From the ethanol in sanitiser and fuel blends to the ethanoic acid in vinegar, from sweet-smelling esters to the soap that lifts oil off your hands — study the two most important carbon compounds, how they react, and how soaps and detergents clean through micelles.
Two carbon compounds are part of almost every home. Ethanol is in the hand sanitiser by the door, in the cough syrup in the cupboard and, increasingly, in the petrol you buy; ethanoic acid (acetic acid) gives vinegar its sour taste and pickles their keeping quality. And every time you wash your hands with soap you use a carbon compound with a very special shape that can drag oil into water.
In this section you will study the properties and reactions of ethanol and ethanoic acid, see how they join to make sweet-smelling esters and how the reverse reaction with sodium hydroxide, saponification, is used to make soap. You will then see why soap molecules clean by forming micelles, why soap fails in hard water, and how detergents solve the problem.
Ethanol
Ethanol, $\mathrm{CH_{3}CH_{2}OH}$ or $\mathrm{C_{2}H_{5}OH}$, is the alcohol of two carbons. It is a colourless liquid with a pleasant smell that boils at $351\ \mathrm{K}$ ($78\ ^{\circ}\mathrm{C}$) and freezes at $156\ \mathrm{K}$ ($-117\ ^{\circ}\mathrm{C}$). It mixes with water in all proportions, and its solution is neutral: it does not change blue or red litmus, even though it contains $-\mathrm{OH}$ — unlike a base such as $\mathrm{NaOH}$, it does not give $\mathrm{OH^{-}}$ ions in water.
📐 Principle 4.13 — Reactions of ethanol
With sodium: a brisk reaction giving hydrogen gas (which burns with a pop) and sodium ethoxide: $$\mathrm{2C_{2}H_{5}OH(l) + 2Na(s) \rightarrow 2C_{2}H_{5}ONa(s) + H_{2}(g)}$$ This is like the reaction of water with sodium, but gentler.
Dehydration: heated at $443\ \mathrm{K}$ with excess concentrated sulphuric acid, ethanol loses a molecule of water and gives ethene: $$\mathrm{CH_{3}CH_{2}OH \xrightarrow[443\ \mathrm{K}]{\text{conc. } \mathrm{H_{2}SO_{4}}} CH_{2}{=}CH_{2} + H_{2}O}$$ Sulphuric acid acts as a dehydrating agent.
Oxidation and combustion (Section 4.3): oxidised to ethanoic acid, and burns in air with a clean blue flame to $\mathrm{CO_{2}}$ and $\mathrm{H_{2}O}$.
📘 Definition 4.13 — Dehydration
Dehydration is the removal of water (the elements of a water molecule) from a compound, usually with a dehydrating agent such as hot concentrated sulphuric acid. Dehydration of ethanol gives ethene.
✏️ Example 19 — Ethanol and sodium
$4.6\ \mathrm{g}$ of ethanol reacts completely with sodium. With $\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16$, $\mathrm{Na}=23\ \mathrm{u}$, find the masses of sodium used, sodium ethoxide formed and hydrogen evolved. Check that mass is conserved.
💡 Show solution▶
Step 1 Equation masses: $2\,\mathrm{C_{2}H_{5}OH}=92$; $2\,\mathrm{Na}=46$; $2\,\mathrm{C_{2}H_{5}ONa}=136$; $\mathrm{H_{2}}=2\ \mathrm{u}$. Atom check: C $4=4$; H $12=10+2$; O $2=2$; Na $2=2$ ✓.
Step 2 $4.6\ \mathrm{g}$ is $\tfrac{4.6}{92}=0.05$ of the equation mass. Sodium: $0.05\times46=2.3\ \mathrm{g}$; sodium ethoxide: $0.05\times136=6.8\ \mathrm{g}$; hydrogen: $0.05\times2=0.10\ \mathrm{g}$.
Check $4.6+2.3=6.9\ \mathrm{g}$ before; $6.8+0.1=6.9\ \mathrm{g}$ after ✓.
