Carboxylic acids

1. Overview

Carboxylic acids are a homologous series of organic compounds characterized by the functional group –COOH (the carboxyl group). These compounds are essential in both industrial chemistry and biological processes, famously known for giving vinegar its sour taste and sharp smell. Understanding their reactions is vital for grasping how organic acids behave similarly to, yet differently from, mineral acids like HCl.

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Key Definitions

• Carboxylic Acid: A homologous series of organic compounds containing the –COOH functional group.
• Weak Acid: An acid that only partially dissociates (ionizes) into ions when dissolved in water, resulting in a pH typically between 3 and 5.
• Homologous Series: A family of compounds with the same functional group, similar chemical properties, and a trend in physical properties.
• Esterification: A condensation reaction between a carboxylic acid and an alcohol to form an ester and water.
• Oxidizing Agent: A substance (like acidified potassium manganate(VII)) that provides oxygen or removes electrons from another reactant.

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Core Content

Ethanoic acid ($CH_3COOH$) is the most common carboxylic acid studied. In aqueous solution, it behaves as a typical acid, though it is weaker than mineral acids.

(a) Reaction with Metals

Ethanoic acid reacts slowly with reactive metals (like Magnesium) to produce a salt and hydrogen gas.

• Word Equation: Ethanoic acid + Magnesium → Magnesium ethanoate + Hydrogen
• Symbol Equation: $2CH_3COOH(aq) + Mg(s) \rightarrow (CH_3COO)_2Mg(aq) + H_2(g)$
• Observation: Effervescence (bubbles of gas) is seen, and the metal slowly dissolves.

(b) Reaction with Bases

Ethanoic acid reacts with bases (like Sodium Hydroxide) in a neutralization reaction to produce a salt and water.

• Word Equation: Ethanoic acid + Sodium hydroxide → Sodium ethanoate + Water
• Symbol Equation: $CH_3COOH(aq) + NaOH(aq) \rightarrow CH_3COONa(aq) + H_2O(l)$

(c) Reaction with Carbonates

Ethanoic acid reacts with carbonates to produce a salt, water, and carbon dioxide gas.

• Word Equation: Ethanoic acid + Sodium carbonate → Sodium ethanoate + Water + Carbon dioxide
• Symbol Equation: $2CH_3COOH(aq) + Na_2CO_3(s) \rightarrow 2CH_3COONa(aq) + H_2O(l) + CO_2(g)$
• Observation: Rapid fizzing/effervescence. The gas turns limewater milky.

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Extended Content (Extended Curriculum Only)

Formation of Ethanoic Acid

Ethanoic acid is produced by the oxidation of ethanol ($CH_3CH_2OH$). This can happen in two ways:

1. Oxidation with Acidified Aqueous Potassium Manganate(VII) Ethanol is heated with $KMnO_4$ in the presence of dilute sulfuric acid ($H_2SO_4$).

• Observation: The purple solution turns colorless.
• Symbol Equation: $CH_3CH_2OH(aq) + 2[O] \rightarrow CH_3COOH(aq) + H_2O(l)$ (Note: $[O]$ represents oxygen from the oxidizing agent).

2. Bacterial Oxidation When ethanol (in wine or beer) is left exposed to the air, microbes (bacteria) use atmospheric oxygen to oxidize the ethanol.

• Context: This is how vinegar is produced commercially.
• Equation: $CH_3CH_2OH(aq) + O_2(g) \rightarrow CH_3COOH(aq) + H_2O(l)$

Formation of Esters (Esterification)

Carboxylic acids react with alcohols to form esters and water. This requires a concentrated sulfuric acid catalyst and heat.

• Example: Ethanoic acid + Ethanol $\rightleftharpoons$ Ethyl ethanoate + Water
• Symbol Equation: $CH_3COOH(l) + CH_3CH_2OH(l) \rightleftharpoons CH_3COOCH_2CH_3(l) + H_2O(l)$
• Structural Formula of Ethyl ethanoate: $CH_3-C(=O)-O-CH_2-CH_3$
• Characteristics: Esters have sweet, fruity smells and are used in perfumes and food flavorings.

