Which one among the following statements is not correct?
- (a)The reaction of methane and chlorine in sunlight produces hydrogen gas.
- (b)Palladium catalyzes the addition of hydrogen gas on an alkene.
- (c)Ethanol burns in oxygen to give carbon dioxide, water and releases heat and light.
- (d)Unsaturated hydrocarbons burn with a sooty flame.
Correct — A, The reaction of methane and chlorine in sunlight produces hydrogen gas. The question asks for the statement that is not correct, and this is it. Methane and chlorine in sunlight undergo free-radical substitution: a chlorine atom replaces a hydrogen atom on the methane, giving chloromethane and hydrogen chloride, and successive substitutions give dichloromethane, trichloromethane and finally tetrachloromethane. The hydrogen that leaves the methane departs bonded to chlorine as HCl gas, not as free H2. The other three statements are all correct as written.
- (b)Palladium catalyzes the addition of hydrogen gas on an alkene. — Correct. Palladium, along with nickel and platinum, catalyses the addition of hydrogen across a carbon-carbon double bond. This is the reaction that converts unsaturated vegetable oils into semi-solid fats.
- (c)Ethanol burns in oxygen to give carbon dioxide, water and releases heat and light. — Correct, and a completely ordinary combustion equation. Ethanol burns with a clean blue flame to carbon dioxide and water, which is why it is used as a fuel and blended into petrol.
- (d)Unsaturated hydrocarbons burn with a sooty flame. — Correct. A high proportion of carbon to hydrogen means combustion is often incomplete in ordinary air, leaving unburnt carbon particles that glow yellow and deposit soot — the standard laboratory distinction between saturated and unsaturated hydrocarbons.
Substitution and addition are the two reaction types that divide the hydrocarbon chapter. Saturated hydrocarbons have no spare bonding capacity, so they react by substitution: in sunlight, chlorine replaces hydrogen on methane one atom at a time, and each replaced hydrogen leaves as hydrogen chloride. Unsaturated hydrocarbons have double or triple bonds and react by addition: hydrogen, halogens or halogen acids add across the multiple bond, with nickel, platinum or palladium catalysing the hydrogen case. Combustion is common to both, but the sootiness of the flame differs with the carbon-to-hydrogen ratio.
Three of the four statements here are standard textbook sentences reproduced almost word for word, and the fourth has been altered by changing only its product. That is the pattern to look for in a not-correct item: the false option is rarely absurd, it is usually a familiar reaction with one term swapped. Substitution chemistry is a good place to check, because the second product of a chlorination is easy to misremember, and HCl and H2 look similar in a hurried reading. This card marks a different letter from the answer we hold on file; the working above is the reason.
- Methane and chlorine in sunlight give chloromethane and hydrogen chloride by free-radical substitution.
- Further substitution gives dichloromethane, trichloromethane and tetrachloromethane.
- Nickel, platinum and palladium catalyse the addition of hydrogen across a carbon-carbon double bond.
- Hydrogenation of vegetable oils turns unsaturated oils into semi-solid fats such as vanaspati.
- Unsaturated hydrocarbons burn with a yellow sooty flame because of incomplete combustion.
In a not-correct item the false statement is usually a real reaction with one product changed, which is exactly what has been done to (a).
- Reading HCl as H2 in a chlorination equation.
- Assuming a catalyst changes the products; palladium only speeds the hydrogen addition.
- Thinking a sooty flame indicates a saturated hydrocarbon; it is the unsaturated ones that soot.
A not-correct item where the false statement is a familiar reaction with one product swapped, so it rewards knowing equations rather than topics.
Naphthalene burns with a yellow sooty flame. This is because
- (a) carbon to hydrogen ratio is low
- (b) there is incomplete combustion
- (c) there is excess supply of air
- (d) of presence of impurities of nitrogen and sulfur
Answer(b) there is incomplete combustion
Explains statement (d) of this question. A sooty flame is the visible sign of incomplete combustion, which is what a carbon-rich unsaturated compound tends to give in ordinary air.
Hydrogenation of alkenes can be carried out in the presence of
- (a) copper
- (b) zinc
- (c) aluminium
- (d) nickel
Answer(d) nickel
The same reaction as statement (b) of this question, keyed on an official CAPF paper. Nickel is the catalyst that item names; palladium and platinum do the same job, which is why (b) here is correct.
- practice — not a real PYQ
The reaction of methane with chlorine in the presence of sunlight is an example of
- (a)an addition reaction
- (b)a substitution reaction
- (c)an oxidation reaction
- (d)a neutralisation reaction
Answer(b) a substitution reaction — a chlorine atom takes the place of a hydrogen atom on the saturated methane molecule.
- practice — not a real PYQ
Vegetable oils are converted to vanaspati ghee by
- (a)oxidation with alkaline KMnO4
- (b)addition of hydrogen in the presence of a nickel catalyst
- (c)dehydration with concentrated sulphuric acid
- (d)reaction with sodium metal
Answer(b) addition of hydrogen in the presence of a nickel catalyst — hydrogen adds across the double bonds, saturating the oil.