Edexcel International A Level Chemistry: Introductory organic chemistry and alkanes (YCH11) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

This set tests Topic 4, Introductory Organic Chemistry and Alkanes, of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017. They test outcomes 4.1 to 4.18, which sit in Unit 1: Structure, Bonding and Introduction to Organic Chemistry, an International AS (IAS) unit.

Learn the content first with the study guide or the revision notes. The course hub and printable checklist set out the whole course; the free 10-minute diagnostics point you to weak areas.

Use C = 12.0, H = 1.0, O = 16.0 and Cl = 35.5 wherever you need relative atomic masses.

Questions

1. Explain how a hazard differs from a risk. [2]

2. This question is about bond breaking.

(a) Define the term free radical. [1] (b) Define the term electrophile. [1] (c) In ultraviolet light, the Cl–Cl bond breaks. State the type of bond fission and identify the products. [2]

3. Classify each reaction as addition, substitution, oxidation, reduction or polymerisation.

(a) CH₂=CHCH₃ + Br₂ → CH₂BrCHBrCH₃ [1] (b) C₃H₈ + Cl₂ → C₃H₇Cl + HCl [1] (c) n CH₂=CHCl → –(CH₂–CHCl)ₙ– [1] (d) CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O [1]

4. This question is about naming alkanes.

(a) Give the IUPAC name of CH₃CH₂CH(CH₃)CH(CH₃)CH₃. [1] (b) A student names a compound 2-ethylhexane. Explain why this name is wrong and give the correct name. [2] (c) Draw the displayed formula of 2,2-dimethylpropane. [1]

5. This question is about structural isomers.

(a) State what is meant by structural isomers. [1] (b) Name the five cycloalkanes with the molecular formula C₅H₁₀. [3] (c) Explain why no alkane has the formula C₅H₁₀. [1]

6. A refinery processes the fractions from crude oil.

(a) Undecane, C₁₁H₂₄, is cracked to give heptane and ethene only. Write the equation. [1] (b) Give two reasons why refineries crack heavy fractions. [2] (c) Pentane can be reformed into cyclopentane. Write the equation and state why reformed hydrocarbons are useful in petrol. [2]

7. Ethylcyclohexane, C₈H₁₆, is a cycloalkane that can be burned as a fuel.

(a) Write an equation for the complete combustion of ethylcyclohexane. [1] (b) In a limited supply of oxygen, ethylcyclohexane burns to give carbon monoxide and water only. Write the equation. [1] (c) Explain why carbon monoxide is toxic. [2] (d) Explain how oxides of nitrogen form when ethylcyclohexane is burned in an engine, and why they are harmful to the environment. Include an equation. [3]

8. A standby generator burns 3.42 kg of 2,2,4-trimethylpentane, C₈H₁₈, completely.

(a) Write the equation for the complete combustion of C₈H₁₈. [1] (b) Calculate the mass of carbon dioxide produced, in kg, to three significant figures. [3] (c) Calculate the volume of oxygen used, in dm³. Take the molar volume of a gas as 24.0 dm³ mol⁻¹. [2]

9. A city council is comparing petrol, bioethanol and hydrogen as fuels for its buses. Discuss the extent to which each fuel can be described as carbon-neutral. Include equations for the formation and the combustion of bioethanol. [6]

10. 2-methylpropane, (CH₃)₃CH, reacts with chlorine in ultraviolet light. One product is 2-chloro-2-methylpropane, (CH₃)₃CCl.

