Why does a gas stove burn with a blue flame but a candle with a yellow, sooty one? How is ethanol turned into vinegar's acid, and a liquid oil into solid vanaspati? Learn the four main reactions of carbon compounds — combustion, oxidation, addition and substitution — with balanced equations, conditions and mass calculations.
Every day you use carbon compounds as fuels: the LPG in your kitchen, the petrol in a scooter, the wax of a candle, the camphor lit at a puja. You can see at once that they do not burn the same way — some give a clean blue flame, others a smoky yellow one that blackens a vessel. You also know that a bottle of ethanol-containing wine left open turns sour, and that the oil you fry in is a liquid while vanaspati ghee is a solid. All of these are chemical reactions of carbon compounds.
Carbon compounds do not react in random ways: because the same functional groups appear again and again, a small number of reaction types explains most of what happens. In this section you will study four of them. Combustion (burning) of carbon compounds, with the difference between clean and sooty flames; oxidation of an alcohol to a carboxylic acid; addition of hydrogen to unsaturated compounds; and substitution of hydrogen by chlorine in a saturated hydrocarbon. Each comes with equations that you must be able to balance, and with calculations that follow the law of conservation of mass (Section 1.1).
Combustion and the Colour of Flames
Carbon and all its compounds burn in oxygen, giving out heat and light. This is why almost every fuel — wood, coal, petroleum, natural gas — is a carbon compound.
📘 Definition 4.9 — Combustion
Combustion is the burning of a substance in oxygen (or air) with the release of heat and usually light. When a carbon compound burns completely, all of its carbon becomes carbon dioxide and all of its hydrogen becomes water. If the supply of oxygen is limited, the combustion is incomplete: carbon monoxide and unburnt carbon (soot) are formed as well.
📐 Principle 4.8 — Equations for complete combustion
A compound of carbon and hydrogen (or of carbon, hydrogen and oxygen) burns completely as
$$\mathrm{\text{fuel} + O_{2} \rightarrow CO_{2} + H_{2}O + \text{heat and light}}$$
Examples, balanced: $$\mathrm{C(s) + O_{2}(g) \rightarrow CO_{2}(g)}$$ $$\mathrm{CH_{4}(g) + 2O_{2}(g) \rightarrow CO_{2}(g) + 2H_{2}O(l)}$$ $$\mathrm{C_{2}H_{5}OH(l) + 3O_{2}(g) \rightarrow 2CO_{2}(g) + 3H_{2}O(l)}$$ $$\mathrm{C_{2}H_{4}(g) + 3O_{2}(g) \rightarrow 2CO_{2}(g) + 2H_{2}O(l)}$$ To balance: first the carbon atoms (one $\mathrm{CO_{2}}$ for each C), then the hydrogen atoms (one $\mathrm{H_{2}O}$ for each two H), then count the oxygen atoms on the right and supply them as $\mathrm{O_{2}}$ — using a fraction, then doubling the whole equation if the result is odd.
✏️ Example 13 — Balancing combustion equations
Write balanced equations, with state symbols, for the complete combustion of (a) propane $\mathrm{C_{3}H_{8}}$; (b) ethyne $\mathrm{C_{2}H_{2}}$; (c) butane $\mathrm{C_{4}H_{10}}$ (the main gas in LPG).
💡 Show solution▶
(a) 3 C give $3\,\mathrm{CO_{2}}$; 8 H give $4\,\mathrm{H_{2}O}$; oxygen atoms needed $=6+4=10$, i.e. $5\,\mathrm{O_{2}}$: $$\mathrm{C_{3}H_{8}(g) + 5O_{2}(g) \rightarrow 3CO_{2}(g) + 4H_{2}O(l)}$$
(b) 2 C give $2\,\mathrm{CO_{2}}$; 2 H give $1\,\mathrm{H_{2}O}$; oxygen atoms $=4+1=5$, i.e. $2\tfrac{1}{2}\,\mathrm{O_{2}}$. Double everything: $$\mathrm{2C_{2}H_{2}(g) + 5O_{2}(g) \rightarrow 4CO_{2}(g) + 2H_{2}O(l)}$$
(c) 4 C give $4\,\mathrm{CO_{2}}$; 10 H give $5\,\mathrm{H_{2}O}$; oxygen atoms $=8+5=13$, i.e. $6\tfrac{1}{2}\,\mathrm{O_{2}}$. Double: $$\mathrm{2C_{4}H_{10}(g) + 13O_{2}(g) \rightarrow 8CO_{2}(g) + 10H_{2}O(l)}$$
Check (a) C $3=3$, H $8=8$, O $10=10$; (b) C $4=4$, H $4=4$, O $10=10$; (c) C $8=8$, H $20=20$, O $26=26$. ✓
Look at a flame closely. In a gas stove with its air holes open, the flame is blue: the fuel is mixed with plenty of air and burns completely, with no soot. If the air holes get blocked, the flame turns yellow and soot blackens the bottom of the vessel. A candle, a kerosene lamp or a burning piece of camphor burn with yellow, smoky flames: much of the carbon is not fully burnt and glows as tiny solid particles of soot.
