Question

Determine whether energy as heat is evolved or required, and whether work was done on the system or whether the system does work on the surroundings, in the following processes at constant pressure: (a) Ozone, \(\mathrm{O}_{3}\), decomposes to form \(\mathrm{O}_{2}\) (b) Methane burns: \(\mathrm{CH}_{4}(\mathrm{g})+2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})+2 \mathrm{H}_{2} \mathrm{O}(\ell)\)

Short answer

Both processes release heat (exothermic) and do work on the surroundings.

Step-by-step solution

Analyze the Reaction for Heat Exchange - Ozone Decomposition

When ozone (\( O_3 \)) decomposes to oxygen (\( O_2 \)), the process is generally exothermic. This means that energy as heat is evolved or released into the surroundings as the chemical bonds in ozone are broken to form oxygen, a more stable molecule.

Determine Work Done in Ozone Decomposition

The decomposition of ozone involves a decrease in the number of gas molecules (3 moles of ozone produce 2 moles of oxygen). At constant pressure, if the volume of gas decreases, the system does work on the surroundings as it contracts.

Analyze the Reaction for Heat Exchange - Methane Combustion

The combustion of methane (\( CH_4 \)) is an exothermic reaction, meaning energy as heat is released. This is because the products (\( CO_2 \) and \( H_2O \) ) are more stable with lower energy than the reactants (methane and oxygen).

Determine Work Done in Methane Combustion

In this reaction, the number of gas moles decreases from 3 (methane and oxygen) to 1 (carbon dioxide) because water is formed in the liquid state, which condenses from the gas phase. Therefore, under constant pressure, the system does work on the surroundings.

Key concepts

Heat Exchange

Heat exchange is a central concept in thermochemistry, describing how heat is either absorbed or released by a system during a chemical reaction.
When we talk about reactions at constant pressure, like the decomposition of ozone or the combustion of methane, we characterize them in terms of being exothermic or endothermic.

• Exothermic Reactions: These reactions release heat to the surroundings. In both ozone decomposition and methane combustion, the reactions are exothermic.


• In ozone decomposition, bonds in ozone are broken to form oxygen, releasing heat as the larger energy bonds convert to more stable, lower-energy bonds.


• Similarly, during methane combustion, heat is released as methane and oxygen form carbon dioxide and water—products that have less energy, hence stable.

Exothermic Reactions

Understanding why a reaction is exothermic helps in visualizing energy changes in chemical reactions. Exothermic reactions, by nature, emit heat energy.
Such reactions occur when the energy required to break bonds in the reactants is less than the energy released when new bonds form in the products.
For instance:

• Ozone ( O_3 ) to Oxygen ( O_2 ): The energy released as ozone decomposes to oxygen signifies that the environment is gaining heat, making more energy available for the surroundings.


• Combustion of Methane: This involves burning in the presence of oxygen, forming carbon dioxide and water, and again, it's about converting high-energy particles into low-energy, stable counterparts.

Essentially, exothermic reactions denote a flow of energy out of the system, contributing to warmth in the surroundings.

Work Done by System

In thermochemistry, the concept of work done is primarily related to volume changes in gases during reactions at constant pressure.
When the number of moles of gases reduces, the system does work on the surroundings by contracting.
This means:

• Ozone Decomposition: Gaseous volume decreases from 3 moles of ozone to 2 moles of oxygen. This contraction means the system does work on the surroundings as it lets out the pressure around it.


• Methane Combustion: Here, the reaction results in the condensation of water, forming liquid from gaseous reactants. Again, by reducing moles from 3 to 1 gas unit, it reflects work done on the environment.