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O LevelChemistryCambridgeExam Questions11. Organic Chemistry11.7 Carboxylic Acids

Carboxylic Acids (Cambridge O Level Chemistry)

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11.7 Carboxylic Acids

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11.7 Carboxylic Acids

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1a7 marks

Esters, polyesters and fats all contain the ester linkage.

Esters can be made from alcohols and carboxylic acids. For example, the ester ethyl ethanoate can be made by the following reaction.

CH3COOH + CH3CH2OH → CH3COOCH2CH3 + H2O

i)
Name the carboxylic acid and the alcohol from which the following ester could be made.

ethyl-ethanoate

[2]

ii)
6.0 g of ethanoic acid, Mr = 60, was reacted with 5.5 g of ethanol, Mr = 46. 
Determine which is the limiting reagent and the maximum yield of ethyl ethanoate, Mr = 88.

Number of moles of ethanoic acid =
[1]
Number of moles of ethanol =
[1]
The limiting reagent is
[1]
Number of moles of ethyl ethanoate formed =
[1]
Maximum yield of ethyl ethanoate =
[1]

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1b3 marks
The following two monomers can form a polyester.

diol-and-dioic-acid

   
   Draw the structural formula of this polyester. Include two ester linkages.

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1c5 marks
Fats and vegetable oils are esters. The formulae of two examples of natural esters are given below.

natural-esters

i)
One ester is saturated, the other is unsaturated.

Describe a test to distinguish between them.

Test:
Result with unsaturated ester:
Result with saturated ester:

[3]

ii)
Deduce which one of the above esters is unsaturated. Give a reason for your choice.
[2]

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1a2 marks

Methanoic acid is the first member of the homologous series of carboxylic acids.


Give two general characteristics of a homologous series.

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1b5 marks

In some areas when water is boiled, the inside of kettles become coated with a layer of calcium carbonate. This can be removed by adding methanoic acid.

i)
Complete the equation.

............ HCOOH + CaCO3 → Ca(HCOO)2 + ................. + .................

[2]

ii)
Methanoic acid reacts with most metals above hydrogen in the reactivity series.
Complete the word equation.

zinc + methanoic acid → ............................................ + ............................................

[2]

iii)
Aluminium is also above hydrogen in the reactivity series.
Why does methanoic acid not react with an aluminium kettle?

[1]

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1c3 marks

Give the name, molecular formula and empirical formula of the fourth acid in this series.


name ...........................................................................................................................
[1]
molecular formula .......................................................................................................
[1]
empirical formula ........................................................................................................
[1]

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2a2 marks

The ester linkage showing all the bonds is drawn as

ester-linkage

or more simply it can be written as –COO–.

i)
Give the structural formula of the ester ethyl ethanoate.

[1]

ii)
Deduce the name of the ester formed from methanoic acid and butanol.

[1]

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2b3 marks

Draw the structural formula of the polyester formed from the following monomers.

HOOCC6H4COOH and HOCH2CH2OH

You are advised to use the simpler form of the ester linkage.

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2c5 marks

Esters can be used as solvents in chromatography. The following shows a chromatogram of plant acids.

chromatogram-of-plant-acid

An ester was used as the solvent and the chromatogram was sprayed with bromothymol blue.

i)
Suggest why it was necessary to spray the chromatogram.

[2]

ii)
Explain what is meant by the Rf value of a sample.

[1]

iii)
Calculate the Rf values of the two samples and use the data in the table to identify the plant acids.

Plant acid Rf value
tartaric acid 0.22
citric acid 0.30
oxalic acid 0.36
malic acid 0.46
succinic acid 0.60

[2]

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3a6 marks

The alcohols form a homologous series. The first member of this series is methanol, CH3OH.

i)
Give the general formula of the alcohols.

[1]

ii)
The mass of one mole of an alcohol is 116 g. What is its formula?
Show your reasoning.

[2]

ii)
Draw a diagram showing the arrangement of the outer (valency) electrons in one
molecule of methanol.

Use x to represent an electron from a carbon atom.
Use o to represent an electron from a hydrogen atom.
Use ● to represent an electron from an oxygen atom.

[3]

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3b5 marks

Methanol is manufactured using the following method.

CH4 (g) + H2O (g) → CO (g) + 3H2 (g)   reaction 1

CO (g) + 2H2 (g) rightwards harpoon over leftwards harpoon CH3OH (g)    reaction 2

The conditions for reaction 2 are:

pressure 100 atmospheres
catalyst a mixture of copper, zinc oxide and aluminium oxide
temperature 250°C


The forward reaction is exothermic.

i)
Why is high pressure used in reaction 2?

[2]

ii)
Explain why using a catalyst at 250 °C is preferred to using a higher temperature of 350°C and no catalyst.

[3]

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3c3 marks

Methanol is oxidised by atmospheric oxygen. This reaction is catalysed by platinum.

i)
The products of this reaction include a carboxylic acid. Give its name and structural formula.

[2]

ii)
Deduce the name of the ester formed by the reaction of methanol with the carboxylic acid named in (i).

[1]

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4a4 marks

Propenoic acid is an unsaturated carboxylic acid. The structural formula of propenoic acid is given below.

propanoic-acid

i)
Describe how you could show that propenoic acid is an unsaturated compound. 