📐 Principle 4.9 — Saturated and unsaturated fuels burn differently
Saturated hydrocarbons (methane, propane, butane) generally burn with a clean blue flame in a good supply of air. Unsaturated hydrocarbons (ethene, ethyne) burn with a yellow, sooty flame with black smoke, because they contain a larger percentage of carbon and the oxygen of the air is not enough to burn all of it completely. Incomplete combustion also gives carbon monoxide, a poisonous gas. Burning ethyne with pure oxygen instead of air gives a very hot, clean flame (about $3000\ ^{\circ}\mathrm{C}$), used in welding.
✏️ Example 14 — The carbon content and the flame
With $\mathrm{C}=12$, $\mathrm{H}=1\ \mathrm{u}$, find the percentage of carbon by mass in methane $\mathrm{CH_{4}}$, ethane $\mathrm{C_{2}H_{6}}$, ethene $\mathrm{C_{2}H_{4}}$ and ethyne $\mathrm{C_{2}H_{2}}$. Which will burn with the sootiest flame in air?
Answer The carbon percentage rises with unsaturation, and ethyne (92.3 %) is the richest in carbon, so it burns with the sootiest flame in air.
✏️ Example 15 — A mass balance for natural gas
Natural gas is mostly methane. With $\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16\ \mathrm{u}$, find the masses of oxygen used and of carbon dioxide and water formed when $16\ \mathrm{g}$ of methane burns completely. Show that mass is conserved.
Step 2 $16\ \mathrm{g}$ of methane is exactly the equation mass, so: oxygen used $=64\ \mathrm{g}$; $\mathrm{CO_{2}}=44\ \mathrm{g}$; $\mathrm{H_{2}O}=36\ \mathrm{g}$.
Choose a fuel and an oxygen supply, then watch the flame. A cold pan above the flame collects soot. Predict first: which fuels give a yellow sooty flame with the air holes open? The readouts give the carbon percentage and the balanced equation for complete combustion.
fuel—
carbon by mass—
flame—
products—
Oxidation: Ethanol into Ethanoic Acid
Burning is oxidation in its fastest form: the carbon compound gains oxygen. Some oxidising agents bring about a gentler, controlled oxidation in which a functional group changes but the carbon chain stays. The best-known example turns an alcohol into a carboxylic acid.
📘 Definition 4.10 — Oxidation of a carbon compound
Oxidation of a carbon compound is a reaction in which it gains oxygen (or loses hydrogen) (Section 1.3). Substances that supply the oxygen are oxidising agents; two of them are alkaline potassium permanganate ($\mathrm{KMnO_{4}}$) and acidified potassium dichromate ($\mathrm{K_{2}Cr_{2}O_{7}}$).
📐 Principle 4.10 — Oxidation of ethanol
Ethanol is oxidised to ethanoic acid when it is heated with alkaline $\mathrm{KMnO_{4}}$ or acidified $\mathrm{K_{2}Cr_{2}O_{7}}$; writing $[\mathrm{O}]$ for the oxygen supplied by the oxidising agent:
$$\mathrm{CH_{3}CH_{2}OH + 2[O] \xrightarrow[\text{or acidified } \mathrm{K_{2}Cr_{2}O_{7}}]{\text{alkaline } \mathrm{KMnO_{4}},\ \text{heat}} CH_{3}COOH + H_{2}O}$$
The purple colour of $\mathrm{KMnO_{4}}$ is lost (it is used up) and the orange of dichromate turns green. The same change happens, slowly, when wine or fermented juice is left open in air: bacteria oxidise ethanol to ethanoic acid, and the liquid turns into vinegar.
✏️ Example 16 — How much acid from ethanol?
