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11.7.1 Reactions of ethanoic acid

Ethanoic acid as an acid

Definition

Salt

A compound formed when the hydrogen in an acid is replaced by a metal ion or an ammonium ion.

  1. Ethanoic acid, CH3COOH\text{CH}_3\text{COOH}CH3​COOH, is a carboxylic acid, so its molecule contains the −COOH-\text{COOH}−COOH functional group.
  2. When ethanoic acid dissolves in water, some of its molecules release hydrogen ions, H+\text{H}^+H+, so the solution is acidic.
  3. Only the hydrogen atom in the −COOH-\text{COOH}−COOH group is released, while the three hydrogen atoms in the CH3\text{CH}_3CH3​ group stay attached to carbon.
  4. Aqueous ethanoic acid has a pH of about 3, turns blue litmus red and turns methyl orange red.
  5. Ethanoic acid is a weak acid, so its reactions are the same types as those of hydrochloric acid but happen more slowly at the same concentration.
  6. Ethanoic acid reacts with metals, bases and carbonates, and in every case it forms a salt.
  7. Vinegar is a dilute solution of ethanoic acid, which is why vinegar shows these reactions too.

Naming ethanoate salts

Definition

Ethanoate

A salt of ethanoic acid, formed when the hydrogen of the acid group is replaced by a metal ion.

  1. Salts made from ethanoic acid are called ethanoates.
  2. When the hydrogen ion leaves the −COOH-\text{COOH}−COOH group, the ethanoate ion, CH3COO−\text{CH}_3\text{COO}^-CH3​COO−, is left behind with a charge of 1−1-1−.
  3. The name of the salt is the name of the metal followed by ethanoate, such as sodium ethanoate or calcium ethanoate.
  4. The formula is worked out by balancing the charges so that the salt has no overall charge.
    1. A metal ion with a 1+1+1+ charge needs one ethanoate ion.
    2. A metal ion with a 2+2+2+ charge needs two ethanoate ions, so brackets are placed around the ethanoate group with a 2 outside.
  5. The formulae of common ethanoate salts are shown below.
    1. Sodium ethanoate, CH3COONa\text{CH}_3\text{COONa}CH3​COONa
    2. Potassium ethanoate, CH3COOK\text{CH}_3\text{COOK}CH3​COOK
    3. Magnesium ethanoate, (CH3COO)2Mg(\text{CH}_3\text{COO})_2\text{Mg}(CH3​COO)2​Mg
    4. Calcium ethanoate, (CH3COO)2Ca(\text{CH}_3\text{COO})_2\text{Ca}(CH3​COO)2​Ca
    5. Zinc ethanoate, (CH3COO)2Zn(\text{CH}_3\text{COO})_2\text{Zn}(CH3​COO)2​Zn
    6. Copper(II) ethanoate, (CH3COO)2Cu(\text{CH}_3\text{COO})_2\text{Cu}(CH3​COO)2​Cu
  6. The metal symbol may also be written first, as in Mg(CH3COO)2\text{Mg(CH}_3\text{COO)}_2Mg(CH3​COO)2​, and both forms show the same salt.
Example

Formula of calcium ethanoate

  • A calcium ion is Ca2+\text{Ca}^{2+}Ca2+ and an ethanoate ion is CH3COO−\text{CH}_3\text{COO}^-CH3​COO−.
  • Two ethanoate ions, each with a 1−1-1− charge, are needed to cancel the 2+2+2+ charge on one calcium ion.
  • The ethanoate group is placed in brackets with a 2 after it, so the whole group is doubled.
(CH3COO)2Ca (\text{CH}_3\text{COO})_2\text{Ca} (CH3​COO)2​Ca
  • In the same way, sodium ions have a 1+1+1+ charge, so sodium ethanoate needs only one ethanoate ion and no brackets.
CH3COONa \text{CH}_3\text{COONa} CH3​COONa

Reaction with metals

Definition

Effervescence

The rapid release of bubbles of gas from a liquid during a chemical reaction.

  1. Ethanoic acid reacts with reactive metals such as magnesium, zinc and calcium to form a salt and hydrogen.

    metal+ethanoic acid→metal ethanoate+hydrogen \text{metal} + \text{ethanoic acid} \rightarrow \text{metal ethanoate} + \text{hydrogen} metal+ethanoic acid→metal ethanoate+hydrogen

  2. You see effervescence, because bubbles of hydrogen gas are given off.

  3. The metal gets smaller and finally disappears, leaving a colourless solution of the ethanoate salt.

  4. The fizzing is less vigorous than with hydrochloric acid of the same concentration, because ethanoic acid is a weak acid.

