6.1.3 Heat of Reaction

This topic explains the heat of reaction — the amount of heat evolved or absorbed in a chemical reaction. It covers standard enthalpy change, the relationship between enthalpy change and heat of reaction, and how reversing a reaction affects the sign of ΔH.

Definition

Heat of Reaction: The amount of heat evolved or absorbed in a chemical reaction, when molar quantities of reactants and products are the same as shown in a chemical reaction.

Standard Enthalpy Change (ΔH°): The heat of reaction measured at 25°C (298 K) and one atmospheric pressure.

Main Idea

The amount of energy released or absorbed depends on the amounts of substances that react. For example, as more fuel burns, more energy is released. Thus, when we report an energy change, we must also report the amounts of the chemical substances that generate the energy change. For this purpose, energy changes are expressed in terms of molar quantities of reactants and products.

Core Concept

• Heat of reaction: Heat evolved or absorbed when molar quantities react.
• Standard enthalpy change (ΔH°): Heat of reaction at 25°C and 1 atm.
• Exothermic: ΔH° is negative.
• Endothermic: ΔH° is positive.
• Reversing a reaction: Changes the sign of ΔH° but not its magnitude.

Detailed Explanation

What is Heat of Reaction?

• The amount of energy released or absorbed depends on the amounts of substances that react.
• For example, as more fuel burns, more energy is released.
• Thus, when we report an energy change, we must also report the amounts of the chemical substances that generate the energy change.
• For this purpose, energy changes are expressed in terms of molar quantities of reactants and products.
• The amount of heat evolved or absorbed in a chemical reaction, when molar quantities of reactants and products are same as shown in a chemical reaction, is called heat of reaction.
• Heat of reaction measured at 25°C or 298 K and one atmospheric pressure is known as standard enthalpy change.
• It is denoted by ΔH°.

Examples

(i) C_(s) + O_2(g) → CO_2(g) Δ H^° = -393.5 kJ

• 1 mole of solid carbon (12 g) reacts with 1 mole of oxygen gas (32 g).
• To give 1 mole of CO₂ gas (44 g) at 25°C and 1 atmospheric pressure.
• 393.5 kJ heat is evolved.

(ii) H_2(g) + I_2(g) → 2HI_(g) Δ H^° = +53.8 kJ

• 1 mole of H₂ reacts with 1 mole of I₂.
• To give 2 moles of HI.
• 53.8 kJ heat is absorbed.

Reversing a Reaction

• If a reaction is exothermic when going in one direction, it will be endothermic in the reverse direction.
• When a reaction is reversed, the magnitude of ΔH° remains the same but its sign changes.

Thus, the above two thermochemical reactions in the reverse direction would be represented as follows:

CO_2(g) → C_(s) + O_2(g) Δ H^° = +393.5 kJ

2HI_(g) → H_2(g) + I_2(g) Δ H^° = -53.8 kJ