✏️ Example 20 — Dehydration of ethanol
What masses of ethene and water are obtained from $4.6\ \mathrm{g}$ of ethanol, if it is completely dehydrated? Name the conditions and say how to test for the unsaturated product.
💡 Show solution▶
Step 1 $\mathrm{C_{2}H_{5}OH}=46$; $\mathrm{C_{2}H_{4}}=28$; $\mathrm{H_{2}O}=18\ \mathrm{u}$. Atoms: C $2=2$; H $6=4+2$; O $1=1$ ✓.
Step 2 $4.6\ \mathrm{g}$ is $0.1$ of $46$: ethene $0.1\times28=2.8\ \mathrm{g}$; water $0.1\times18=1.8\ \mathrm{g}$. Check: $2.8+1.8=4.6\ \mathrm{g}$ ✓.
Conditions Excess concentrated $\mathrm{H_{2}SO_{4}}$ at $443\ \mathrm{K}$. Ethene is unsaturated: it burns with a luminous, sooty flame (Section 4.3) and adds hydrogen with nickel.
📐 Principle 4.14 — Uses and effects of ethanol
Ethanol is used as a solvent (tinctures, cough syrups, tonics, perfumes, sanitisers) and as a fuel: it can be blended with petrol; it is made from sugarcane and is a renewable fuel. Drinking alcohol depresses the nervous system: it slows reactions, harms the liver and stomach and leads to accidents and addiction. To stop industrial alcohol from being drunk, it is made unfit by adding poisonous substances.
📘 Definition 4.14 — Denatured alcohol
Denatured alcohol is ethanol to which small amounts of poisonous methanol and a bad-tasting, coloured substance (such as pyridine or copper sulphate) are added so that it cannot be drunk. Drinking methanol damages the optic nerve and can cause blindness or death, as the body oxidises it to a much more poisonous compound.
Ethanoic Acid
Ethanoic acid, $\mathrm{CH_{3}COOH}$ (acetic acid), is the carboxylic acid of two carbons. It is a colourless liquid with a sharp smell. A $5$–$8\,\%$ solution in water is vinegar, used as a preservative in pickles. Pure ethanoic acid freezes at $290\ \mathrm{K}$ ($17\ ^{\circ}\mathrm{C}$), so on cold winter days it freezes into ice-like crystals in the bottle — which is why it is called glacial ethanoic acid.
Ethanoic acid is a weak acid: its solution turns blue litmus red but is less acidic than a hydrochloric acid solution of the same concentration (pH about 3 for $0.1\ \mathrm{mol/L}$ against 1), because only a small fraction of its molecules gives $\mathrm{H^{+}}$ ions (Section 2.2).
📐 Principle 4.15 — Acid reactions of ethanoic acid
With a base (neutralisation): $\mathrm{CH_{3}COOH(aq) + NaOH(aq) \rightarrow CH_{3}COONa(aq) + H_{2}O(l)}$, giving sodium ethanoate (a salt) and water.
With a carbonate: $\mathrm{2CH_{3}COOH(aq) + Na_{2}CO_{3}(s) \rightarrow 2CH_{3}COONa(aq) + H_{2}O(l) + CO_{2}(g)}$.
With a hydrogencarbonate: $\mathrm{CH_{3}COOH(aq) + NaHCO_{3}(s) \rightarrow CH_{3}COONa(aq) + H_{2}O(l) + CO_{2}(g)}$.
The gas $\mathrm{CO_{2}}$ turns lime water milky. These reactions, with the fizzing of the carbonate, distinguish a carboxylic acid from an alcohol: ethanol does not react with sodium hydrogencarbonate.
✏️ Example 21 — Ethanoic acid and baking soda
$6.0\ \mathrm{g}$ of ethanoic acid reacts completely with sodium hydrogencarbonate. With $\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16$, $\mathrm{Na}=23\ \mathrm{u}$, find the masses of $\mathrm{NaHCO_{3}}$ used and of sodium ethanoate, water and carbon dioxide formed, and check the mass balance.
