Carbon (C) - Chemical Properties & Reactions

Chemical Properties Overview

Carbon (C) is a non-metallic element belonging to Group 14 of the periodic table. Its unique properties, such as catenation and tetravalency, are fundamental to organic chemistry.

Reactivity

• Reactivity Series Position: Carbon is not typically included in the standard metal reactivity series. As a non-metal, its reactivity is described by its tendency to form covalent bonds.
• Electronegativity: With a Pauling electronegativity of 2.55, carbon has a moderate electronegativity, allowing it to form stable covalent bonds with both more and less electronegative elements.
• General Reactivity: Carbon is generally unreactive at room temperature but becomes highly reactive at elevated temperatures. It primarily exhibits +4 and +2 oxidation states in its compounds. Due to its small size, it forms stable multiple bonds (double and triple) with other small atoms, including itself.

Action of Air and Oxygen

Carbon reacts with oxygen upon heating, leading to combustion. The products depend on the availability of oxygen.

Oxidation/Combustion Reactions

• Complete Combustion (Sufficient Oxygen): When carbon burns in a plentiful supply of oxygen, it forms carbon dioxide, a non-toxic gas. $$ \text{C} (\text{s}) + \text{O}_2 (\text{g}) \xrightarrow{\text{heat}} \text{CO}_2 (\text{g}) $$
• Incomplete Combustion (Insufficient Oxygen): When carbon burns in a limited supply of oxygen, it forms carbon monoxide, a highly toxic gas. $$ 2\text{C} (\text{s}) + \text{O}_2 (\text{g}) \xrightarrow{\text{heat}} 2\text{CO} (\text{g}) $$

Action of Water and Steam

Elemental carbon does not react with liquid water, even when hot. However, it reacts with steam at very high temperatures.

Reaction with Steam

• At High Temperatures (1000°C - 1200°C): When steam is passed over red-hot carbon (coke), a mixture of carbon monoxide and hydrogen gas is produced. This mixture is known as water gas or synthesis gas, an important industrial fuel and raw material. $$ \text{C} (\text{s}) + \text{H}_2\text{O} (\text{g}) \xrightarrow{1000-1200^\circ\text{C}} \text{CO} (\text{g}) + \text{H}_2 (\text{g}) $$

Action of Acids and Bases

Elemental carbon is generally unreactive with non-oxidizing acids and bases. It reacts only with strong oxidizing concentrated acids upon heating.

Action of Acids

• Non-oxidizing Acids (e.g., Dilute HCl, Dilute H$_2$SO$_4$): Elemental carbon (e.g., graphite, charcoal) does not react with dilute acids.
• Concentrated Oxidizing Acids:
  • Concentrated Nitric Acid ($\text{HNO}_3$): When heated with concentrated nitric acid, carbon is oxidized to carbon dioxide, while nitric acid is reduced to nitrogen dioxide. $$ \text{C} (\text{s}) + 4\text{HNO}_3 (\text{conc.}) \xrightarrow{\text{heat}} \text{CO}_2 (\text{g}) + 4\text{NO}_2 (\text{g}) + 2\text{H}_2\text{O} (\text{l}) $$
  • Concentrated Sulfuric Acid ($\text{H}_2\text{SO}_4$): When heated with concentrated sulfuric acid, carbon is oxidized to carbon dioxide, and sulfuric acid is reduced to sulfur dioxide. $$ \text{C} (\text{s}) + 2\text{H}_2\text{SO}_4 (\text{conc.}) \xrightarrow{\text{heat}} \text{CO}_2 (\text{g}) + 2\text{SO}_2 (\text{g}) + 2\text{H}_2\text{O} (\text{l}) $$

Action of Bases (Alkalis)

• Elemental carbon does not react with bases (alkalis).

Key Laboratory Test/Identification Reactions

The presence of carbon in organic compounds is typically identified by combustion. The most common test is for the gaseous product, carbon dioxide. For identifying carbonate ions, a different approach is used.

Identification of Carbon (via $\text{CO}_2$ production)

• Combustion and Limewater Test: When a substance containing carbon is heated in air or oxygen, it forms carbon dioxide. The evolved $\text{CO}2$ gas is then passed through limewater (calcium hydroxide solution), which turns milky due to the formation of insoluble calcium carbonate. $$ \text{C}{\text{(in compound)}} + \text{O}_2 (\text{g}) \xrightarrow{\text{heat}} \text{CO}_2 (\text{g}) $$ $$ \text{CO}_2 (\text{g}) + \text{Ca(OH)}_2 (\text{aq}) \rightarrow \text{CaCO}_3 (\text{s}) \downarrow + \text{H}_2\text{O} (\text{l}) $$
  • Note: If excess $\text{CO}_2$ is passed through the milky solution, the milkiness disappears due to the formation of soluble calcium bicarbonate. $$ \text{CaCO}_3 (\text{s}) + \text{CO}_2 (\text{g}) + \text{H}_2\text{O} (\text{l}) \rightarrow \text{Ca(HCO}_3)_2 (\text{aq}) $$

Identification of Carbonate Ions ($\text{CO}_3^{2-}$)

• Dilute Acid Test: Carbonate-containing compounds (e.g., salts) react with dilute acids (like HCl or $\text{H}_2\text{SO}_4$) to produce a brisk effervescence of colorless, odorless carbon dioxide gas. $$ \text{Na}_2\text{CO}_3 (\text{s}) + 2\text{HCl} (\text{aq}) \rightarrow 2\text{NaCl} (\text{aq}) + \text{H}_2\text{O} (\text{l}) + \text{CO}_2 (\text{g}) $$ The evolved $\text{CO}_2$ gas is then confirmed using the limewater test as described above.