With $\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16\ \mathrm{u}$, find the mass of ethanoic acid obtained by complete oxidation of $4.6\ \mathrm{g}$ of ethanol, the mass of oxygen $[\mathrm{O}]$ used, and the mass of water formed. Check the conservation of mass.
💡 Show solution▶
Step 1 $\mathrm{C_{2}H_{5}OH}=46\ \mathrm{u}$; $2[\mathrm{O}]=32\ \mathrm{u}$; $\mathrm{CH_{3}COOH}=60\ \mathrm{u}$; $\mathrm{H_{2}O}=18\ \mathrm{u}$. Atom check: C $2=2$; H $6=4+2$; O $1+2=2+1$ ✓.
Step 2 $4.6\ \mathrm{g}$ is $\tfrac{4.6}{46}=0.1$ of the equation mass. Ethanoic acid: $0.1\times60=6.0\ \mathrm{g}$. Oxygen: $0.1\times32=3.2\ \mathrm{g}$. Water: $0.1\times18=1.8\ \mathrm{g}$.
Check $4.6+3.2=7.8\ \mathrm{g}$ before; $6.0+1.8=7.8\ \mathrm{g}$ after ✓.
Addition Reactions: Hydrogenation
Unsaturated compounds have double or triple bonds, which can open up to take on more atoms. Saturated compounds have no spare bonds and cannot do this.
📘 Definition 4.11 — Addition reaction
An addition reaction is one in which atoms are added across a double or triple bond of an unsaturated compound, which becomes saturated (or less unsaturated). The addition of hydrogen, hydrogenation, takes place in the presence of a finely divided nickel or palladiumcatalyst (a substance that speeds up a reaction without being used up).
📐 Principle 4.11 — Hydrogenation
$$\mathrm{CH_{2}{=}CH_{2} + H_{2} \xrightarrow{\text{Ni or Pd}} CH_{3}CH_{3}}\qquad \text{(ethene to ethane)}$$
$$\mathrm{HC{\equiv}CH + 2H_{2} \xrightarrow{\text{Ni}} CH_{3}CH_{3}}\qquad \text{(ethyne to ethane)}$$
Industrial use: vegetable oils such as groundnut, mustard and sunflower oil have long unsaturated carbon chains and are liquids. Hydrogenation with a nickel catalyst turns them into saturated fats, solid at room temperature — vanaspati ghee. Animal fats such as butter and ghee are already saturated; oils with unsaturated chains are healthier to eat than saturated fats, which is why oils are sold as “cholesterol free”.
✏️ Example 17 — Hydrogen for hydrogenation
(a) What mass of hydrogen is needed to convert $2.8\ \mathrm{g}$ of ethene to ethane, and what mass of ethane forms? (b) What mass of hydrogen converts $2.6\ \mathrm{g}$ of ethyne to ethane? ($\mathrm{C}=12$, $\mathrm{H}=1\ \mathrm{u}$.) (c) Why can ethane not take up hydrogen?
💡 Show solution▶
(a) $\mathrm{C_{2}H_{4}}=28$, $\mathrm{H_{2}}=2$, $\mathrm{C_{2}H_{6}}=30\ \mathrm{u}$. $2.8\ \mathrm{g}$ is $0.1$ of $28$: hydrogen $0.1\times2=0.2\ \mathrm{g}$; ethane $0.1\times30=3.0\ \mathrm{g}$. ✓ ($2.8+0.2=3.0$)
(b) $\mathrm{C_{2}H_{2}}=26\ \mathrm{u}$ and two $\mathrm{H_{2}}$ $=4\ \mathrm{u}$. $2.6\ \mathrm{g}$ is $0.1$ of $26$: hydrogen $0.1\times4=0.4\ \mathrm{g}$, giving $3.0\ \mathrm{g}$ of ethane. ✓ ($2.6+0.4=3.0$)
(c) Ethane is saturated: all carbon–carbon bonds are single, so there is no double or triple bond to open.
Substitution Reactions
Saturated hydrocarbons are rather unreactive: they have no multiple bond to open. But in sunlight they react with chlorine.
📘 Definition 4.12 — Substitution reaction
A substitution reaction is one in which an atom or group of a compound is replaced by another atom or group. In the chlorination of a saturated hydrocarbon, hydrogen atoms are replaced one at a time by chlorine atoms, and hydrogen chloride is the other product.
