SPI Chemistry · Carbon and Its Compounds

Reactions, Ethanol and Ethanoic Acid

Carbon compounds burn, are oxidised, add hydrogen or swap atoms, and two of them, ethanol and ethanoic acid, have reactions you must know by name.

All Carbon and Its Compounds notes

Why this matters

Five questions in the bank, three EASY and two MODERATE, one of them from the 2022 paper (the boiling point of ethanol). The rest ask which hydrocarbons add, what an aldehyde becomes when oxidised, what you see when sodium bicarbonate meets ethanoic acid, and which equation is saponification.

Concept 1 of 4: Combustion, oxidation, addition and substitution

Four kinds of reaction cover almost everything a Class 10 carbon compound does. Which one happens depends on the compound: anything burns, alcohols are oxidised, unsaturated compounds add, and saturated ones can only swap atoms.

Definition

Combustion: burning in oxygen gives carbon dioxide, water, heat and light. Saturated compounds burn with a clean blue flame, unsaturated ones with a sooty yellow flame.
Oxidation: alkaline potassium permanganate or acidified potassium dichromate turns an alcohol into a carboxylic acid (by way of an aldehyde). Ethanol becomes ethanoic acid.
Addition: an unsaturated compound (double or triple bond) adds hydrogen with a nickel catalyst. This hydrogenation turns vegetable oils into vanaspati ghee.
Substitution: in sunlight, chlorine replaces the hydrogen atoms of a saturated hydrocarbon one by one.

Reaction	Which compounds	Example
Combustion	All hydrocarbons	$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$
Oxidation	Alcohols, then aldehydes	Ethanol to ethanoic acid, which turns blue litmus red
Addition	Unsaturated only	$\mathrm{C_2H_4 + H_2 \rightarrow C_2H_6}$ (nickel catalyst) Alkenes and alkynes add; alkanes cannot.
Substitution	Saturated hydrocarbons	$\mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}$ (sunlight)

Unsaturated means a double or triple bond: that is what lets a compound add.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionEasy

Example 1 · Carbon and Its Compounds · Reactions, Ethanol and Ethanoic Acid

Which of the following hydrocarbons undergo addition reactions?

\text{C}_3\text{H}_8

\text{C}_3\text{H}_6

\text{C}_2\text{H}_5\text{OH}

\text{CH}_4

\text{C}_2\text{H}_2

Expecting an alkane to add

Addition needs a double or triple bond to open. Alkanes are saturated, so with chlorine they substitute, and with hydrogen they do nothing.

Calling ethanol a hydrocarbon

A hydrocarbon contains only carbon and hydrogen. Ethanol,

\mathrm{C_2H_5OH}

, contains oxygen, so it is an alcohol, not a hydrocarbon.

Drill 1 more on combustion, oxidation, addition and substitution

Concept 2 of 4: Ethanol

Ethanol is the alcohol in drinks and medicines and a common solvent. Two reactions matter: with sodium it gives off hydrogen, and with hot concentrated sulphuric acid it loses water and becomes ethene.

Definition

Formula $\mathrm{C_2H_5OH}$ : a colourless liquid that mixes with water in any proportion.
It boils at about 78 °C (351 K), below water's 100 °C.
With sodium it gives hydrogen gas and sodium ethoxide.
Heated to 443 K with excess concentrated sulphuric acid, it loses water and gives ethene. The acid acts as a dehydrating agent.

Fact	Ethanol
Formula	$\mathrm{C_2H_5OH}$
Boiling point	About 78 °C (351 K)
With sodium	$\mathrm{2Na + 2C_2H_5OH \rightarrow 2C_2H_5ONa + H_2}$ The gas is hydrogen: it burns with a pop.
With hot concentrated sulphuric acid	Loses water to give ethene, $\mathrm{C_2H_4}$
Uses	Solvent, medicines such as cough syrups, fuel

Ethanol is the second member of the alcohols, after methanol.

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The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q99Easy

Example 2 · Carbon and Its Compounds · Reactions, Ethanol and Ethanoic Acid

Boiling point of ethanol is____degrees.

Mixing up the boiling points

Ethanol boils at about 78 °C, water at 100 °C and ethanoic acid at about 118 °C. Ethanol is the lowest of the three.

Expecting carbon dioxide from sodium and ethanol

A metal such as sodium gives hydrogen gas with ethanol. Carbon dioxide comes from an acid reacting with a carbonate or a hydrogencarbonate.

Concept 3 of 4: Ethanoic acid

Ethanoic acid (acetic acid) is the acid in vinegar. It behaves like any acid, only weaker: it turns blue litmus red, makes a salt and water with a base, and gives off carbon dioxide with a carbonate.

Definition

Formula $\mathrm{CH_3COOH}$ . Vinegar is a 5 to 8% solution of it in water.
Pure ethanoic acid melts at 290 K (17 °C), so it freezes in cold weather: it is called glacial acetic acid.
It is a weak acid: it turns blue litmus red, but it has a higher pH than hydrochloric acid of the same strength.
With a base: salt + water. With a carbonate or hydrogencarbonate: salt + water + carbon dioxide, which turns lime water milky.