💡 Show solution▶
Step 1 $\mathrm{CH_{3}COOH}=60$; $\mathrm{NaHCO_{3}}=84$; $\mathrm{CH_{3}COONa}=82$; $\mathrm{H_{2}O}=18$; $\mathrm{CO_{2}}=44\ \mathrm{u}$. Atoms: C $2+1=2+1$; H $4+1=3+2$; O $2+3=2+1+2$; Na $1=1$ ✓.
Step 2 $6.0\ \mathrm{g}$ is $0.1$ of $60$. $\mathrm{NaHCO_{3}}$: $8.4\ \mathrm{g}$; $\mathrm{CH_{3}COONa}$: $8.2\ \mathrm{g}$; $\mathrm{H_{2}O}$: $1.8\ \mathrm{g}$; $\mathrm{CO_{2}}$: $4.4\ \mathrm{g}$.
Interactive 1 — Reaction Lab: Ethanol and Ethanoic Acid
Choose a reaction and press ▶ Play to run it in the test tube. Watch for gases, the lime-water test and the smell of an ester. The readouts count the atoms on both sides, so you can check that the equation is balanced. Predict what you will observe before you play.
type of reaction—
you observe—
atoms: left = right—
mass: left = right—
Esters and Saponification
If you warm ethanoic acid with ethanol and a few drops of concentrated sulphuric acid, the mixture soon smells sweet and fruity. A new compound has formed: an ester. Esters give many fruits and flowers their smell and are used in perfumes and as flavourings.
📘 Definition 4.14 — Esterification
Esterification is the reaction of a carboxylic acid with an alcohol to form an ester and water, in the presence of a few drops of concentrated sulphuric acid as a catalyst:
$$\mathrm{CH_{3}COOH + C_{2}H_{5}OH \underset{\text{(heat)}}{\overset{\text{conc. } \mathrm{H_{2}SO_{4}}}{\rightleftharpoons}} CH_{3}COOC_{2}H_{5} + H_{2}O}$$
The ester is named from the alcohol (first word, ending -yl) and the acid (second word, ending -oate): ethyl ethanoate. The reaction is reversible, so only part of the reactants is converted.
📘 Definition 4.15 — Saponification
Saponification is the reaction of an ester with a base such as sodium hydroxide to give the alcohol and the sodium salt of the acid:
$$\mathrm{CH_{3}COOC_{2}H_{5} + NaOH \rightarrow CH_{3}COONa + C_{2}H_{5}OH}$$
When the ester is a fat or oil, this reaction is used to make soap.
✏️ Example 22 — Making an ester
(a) Name the esters formed from (i) methanoic acid and ethanol; (ii) ethanoic acid and methanol. (b) $6.0\ \mathrm{g}$ of ethanoic acid reacts with $4.6\ \mathrm{g}$ of ethanol. Find the greatest mass of ethyl ethanoate and of water that can form ($\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16\ \mathrm{u}$). (c) If only $70\,\%$ of that is obtained, what is the mass of ester?
(b) $\mathrm{CH_{3}COOH}=60$; $\mathrm{C_{2}H_{5}OH}=46$; $\mathrm{CH_{3}COOC_{2}H_{5}}=88$; $\mathrm{H_{2}O}=18\ \mathrm{u}$. The masses $6.0$ and $4.6\ \mathrm{g}$ are $0.1$ of $60$ and $46$, so they match exactly: ester $0.1\times88=8.8\ \mathrm{g}$; water $0.1\times18=1.8\ \mathrm{g}$. Check: $6.0+4.6=10.6=8.8+1.8$ ✓.
Fats and oils are esters of long-chain acids with glycerol (propane-1,2,3-triol). Boiled with sodium hydroxide they give soap and glycerol. For the fat tristearin:
$$\mathrm{(C_{17}H_{35}COO)_{3}C_{3}H_{5} + 3NaOH \rightarrow 3C_{17}H_{35}COONa + C_{3}H_{5}(OH)_{3}}$$
Sodium chloride is added to make the soap separate and float, and the soap is then collected, pressed and moulded.
✏️ Example 23 — Soap from a fat
$8.9\ \mathrm{g}$ of tristearin ($890\ \mathrm{u}$) is saponified with sodium hydroxide. With $\mathrm{NaOH}=40$, sodium stearate $=306$ and glycerol $=92\ \mathrm{u}$, find the masses of sodium hydroxide used, of soap and of glycerol formed. Check mass conservation.
