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.
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.
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.
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.
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)
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.
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.