The 2018 Chemistry paper focuses heavily on organic chemistry reaction pathways, structural formulas, and fundamental physical chemistry calculations. Prominent recurring themes include the properties and reactions of hydrocarbons and oxygen-containing organic compounds, gas laws alongside rates of diffusion, and acid-base neutralization or pH calculations. To get the most out of practicing this page, carefully read the step-by-step explanations for any calculation or reaction mechanism you miss to reinforce your conceptual understanding.
Topics in this paper
The fraction of crude oil used as jet fuel is
Distractor check: Options A, B, and D mention other crude oil distillation products like refinery gas, diesel, or gasoline, which have different boiling ranges and engine applications. Reasoning to the answer: Jet fuel consists primarily of kerosene, a petroleum fraction recovered during fractional distillation within a boiling range of roughly 150–275°C that provides the necessary stability and energy for commercial and military aircraft engines. Common mistake: Confusing jet fuel with heavier diesel fractions or lighter gasoline.
What is the IUPAC name of the compound CH₃-CH₂-CH(CH₃)-C=C(CH₃)-CH₂-CH₃?
A student might incorrectly choose B or D by miscounting the carbon chain as a hex-3-ene or hex-2-ene derivative rather than identifying the longest 7-carbon parent chain. To determine the correct IUPAC name, first identify the principal chain of seven carbons forming a heptane skeleton, then locate the double bond so it receives the lowest possible locant, placing it at position 3 (hept-3-ene). Afterward, assign the methyl substituents at carbons 3 and 5 in ascending order. Common mistake: failing to number the longest continuous carbon chain from the end that gives the double bond the lowest locant.
It is not desirable to use lead tetraethyl as an anti-knock agent because
Option A or D might look plausible if a student confuses an anti-knock agent's role with altering fuel volatility or octane numbers. Historically used to suppress engine knocking, lead tetraethyl releases harmful lead particles through tailpipe exhausts that cause severe environmental contamination and neurological risks, making exhaust pollution the primary reason for its discontinuation. Common mistake: confusing fuel performance metrics with environmental toxicity outcomes.
The carbon atoms in ethene (C₂H₄) are
A student might incorrectly guess A or C by confusing the hybridization of an alkene with single-bonded alkanes or triple-bonded alkynes. In ethene (C₂H₄), each carbon atom forms a double bond consisting of one sigma and one pi bond, alongside two single sigma bonds, which necessitates three hybrid orbitals. This structural requirement yields sp² hybridization and establishes a trigonal planar geometry with characteristic bond angles of approximately 120°. Common mistake: counting individual bonds rather than regions of electron density to determine hybridization state.
Catalytic hydrogenation of benzene (C₆H₆) produces
Option A or B might be selected if a student mistakes the benzene ring for a generic aromatic or lipid-like substance like margarine. The catalytic hydrogenation of benzene (C₆H₆) involves introducing hydrogen gas (H₂) over a nickel catalyst, which breaks the aromatic stability and transforms the cyclic structure into a saturated cycloalkane. Consequently, the reaction C₆H₆ + 3H₂ yields cyclohexane, representing a six-carbon ring containing only single bonds. Common mistake: forgetting that adding hydrogen to an aromatic ring eliminates unsaturation and yields a cycloalkane.
The functional group present in an organic compound that reacts with a saturated solution of NaHCO₃ to produce CO₂ gas is
A student could mistakenly select option A or C by confusing the reactions of alcohols or carbonyls with standard acid-base tests. Carboxylic acids feature the carboxyl group (-COOH), which contains an acidic proton capable of reacting with a sodium bicarbonate (NaHCO₃) solution to release carbon dioxide gas, water, and a salt via the reaction RCOOH + NaHCO₃ → RCOONa + H₂O + CO₂. Common mistake: assuming all oxygen-containing organic functional groups react with bicarbonate to release carbon dioxide.
The characteristic reaction of carbonyl compounds (aldehydes and ketones) is
Option A or B might be chosen by a student who confuses carbonyl reactivity with alkane or alkyl halide reaction pathways. Aldehydes and ketones contain a polar carbon-oxygen double bond where the carbon atom is electrophilic, making them prone to nucleophilic attack. Because of this electron deficiency, they engage in nucleophilic addition reactions with reagents like HCN or NaHSO₃ across the π-bond to form cyanohydrins or bisulfite addition products. Common mistake: assuming C=O double bonds undergo substitution instead of addition.
Palm wine turns sour with time because
A student might pick option A if they think the sugar content turning into alcohol makes the wine taste sour, overlooking the subsequent chemical conversion. Palm wine becomes sour over time because bacteria such as Acetobacter drive a microbial oxidation process that converts ethanol into acetic acid through the reaction C₂H₅OH + O₂ → CH₃COOH + H₂O. Common mistake: stopping at alcohol fermentation and missing the secondary microbial oxidation step that produces organic acids.
Which of the following represents saponification?
Option A might seem correct if a student confuses general carboxylic acid neutralization with the alkaline hydrolysis of an ester. Saponification is specifically the base-promoted cleavage of an ester using a base such as sodium hydroxide (NaOH), which yields an alcohol and a carboxylate salt (soap) following the general equation RCOOR' + NaOH → RCOONa + R'OH. Common mistake: confusing standard acid-base neutralization with ester hydrolysis.