💡 Show solution▶
Step 1 Atom check on the equation: C $57=54+3$; H $110+3=105+8$ (that is $113=113$); O $6+3=6+3$; Na $3=3$ ✓. Equation masses: $890+3\times40=1010\ \mathrm{u}$ and $3\times306+92=1010\ \mathrm{u}$ ✓.
Step 2 $8.9\ \mathrm{g}$ is $0.01$ of $890$. NaOH: $0.01\times120=1.2\ \mathrm{g}$; soap: $0.01\times918=9.18\ \mathrm{g}$; glycerol: $0.01\times92=0.92\ \mathrm{g}$.
Oil and grease do not dissolve in water, so water alone cannot wash them off your clothes. Soap solves this, because each soap molecule has two different ends.
📘 Definition 4.16 — Soap
Soaps are sodium (or potassium) salts of long-chain carboxylic acids, such as sodium stearate $\mathrm{C_{17}H_{35}COONa}$. A soap molecule has an ionic head, $-\mathrm{COO^{-}Na^{+}}$, which dissolves in water (hydrophilic, “water-loving”), and a long hydrocarbon tail which dissolves in oil but not in water (hydrophobic, “water-hating”).
A soap molecule: an oil-loving tail and a water-loving ionic head. Each corner of the zig-zag is a carbon atom with its hydrogens (not shown).
📘 Definition 4.17 — Micelle
A micelle is a spherical cluster of soap (or detergent) molecules in water, with the hydrophobic tails pointing inwards, together, and the hydrophilic heads on the outside, in contact with water. The tails can surround a droplet of oil, which sits in the middle of the micelle.
📐 Principle 4.17 — Cleansing action of soap
The dirt on cloth or skin is held by a film of oil or grease. In soap solution the tails of soap molecules dissolve in the oil and the heads stay in the water, so the oil is surrounded by soap molecules, forming a micelle. The charged heads on the surface of the micelle repel those of other micelles, so the oily dirt stays dispersed in the water (an emulsion) and does not settle back. Rubbing or beating the clothes lifts the micelles from the surface and rinsing washes them away. A micelle scatters light, so soap solution looks cloudy.
📐 Principle 4.18 — Hard water, scum and detergents
Hard water contains calcium and magnesium salts. Soap reacts with these ions to give an insoluble precipitate, the scum, so a lot of soap is wasted and cloth does not come clean:
$$\mathrm{2C_{17}H_{35}COONa(aq) + CaCl_{2}(aq) \rightarrow (C_{17}H_{35}COO)_{2}Ca(s) + 2NaCl(aq)}$$
Detergents are sodium salts of sulphonic acids (or ammonium salts with chloride or bromide ions) with long hydrocarbon chains. Their calcium and magnesium salts are soluble, so no scum forms and they work in hard water as well as soft. Detergents are used in shampoos and washing powders; the long straight chains are broken down by bacteria (biodegradable), whereas some branched chains were not and polluted rivers.
✏️ Example 24 — Scum from hard water
(a) How many carbon atoms are in the hydrocarbon tail and in the whole stearate ion $\mathrm{C_{17}H_{35}COO^{-}}$? (b) $6.12\ \mathrm{g}$ of sodium stearate ($306\ \mathrm{u}$) is dissolved in hard water containing calcium chloride ($\mathrm{CaCl_{2}}=111$, calcium stearate $=606$, $\mathrm{NaCl}=58.5\ \mathrm{u}$). Find the masses of scum and of sodium chloride formed, and of $\mathrm{CaCl_{2}}$ needed. Check the mass balance.
A drop of oil sits on a piece of cloth. Choose the cleaning agent and the water, then press ▶ Play. Each molecule is a ● water-loving head with a tail. Watch the tails push into the oil and the micelle lift the oil off. Then try soap in hard water — predict what the calcium ions do.
cleaning agent · water—
micelle formed?—
scum formed?—
oil removed from cloth?—
⚠️ Common mistakes about ethanol, ethanoic acid and soaps
1. “Ethanol is a base because it has –OH.” It does not give $\mathrm{OH^{-}}$ ions and is neutral to litmus.
