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Questions on carboxylic acids, esters and biodiesel
Carboxylic acids as weak acids and their reactions with carbonates, bases and metals, the relative acidities of acids, phenols, alcohols and chloro-substituted acids, carboxylic acids from nitriles, methanoic acid as a reducing agent, LiAlH₄ reduction to alcohols, esterification with an alcohol and a concentrated sulfuric acid catalyst, naming esters, uses of esters, acid and alkaline hydrolysis of esters, fats and oils as triglycerides, soap by alkaline hydrolysis, and biodiesel by transesterification with methanol.
21 original questions · 65 marks · the carboxylic acids, esters and biodiesel notes · Carbonyls and carboxylic acids · Flashcards for this unit
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Evaluate biodiesel made from rapeseed oil as a replacement for diesel from crude oil, including the claim that it is carbon-neutral.
Mark scheme
Indicative content
- the carbon dioxide released on burning was taken in by the rapeseed plants during photosynthesis, which supports the claim of carbon neutrality
- fossil fuels are burned in farming, making fertiliser, transport and processing, so extra carbon dioxide is released overall
- the methanol used in transesterification is usually made from natural gas, a fossil fuel, so the claim is not fully supported
- the oil is a renewable resource, where crude oil is finite, and biodiesel contains almost no sulfur, so little sulfur dioxide forms
- land used to grow oil crops is land not growing food, which can raise food prices or lead to forest clearance
- a justified conclusion: biodiesel releases less fossil carbon than diesel but is not carbon-neutral, so it reduces rather than removes the impact
Levels
- Level 3 (5–6 marks) the basis of the carbon-neutral claim is explained through photosynthesis, the fossil-fuel inputs including the methanol are identified, the renewability and low sulfur of biodiesel and the land-use cost are all weighed, and a justified conclusion is drawn that it reduces rather than removes the impact
- Level 2 (3–4 marks) the claim is explained and at least one fossil-fuel input named, with one more advantage or drawback; the conclusion is absent or does not follow from the points made
- Level 1 (1–2 marks) biodiesel is accepted as carbon-neutral because it comes from plants, with no fossil-fuel input such as the methanol or the fertiliser considered
Describe a chemical test that shows an unknown liquid is ethanoic acid rather than ethanol, giving the observations, an equation for the reaction and the name of the salt formed.
Mark scheme
- B1 add solid sodium carbonate or sodium hydrogencarbonate to each liquid
- B1 ethanoic acid effervesces, giving a gas that turns limewater milky; ethanol gives no change
- B1 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂; allow the hydrogencarbonate equation
- B1 the salt is sodium ethanoate
Compare the hydrolysis of methyl ethanoate by dilute sulfuric acid with its hydrolysis by aqueous sodium hydroxide.
Mark scheme
- B1 both are carried out by heating under reflux, and both give methanol
- B1 acid hydrolysis gives ethanoic acid, while alkaline hydrolysis gives sodium ethanoate
- B1 acid hydrolysis is reversible and incomplete, while alkaline hydrolysis goes to completion
- B1 in acid, H₂SO₄ is a catalyst, while in alkali the hydroxide is used up in the reaction
Outline how a pure sample of ethyl ethanoate is obtained from the mixture left after heating ethanoic acid and ethanol under reflux.
Mark scheme
- B1 distil the mixture and collect the crude ester
- B1 shake the distillate with sodium carbonate solution in a separating funnel to remove acid, releasing the pressure of carbon dioxide
- B1 run off the aqueous layer and dry the ester with anhydrous calcium chloride
- B1 redistil, collecting the fraction boiling close to 77 °C
Name the esters CH₃CH₂COOCH₂CH₃, HCOOCH₃ and CH₃CH₂CH₂COOCH₃.
Mark scheme
- B1 CH₃CH₂COOCH₂CH₃ is ethyl propanoate
- B1 HCOOCH₃ is methyl methanoate
- B1 CH₃CH₂CH₂COOCH₃ is methyl butanoate; reject butyl methanoate
Give the reagent and condition needed to make propyl ethanoate from ethanoic acid, and give the equation for the reaction.
Mark scheme
- B1 propan-1-ol, with concentrated sulfuric acid as the catalyst
- B1 warm, or heat under reflux
- B1 CH₃COOH + CH₃CH₂CH₂OH ⇌ CH₃COOCH₂CH₂CH₃ + H₂O
Explain why the hydrolysis of an ester by aqueous sodium hydroxide goes to completion, whereas hydrolysis by dilute acid does not.
Mark scheme
- B1 acid hydrolysis is the reverse of esterification and reaches the same equilibrium, so some ester remains
- B1 in alkali the carboxylic acid formed is at once neutralised to a carboxylate ion
- B1 the carboxylate ion does not react with the alcohol, so the reverse reaction cannot occur
Explain how adding an ester plasticiser makes PVC flexible.
Mark scheme
- B1 the plasticiser molecules fit between the polymer chains
- B1 they push the chains further apart, weakening the intermolecular forces between them
- B1 the chains can then slide past each other, so the polymer bends instead of being rigid
Calculate the percentage yield of ethyl propanoate when ethanol is heated with excess propanoic acid and concentrated sulfuric acid, using the data given in the table.