Test:
Result:

[2]

ii)
Without using an indicator, describe how you could show that a compound is an acid.

Test:
Result:

[2]

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4b3 marks

Propenoic acid reacts with ethanol to form an ester. Deduce the name of this ester. Draw its structural formula.

Name of ester:

Structural formula showing all bonds:

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4c5 marks

An organic compound has a molecular formula C6H8O4. It is an unsaturated carboxylic acid. One mole of the compound reacts with two moles of sodium hydroxide.

i)
Explain the phrase molecular formula.

[2]

ii)
One mole of this carboxylic acid reacts with two moles of sodium hydroxide.
How many moles of –COOH groups are there in one mole of this compound?

[1]

iii)
What is the formula of another functional group in this compound?

[1]

iv)
Deduce a structural formula of this compound.

[1]

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1a5 marks

Propanoic acid is a carboxylic acid. Its formula is CH3–CH2–COOH.

 
i)
Propanoic acid is the third member of the homologous series of carboxylic acids.
 
Give the name and structural formula of the fourth member of this series.
 
Name ..............................................................
 
Formula ..............................................................

[2]

ii)
Members of a homologous series have very similar chemical properties.
 
State three other characteristics of a homologous series.

[3]

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1b3 marks

Carboxylic acids can be made by the oxidation of alcohols.

 
i)
Draw the structural formula of the alcohol which can be oxidised to propanoic acid.
Show all atoms and bonds.

[1]

ii)
Name a reagent, other than oxygen, which can oxidise alcohols to carboxylic acids.

[2]

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1c3 marks

Complete the following equations for some of the reactions of propanoic acid.

The salts of this acid are called propanoates.

 
i)
Zinc + propanoic acid → ........................... ........................... + hydrogen
[1]
ii)
Calcium oxide + propanoic acid →........................... + ...........................
[1]
iii)
LiOH + CH3CH2COOH → ............................... + ...........................
[1]

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1d7 marks

A piece of magnesium was added to 100 cm3 of an aqueous acid. The time taken for the metal to react completely was measured.

This experiment was repeated using different aqueous acids. The same volume of acid was used in each experiment and the pieces of magnesium used were identical.

In one experiment the reaction was carried out at a different temperature.

experiment acid concentration
in mol / dm3		 temperature / oC time / minutes
A propanoic 1.0 20 5
B propanoic 1.0 30 3
C propanoic 0.5 20 8
D hydrochloric 1.0 20 1
 
 

Explain the following in terms of collision rate between reacting particles.

 
i)
Why is the rate in experiment C slower than the rate in experiment A? 

[2]

ii)
Why is the rate in experiment B faster than the rate in experiment A?

[2]

iii)
Why is the rate in experiment D faster than the rate in experiment A?

[3]

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2a2 marks

Structural formulae are an essential part of Organic Chemistry.

Draw the structural formula of each of the following. Show all the bonds in the structure.

i)
ethanoic acid

[1]

ii)
ethanol

[1]

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2b6 marks

Ethanoic acid and ethanol react to form an ester.

i)
What is the name of this ester?

[1]

ii)
The same linkage is found in polyesters. Draw the structure of the polyester which can be formed from the monomers shown below.

HOOC—C6H4—COOH and HO—CH2—CH2—OH

[3]

iii)
Describe the pollution problems caused by non-biodegradable polymers.

[2]

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2c2 marks

Two macromolecules have the same amide linkage.
Nylon, a synthetic polymer, has the following structure.

nylon

Protein, a natural macromolecule, has the following structure.

protein

How are they different?

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3a5 marks

Carboxylic acids contain the group

-cooh

Ethanoic acid is a typical carboxylic acid. It forms ethanoates.

i)
Complete the following equations.

Mg + ......CH3COOH → ............................... + ...............................
[2]

sodium hydroxide + ethanoic acid → ........................  ........................ + ........................

[1]

ii)
Ethanoic acid reacts with ethanol to form an ester. Give the name of the ester and draw its structural formula. Show all of the bonds.
[2]

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3b
Sme Calculator8 marks

Maleic acid is an unsaturated acid. 5.8 g of this acid contained 2.4 g of carbon, 0.2 g of hydrogen and 3.2 g of oxygen.

i)
How do you know that the acid contained only carbon, hydrogen and oxygen?
[1]
ii)
Calculate the empirical formula of maleic acid.

Number of moles of carbon atoms = ................................
Number of moles of hydrogen atoms = ................................
Number of moles of oxygen atoms = ................................
The empirical formula is ......................................................................................

[3]

iii)
The mass of one mole of maleic acid is 116 g. What is its molecular formula?

[2]

iv)
Maleic acid is dibasic. One mole of acid produces two moles of H+. Deduce its structural formula.

[2]

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4a4 marks

Esters, fats and polyesters all contain the ester linkage.

The structural formula of an ester is given below.

 
I4iZm0aV_butyl-ethanoate
 

Name two chemicals that could be used to make this ester and draw their structural formulae. Show all bonds.

 
names ......................................................... and ..............................................................


[2]

   structural formulae




[2]

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4b3 marks
i)
Draw the structural formula of a polyester such as Terylene.

[2]

ii)
Suggest a use for this polymer.

[1]

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