2. Mixing up dehydration and combustion. Hot concentrated $\mathrm{H_{2}SO_{4}}$ removes water to give ethene; burning gives $\mathrm{CO_{2}}$ and $\mathrm{H_{2}O}$.
3. Calling ethanoic acid a strong acid. It is a weak acid, and glacial ethanoic acid is the pure acid, not a strong solution.
4. Naming an ester the wrong way round. Alcohol first, acid second: ethanol + ethanoic acid gives ethyl ethanoate.
5. Forgetting the catalyst. Esterification needs a few drops of concentrated sulphuric acid.
6. Saying detergents clean “by a different mechanism”. They also form micelles; the difference is that their calcium and magnesium salts are soluble, so there is no scum in hard water.
Exercises
I
Ethanol.
1 State four physical properties of ethanol. Is its solution acidic, basic or neutral?
2 What happens when a small piece of sodium is dropped into ethanol? Write the equation and name the gas and its test.
3 Write the equation for the dehydration of ethanol. What is the role of concentrated sulphuric acid and what is the temperature?
4 What is denatured alcohol and why is ethanol denatured?
5 Give two harmful effects of drinking ethanol and one effect of drinking methanol.
6 $9.2\ \mathrm{g}$ of ethanol reacts completely with sodium ($\mathrm{Na}=23$, $\mathrm{C_{2}H_{5}ONa}=68\ \mathrm{u}$). Find the masses of sodium used, sodium ethoxide formed and hydrogen evolved.
7 What mass of ethene forms from $23\ \mathrm{g}$ of ethanol on complete dehydration? How much water?
8Applied. Sugarcane juice is fermented to ethanol: $\mathrm{C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2}}$. Balance-check the equation, and find the masses of ethanol and $\mathrm{CO_{2}}$ from $180\ \mathrm{g}$ of glucose.
💡 Show solutions▶
1 Colourless liquid, pleasant smell, boiling point $351\ \mathrm{K}$, freezing point $156\ \mathrm{K}$, mixes with water in all proportions. Neutral to litmus.
2 Brisk bubbling: $\mathrm{2C_{2}H_{5}OH + 2Na \rightarrow 2C_{2}H_{5}ONa + H_{2}}$; hydrogen, which burns with a pop.
3 $\mathrm{C_{2}H_{5}OH \xrightarrow[443\ \mathrm{K}]{\text{conc. H}_{2}\text{SO}_{4}} C_{2}H_{4} + H_{2}O}$; the acid is a dehydrating agent; $443\ \mathrm{K}$.
4 Ethanol made unfit for drinking by adding poisonous methanol and a bad-tasting dye (e.g. copper sulphate), to stop misuse of industrial alcohol.
5 Liver and stomach damage, slowed reactions and accidents, addiction (any two). Methanol damages the optic nerve and can cause blindness or death.
6 $9.2\ \mathrm{g}$ is $\tfrac{9.2}{92}=0.1$ of the equation mass. Sodium $0.1\times46=4.6\ \mathrm{g}$; $\mathrm{C_{2}H_{5}ONa}$ $0.1\times136=13.6\ \mathrm{g}$; $\mathrm{H_{2}}$ $0.1\times2=0.2\ \mathrm{g}$. Check: $9.2+4.6=13.8=13.6+0.2$ ✓.
8 C $6=2\times2+2$; H $12=2\times6$; O $6=2+4$ ✓. $180\ \mathrm{g}$ is the equation mass ($180\ \mathrm{u}$): ethanol $2\times46=92\ \mathrm{g}$; $\mathrm{CO_{2}}$ $2\times44=88\ \mathrm{g}$ ($92+88=180$ ✓).
II
Ethanoic acid.
