Ch 7 · Carboxylic Acids · O Level Chemistry 5070
Topic 7 of 8 in Organic Chemistry — O Level Chemistry 5070 — 48 lessons.
Introduction
Ethanoic acid is the acid in vinegar. It behaves as a typical acid with metals, bases and carbonates, so everything you learned about acids applies, with one twist: its salts are ethanoates and their formulae need care. This also covers how ethanoic acid is made from ethanol and how carboxylic acids react with alcohols to make sweet-smelling esters.
Key ideas — Ethanoic acid as an acid
Carboxylic acids contain the -COOH functional group and have the general formula mathrmC_nH_2n+1COOH. Ethanoic acid, CH_3COOH, is a colourless liquid with a sharp smell. In water it is a weak acid: only a small fraction of its molecules ionise to release hydrogen ions, so vinegar has a pH of about 3.
🧑🏫 Teacher's explanation
Think of a carboxylic acid like a group of people in a room where only a few people are willing to shout (release H^+ ions) at any given time. Because most molecules stay together, the 'shouting' is quiet, making it a weak acid. This is why vinegar, which is ethanoic acid, is only mildly acidic rather than extremely corrosive like hydrochloric acid.
📖 Definitions
🔍 Worked example — Easy
Write the balanced equation for the reaction between ethanoic acid and magnesium metal.
🔍 Worked example — Standard
Calculate the mass of ethanoic acid needed to produce 5.00 g of sodium ethanoate when reacted with excess sodium carbonate.
🔍 Worked example — Exam-style
Ethanol is oxidized to ethanoic acid. Write the balanced equation for the oxidation of ethanol using acidified potassium manganate(VII).
⚠️ Common errors
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🎯 Exam question
Key ideas — Reactions with metals, bases and carbonates
Ethanoic acid gives the same three reaction types as any dilute acid, just more slowly than hydrochloric acid of the same concentration.
🧑🏫 Teacher's explanation
Think of carboxylic acids like a slightly 'lazy' version of strong acids like hydrochloric acid. They have the same chemical 'personality'—they want to swap their hydrogen ions for something else—but they aren't as aggressive. Because the bond between the carbon and the oxygen is quite strong, they don't let go of that hydrogen atom as easily as HCl does.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of magnesium ethanoate produced when 0.1 mol of magnesium reacts completely with ethanoic acid. (M_r of CH_3 extCOONa = 82, Mg = 24, C = 12, H = 1, O = 16)
🔍 Worked example — Standard
A student reacts 25 cm^3 of 1.0 mol/dm^3 ethanoic acid with excess sodium carbonate. Calculate the volume of CO_2 produced at RTP. (Molar volume at RTP = 24 dm^3/mol)
🔍 Worked example — Exam-style
Identify the products of the reaction between ethanoic acid and copper(II) oxide (CuO). Write the balanced equation.
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🎯 Exam question
Key ideas — Making ethanoic acid
oxidation of ethanol. Ethanoic acid is formed by oxidising ethanol. In the laboratory, ethanol is heated with acidified aqueous potassium manganate(VII). The manganate(VII) is the oxidising agent and its purple colour is decolourised as it is reduced:
🧑🏫 Teacher's explanation
Think of oxidation in organic chemistry like a controlled 'burning' process. Just as a piece of wood burns to become ash and smoke, ethanol can be 'burned' chemically to become ethanoic acid. In this process, oxygen atoms are added to the molecule, and hydrogen atoms are removed.
📖 Definitions
🔍 Worked example — Easy
Write a balanced chemical equation for the oxidation of ethanol to ethanoic acid, using [O] to represent oxygen.
🔍 Worked example — Standard
A student oxidises 0.10 mol of ethanol using excess acidified potassium manganate(VII). Calculate the mass of ethanoic acid produced, assuming 100\% yield.
🔍 Worked example — Exam-style
In the production of vinegar, ethanol is oxidized by bacteria. If the reaction is ceC2H5OH + [O] -> C2H4O2 + H2O, what is the ratio of ethanol to ethanoic acid produced?
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🎯 Exam question
Key ideas — Esters
esterification. A carboxylic acid reacts with an alcohol, when warmed with a few drops of concentrated sulfuric acid as catalyst, to form an ester and water. The reaction is reversible:
🧑🏫 Teacher's explanation
Think of esterification like a molecular 'handshake' between two different molecules. An alcohol and a carboxylic acid come together, and in the process, they both lose a small piece—the acid loses an -OH group and the alcohol loses an -H atom. These two pieces join to form water (H_2O), while the remaining parts of the acid and alcohol fuse together to form the ester.
📖 Definitions
🔍 Worked example — Easy
Name the ester formed from the reaction between propanoic acid and methanol.
🔍 Worked example — Standard
Calculate the molar mass (M_r) of ethyl ethanoate, CH_3COOCH_2CH_3. (Use C=12, H=1, O=16)
🔍 Worked example — Exam-style
A reaction is carried out between 0.50 mol of ethanoic acid and 0.50 mol of ethanol using concentrated sulfuric acid. Write the balanced chemical equation for the formation of the ester and state the catalyst used.
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🎯 Exam question
Worked example
Question. A student adds 5.3 g of sodium carbonate to an excess of dilute ethanoic acid. (a) Write the equation for the reaction and name the salt formed. [3] (b) Calculate the volume of gas produced at room temperature and pressure. (M_r of Na_2CO_3 = 106; one mole of gas occupies 24 dm³ at r.t.p.) [3] (c) Name the ester formed when propanoic acid reacts with methanol and state the catalyst needed. [2]
🔍 Worked example — Easy
Write the balanced equation for the reaction between ethanoic acid and sodium hydroxide (NaOH). Name the salt produced.
🔍 Worked example — Standard
Calculate the mass of sodium ethanoate (CH_3COONa, M_r = 82) produced when 0.2 mol of ethanoic acid reacts completely with excess sodium carbonate (Na_2CO_3).
🔍 Worked example — Exam-style
A student reacts 12.0 g of ethanol (C_2H_5OH, M_r = 46) with acidified potassium manganate(VII) to produce ethanoic acid. Calculate the mass of ethanoic acid (CH_3COOH, M_r = 60) produced, assuming 100\% yield.
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Practice
Answers Effervescence (bubbles of gas) and the magnesium disappears [B1]; 2CH_3COOH + Mg arrow (CH_3COO)_2Mg + H_2: formulae [B1], balanced [B1]. Potassium ethanoate [B1]; CH_3COOK [B1]. 2CH_3COOH + CaCO_3 arrow (CH_3COO)_2Ca + H_2O + CO_2: formulae [B1], balanced [B1]. Heat ethanol with acidified aqueous potassium manganate(VII) [B1], which is decolourised as the ethanol is oxidised [B1]; or leave ethanol (in wine or beer) exposed to air so that bacteria oxidise it to ethanoic acid, as in vinegar production [B1]. HCOOH + CH_3CH_2OH leftharpoons HCOOCH_2CH_3 + H_2O [B1]; any two of flavourings, perfumes, solvents [B1 each]. A CH_3 carbon (three C–H bonds) bonded to a second carbon [B1]; that carbon has a C=O double bond and a single C–O bond [B1]; the single-bonded oxygen is bonded to a terminal CH_3 carbon (three C–H bonds), with no O–H bond remaining [B1].