  5. The gas can be identified because it burns with a squeaky pop when a lighted splint is held at the mouth of the tube.

  6. Metals of low reactivity, such as copper, do not react with ethanoic acid.

Example

Magnesium and ethanoic acid

  • Magnesium forms Mg2+\text{Mg}^{2+}Mg2+ ions, so it reacts with two molecules of ethanoic acid.
magnesium+ethanoic acid→magnesium ethanoate+hydrogen \text{magnesium} + \text{ethanoic acid} \rightarrow \text{magnesium ethanoate} + \text{hydrogen} magnesium+ethanoic acid→magnesium ethanoate+hydrogen
  • The two acidic hydrogen atoms released from the two acid molecules join to form one H2\text{H}_2H2​ molecule.
2CH3COOH(aq)+Mg(s)→(CH3COO)2Mg(aq)+H2(g) 2\text{CH}_3\text{COOH(aq)} + \text{Mg(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg(aq)} + \text{H}_2\text{(g)} 2CH3​COOH(aq)+Mg(s)→(CH3​COO)2​Mg(aq)+H2​(g)
  • Zinc also forms 2+2+2+ ions, so it follows the same pattern to give zinc ethanoate.
2CH3COOH(aq)+Zn(s)→(CH3COO)2Zn(aq)+H2(g) 2\text{CH}_3\text{COOH(aq)} + \text{Zn(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Zn(aq)} + \text{H}_2\text{(g)} 2CH3​COOH(aq)+Zn(s)→(CH3​COO)2​Zn(aq)+H2​(g)

Reaction with bases

  1. Ethanoic acid reacts with bases, which are metal oxides and metal hydroxides, to form a salt and water only.

    base+ethanoic acid→metal ethanoate+water \text{base} + \text{ethanoic acid} \rightarrow \text{metal ethanoate} + \text{water} base+ethanoic acid→metal ethanoate+water

  2. This is a neutralisation reaction, in which the hydrogen ions from the acid react with the oxide or hydroxide ions of the base to form water.

  3. No gas is produced, so there is no fizzing.

  4. With sodium hydroxide solution, both reactants and products are colourless, so the only sign of reaction is a small rise in temperature.

  5. When black copper(II) oxide is warmed with ethanoic acid, the black solid disappears and the solution turns blue as copper(II) ethanoate forms.

Example

Sodium hydroxide and copper(II) oxide

  • Sodium ions have a 1+1+1+ charge, so one molecule of ethanoic acid reacts with one unit of sodium hydroxide.
sodium hydroxide+ethanoic acid→sodium ethanoate+water \text{sodium hydroxide} + \text{ethanoic acid} \rightarrow \text{sodium ethanoate} + \text{water} sodium hydroxide+ethanoic acid→sodium ethanoate+water CH3COOH(aq)+NaOH(aq)→CH3COONa(aq)+H2O(l) \text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} CH3​COOH(aq)+NaOH(aq)→CH3​COONa(aq)+H2​O(l)
  • Copper(II) ions have a 2+2+2+ charge, so two molecules of ethanoic acid react with one unit of copper(II) oxide.
2CH3COOH(aq)+CuO(s)→(CH3COO)2Cu(aq)+H2O(l) 2\text{CH}_3\text{COOH(aq)} + \text{CuO(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Cu(aq)} + \text{H}_2\text{O(l)} 2CH3​COOH(aq)+CuO(s)→(CH3​COO)2​Cu(aq)+H2​O(l)

Reaction with carbonates

  1. Ethanoic acid reacts with carbonates to form a salt, water and carbon dioxide.

    carbonate+ethanoic acid→metal ethanoate+water+carbon dioxide \text{carbonate} + \text{ethanoic acid} \rightarrow \text{metal ethanoate} + \text{water} + \text{carbon dioxide} carbonate+ethanoic acid→metal ethanoate+water+carbon dioxide

