Carboxylic Acids (Cambridge (CIE) O Level Chemistry): Theory Questions

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]

iii) 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]

Methanol is manufactured using the following method.

CH₄ (g) + H₂O (g) → CO (g) + 3H₂ (g) reaction 1
CO (g) + 2H₂ (g) $\rightleftharpoons$ CH₃OH (g) reaction 2

The conditions for reaction 2 are:

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]

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. Show all of the bonds.

[2]

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

[1]

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

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]

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:

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]

Table 2.1 shows some information about the homologous series of unbranched carboxylic acids.

Table 2.1

One of the characteristics of a homologous series is that it has a general formula.

i) the general formula for the homologous series of unbranched carboxylic acids.

[1]

ii) Describe two other characteristics of a homologous series.

[2]

An aqueous solution of propanoic acid is a weak acid.

Define the term acid.

Butanoic acid reacts with ethanol to make an ester.

State the name of the ester made and draw the displayed formula of the ester linkage.

Ethanoic acid is a liquid at room temperature.

Describe the changes in the arrangement and movement of the molecules of ethanoic acid

when it is heated from room temperature to 120°C.

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

Give two general characteristics of a homologous series.

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]

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

name .................................................. 

[1]

 molecular formula .................................................. 

[1]

 empirical formula .................................................. 

[1]

Esters are made by reacting a carboxylic acid with an alcohol.

A student is asked to name the ester formed when propanoic acid reacts with ethanol.

The student answers propyl ethanoate.

Explain what the student has done wrong.

Give the correct name of the ester.

Butanoic acid reacts with methanol to form an ester.

Give the name of this ester.

Draw its structural formula.

Methanoic acid reacts with butan-1-ol to form a different ester.

Write the structural formula of this ester.

Use the two structural formulae to explain why these two esters are not the same compound.

Carboxylic acids contain the group

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]

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]

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

Propanoic acid is the third member of the homologous series of carboxylic acids.

 i) 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]

Ethanol can be oxidised to ethanoic acid. Other alcohols react in the same way to form carboxylic acids.

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]

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]

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.

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]

Butanoic acid, CH₃CH₂CH₂COOH, takes part in the three reactions shown.

Reaction 1 butanoic acid + magnesium → compound X + gas Y

Reaction 2 butanoic acid + magnesium carbonate → compound X + gas Z + water

Reaction 3 butanoic acid + butan-1-ol, with a catalyst → compound W + water

State the name and the formula of compound X.

Name gas Y and gas Z.

Name compound W.

Write the structural formula of compound W.

A student writes three equations for reactions of ethanoic acid, CH₃COOH.

Equation 1 CH₃COOH + Mg → CH₃COOMg + H₂

Equation 2 2CH₃COOH + ZnO → (CH₃COO)₂Zn + H₂O

Equation 3 2CH₃COOH + 2NaOH → 2CH₃COONa + H₂

Equation 1 is incorrect.

Write the correct equation for this reaction.

Equation 3 is also incorrect.

Explain what is wrong with equation 3.

The student suggests that ethanoic acid will react with copper to form copper ethanoate and hydrogen.

Explain why this reaction does not take place.

A colourless liquid X mixes completely with water.

A solution of X of concentration 0.1 mol / dm³ has a pH of 3.

When solid sodium carbonate is added to this solution, effervescence is seen.

0.010 mol of X reacts exactly with 0.0050 mol of magnesium.

The M_r of X is 60.

Explain how the pH of the solution shows that X is a weak acid.

State what the reacting ratio of X with magnesium shows about one molecule of X.

Deduce the name and the structural formula of X.

Write the equation for the reaction of X with sodium carbonate.