Organic Chemistry Prac Qns ANS

SECONDARY 4 EXPRESS – COMBINED CHEMISTRY Class Index No NAME ANSWERS 0 6 7 6 7 SCIENCE (CHEMISTRY) Topical Revision – Organic Chemistry Candidates answer on the Question Paper. Additional Materials: Periodic Table, Data Sheet 5086/5088 30 minutes Date: _______________ READ THESE INSTRUCTIONS FIRST Write your class, index number and name on ALL the work you hand in You may use a HB pencil for any drawing. Write in dark blue or black pen. Do not use staples, paper clips, glue or correction fluid. Section A – Multiple Choice Questions (5 marks) There are 5 questions in this Section. Answer ALL questions. For each question, there are four possible answers A, B, C and D . Choose the one you consider correct and write your answer in the given spaces. Each question scores 1 mark. A mark will not be deducted for a wrong answer. Any rough working should be done in this booklet. Section B – Structured Questions (15 marks) Answer ALL questions. The number of marks is given in the brackets [ ] at the end of each question or part question. The total marks for this paper is 20 . FOR EXAMINERS’ USE Target marks Your marks Full Marks TOTAL 20 20 20 Adapted from past year prelim papers This document consists of 6 pages and 0 blank pages.

2 Section A – Multiple Choice Questions (5 marks) Write your answers in the given boxes on page 3. 1 An organic compound, X, decolourises red-brown aqueous bromine and extinguishes a lighted splint with a 'pop' sound when it reacts with magnesium. Which of the following is X? (AES 2024 Q19) A B C D 2 The structure below shows a monomer. Which of the following shows three repeating units of the polymer chain formed? (GMSS 2024 Q19) A B C D © chiasamuel 2025

3 3 Bitumen contains an organic compound with the molecular formula of C ₂₀ H ₄₂ . It undergoes a process called catalytic cracking, to form hydrocarbons with shorter carbon chains which are higher in demand. The chemical equation is given as shown: C ₂₀ H ₄₂ →2C ₆ H ₁₂ + compound K +H ₂ What is the identity of compound K ? (AMKSS 2024 Q17 - edited) A C ₄ H ₈ B C ₄ H ₁₀ C C ₈ H ₁₆ D C ₈ H ₁₈ 4 Many molecules of X combine to form a single molecule Y, as shown in the equation. n X → Y (n is a large number) Which terms best describe X and Y in this reaction? (GESS 2024 Q19) X Y A fraction monomer B monomer fraction C monomer polymer D polymer fraction 5 What reagent is used to differentiate a sample of ethanoic acid from ethanol? A Aqueous bromine B Acidified potassium manganate (VII) C Limewater D Lighted splint WRITE YOUR ANSWERS IN THE GIVEN BOXES BELOW Q1 Q2 Q3 Q4 Q5 D B B C B © chiasamuel 2025

4 Section B – 15 marks Answer all questions. 6 (a) Naphtha is a fraction of crude oil. Naphtha is processed by cracking in an oil refinery. One of the molecules in naphtha is a saturated hydrocarbon X containing 5 carbon atoms. When this molecule is cracked it produces propane, C ₃ H ₈ and one other hydrocarbon Y. (i) Why is cracking of naphtha an important process in an oil refinery? Cracking of naphtha is an important process as hydrocarbons with larger carbon chains can be reduced into hydrocarbons with smaller carbon chains, which is economically more viable. [2] (ii) Draw a dot-and-cross diagram of propane. [2] (b) Deduce the general formula of the organic compound with a functional formula of –COOH. 𝐶 𝑛 − 1 𝐻 2 𝑛 − 1 𝐶𝑂𝑂𝐻 [1] [Total: 5] © chiasamuel 2025

5 7 In Formula One, octane is used as fuel. Octane belongs to a family of organic compounds called alkanes, which is also known as a hydrocarbon. Table 2.1 shows the melting points and boiling points of a few alkanes. Table 2.1 Name of alkane Formula Melting point / ºC Boiling point / ºC ethane C ₂ H ₆ -183 -89 propane C ₃ H ₈ -188 -42 butane C ₄ H ₁₀ -138 -1 pentane C ₅ H ₁₂ -130 36 hexane C ₆ H ₁₄ -95 69 octane C ₈ H ₁₈ -57 126 (a) What is the type of bonding in octane? Covalent [1] (b) Describe why a molecule of octane has a low boiling point. Octane has a simple covalent structure. Small amounts of energy is used to overcome the weak intermolecular forces of attraction between molecules. Thus, octane has a low boiling point. [3] (c) (i) Describe how you would differentiate a sample of octane with a sample of octene. I would add aq. bromine into both samples. The sample of octene decolourises the aq. bromine, changing its colour from purple to colourless. There will not be any observed change for the sample of octene when aq. bromine is added. [3] © chiasamuel 2025

6 (ii) Describe how you would differentiate a sample of ethanol with a sample of ethanoic acid. I would add a few drops of acidified potassium manganate (VII) to both samples. The sample of ethanol decolourises the acidified potassium manganate (VII), changing its colour from purple to colourless. The sample of ethanoic acid does not decolourise the acidified potassium manganate (VII), hence no colour change. [3] [Total: 10] [END OF PAPER] © chiasamuel 2025