1 What is glacial ethanoic acid and why is it so called? What is vinegar?
2 Write the balanced equation for ethanoic acid with sodium hydroxide and name the products.
3 Write the equations of ethanoic acid with sodium carbonate and sodium hydrogencarbonate. How is the gas tested?
4 Why is ethanoic acid called a weak acid?
5 Describe three ways to distinguish ethanol from ethanoic acid.
6 What mass of carbon dioxide is obtained when $12\ \mathrm{g}$ of ethanoic acid reacts with excess sodium hydrogencarbonate?
7 What mass of sodium hydroxide neutralises $3.0\ \mathrm{g}$ of ethanoic acid, and what mass of sodium ethanoate ($82\ \mathrm{u}$) forms?
8 A laboratory is at $10\ ^{\circ}\mathrm{C}$. Will pure ethanoic acid (freezing point $290\ \mathrm{K}$) be a liquid or a solid there? Give your reasoning.
💡 Show solutions▶
1 Pure ethanoic acid, which freezes to ice-like crystals below $290\ \mathrm{K}$ ($17\ ^{\circ}\mathrm{C}$). Vinegar is a $5$–$8\,\%$ solution of ethanoic acid in water.
3 $\mathrm{2CH_{3}COOH + Na_{2}CO_{3} \rightarrow 2CH_{3}COONa + H_{2}O + CO_{2}}$; $\mathrm{CH_{3}COOH + NaHCO_{3} \rightarrow CH_{3}COONa + H_{2}O + CO_{2}}$. The gas turns lime water milky.
4 Only a small fraction of its molecules gives $\mathrm{H^{+}}$ ions in water, so it is less acidic than a strong acid like HCl of the same concentration (higher pH).
5 (i) Add sodium hydrogencarbonate: the acid fizzes, ethanol does not. (ii) Blue litmus turns red with the acid, not with ethanol. (iii) Smell: sharp vinegar smell versus the pleasant smell of ethanol.
6 $12\ \mathrm{g}$ is $0.2$ of $60$: $\mathrm{CO_{2}}$ $0.2\times44=8.8\ \mathrm{g}$.
7 $3.0\ \mathrm{g}$ is $0.05$ of $60$: NaOH $0.05\times40=2.0\ \mathrm{g}$; sodium ethanoate $0.05\times82=4.1\ \mathrm{g}$ (and $0.9\ \mathrm{g}$ water: $3.0+2.0=5.0=4.1+0.9$ ✓).
8 $10\ ^{\circ}\mathrm{C}=283\ \mathrm{K}$, below $290\ \mathrm{K}$, so it is a solid (glacial).
III
Esters and saponification.
1 What is esterification? What catalyst is used?
2 Write the equation for ethanoic acid with ethanol and name the ester. How would you detect it?
3 Name the ester from (a) ethanoic acid + methanol; (b) methanoic acid + ethanol; (c) propanoic acid + ethanol.
4 Write the condensed formula and molecular formula of ethyl propanoate.
5 Define saponification and write the equation for the saponification of ethyl ethanoate with sodium hydroxide.
6 What is the greatest mass of ethyl ethanoate from $3.0\ \mathrm{g}$ of ethanoic acid and excess ethanol? How much water forms?
7 Give two uses of esters.
8Challenging. $8.8\ \mathrm{g}$ of ethyl ethanoate is boiled with excess sodium hydroxide. Find the masses of sodium ethanoate and ethanol formed and of sodium hydroxide used.
💡 Show solutions▶
1 Reaction of a carboxylic acid with an alcohol to form an ester and water; a few drops of concentrated sulphuric acid.
2 $\mathrm{CH_{3}COOH + C_{2}H_{5}OH \rightleftharpoons CH_{3}COOC_{2}H_{5} + H_{2}O}$; ethyl ethanoate; by its sweet, fruity smell.
5 Reaction of an ester with a base giving an alcohol and the salt of the acid: $\mathrm{CH_{3}COOC_{2}H_{5} + NaOH \rightarrow CH_{3}COONa + C_{2}H_{5}OH}$.
6 $3.0\ \mathrm{g}$ is $0.05$ of $60$. Ester $0.05\times88=4.4\ \mathrm{g}$; water $0.05\times18=0.9\ \mathrm{g}$. (The ethanol used is $2.3\ \mathrm{g}$: $3.0+2.3=5.3=4.4+0.9$ ✓.)