(a) Write an equation for the initiation step and state what the ultraviolet light does. [2] (b) Write the two propagation steps that form 2-chloro-2-methylpropane. [2] (c) Write a termination step that forms an alkane with eight carbon atoms, and name that alkane. [1] (d) A student reacts 5.80 g of 2-methylpropane and obtains 3.70 g of 2-chloro-2-methylpropane. Calculate the percentage yield. [3] (e) Give two reasons why this reaction has limited use for making 2-chloro-2-methylpropane. [2]

11. A student cracks liquid paraffin by passing its vapour over heated aluminium oxide and collects the gas over water.

(a) Describe a test that shows the gas contains an alkene, giving the result. [2] (b) The student must lift the delivery tube out of the water before stopping heating. Explain why. [2] (c) Identify one other hazard in this experiment and suggest how the risk can be reduced. [2] (d) One alkane in liquid paraffin is C₁₈H₃₈. Write an equation for its cracking into decane and butene only. [1]

Answers

1. A hazard is something with the potential to cause harm [1]. A risk is the likelihood that the hazard causes harm in the way the substance is used [1]. [2] Examiner insight: An answer that only says “risk is how dangerous it is” tends to lose the second mark, because that describes the hazard; the word likelihood (or chance) carries the point.

2. (a) A species with an unpaired electron [1]. (b) A species that accepts a pair of electrons to form a new covalent bond [1]. (c) Homolytic fission [1], giving two chlorine free radicals, 2Cl• [1]. Examiner insight: “Accepts electrons” is too vague for an electrophile; include the word pair, because that is what makes the definition complete.

3. (a) Addition [1] (b) Substitution [1] (c) Polymerisation [1] (d) Oxidation [1] Examiner insight: Give one type per equation as asked; listing two types for the same equation can be read as a contradiction and lose the mark.

4. (a) 2,3-dimethylpentane [1] (b) The longest chain has seven carbons, running through the ethyl group, so hexane is not the parent chain [1]. The correct name is 3-methylheptane [1]. (c) All atoms and bonds shown, with four CH₃ groups on a central carbon [1]:

         H
         |
      H--C--H
    H    |    H
    |    |    |
 H--C----C----C--H
    |    |    |
    H    |    H
      H--C--H
         |
         H

Examiner insight: A displayed formula must show every C–H bond; writing CH₃ on the diagram turns it into a structural formula and usually loses the mark.

5. (a) They share a molecular formula but differ in structural formula [1]. (b) Cyclopentane and methylcyclobutane [1]; ethylcyclopropane [1]; 1,1-dimethylcyclopropane and 1,2-dimethylcyclopropane [1]. (c) An alkane with five carbons has the formula CₙH₂ₙ₊₂, which is C₅H₁₂ [1]. Examiner insight: For the isomer list, two names for the same structure (such as dimethylcyclopropane twice with no locants) count once, so always give the numbers.

6. (a) C₁₁H₂₄ → C₇H₁₆ + 2C₂H₄ [1] (b) Demand for short-chain fractions such as petrol is greater than the supply from distillation [1]. Cracking also produces alkenes, which are used to make polymers [1]. (c) C₅H₁₂ → C₅H₁₀ + H₂ [1]. Cyclic and branched hydrocarbons burn more smoothly in petrol engines, so they promote efficient combustion [1]. Examiner insight: “Cracking makes more useful products” alone is unlikely to score; name the product and its use, and tie it to demand.

7. (a) C₈H₁₆ + 12O₂ → 8CO₂ + 8H₂O [1] (b) C₈H₁₆ + 8O₂ → 8CO + 8H₂O [1] (c) Carbon monoxide binds to the haemoglobin in red blood cells, more strongly than oxygen does [1]. This reduces the amount of oxygen carried to the body’s cells [1]. (d) The engine’s very high temperature makes nitrogen from the air combine with oxygen [1]: N₂ + O₂ → 2NO [1]. Oxides of nitrogen are acidic; NO₂ dissolves in rain to form nitric acid, causing acid rain [1]. Examiner insight: For part (d), answers that say the nitrogen comes from the fuel lose credit; the nitrogen is from air, and the high temperature is the condition you must state.