  2. You see effervescence, because carbon dioxide gas is given off.

  3. A solid carbonate, such as calcium carbonate, gets smaller and disappears if enough acid is present.

  4. The gas turns limewater milky, which shows that it is carbon dioxide.

  5. The fizzing stops when either the carbonate or the ethanoic acid has been used up.

Example

Sodium carbonate and calcium carbonate

  • Sodium carbonate, Na2CO3\text{Na}_2\text{CO}_3Na2​CO3​, contains two sodium ions, so it gives two units of sodium ethanoate.
2CH3COOH(aq)+Na2CO3(s)→2CH3COONa(aq)+H2O(l)+CO2(g) 2\text{CH}_3\text{COOH(aq)} + \text{Na}_2\text{CO}_3\text{(s)} \rightarrow 2\text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} 2CH3​COOH(aq)+Na2​CO3​(s)→2CH3​COONa(aq)+H2​O(l)+CO2​(g)
  • Calcium carbonate, CaCO3\text{CaCO}_3CaCO3​, contains one Ca2+\text{Ca}^{2+}Ca2+ ion, so it gives one unit of calcium ethanoate.
calcium carbonate+ethanoic acid→calcium ethanoate+water+carbon dioxide \text{calcium carbonate} + \text{ethanoic acid} \rightarrow \text{calcium ethanoate} + \text{water} + \text{carbon dioxide} calcium carbonate+ethanoic acid→calcium ethanoate+water+carbon dioxide 2CH3COOH(aq)+CaCO3(s)→(CH3COO)2Ca(aq)+H2O(l)+CO2(g) 2\text{CH}_3\text{COOH(aq)} + \text{CaCO}_3\text{(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} 2CH3​COOH(aq)+CaCO3​(s)→(CH3​COO)2​Ca(aq)+H2​O(l)+CO2​(g)
Practical

Test-tube reactions of ethanoic acid

  • Aim: to observe how dilute ethanoic acid reacts with a metal, a base and a carbonate, and to identify any gas given off.
  • Apparatus: test tubes in a rack, test-tube holder, dilute ethanoic acid, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, 2 cm2\ \text{cm}2 cm strips of magnesium ribbon, copper(II) oxide powder, sodium carbonate powder, spatula, bung fitted with a delivery tube, limewater, wooden splints, Bunsen burner, heat-resistant mat and eye protection.
  • Method, acid and metal:
    • Pour 3 cm33\ \text{cm}^33 cm3 of dilute ethanoic acid into a test tube in the rack.
    • Add one strip of magnesium ribbon and watch the surface of the metal.
    • Hold a lighted splint at the mouth of the tube once bubbles have been forming for a short time.
  • Method, acid and base:
    • Add half a spatula of black copper(II) oxide to 3 cm33\ \text{cm}^33 cm3 of the acid in a clean tube.
    • Hold the tube in a test-tube holder and warm it gently in a Bunsen flame, shaking it and pointing the mouth away from everyone.
    • Stop heating before the liquid boils and note the colour of the solution.
  • Method, acid and carbonate:
    • Pour 3 cm33\ \text{cm}^33 cm3 of limewater into a second tube and stand it in the rack.
    • Put one spatula of sodium carbonate into a clean tube, add 5 cm35\ \text{cm}^35 cm3 of the acid and fit the bung straight away.
    • Let the gas bubble through the limewater.
  • Results:
    • Magnesium fizzes steadily, though less vigorously than in hydrochloric acid, and gradually disappears, and the gas pops with a lighted splint.
    • Copper(II) oxide disappears and the solution turns blue, with no bubbles at any stage.
    • Sodium carbonate fizzes and dissolves, and the limewater turns milky.
  • Conclusion: ethanoic acid behaves as a typical acid, forming an ethanoate salt with hydrogen from a metal, water from a base, and water and carbon dioxide from a carbonate.
  • Safety: wear eye protection, avoid getting the acid on skin or in eyes, keep splint flames away from reagent bottles, and never point a heated tube at anyone.
Common Mistake
  • Do not call the salt an "ethanoic salt" or "sodium ethanoic", because salts of ethanoic acid are named ethanoates.
  • Do not write the formula of magnesium ethanoate as CH3COOMg\text{CH}_3\text{COOMg}CH3​COOMg, because a 2+2+2+ ion needs two ethanoate ions.
  • Do not include hydrogen as a product of a reaction with a base or a carbonate, because hydrogen forms only when the acid reacts with a metal.
  • Do not leave water out of the products of the reactions with bases and carbonates.
Exam technique

Products, formulae and observations

  • Identify what the acid reacts with first, then list the products: metal gives salt + hydrogen, base gives salt + water, carbonate gives salt + water + carbon dioxide.
  • Work out the charge on the metal ion before writing the formula of the ethanoate, and use brackets when there are two ethanoate ions.
  • When asked what you would see, write bubbles or fizzing, the solid disappears, or a stated colour change, rather than naming the gas.
  • Give the test for the gas in full, such as "limewater turns milky" or "lighted splint gives a squeaky pop".
Self review
  • What are the products when ethanoic acid reacts with zinc?
  • Write the formula of sodium ethanoate and of calcium ethanoate.
  • What would you see when ethanoic acid is added to sodium carbonate?
  • Name the salt formed when ethanoic acid reacts with copper(II) oxide.
  • Why does magnesium fizz less vigorously in ethanoic acid than in hydrochloric acid of the same concentration?
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Revision notes for CIE IGCSE Chemistry 11.7.1 Reactions of ethanoic acid: explanations and worked examples.

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