Question

Indicate the direction of heat transfer between the system and the surroundings, classify the following processes as endo- or exothermic, and give the sign of \(\Delta H^{\circ}\). (a) \(\mathrm{N}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}(g) \quad \Delta H^{\circ}=+182.6 \mathrm{~kJ}\) (b) \(2 \mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \quad \Delta H^{\circ}=+483.6 \mathrm{~kJ}\) (c) \(\mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{HCl}(g) \quad \Delta H^{\circ}=-184.6 \mathrm{~kJ}\)

Short answer

(a) Endothermic, \( \Delta H^{\circ} = +182.6 \) kJ; (b) Endothermic, \( \Delta H^{\circ} = +483.6 \) kJ; (c) Exothermic, \( \Delta H^{\circ} = -184.6 \) kJ.

Step-by-step solution

Analyze Heat Transfer for Reaction (a)

For the reaction \( \mathrm{N}_{2}(g) + \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{NO}(g) \), the standard enthalpy change \( \Delta H^{\circ} = +182.6 \) kJ. A positive \( \Delta H^{\circ} \) indicates that the system absorbs heat from the surroundings. Therefore, this process is endothermic.

Analyze Heat Transfer for Reaction (b)

For the reaction \( 2 \mathrm{H}_{2} \mathrm{O}(g) \longrightarrow 2 \mathrm{H}_{2}(g) + \mathrm{O}_{2}(g) \), the standard enthalpy change \( \Delta H^{\circ} = +483.6 \) kJ. A positive \( \Delta H^{\circ} \) suggests that heat is absorbed by the system from the surroundings. Thus, this process is endothermic.

Analyze Heat Transfer for Reaction (c)

For the reaction \( \mathrm{H}_{2}(g) + \mathrm{Cl}_{2}(g) \longrightarrow 2 \mathrm{HCl}(g) \), the standard enthalpy change \( \Delta H^{\circ} = -184.6 \) kJ. A negative \( \Delta H^{\circ} \) means the system releases heat to the surroundings, classifying it as an exothermic process.

Key concepts

Endothermic Reactions

Endothermic reactions are processes where the system captures heat from its surroundings. This means that energy is absorbed to proceed with the reaction. We can identify endothermic reactions by a positive enthalpy change ( \( \Delta H^{\circ} > 0 \) ).

Some key aspects of endothermic reactions are:

• The temperature of the surroundings often drops because the system absorbs heat.
• Energy is required for breaking chemical bonds.
• Endothermic reactions are often not spontaneous.

A perfect example of an endothermic reaction, as seen in the homework exercise, is the combination of nitrogen gas and oxygen gas to form nitric oxide (\( \mathrm{N}_2(g) + \mathrm{O}_2(g) \longrightarrow 2 \mathrm{NO}(g) \) with \( \Delta H^{\circ} = +182.6 \text{ kJ} \)). This shows that the system absorbs 182.6 kJ of energy from its surroundings.

Exothermic Reactions

Exothermic reactions, on the other hand, release energy, usually in the form of heat, into the surroundings. This is evidenced by a negative enthalpy change ( \( \Delta H^{\circ} < 0 \) ).These reactions are characterized by:

• The surrounding's temperature increases, as the energy is discharged from the system.
• Energy is often released by forming chemical bonds.
• Exothermic reactions tend to be spontaneous as they naturally move towards releasing energy.

For instance, in the homework solution, the reaction between hydrogen gas and chlorine gas to produce hydrogen chloride (\( \mathrm{H}_2(g) + \mathrm{Cl}_2(g) \longrightarrow 2 \mathrm{HCl}(g) \)) has \( \Delta H^{\circ} = -184.6 \text{ kJ} \), indicating a release of 184.6 kJ into the surroundings.

Enthalpy Change

Enthalpy change, denoted as \( \Delta H \), measures the total heat content of a system during a chemical reaction, occurring at constant pressure. It's an essential concept when determining whether a reaction is endo- or exothermic.Here's what you need to understand about enthalpy change:

• It determines if heat is absorbed or released, influencing the classification of the reaction.
• A positive \( \Delta H \) means heat is absorbed (endothermic), while a negative \( \Delta H \) indicates heat is released (exothermic).
• \( \Delta H \) is measured in kilojoules per mole (\text{ kJ/mol}), providing an idea of how much energy is involved in the reaction.

Knowing the enthalpy change helps in designing processes and understanding energy profitability in chemical reactions. In the exercises mentioned, understanding \( \Delta H \) allows the classification of reactions and guidance on the direction of heat flow.