📐 Principle 4.12 — Chlorination of methane
In sunlight, methane and chlorine react in steps:
$$\mathrm{CH_{4} + Cl_{2} \xrightarrow{\text{sunlight}} CH_{3}Cl + HCl}\qquad\text{(chloromethane)}$$
$$\mathrm{CH_{3}Cl + Cl_{2} \xrightarrow{\text{sunlight}} CH_{2}Cl_{2} + HCl}\qquad\text{(dichloromethane)}$$
$$\mathrm{CH_{2}Cl_{2} + Cl_{2} \xrightarrow{\text{sunlight}} CHCl_{3} + HCl}\qquad\text{(trichloromethane)}$$
$$\mathrm{CHCl_{3} + Cl_{2} \xrightarrow{\text{sunlight}} CCl_{4} + HCl}\qquad\text{(tetrachloromethane)}$$
Without sunlight the reaction does not occur. Compare: addition happens to unsaturated compounds; substitution is typical of saturated ones.
✏️ Example 18 — Chlorinating methane
(a) Write the balanced equation for the first step of the chlorination of ethane in sunlight and name the product. (b) $1.6\ \mathrm{g}$ of methane reacts with $7.1\ \mathrm{g}$ of chlorine to make chloromethane. Find the masses of chloromethane and hydrogen chloride formed ($\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{Cl}=35.5\ \mathrm{u}$) and check the mass balance.
💡 Show solution▶
(a) $$\mathrm{C_{2}H_{6} + Cl_{2} \xrightarrow{\text{sunlight}} C_{2}H_{5}Cl + HCl}$$ The product is chloroethane. Check: C $2=2$; H $6=5+1$; Cl $2=1+1$ ✓.
(b) $\mathrm{CH_{4}}=16$, $\mathrm{Cl_{2}}=71$, $\mathrm{CH_{3}Cl}=50.5$, $\mathrm{HCl}=36.5\ \mathrm{u}$. $1.6\ \mathrm{g}$ is $0.1$ of $16$, and $7.1\ \mathrm{g}$ is $0.1$ of $71$, so the quantities fit exactly: $\mathrm{CH_{3}Cl}=0.1\times50.5=5.05\ \mathrm{g}$; $\mathrm{HCl}=0.1\times36.5=3.65\ \mathrm{g}$.
Check $1.6+7.1=8.7\ \mathrm{g}$ before; $5.05+3.65=8.7\ \mathrm{g}$ after ✓.
Interactive 2 — Reaction Lab
Choose a reaction. Press ▶ Play to switch between the reactants and the products; the part of the molecules that changes is circled in red. The readouts count every atom on each side, so you can see the equation balance.
type · condition—
atoms on the left—
atoms on the right—
mass (left = right)—
⚠️ Common mistakes with reactions of carbon compounds
1. Forgetting that unsaturated compounds burn with a sooty flame. Ethyne and ethene give black smoke in air; methane and LPG give a blue flame.
2. Writing hydrogen chloride as a product of addition. Addition of $\mathrm{H_{2}}$ gives only one product; substitution gives two (the substituted compound and HCl).
3. Leaving out the catalyst or condition. Hydrogenation needs Ni or Pd; chlorination of methane needs sunlight; oxidation of ethanol needs an oxidising agent and heat.
4. Not balancing with half molecules properly. If $\tfrac{5}{2}\,\mathrm{O_{2}}$ appears, double the whole equation.
5. Confusing combustion with oxidation by $\mathrm{KMnO_{4}}$. Burning destroys the whole molecule (to $\mathrm{CO_{2}}$ and $\mathrm{H_{2}O}$); controlled oxidation changes only the functional group (ethanol to ethanoic acid).
Exercises
I
Combustion and flames.
1 What is combustion? Name the products of complete combustion of a hydrocarbon.
3 Write the balanced equation for the complete combustion of ethane, $\mathrm{C_{2}H_{6}}$.
4 Why does a gas stove burn with a blue flame, and why does the flame turn yellow when the air holes are blocked?
5 Why do unsaturated hydrocarbons burn with a sooty flame while saturated ones burn with a clean flame?
6 Find the percentage of carbon in ethane and in ethyne ($\mathrm{C}=12$, $\mathrm{H}=1\ \mathrm{u}$).
7 What mass of carbon dioxide and of water forms when $11\ \mathrm{g}$ of propane burns completely? Check the mass balance ($\mathrm{O}=16\ \mathrm{u}$).