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Key Equations

Reaction Type	Balanced Symbol Equation
Neutralization	$CH_3COOH(aq) + KOH(aq) \rightarrow CH_3COOK(aq) + H_2O(l)$
Metal Reaction	$2CH_3COOH(aq) + Zn(s) \rightarrow (CH_3COO)_2Zn(aq) + H_2(g)$
Oxidation	$C_2H_5OH(l) + O_2(g) \rightarrow CH_3COOH(aq) + H_2O(l)$
Esterification	$R-COOH + R'-OH \xrightarrow{H_2SO_4} R-COOR' + H_2O$

Units & Symbols:

• $(aq)$: aqueous (dissolved in water)
• $(l)$: liquid
• $(g)$: gas
• $(s)$: solid
• $H_2SO_4$: Catalyst (not consumed in the reaction)

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Common Mistakes to Avoid

• ❌ Wrong: Writing the salt of ethanoic acid as $NaCH_3COO$.
• ✅ Right: Always write the organic part first: $CH_3COONa$.
• ❌ Wrong: Forgetting that carboxylic acids are "weak" acids.
• ✅ Right: Carboxylic acids have a higher pH (e.g., pH 3) than strong acids like HCl (e.g., pH 1) at the same concentration.
• ❌ Wrong: Forgetting the water molecule in esterification.
• ✅ Right: Esterification is a condensation reaction; water is always a product.

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Exam Tips

• Command Word "Name": If a question asks to "name the salt," write Sodium ethanoate, not the formula $CH_3COONa$.
• Command Word "State": If asked to "state the observation" for the reaction with carbonates, write "effervescence" or "fizzing."
• Typical Values: Be prepared to identify ethanoic acid by its pH; it is usually around pH 3.0 in 1.0 mol/dm³ solutions.
• Ester Identification: If you see a molecule with the $-COO-$ linkage in the middle of a chain, it is an ester. The name always ends in "-oate" (e.g., ethyl methanoate).
• Real-world context: Vinegar production is the most common context for bacterial oxidation questions. Just remember: Ethanol + Oxygen → Ethanoic acid.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Ethanoic acid, $CH_3COOH$, is a carboxylic acid commonly found in vinegar.

(a) State the name of the functional group present in ethanoic acid. [1]

(b) Ethanoic acid reacts with magnesium to produce a salt and a gas. (i) Name the salt formed in this reaction. [1] (ii) State a test for the gas produced, including the observation. [2]

(c) Complete the chemical equation for the reaction between ethanoic acid and magnesium: [1]

$Mg + CH_3COOH \rightarrow$

Worked Solution:

(a)

1. Carboxyl group The functional group of carboxylic acids is the carboxyl group.

How to earn full marks:

• Correct spelling of 'carboxyl' is essential.

(b)

1. (i) Magnesium ethanoate The metal replaces the hydrogen in the acid to form the salt.

How to earn full marks:

• Correct spelling and name are essential.

2. (ii) Test: Use a burning splint. Observation: A squeaky pop sound is heard. Hydrogen gas is flammable and produces a characteristic 'squeaky pop' sound when ignited.

How to earn full marks:

• Must include both the correct test AND the correct observation for 2 marks.

(c)

1. $Mg + 2CH_3COOH \rightarrow (CH_3COO)_2Mg + H_2$ The magnesium replaces both acidic hydrogens in two ethanoic acid molecules, and hydrogen gas is released.

How to earn full marks:

• Correct products and balanced equation are required for the mark.

Common Pitfall: Many students confuse the test for hydrogen with the test for carbon dioxide. Remember that hydrogen produces a 'squeaky pop' with a burning splint, while carbon dioxide turns limewater milky. Also, make sure your chemical formulas are correct, especially the subscripts.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

Ethanol can be oxidised to form ethanoic acid.

(a) State the name of the oxidising agent commonly used in the laboratory to oxidise ethanol to ethanoic acid. [1]

(b) Describe two observations you would make when ethanol is oxidised to ethanoic acid using this oxidising agent. [2]

(c) Name the other method of producing ethanoic acid. [1]

(d) Write a balanced chemical equation for the oxidation of ethanol to ethanoic acid using oxygen. [2]

Worked Solution:

(a)

1. Acidified potassium manganate(VII) (or $KMnO_4$ with $H^+$) Potassium manganate(VII) is a strong oxidizing agent.

How to earn full marks:

• The 'acidified' part is important, or the chemical formula with $H^+$.