7 Perfumes, flavourings and food essences, solvents (any two).
8 $8.8\ \mathrm{g}$ is $0.1$ of $88$. Sodium ethanoate $0.1\times82=8.2\ \mathrm{g}$; ethanol $0.1\times46=4.6\ \mathrm{g}$; NaOH $0.1\times40=4.0\ \mathrm{g}$. Check: $8.8+4.0=12.8=8.2+4.6$ ✓.
IV
Soaps, detergents and micelles.
1 What is soap? Give the formula of a soap.
2 Describe the two ends of a soap molecule and say what each is attracted to.
3 How is soap prepared from a fat or oil? Name the second product.
4 Explain the cleansing action of soap using a micelle.
5 What is scum? Write the equation for its formation with calcium chloride.
6 What are detergents? Why do they work in hard water?
7 Find the mass of soap, glycerol and the mass of sodium hydroxide needed to saponify $17.8\ \mathrm{g}$ of tristearin ($890\ \mathrm{u}$; soap $306\ \mathrm{u}$; glycerol $92\ \mathrm{u}$).
8Applied. A family finds that soap leaves a grey film and lathers poorly with their borewell water. Explain, and suggest what could help.
9Challenging. Why does a micelle in water have its tails inside and heads outside, and not the other way round?
💡 Show solutions▶
1 The sodium or potassium salt of a long-chain carboxylic acid, e.g. sodium stearate $\mathrm{C_{17}H_{35}COONa}$.
2 An ionic head $-\mathrm{COO^{-}Na^{+}}$, hydrophilic (water-loving); and a hydrocarbon tail, hydrophobic (oil-loving).
3 By saponification: boil the fat or oil with sodium hydroxide; the products are soap and glycerol.
4 Tails dissolve in the oil on the cloth, heads stay in water; the oil is enclosed in a micelle with the heads outside; the micelles repel each other and are washed away.
5 The insoluble precipitate formed by soap with calcium or magnesium ions: $\mathrm{2C_{17}H_{35}COONa + CaCl_{2} \rightarrow (C_{17}H_{35}COO)_{2}Ca + 2NaCl}$.
6 Sodium salts of sulphonic acids (or ammonium salts) with long chains. Their calcium and magnesium salts are soluble, so no scum forms.
7 $17.8\ \mathrm{g}$ is $0.02$ of $890$: soap $0.02\times918=18.36\ \mathrm{g}$; glycerol $0.02\times92=1.84\ \mathrm{g}$; NaOH $0.02\times120=2.4\ \mathrm{g}$. Check: $17.8+2.4=20.2=18.36+1.84$ ✓.
8 The water is hard: soap reacts with its calcium and magnesium ions and forms scum. Using a detergent, or removing the hardness (for example with washing soda, Section 2.3) helps.
9 The head is attracted to water and the tail is not, so in water the tails come together away from water (inside) while the heads face the water (outside).
E1.Board · 1 mark When ethanol is heated at $443\ \mathrm{K}$ with excess concentrated sulphuric acid, the product is (A) ethane (B) ethene (C) ethanoic acid (D) ethyl ethanoate
💡 Show solution▶
Idea Hot concentrated $\mathrm{H_{2}SO_{4}}$ dehydrates ethanol: $\mathrm{C_{2}H_{5}OH \rightarrow C_{2}H_{4} + H_{2}O}$.
Answer (B).
E2.Board · A–RAssertion (A): Detergents can be used in hard water but soaps cannot. Reason (R): The calcium and magnesium salts of detergents are soluble in water. 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 Soap gives an insoluble scum with $\mathrm{Ca^{2+}}$ and $\mathrm{Mg^{2+}}$, but detergent salts of these ions are soluble, so detergents keep working. R explains A.
Answer (A).
E3.Board · 3 marks (a) What is esterification? (b) Write the equation for the formation of ethyl ethanoate, with the catalyst. (c) What happens when this ester is heated with sodium hydroxide? Name the reaction.
