0620 · 11.7Carboxylic acids

Carboxyl group and ethanoate ions

0:00Vinegar contains ethanoic acid, which gives it its characteristic acidic taste. This substance connects familiar acid reactions with the chemistry of alcohols and esters. Its behaviour comes from one particular group of atoms within its molecules. By understanding that group, you can predict its reactions rather than memorise separate answers. We will connect three ideas throughout this video. Ethanoic acid forms salts when it reacts with metals, bases and carbonates.

0:27Oxidation can produce it from ethanol, including during vinegar production. A reaction with an alcohol gives an ester, showing that the same acid can take part in very different chemical changes. Carboxylic acids contain the carboxyl functional group. A functional group is an atom or group of atoms that determine the chemical properties of a homologous series. Here, one carbon has a double bond to oxygen and a single bond to an oxygen attached to hydrogen.

0:51The highlighted group is responsible for the characteristic reactions we will study. Ethanoic acid has two carbon atoms altogether, including the carbon within its carboxyl group. Its condensed formula on screen keeps that arrangement visible. The hydrogen attached to oxygen is the one released as a hydrogen ion during acid reactions. The three hydrogens attached to the other carbon are not released in this way. In aqueous solution, ethanoic acid is a weak acid because it only partially dissociates into ions.

1:20Only some molecules release hydrogen ions, while many remain unchanged. Weak does not mean dilute, because concentration describes how much acid is dissolved in a given volume. Despite partial dissociation, ethanoic acid still undergoes the characteristic reactions of acids. After losing that hydrogen ion, the remaining group is an ethanoate ion with a 1− charge. This explains why salts made from ethanoic acid are called ethanoates. The other part of the salt name identifies the positive ion.

1:48Sodium gives sodium ethanoate, while magnesium produces magnesium ethanoate.

Acid reactions and salt equations

1:53Magnesium reacts with ethanoic acid to form magnesium ethanoate and hydrogen. You would observe bubbles as hydrogen gas forms, and the magnesium gradually dissolves. This follows the familiar pattern of a reactive metal with an acid. Copper does not react this way because it is below hydrogen in the reactivity series. The balanced symbol equation is on the board. Each magnesium ion has a 2+ charge, so it needs two ethanoate ions to balance its charge.

2:19Two ethanoic acid molecules therefore react for each magnesium atom. Their released hydrogen forms hydrogen gas, while the ethanoate ions become part of the salt. The brackets in magnesium ethanoate show that the whole ethanoate group occurs twice. They do not mean that only the final oxygen is doubled. When writing a salt formula, balance the charges of its positive and negative ions. This is the same principle used for inorganic salts, even though the ethanoate ion contains several atoms.

2:46With a base, ethanoic acid undergoes neutralisation. Ethanoic acid plus sodium hydroxide gives sodium ethanoate plus water. Hydrogen ions from the acid combine with hydroxide ions from the alkali to make water. Sodium ions and ethanoate ions remain in solution, giving the dissolved salt. Sodium ethanoate contains one sodium ion for each ethanoate ion because their opposite charges have equal sizes. Its formula is shown on the board. Notice that no hydrogen gas forms in this reaction.

3:16A base gives salt and water, whereas a reactive metal produces salt and hydrogen. Let us work through the reaction with sodium carbonate rather than jump straight to a balanced equation. An acid reacting with a carbonate gives a salt, water and carbon dioxide. The salt here must be sodium ethanoate because the acid supplies ethanoate ions. We can now write the correct substances before choosing any balancing numbers. Sodium carbonate contains two sodium ions in each formula unit.

3:44We therefore need two units of sodium ethanoate on the product side. Each ethanoic acid molecule supplies one ethanoate group, so we also need two acid molecules. The completed equation has two ethanoic acid molecules reacting with one sodium carbonate formula unit. Those two acid molecules supply the hydrogen needed to form one water molecule. The carbonate part produces one carbon dioxide molecule, with its remaining oxygen entering the water.