Reacts with	Products
Sodium hydroxide	$\mathrm{CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O}$
Sodium carbonate	$\mathrm{2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2}$ The fizzing is carbon dioxide: it turns lime water milky.
Blue litmus	Turns red
Ethanol (concentrated sulphuric acid)	Ethyl ethanoate, a sweet-smelling ester

Sodium ethanoate,

\mathrm{CH_3COONa}

, is the salt in every row that makes one.

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The same idea in a practice question:

Practice questionEasy

Example 3 · Carbon and Its Compounds · Reactions, Ethanol and Ethanoic Acid

Sodium bicarbonate solution is added to dilute ethanoic acid. It is observed that

Taking ethanoic acid for a strong acid

Ethanoic acid is weak: only a little of it splits into ions. At the same concentration its pH is higher than that of hydrochloric acid.

Saying the gas is hydrogen

An acid with a carbonate or a hydrogencarbonate gives carbon dioxide. Hydrogen comes from an acid with a reactive metal.

Concept 4 of 4: Esters, saponification and soap

An acid and an alcohol join to make an ester, which smells sweet. An alkali can break the ester again into the salt of the acid and the alcohol. When the ester is a fat or oil, the salt is soap: that is why the reaction is called saponification.

Definition

Esterification: carboxylic acid + alcohol, with a little concentrated sulphuric acid as catalyst, gives an ester + water.
Saponification: ester + sodium hydroxide gives the sodium salt of the acid + the alcohol.
Soap is made by saponification of oils and fats: it is the sodium salt of a long-chain acid.
Soap forms an insoluble scum with the calcium and magnesium salts of hard water. Detergents do not.

Reaction	Reactants	Products
Esterification	$\mathrm{CH_3COOH + C_2H_5OH}$ , with concentrated $\mathrm{H_2SO_4}$	$\mathrm{CH_3COOC_2H_5 + H_2O}$
Saponification	Ester + sodium hydroxide	Sodium salt of the acid + alcohol Saponification starts from an ester and an alkali.
Soap making	Oil or fat + sodium hydroxide	Soap + glycerol

Ethyl ethanoate,

\mathrm{CH_3COOC_2H_5}

, is the ester of ethanoic acid and ethanol.

Practice this conceptself-check · 4 quick reps

The same idea in a practice question:

Practice questionModerate

Example 4 · Carbon and Its Compounds · Reactions, Ethanol and Ethanoic Acid

Which of the following represents a saponification reaction?

Swapping esterification and saponification

Acid + alcohol giving an ester is esterification. Ester + alkali giving a salt and an alcohol is saponification. One builds the ester, the other breaks it.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (4)

Combustion, oxidation, addition and substitution4 rows

Reaction	Which compounds	Example
Combustion	All hydrocarbons	$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$
Oxidation	Alcohols, then aldehydes	Ethanol to ethanoic acid, which turns blue litmus red
Addition	Unsaturated only	$\mathrm{C_2H_4 + H_2 \rightarrow C_2H_6}$ (nickel catalyst) Alkenes and alkynes add; alkanes cannot.
Substitution	Saturated hydrocarbons	$\mathrm{CH_4 + Cl_2 \rightarrow CH_3Cl + HCl}$ (sunlight)

Unsaturated means a double or triple bond: that is what lets a compound add.

Ethanol5 rows

Fact	Ethanol
Formula	$\mathrm{C_2H_5OH}$
Boiling point	About 78 °C (351 K)
With sodium	$\mathrm{2Na + 2C_2H_5OH \rightarrow 2C_2H_5ONa + H_2}$ The gas is hydrogen: it burns with a pop.
With hot concentrated sulphuric acid	Loses water to give ethene, $\mathrm{C_2H_4}$
Uses	Solvent, medicines such as cough syrups, fuel

Ethanol is the second member of the alcohols, after methanol.

Ethanoic acid4 rows

Reacts with	Products
Sodium hydroxide	$\mathrm{CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O}$
Sodium carbonate	$\mathrm{2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2}$ The fizzing is carbon dioxide: it turns lime water milky.
Blue litmus	Turns red
Ethanol (concentrated sulphuric acid)	Ethyl ethanoate, a sweet-smelling ester

Sodium ethanoate,

\mathrm{CH_3COONa}

, is the salt in every row that makes one.

Esters, saponification and soap3 rows

Reaction	Reactants	Products
Esterification	$\mathrm{CH_3COOH + C_2H_5OH}$ , with concentrated $\mathrm{H_2SO_4}$	$\mathrm{CH_3COOC_2H_5 + H_2O}$
Saponification	Ester + sodium hydroxide	Sodium salt of the acid + alcohol Saponification starts from an ester and an alkali.
Soap making	Oil or fat + sodium hydroxide	Soap + glycerol

Ethyl ethanoate,

\mathrm{CH_3COOC_2H_5}

, is the ester of ethanoic acid and ethanol.

Watch out for (7)

Expecting an alkane to add
→ Combustion, oxidation, addition and substitution
Calling ethanol a hydrocarbon
→ Combustion, oxidation, addition and substitution
Mixing up the boiling points
→ Ethanol
Expecting carbon dioxide from sodium and ethanol
→ Ethanol
Taking ethanoic acid for a strong acid
→ Ethanoic acid
Saying the gas is hydrogen
→ Ethanoic acid
Swapping esterification and saponification
→ Esters, saponification and soap

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