8 Write the equation for incomplete combustion of methane to carbon monoxide and water. Why is it dangerous to run a coal or gas heater in a closed room?
💡 Show solutions▶
1 Burning in oxygen with release of heat and light; carbon dioxide and water.
3 $\mathrm{2C_{2}H_{6}(g) + 7O_{2}(g) \rightarrow 4CO_{2}(g) + 6H_{2}O(l)}$. Check: C $4=4$; H $12=12$; O $14=8+6$ ✓.
4 With the air holes open, enough air mixes with the gas for complete combustion (blue flame). With the holes blocked, the oxygen is insufficient, so unburnt carbon glows yellow and is deposited as soot.
5 Unsaturated hydrocarbons contain a larger percentage of carbon; the oxygen in air is not enough to burn it all, so soot (and carbon monoxide) forms.
7 $\mathrm{C_{3}H_{8}}=44\ \mathrm{u}$: $11\ \mathrm{g}$ is $0.25$ of the equation mass. $\mathrm{CO_{2}}$: $0.25\times132=33\ \mathrm{g}$; $\mathrm{H_{2}O}$: $0.25\times72=18\ \mathrm{g}$; oxygen used: $0.25\times160=40\ \mathrm{g}$. Check: $11+40=51=33+18$ ✓.
8 $\mathrm{2CH_{4} + 3O_{2} \rightarrow 2CO + 4H_{2}O}$. Carbon monoxide is poisonous (it combines with the haemoglobin of blood), and it forms when the oxygen supply is poor.
II
Oxidation and addition.
1 Write the equation for the oxidation of ethanol to ethanoic acid. Name two oxidising agents that can be used.
2 What colour changes do you see when ethanol is warmed with (a) alkaline potassium permanganate; (b) acidified potassium dichromate?
3 What is an addition reaction? Give the equation for the addition of hydrogen to ethene and name the catalyst.
4 What is the industrial use of hydrogenation of vegetable oils? How does the physical state change?
5 What mass of hydrogen is needed to hydrogenate $5.6\ \mathrm{g}$ of ethene, and what mass of ethane forms?
6 What mass of hydrogen converts $13\ \mathrm{g}$ of ethyne to ethane, and how much ethane results?
7 Will butane react with hydrogen in the presence of nickel? Explain.
8Applied. A food label says “contains hydrogenated vegetable oil”. What does this mean for the carbon chains of the fat, and why is the fat a solid?
2 (a) The purple colour of permanganate disappears. (b) The orange of dichromate turns green.
3 Atoms are added across a double or triple bond of an unsaturated compound. $\mathrm{CH_{2}{=}CH_{2} + H_{2} \xrightarrow{\text{Ni}} CH_{3}CH_{3}}$; catalyst nickel (or palladium).
4 Unsaturated liquid oils are converted into saturated solid fats (vanaspati ghee): liquid $\to$ solid.
5 $5.6\ \mathrm{g}$ is $0.2$ of $28$: hydrogen $0.2\times2=0.4\ \mathrm{g}$; ethane $0.2\times30=6.0\ \mathrm{g}$ ($5.6+0.4=6.0$ ✓).
6 $13\ \mathrm{g}$ is $0.5$ of $26$: hydrogen $0.5\times4=2.0\ \mathrm{g}$; ethane $0.5\times30=15\ \mathrm{g}$ ($13+2=15$ ✓).
7 No: butane is saturated, with only single bonds, so there is nothing to add to.
8 Hydrogen has been added to the double bonds, making the chains saturated. Saturated chains pack together closely, so the fat is a solid at room temperature.
III
Substitution and classifying reactions.
1 What is a substitution reaction? What condition is needed for the reaction of methane with chlorine?
2 Write the first two steps of the chlorination of methane.
3 Name $\mathrm{CH_{3}Cl}$, $\mathrm{CH_{2}Cl_{2}}$, $\mathrm{CHCl_{3}}$ and $\mathrm{CCl_{4}}$.
4 Why does ethene undergo addition but methane substitution?
5 Write the equation for the reaction of ethane with chlorine in sunlight to give one chlorine-containing organic product.