4:10All atoms are now accounted for on both sides. Balancing changes the numbers in front of formulae, never the formulae of the substances themselves. Effervescence, meaning bubbling, occurs as carbon dioxide escapes. In a practical paper, you may need to describe an observation and identify the gas. Carbon dioxide turns limewater milky, whereas hydrogen gives a squeaky pop with a lighted splint. Bubbles alone cannot distinguish them, so connect the gas test to the reactant used.

4:38These reactions share one organising idea: the acid forms an ethanoate salt. The other reactant determines the additional products. A reactive metal produces hydrogen, a base forms water, and a carbonate gives water and carbon dioxide. Keeping those patterns separate prevents the common mistake of writing the same gas for every acid reaction.

Oxidising ethanol to ethanoic acid

4:58This next part is for the Extended paper. Ethanoic acid can be formed by oxidising ethanol with acidified aqueous potassium manganate(VII). The reagent acts as an oxidising agent, causing ethanol to become ethanoic acid. During this reaction, the potassium manganate(VII) solution changes from purple to colourless. Compare the formulae rather than seeing this as a completely new molecule appearing. The compound still contains two carbon atoms, but the alcohol group is converted into a carboxyl group.

5:27Overall, oxidation adds oxygen and removes hydrogen from the organic molecule. On the board, each O in square brackets stands for oxygen from the oxidising agent, and two of them are needed. The acidified condition matters because the reagent must be used in acidic aqueous solution. In an exam, state its full name rather than simply writing potassium manganate(VII). The colour change is evidence that the oxidising agent has reacted.

5:53Do not reverse the change, because the purple colour belongs to the manganate(VII) before it reacts. Bacterial oxidation provides another route from ethanol to ethanoic acid. During vinegar production, bacteria help oxidise ethanol using oxygen from the air. Ethanoic acid forms and remains dissolved in the liquid. Unlike the laboratory route, this process does not require potassium manganate(VII) as the oxidising agent. Air must be available, because the bacteria use its oxygen to oxidise the ethanol.

6:21The overall products are ethanoic acid and water. Keep this process distinct from fermentation, which produces ethanol rather than oxidising it into an acid. Both routes described here reach the same acid, but they use different conditions.

Esterification and exam connections

6:35This next part is for the Extended paper. A carboxylic acid reacts with an alcohol, using an acid catalyst, to form an ester and water. This reaction is called esterification. An ester contains a carbon to oxygen double bond beside an oxygen joined to another carbon group. Ethanoic acid plus ethanol reacts reversibly to give ethyl ethanoate plus water. An acid catalyst, commonly concentrated sulfuric acid, increases the reaction rate without being used up overall.

7:03The two reactants become joined within the ester molecule. Water is also formed, so it must appear in both the word and symbol equations. The ester name tells you where its two organic parts came from. Ethanol supplies the ethyl part, while ethanoic acid provides the ethanoate part. Read ethyl ethanoate as a record of both starting substances. This helps you work backwards from the product name instead of memorising an isolated reaction.

7:28The reversible arrow means the reaction can occur in both directions. Ethyl ethanoate and water can react to form the original acid and alcohol. Do not assume that all the starting substances must become products. Also distinguish the ester from sodium ethanoate, which is an ionic salt rather than a molecule containing an ethyl group. An exam question could ask you to describe the chemical oxidation of ethanol, then state the products of esterification.

7:53First identify which transformation each part describes. Oxidation changes an alcohol into a carboxylic acid, while esterification combines an acid with an alcohol. Match the conditions and observations to the correct process before writing your answer. Oxidise ethanol using acidified aqueous potassium manganate(VII). The solution changes from purple to colourless. Ethanoic acid reacts with ethanol to form ethyl ethanoate and water. An acid catalyst is required. Each sentence supplies a separate creditworthy point without mixing the conditions of the two reactions.

8:26You should now see ethanoic acid as one substance with several connected behaviours. Its acidic hydrogen allows it to form ethanoate salts with metals, bases and carbonates. Oxidation of ethanol produces the carboxyl group, either chemically or through bacterial action. With an alcohol and an acid catalyst, the acid forms an ester and water.