💡 Show solution▶
(a) The reaction of a carboxylic acid with an alcohol, giving an ester and water.
(c) It gives sodium ethanoate and ethanol: $\mathrm{CH_{3}COOC_{2}H_{5} + NaOH \rightarrow CH_{3}COONa + C_{2}H_{5}OH}$ — saponification.
E4.Board · Case-based A student adds sodium hydrogencarbonate to two colourless liquids, P and Q. P fizzes and the gas turns lime water milky; Q does not react. Q reacts with sodium metal giving a gas that burns with a pop. P and Q are warmed together with a few drops of concentrated sulphuric acid and a sweet smell is noticed. (i) Identify P and Q. (1 mark) (ii) Name the gas given off with sodium hydrogencarbonate and the gas with sodium. (1 mark) (iii) Write the equation for the reaction between P and Q and name the product with the sweet smell. (2 marks)
💡 Show solution▶
(i) P is ethanoic acid (carboxylic acid); Q is ethanol (alcohol).
(ii) Carbon dioxide with $\mathrm{NaHCO_{3}}$; hydrogen with sodium.
E5.NEETFoundation. The number of moles of hydrogen gas obtained when 1 mole of ethanol reacts completely with excess sodium is (A) 1 (B) $0.5$ (C) 2 (D) $0.25$
💡 Show solution▶
Idea $\mathrm{2C_{2}H_{5}OH + 2Na \rightarrow 2C_{2}H_{5}ONa + H_{2}}$: 2 mol ethanol give 1 mol $\mathrm{H_{2}}$, so 1 mol ethanol gives $0.5$ mol.
Answer (B).
E6.Board · 2 marks Why does soap not form lather in hard water while a detergent does? Write the equation for what happens to soap in water containing calcium chloride.
💡 Show solution▶
Reason Soap forms an insoluble scum with calcium and magnesium ions of hard water, so little soap is left to lather; detergent salts of these ions are soluble.
🔬 Research–Teaching NexusResearch-tutored · optional, one or two evenings
Research question. How much better does a detergent work than a soap in hard water, and can washing soda or boiling make a soap work better?
Why it matters. Hard water is common in many parts of India and costs households soap, time and water-using appliances (scale in boilers and pipes). Chemists design detergents so that they clean in all kinds of water, and water-treatment plants soften water. Testing a cleaning product's performance in hard and soft water is a basic quality-control experiment.
What to do.
Make soft water (boiled and cooled distilled water or rainwater) and hard water (dissolve a level teaspoon of Epsom salt in a litre of water).
In four clean bottles put $50\ \mathrm{mL}$ of (i) soft water, (ii) hard water, (iii) hard water $+$ a pinch of washing soda, (iv) soft water $+$ a little vinegar. Add $5$ drops of liquid soap, or $\tfrac{1}{4}$ teaspoon of grated soap, to each; repeat in a second set with a detergent.
Shake each bottle for 10 seconds, wait one minute and measure the height of the foam. Note any scum (cloudy grey film).
Rank the six samples and explain with the reactions in this section.
Tools: six identical clean bottles, a ruler, Epsom salt, washing soda, vinegar, soap, detergent, a notebook.
Hand in: A table of foam height and scum for each sample, with the equation for scum formation and a short conclusion.
Going further. Why does adding vinegar to soap solution reduce lather? (Hint: the stearate ions react with an acid.)
Instructor note: expect plenty of foam for soap in soft water, little foam and a grey scum for soap in hard water, improved lather when washing soda precipitates the hardness, and good foam for the detergent in both waters. Vinegar converts soap back into its insoluble fatty acid, so lather is lost. Common errors: tap water already hard, too little soap, bottles not rinsed free of detergent. Use only household substances; do not taste samples.
🧪 Self-check quiz
0/5 correct
Q1. The sweet-smelling compound formed from ethanoic acid and ethanol is
Q2. Which test distinguishes ethanoic acid from ethanol?
Q3. Saponification is the reaction of an ester (such as a fat) with
Q4. In a soap micelle in water, the ionic heads of the soap molecules
Q5. Detergents work in hard water because their calcium and magnesium salts are