(a) The structures of four organic compounds, P, Q, R and S, are shown.
Answer the following questions about these structures. Each structure may be used once, more than once or not at all.
(i) State which structure, P, Q, R or S, has a carboxylic acid functional group.[1]
(ii) State which structure, P, Q, R or S, is in the same homologous series as ethane.[1]
(iii) State which structure, P, Q, R or S, decolourises aqueous bromine.[1]
(iv) Deduce the molecular formula of structure Q to show the number of carbon, hydrogen and oxygen atoms.[1]
(b) Structure S is produced by cracking petroleum fractions.
(i) Complete the sentence using a word from the list.
acids alkenes alcohols nitrogen
During cracking, long-chain alkanes are converted to shorter chain alkanes and[1]
(ii) Cracking is an example of thermal decomposition.
State the meaning of the term thermal decomposition.[2]
(c) Link each petroleum fraction on the left to its use on the right.
The first one has been done for you.
▶️ Answer/Explanation
(a)(i) Q has the carboxylic acid functional group (-COOH), identifiable by the carbonyl (C=O) and hydroxyl (O-H) groups attached to the same carbon.
(a)(ii) S is in the same homologous series as ethane (alkanes), as it contains only single bonds between carbon atoms and follows the general formula CnH2n+2.
(a)(iii) R decolourises aqueous bromine because it contains a C=C double bond (alkene functional group) which reacts with bromine water.
(a)(iv) Q has molecular formula C4H8O2: 4 carbon atoms, 8 hydrogen atoms (accounting for all bonds), and 2 oxygen atoms (one in carbonyl, one in hydroxyl group).
(b)(i) alkenes – These unsaturated hydrocarbons are always produced alongside shorter alkanes during cracking.
(b)(ii) Thermal decomposition means the breakdown of large molecules into smaller ones (1) using heat energy (1). This requires both parts for full marks.
(c) Correct matches:
• Naphtha → making chemicals (already matched)
• Lubricating fraction → waxes and polishes
• Kerosene → jet fuel
All three correct scores 2 marks, one or two correct scores 1 mark.
The table shows the masses of some ions in a 1000 cm3 sample of toothpaste.
(a) Answer these questions using only the information in the table.
(i) State which negative ion has the lowest mass in 1000 cm3 of toothpaste. [1]
(ii) Name the compound that contains NH4+ and Cl– ions. [1]
(iii) Calculate the mass of phosphate ions in 250 cm3 of toothpaste.
mass = g [1]
(b) Describe a test for sulfate ions. [2]
test
observations
(c) Toothpaste also contains cinnamal.
The structure of cinnamal is shown.
Deduce the formula of cinnamal to show the number of atoms of carbon, hydrogen and oxygen. [1]
(d) Cinnamal can be oxidised to a carboxylic acid.
(i) Draw the structure of a carboxylic acid functional group to show all of the atoms and all of the bonds. [1]
(ii) Ethanoic acid is a carboxylic acid.
Ethanoic acid reacts like a typical acid.
Complete the word equation for the reaction of ethanoic acid with sodium carbonate.
[2]
▶️ Answer/Explanation
(a)(i) Ans: Cl– (chloride)
Chloride has the smallest mass (0.2 g) among the negative ions in the table.
(a)(ii) Ans: ammonium chloride
The compound formed from NH4+ and Cl– ions is ammonium chloride (NH4Cl).
(a)(iii) Ans: 6.1 g
Calculation: (24.4 g ÷ 1000 cm3) × 250 cm3 = 6.1 g
(b) Ans: Add acidified barium nitrate solution (1), white precipitate forms (1)
Sulfate ions (SO42-) react with Ba2+ to form insoluble white BaSO4 precipitate.
(c) Ans: C9H8O
Counting atoms in the structure: 9 carbon, 8 hydrogen, and 1 oxygen atom.
(d)(i) Ans:
The carboxyl group (-COOH) contains a carbonyl (C=O) and hydroxyl (O-H) group.
(d)(ii) Ans: sodium ethanoate (1), carbon dioxide (1), water (1)
Complete equation: ethanoic acid + sodium carbonate → sodium ethanoate + carbon dioxide + water
This shows the typical acid-carbonate reaction producing salt, CO2 gas, and water.