The confirmatory test for alkanoic acids in organic qualitative analysis is
Options A or B might be mistakenly chosen because they are general reactions of acids or functional group conversions, but they lack the specificity required for a confirmatory test. Alkanoic acids react with sodium carbonate (Na₂CO₃) to liberate carbon dioxide gas, which produces effervescence and turns lime water milky according to the equation 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂. Common mistake: confusing a general pH indicator color change with a definitive functional group confirmatory test.
Which of the following is a secondary alcohol?
A student might select option A or C by choosing a straight-chain primary alcohol instead of evaluating the carbon attachment points. A secondary alcohol is defined by a hydroxyl (-OH) group bonded to a carbon atom that is itself attached to two other carbon groups, which is precisely the arrangement found in propan-2-ol (CH₃CH(OH)CH₃). Common mistake: counting the total number of carbons in the chain rather than checking the degree of substitution on the hydroxyl-bearing carbon.
Which of the following compounds reacts with both sodium metal and silver nitrate solution?
Option A or D might be mistakenly picked by a student who assumes any alkyne will react with sodium metal and silver nitrate. Only terminal alkynes possess an acidic hydrogen atom bonded to a triple-bonded carbon (≡C-H), allowing CH₃C≡CH (propyne) to react with sodium to form a sodium acetylide and with AgNO₃ to yield a white silver acetylide precipitate. Common mistake: ignoring the position of the triple bond when predicting acidity and metal substitution reactions.
Ethanol and dimethyl ether have the same molecular formula (C₂H₅OH and CH₃OCH₃). This phenomenon is known as
Options B, C, or D might be selected if a student confuses structural formulas with allotropes or elemental isotopes. Ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃) share an identical molecular formula of C₂H₆O but possess entirely different atom arrangements and functional groups, placing them in the category of structural isomers. Common mistake: confusing isomerism in organic compounds with allotropy or isotopic variations.
The reaction of ethene with bromine water is an example of
Option A might be chosen if a student confuses the breaking of a pi bond with the replacement of a hydrogen atom. Ethene (C₂H₄) reacts with bromine water (Br₂/H₂O) via electrophilic addition, wherein the carbon-carbon double bond opens up and bromine atoms attach to adjacent carbons to yield 1,2-dibromoethane (C₂H₄Br₂). Common mistake: assuming multiple bonds undergo substitution rather than addition.
What gas is produced when ethanol is heated with concentrated H₂SO₄ at 170°C?
A student might mistakenly choose option A or C by confusing dehydration products with combustion byproducts or alkane reductions. When ethanol (C₂H₅OH) is subjected to heating at 170°C with concentrated sulfuric acid (H₂SO₄), it undergoes an elimination reaction where a molecule of water is removed to generate ethene (C₂H₄) via the pathway C₂H₅OH → C₂H₄ + H₂O. Common mistake: overlooking the role of concentrated sulfuric acid as a dehydrating agent at high temperatures.
The compound formed when ethanoic acid reacts with ethanol in the presence of a catalyst is
Option B or C might be chosen if a student mixes up the alkyl groups derived from the starting acid and alcohol reactants. Ethanoic acid (CH₃COOH) combines with ethanol (C₂H₅OH) via an acid-catalyzed esterification reaction to produce ethyl ethanoate (CH₃COOC₂H₅) and water, deriving its name from the ethyl group of the alcohol and the ethanoate portion of the acid. Common mistake: reversing the order of the acid and alcohol components when naming the resulting ester.
The oxidation of a primary alcohol with acidified potassium permanganate produces
Option A might be mistakenly selected by a student who thinks strong oxidizing agents stop at the intermediate aldehyde stage. Primary alcohols treated with an excess of a strong oxidizing agent such as acidified potassium permanganate (KMnO₄) undergo complete oxidation past the aldehyde level to yield a carboxylic acid, converting ethanol directly to ethanoic acid via the sequence C₂H₅OH → CH₃CHO → CH₃COOH. Common mistake: stopping the oxidation pathway at the aldehyde stage when using strong oxidizing agents.
Which of the following is a test for unsaturation in organic compounds?
Options B, C, or D might be selected if a student confuses unsaturation tests with functional group tests for acids or terminal alkynes. Unsaturation in alkenes is identified by treating the sample with bromine water (Br₂/H₂O), which causes the characteristic red-brown color of bromine to vanish as it adds across the multiple bond to form a dibromo compound like 1,2-dibromoethane. Common mistake: confusing tests for double bonds with tests for specific functional groups like carboxylic acids.
The reaction of ethanamide (CH₃CONH₂) with bromine and NaOH produces
Option B or C might be mistakenly chosen by a student who forgets that the Hofmann rearrangement alters the carbon chain length. The reaction of ethanamide (CH₃CONH₂) with bromine and sodium hydroxide (NaOH) removes a carbonyl carbon as sodium carbonate, yielding methylamine (CH₃NH₂), which contains one fewer carbon atom than the starting amide. Common mistake: keeping the original carbon chain length when predicting the products of an amide degradation reaction.
Glucose can be distinguished from fructose by
Options A, B, or C might be selected if a student uses general reducing sugar tests that react positively with both monosaccharide types. Seliwanoff’s test successfully differentiates aldoses like glucose from ketoses like fructose because fructose is a ketose that reacts rapidly with resorcinol in hydrochloric acid to yield a red-colored complex. Common mistake: using general tests like Benedict's or Tollen's to distinguish between different classes of monosaccharides.
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