Quantity Value mass of ethanol 4.60 g Mr of ethanol 46.0 mass of ethyl propanoate collected 6.63 g Mr of ethyl propanoate 102.0 Mark scheme
- M1 amount of ethanol = 4.60 ÷ 46.0 = 0.100 mol, so the maximum amount of ester is 0.100 mol
- M1 amount of ester obtained = 6.63 ÷ 102.0 = 0.0650 mol
- A1 percentage yield = 0.0650 ÷ 0.100 × 100 = 65.0%, to 3 significant figures
Calculate the maximum mass of methyl hexadecanoate, Mr 270.0, formed by the transesterification of 403 g of the triglyceride (C₁₅H₃₁COO)₃C₃H₅, Mr 806.0, with excess methanol.
Mark scheme
- M1 amount of triglyceride = 403 ÷ 806.0 = 0.500 mol
- M1 each triglyceride gives three methyl esters, so 3 × 0.500 = 1.50 mol of methyl hexadecanoate
- A1 mass = 1.50 × 270.0 = 405 g, to 3 significant figures
Describe how soap is made from a fat, naming the products.
Mark scheme
- B1 the fat is heated with concentrated aqueous sodium hydroxide
- B1 the three ester links of the triglyceride are hydrolysed
- B1 the products are glycerol, propane-1,2,3-triol, and the sodium salts of the fatty acids, which are the soap
Give three uses of esters, and for each give the property of esters that suits them to it.
Mark scheme
- B1 perfumes or food flavourings, because small esters are volatile with sweet, fruity smells
- B1 solvents such as nail-varnish remover or glue, because they dissolve organic compounds and evaporate easily
- B1 plasticisers in PVC, because their molecules fit between the polymer chains and let them slide; a use with no property is not credited
Suggest two ways of increasing the yield of ethyl ethanoate made from ethanoic acid and ethanol, and explain one of them.
Mark scheme
- B1 use an excess of one reactant, usually the cheaper one
- B1 distil the ester off as it forms, or remove water, for example with the concentrated sulfuric acid
- B1 removing a product or adding a reactant moves the position of equilibrium to the right, by Le Chatelier's principle
Explain why ethanoic acid is a stronger acid than ethanol.
Mark scheme
- B1 the ethanoate ion's negative charge is delocalised over both oxygen atoms
- B1 the ethoxide ion's charge is on one oxygen, and the ethyl group pushes electron density towards it
- B1 so the ethanoate ion is more stable and ethanoic acid dissociates further, releasing more H⁺
Suggest the order of acid strength of ethanoic acid, chloroethanoic acid and trichloroethanoic acid, strongest first, and explain the order.
Mark scheme
- B1 trichloroethanoic acid, then chloroethanoic acid, then ethanoic acid
- B1 chlorine is more electronegative than carbon and draws electron density away from the COO⁻ group, spreading its charge and stabilising the anion
- B1 each extra chlorine adds to the effect, so the more chlorines, the further the acid dissociates
Give the reagent and condition for converting propanenitrile into propanoic acid, and give the equation for the reaction.
Mark scheme
- B1 dilute hydrochloric acid (or another dilute strong acid)
- B1 the mixture is heated under reflux
- B1 CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl
Show that a saturated ester of relative molecular mass 102.0, which gives ethanol as the only alcohol when it is hydrolysed, must be ethyl propanoate.
Mark scheme
- M1 a saturated ester with no ring has the general formula CₙH₂ₙO₂, so Mr = 14n + 32.0 = 102.0 and n = 5: the ester is C₅H₁₀O₂
- M1 hydrolysis gives ethanol, so the alcohol part is the ethyl group, OCH₂CH₃, which accounts for two of the five carbons; the acid part therefore has three carbons and is propanoic acid, CH₃CH₂COOH
- A1 the ester of propanoic acid and ethanol is ethyl propanoate, CH₃CH₂COOCH₂CH₃, whose Mr is 5 × 12.0 + 10 × 1.0 + 2 × 16.0 = 102.0, as required
Explain why a 0.10 mol dm⁻³ solution of ethanoic acid has a higher pH than a 0.10 mol dm⁻³ solution of hydrochloric acid.
Mark scheme
- B1 ethanoic acid is a weak acid, only slightly dissociated into ions, whereas hydrochloric acid is fully dissociated
- B1 the concentration of H⁺ ions in the ethanoic acid solution is therefore lower, so its pH is higher
Explain what the following result shows about esterification: when ethanol labelled with oxygen-18 in its OH group is esterified with ethanoic acid, all of the oxygen-18 is found in the ester.
Mark scheme
- B1 the C–O bond of the alcohol is not broken, so the alcohol's oxygen becomes the ester's single-bonded oxygen
- B1 the water is formed from the OH of the acid and the H of the alcohol
Suggest why the glycerol formed in biodiesel production can be separated from the methyl esters with a separating funnel.
Mark scheme
- B1 glycerol has three OH groups and hydrogen bonds strongly, so it does not mix with the much less polar methyl esters
- B1 it is denser, so it forms a separate lower layer that can be run off
Suggest why methanoic acid gives a silver mirror with Tollens' reagent while ethanoic acid does not.
Mark scheme
- B1 the carbon of methanoic acid carries a hydrogen atom, as an aldehyde's carbonyl carbon does
- B1 so methanoic acid can be oxidised further, to carbon dioxide and water, reducing the silver(I) to silver; ethanoic acid cannot be oxidised by a mild oxidising agent
Practise carboxylic acids, esters and biodiesel one question at a time