6 What mass of chloromethane forms when $3.2\ \mathrm{g}$ of methane is fully used in the first chlorination step? ($\mathrm{CH_{3}Cl}=50.5\ \mathrm{u}$)
7 For the reaction in 6, what mass of chlorine reacts and what mass of hydrogen chloride forms?
1 What mass of carbon dioxide is released on burning $1\ \mathrm{kg}$ of methane? ($\mathrm{CO_{2}}=44$, $\mathrm{CH_{4}}=16\ \mathrm{u}$)
2 Ethanol is burnt as a fuel in some cars. Find the masses of $\mathrm{CO_{2}}$ and $\mathrm{H_{2}O}$ formed from $23\ \mathrm{g}$ of ethanol ($\mathrm{C_{2}H_{5}OH}=46\ \mathrm{u}$, $\mathrm{O_{2}}=32\ \mathrm{u}$), and the oxygen used.
3 How many molecules of $\mathrm{O_{2}}$ are needed to burn 10 molecules of ethene completely?
4 A student adds drops of alkaline $\mathrm{KMnO_{4}}$ to warm ethanol. The purple colour disappears for the first few drops, then stays. Explain.
5 Why does a kerosene lamp smoke while an LPG burner does not?
6 Explain why an ethyne–oxygen torch gives a hotter flame than ethyne burning in air.
7 Why do chemists say saturated hydrocarbons are “unreactive” although they burn so well?
8Challenging. A hydrocarbon of molecular mass $56\ \mathrm{u}$ gives $22\ \mathrm{g}$ of $\mathrm{CO_{2}}$ and $9\ \mathrm{g}$ of $\mathrm{H_{2}O}$ on complete combustion of $7\ \mathrm{g}$. Find its molecular formula and decide whether it is saturated.
💡 Show solutions▶
1 $16\ \mathrm{u}$ gives $44\ \mathrm{u}$, so $1\ \mathrm{kg}$ gives $\tfrac{44}{16}=2.75\ \mathrm{kg}$ of $\mathrm{CO_{2}}$.
3 $\mathrm{C_{2}H_{4} + 3O_{2} \rightarrow \dots}$: 3 molecules per molecule, so 30 molecules.
4 Permanganate is used up while ethanol is still present (colour lost); once all ethanol is oxidised, the extra drops no longer react and the purple colour persists.
5 Kerosene contains large carbon-rich molecules that are not completely burnt in the limited air at the wick, giving soot. LPG is burnt after mixing with air in the burner.
6 Pure oxygen supplies far more oxygen than air (which is mostly nitrogen), so ethyne burns completely and rapidly, giving more heat per second.
7 They have no double or triple bond, so they do not take part in addition reactions and resist most reagents; burning and substitution in sunlight are the main reactions.
8 $22\ \mathrm{g}$ of $\mathrm{CO_{2}}$ contains $\tfrac{12}{44}\times22=6\ \mathrm{g}$ of C; $9\ \mathrm{g}$ of $\mathrm{H_{2}O}$ contains $\tfrac{2}{18}\times9=1\ \mathrm{g}$ of H; $6+1=7\ \mathrm{g}$ ✓. Ratio $\tfrac{6}{12}:\tfrac{1}{1}=0.5:1=1:2$, so the empirical formula is $\mathrm{CH_{2}}$ ($14\ \mathrm{u}$). $56/14=4$: $\mathrm{C_{4}H_{8}}$ — one fewer pair of hydrogens than butane, so it is unsaturated (butene).
E1.Board · 1 mark A hydrocarbon that burns with a yellow sooty flame is most likely (A) methane (B) ethane (C) ethyne (D) propane
💡 Show solution▶
Idea Unsaturated hydrocarbons have a high percentage of carbon and give a sooty flame; ethyne is unsaturated (92.3 % carbon). The others are saturated.
Answer (C).
E2.Board · A–RAssertion (A): Ethene reacts with hydrogen in the presence of nickel, but ethane does not. Reason (R): Ethene contains a carbon–carbon double bond. Choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
💡 Show solution▶
Idea Ethene (unsaturated) adds hydrogen with a Ni catalyst because its double bond opens; ethane has only single bonds and cannot add. R explains A.
Answer (A).
E3.Board · 3 marks Distinguish between an addition reaction and a substitution reaction, giving one balanced equation (with the condition) for each.
💡 Show solution▶
Addition Atoms are added across a double or triple bond of an unsaturated compound: $\mathrm{CH_{2}{=}CH_{2} + H_{2} \xrightarrow{\text{Ni}} CH_{3}CH_{3}}$.