(b)

1. The purple colour of the potassium manganate(VII) disappears. The purple $MnO_4^-$ ion is reduced and becomes colourless when it oxidises the ethanol.
2. The solution may become warm. The reaction is exothermic.

How to earn full marks:

• Must mention the colour change AND the colour that disappears to get the first mark. 'Goes clear' is acceptable.

(c)

1. Bacterial oxidation (or fermentation) This is how vinegar is made.

How to earn full marks:

• Accept 'fermentation'.

(d)

1. $C_2H_5OH + O_2 \rightarrow CH_3COOH + H_2O$ Ethanol reacts with oxygen to form ethanoic acid and water.

How to earn full marks:

• Correct formulas for all reactants and products.
• Balanced equation.

Common Pitfall: Students often forget to balance the equation in part (d). Make sure you double-check that the number of atoms of each element is the same on both sides of the equation. Also, remember to include "acidified" when naming potassium manganate(VII) as the oxidizing agent.

Exam-Style Question 3 — Extended Response [9 marks]

Question:

Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters. Consider the reaction between ethanoic acid and propanol.

(a) Draw the displayed formula of ethanoic acid and propanol. [2]

(b) Name the ester formed when ethanoic acid reacts with propanol. [1]

(c) State the role of the acid catalyst in this reaction. [1]

(d) Describe how you would carry out this reaction in the laboratory and how you would purify the ester formed. [5]

Worked Solution:

(a)

1. 📊Displayed formula of ethanoic acid showing all bonds. Labelled C, H, and O atoms. The structure should clearly show CH3-C(=O)-OH.
   *Each bond must be clearly shown.*
2. 📊Displayed formula of propanol showing all bonds. Labelled C, H, and O atoms. The -OH group must be on the end carbon, showing CH3-CH2-CH2-OH.
   *Each bond must be clearly shown.*

How to earn full marks:

• Correct displayed formula for each compound.
• All bonds and atoms must be clearly shown and labelled.

(b)

1. Propyl ethanoate The ester is named from the alcohol (propyl) and the acid (ethanoate).

How to earn full marks:

• Correct name of the ester.

(c)

1. The acid catalyst speeds up the reaction / increases the rate of reaction / provides an alternative reaction pathway with a lower activation energy. Catalysts increase the rate of reaction.

How to earn full marks:

• Must mention that the catalyst speeds up the reaction.

(d)

1. Mix ethanoic acid and propanol in a flask. The reactants must be mixed.
2. Add a few drops of concentrated sulfuric acid as a catalyst. Sulfuric acid is the common catalyst.
3. Heat the mixture under reflux for approximately 30 minutes. Reflux allows the reaction to proceed at a higher temperature without losing volatile reactants and products.
4. Distill the mixture to collect the ester. Distillation separates the ester based on its boiling point.
5. Add sodium carbonate solution to the distillate in a separating funnel and shake, releasing the pressure by inverting the funnel and opening the tap. Allow the layers to separate. Sodium carbonate neutralizes the unreacted ethanoic acid.
6. Separate the ester layer. The ester is immiscible with the aqueous layer.
7. Dry the ester with a drying agent such as anhydrous calcium chloride or magnesium sulfate. Calcium chloride or magnesium sulfate removes any remaining water.

How to earn full marks:

• Each step must be clear and logical.
• Mentioning the reflux apparatus and separating funnel is important.
• Order of operations matters, but slight variations are allowed.

Common Pitfall: When describing the purification process, many students miss key steps like using sodium carbonate to remove excess acid or drying the ester with a drying agent. Make sure you understand the purpose of each step in the purification process. Also, remember to mention reflux as part of the reaction setup.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A student investigates the reaction between a carboxylic acid, butanoic acid ($C_3H_7COOH$), and an alcohol, ethanol ($C_2H_5OH$), to produce an ester, ethyl butanoate. The reaction is reversible and reaches equilibrium.