8. (a) 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O [1] (b) Mᵣ of C₈H₁₈ = 114.0; amount = 3420 ÷ 114.0 = 30.0 mol [1]. Each mole gives 8 mol CO₂, so 240 mol CO₂ [1]. Mass = 240 × 44.0 = 10 560 g = 10.6 kg [1]. (c) O₂ needed = 30.0 × 12.5 = 375 mol [1]. Volume = 375 × 24.0 = 9000 dm³ [1]. Examiner insight: Convert kg to g before dividing by Mᵣ; using 3.42 instead of 3420 gives an answer a thousand times too small, and clear working gives you the best chance of credit for the steps that are right.

9. Petrol is not carbon-neutral: it releases carbon that was stored underground for millions of years, adding to atmospheric CO₂ [1]. Bioethanol: plants take in CO₂ by photosynthesis, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ [1]. Fermentation, C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, and combustion, C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O, release CO₂ [1]. Six moles of CO₂ are taken in and six released for each mole of glucose, so in theory there is no net release [1]. In practice, energy for farming, fertiliser, distillation and transport often comes from fossil fuels, so bioethanol is not fully carbon-neutral [1]. Hydrogen produces only water when it burns, but it is carbon-neutral only if it is made without releasing CO₂, for example by electrolysis using renewable electricity, not from methane [1]. [6] Examiner insight: A discussion question rewards a judgement on each fuel; three descriptions with no conclusion about how carbon-neutral each one is tend to score well below the full mark.

10. (a) Cl₂ → 2Cl• [1]. The UV supplies enough energy for homolytic fission of the Cl–Cl bond [1]. (b) Cl• + (CH₃)₃CH → (CH₃)₃C• + HCl [1]; (CH₃)₃C• + Cl₂ → (CH₃)₃CCl + Cl• [1]. (c) 2(CH₃)₃C• → (CH₃)₃CC(CH₃)₃, 2,2,3,3-tetramethylbutane [1]. (d) Amount of 2-methylpropane = 5.80 ÷ 58.0 = 0.100 mol [1]. Theoretical mass of C₄H₉Cl = 0.100 × 92.5 = 9.25 g [1]. Percentage yield = 3.70 ÷ 9.25 × 100 = 40.0% [1]. (e) Further substitution gives dichloro and more highly substituted products [1]. Substitution at a CH₃ carbon gives the isomer 1-chloro-2-methylpropane, so a mixture forms that must be separated [1]. Examiner insight: In propagation, the radical dot must sit on the carbon, (CH₃)₃C•, and the first step must form HCl, not H•; a step that makes a hydrogen radical is wrong chemistry, so it cannot gain credit.

11. (a) Shake the gas with bromine water [1]. The colour changes from orange to colourless [1]. (b) If heating stops while the tube is in the water, the gas in the apparatus cools and contracts [1]. Cold water is sucked back into the hot boiling tube, which can crack it [1]. (c) The alkene products are flammable [1], so keep the collected gas away from naked flames and work on a small scale [1]. (d) C₁₈H₃₈ → C₁₀H₂₂ + 2C₄H₈ [1] Examiner insight: Write colourless rather than “clear”, because a clear solution can still be coloured; the full result is orange (or brown) to colourless.

Where marks are usually lost

• Describing risk as the dangerous property of a substance instead of the likelihood of harm.
• Defining an electrophile as an “electron acceptor” without the word pair.
• Naming an alkane from the row as drawn when a longer chain runs through a branch.
• Writing CH₃ inside a “displayed” formula.
• Listing two cycloalkanes that are really the same structure because no locants were given.
• Unbalanced cracking equations, often from forgetting how many alkene molecules are needed.
• Linking CO to “less oxygen in the air” rather than to haemoglobin.
• Saying oxides of nitrogen come from the fuel.
• Calling bioethanol or hydrogen carbon-neutral with no condition attached.
• Dividing a mass in kg by Mᵣ without converting to grams.

Next steps

• Revision notes for this topic
• Study guide for this topic
• Edexcel International A Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, Pearson Education Limited: Unit 1, Topic 4: Introductory Organic Chemistry and Alkanes, outcomes 4.1 to 4.18.