Substitution An atom (usually H) of a saturated compound is replaced by another atom: $\mathrm{CH_{4} + Cl_{2} \xrightarrow{\text{sunlight}} CH_{3}Cl + HCl}$.
Difference Addition gives a single product from an unsaturated compound; substitution gives two products and occurs in saturated compounds.
E4.Board · Case-based A shopkeeper sells two products: a cooking oil “rich in unsaturated fats” and vanaspati ghee. The oil is liquid at room temperature; the ghee is solid. Vanaspati is made from vegetable oil by a process using a nickel catalyst. (i) Name the process and the gas used. (1 mark) (ii) Write an equation using ethene to show what happens to a double bond in it. (1 mark) (iii) Why is the oil liquid and the ghee solid? (2 marks)
💡 Show solution▶
(i) Hydrogenation, with hydrogen gas.
(ii) $\mathrm{CH_{2}{=}CH_{2} + H_{2} \xrightarrow{\text{Ni}} CH_{3}CH_{3}}$.
(iii) The oil has unsaturated chains (double bonds), which do not pack closely, so it is liquid. Hydrogenation makes the chains saturated, so they pack closely and the fat is solid.
E5.NEETFoundation. The number of moles of oxygen needed for the complete combustion of one mole of propane, $\mathrm{C_{3}H_{8}}$, is (A) 3 (B) 4 (C) 5 (D) 7
💡 Show solution▶
Idea $\mathrm{C_{3}H_{8} + 5O_{2} \rightarrow 3CO_{2} + 4H_{2}O}$: oxygen atoms needed $=6+4=10$, i.e. 5 molecules of $\mathrm{O_{2}}$.
Answer (C).
E6.Board · 2 marks (a) What happens when ethanol is heated with alkaline potassium permanganate? (b) Write the equation using $[\mathrm{O}]$.
💡 Show solution▶
(a) Ethanol is oxidised to ethanoic acid; the purple colour of $\mathrm{KMnO_{4}}$ disappears.
🔬 Research–Teaching NexusResearch-tutored · optional, one or two evenings
Research question. How much soot do different household fuels leave behind, and does the amount agree with the percentage of carbon in their molecules?
Why it matters. Soot (particulate matter) from burning fuel is a major cause of air pollution and breathing disease in Indian cities and kitchens. Engineers design burners, stoves and engines so that fuels burn completely; changing from solid fuels to LPG and natural gas has reduced indoor smoke in millions of homes.
What to do.
With an adult present, hold a cold, clean steel spoon or a steel plate for a few seconds in the flame of (i) a candle, (ii) a cooking-gas burner with air holes open, (iii) the same burner with the air holes half closed, and (iv) a spirit (ethanol) lamp.
Photograph or describe the flame colour and the soot layer on the spoon each time (none, light, heavy).
Look up the molecular formula of each fuel (candle wax is a mixture of long alkanes, for which $\mathrm{C_{25}H_{52}}$ is a typical formula; LPG is mainly butane, $\mathrm{C_{4}H_{10}}$; the lamp burns ethanol, $\mathrm{C_{2}H_{5}OH}$), and compute the percentage of carbon.
Compare soot, flame colour and carbon percentage, and explain the role of air.
Tools: candle, gas burner, spirit lamp, a cold steel spoon, tongs, a notebook and a camera.
Hand in: A table of fuel, formula, carbon percentage, flame colour and soot, with a short explanation.
Going further. Why does a lamp that burns kerosene smoke less when its wick is turned lower, and what happens to the flame colour?
Instructor note: expect the open-air-hole burner to leave no soot and a blue flame, the half-closed holes a yellow flame and light soot, the candle yellow with soot, and the spirit lamp a pale blue flame with no soot. Carbon percentages: butane $48/58=82.8\,\%$, candle wax ($\mathrm{C_{25}H_{52}}$) $300/352=85.2\,\%$, ethanol $24/46=52.2\,\%$ — the low value for ethanol (it already contains oxygen) fits its clean flame. Real flames also depend on how well fuel and air mix, so the match is not perfect. Safety: adult supervision, tie back hair, keep water nearby, never leave flames unattended, avoid inhaling smoke.
🧪 Self-check quiz
0/5 correct
Q1. The products of the complete combustion of ethanol are
Q2. Hydrogenation of vegetable oil uses a catalyst of
Q3. Which reaction is a substitution reaction?
Q4. A gas stove flame turns yellow and blackens the pan when