(a) Write the balanced chemical equation for the formation of ethyl butanoate from butanoic acid and ethanol. [2]

(b) The student mixes 8.8 g of butanoic acid with 4.6 g of ethanol and a few drops of concentrated sulfuric acid catalyst. After reaching equilibrium, the mixture contains 5.22 g of ethyl butanoate.

(i) Calculate the number of moles of butanoic acid and ethanol initially used. [2] (ii) Calculate the number of moles of ethyl butanoate formed at equilibrium. [1] (iii) Deduce the number of moles of butanoic acid and ethanol remaining at equilibrium. [2] (iv) Calculate the percentage yield of ethyl butanoate. [1]

(c) Suggest two ways to increase the yield of ethyl butanoate in this reaction. [2]

(Relative atomic masses: H = 1, C = 12, O = 16)

Worked Solution:

(a)

1. $C_3H_7COOH + C_2H_5OH \rightleftharpoons C_3H_7COOC_2H_5 + H_2O$ Butanoic acid and ethanol react to form ethyl butanoate and water.

How to earn full marks:

• Correct formulas for all reactants and products.
• Balanced equation.

(b)

1. (i) $M_r$ of butanoic acid = (4 x 12) + (8 x 1) + (2 x 16) = 88 $n(butanoic \ acid) = \frac{8.8 \ g}{88 \ g/mol} = 0.100 \ mol$ Calculate the relative molecular mass of butanoic acid, then use it to calculate the number of moles.
2. $M_r$ of ethanol = (2 x 12) + (6 x 1) + (1 x 16) = 46 $n(ethanol) = \frac{4.6 \ g}{46 \ g/mol} = 0.100 \ mol$ Calculate the relative molecular mass of ethanol, then use it to calculate the number of moles.

How to earn full marks:

• Correct $M_r$ for both compounds.
• Correct number of moles, with correct units.

(ii)

1. $M_r$ of ethyl butanoate = (6 x 12) + (12 x 1) + (2 x 16) = 116 $n(ethyl \ butanoate) = \frac{5.22 \ g}{116 \ g/mol} = 0.045 \ mol$ Calculate the relative molecular mass of ethyl butanoate, then use it to calculate the number of moles.

How to earn full marks:

• Correct $M_r$.
• Correct number of moles, with correct units.

(iii)

1. Since 1 mole of butanoic acid reacts with 1 mole of ethanol to produce 1 mole of ethyl butanoate, the number of moles of butanoic acid and ethanol used up is equal to the number of moles of ethyl butanoate formed: 0.045 mol. $n(butanoic \ acid \ remaining) = 0.100 \ mol - 0.045 \ mol = 0.055 \ mol$ $n(ethanol \ remaining) = 0.100 \ mol - 0.045 \ mol = 0.055 \ mol$ Subtract the number of moles of ethyl butanoate formed from the initial number of moles of each reactant.

How to earn full marks:

• Correct subtraction for both reactants.

(iv)

1. Theoretical yield of ethyl butanoate is limited by the limiting reactant, which is either ethanol or butanoic acid since they have the same number of moles: 0.100 mol Percentage yield = $\frac{Actual \ yield}{Theoretical \ yield} \times 100 = \frac{0.045 \ mol}{0.100 \ mol} \times 100 = 45%$ Divide the actual yield by the theoretical yield and multiply by 100.

How to earn full marks:

• Correctly identify limiting reactant as ethanol or butanoic acid.
• Correct calculation of percentage yield, with correct units.

(c)

1. Use an excess of one of the reactants (e.g., butanoic acid). Using an excess of one reactant will shift the equilibrium to the right, increasing the yield of ethyl butanoate.
2. Remove the water as it is formed. Removing a product from the reaction mixture will shift the equilibrium to the right, increasing the yield of ethyl butanoate.

How to earn full marks:

• Suggesting valid methods that will shift the equilibrium to the right.

Common Pitfall: Many students struggle with the mole calculations in part (b). Remember to first calculate the relative molecular mass ($M_r$) of each compound before calculating the number of moles. Also, be careful to identify the limiting reactant correctly when calculating the percentage yield. Finally, remember to include units in your answers.