Edexcel Chemistry for A2