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

(a) Heat is evolved, no work done. (b) Heat is evolved, work done on the system.

Step-by-step solution

Understanding Energy Evolved or Required

In any chemical reaction, if energy in the form of heat is released, it is termed as an exothermic reaction. Conversely, if energy is required for the reaction, it's an endothermic process. We'll analyze both processes separately to determine the nature of energy change.

Identifying Heat Change in Ozone Decomposition

The decomposition of ozone, \(\mathrm{O}_3\), into \(\mathrm{O}_2\) typically releases energy, meaning it is an exothermic reaction. Decomposition processes often release energy that was stored in the molecular bonds of the compound.

Analyzing Heat in Methane Combustion

The combustion of methane is a reaction that releases a significant amount of energy as heat. This is typical for combustion reactions, thus it is an exothermic process. The reactants (methane and oxygen) yield products (carbon dioxide and water) while releasing energy.

Understanding Work Done on System or Surroundings

Work done during a process is related to changes in volume at constant pressure. When a system expands, it does work on the surroundings. Conversely, if the surroundings compress the system, work is done on the system.

Work Associated with Ozone Decomposition

In the reaction \( \mathrm{O}_3 \rightarrow \mathrm{O}_2 \), there is a decrease in the number of gas molecules (3 moles of \( \mathrm{O}_2 \) to 2 moles of \( \mathrm{O}_3 \)). The system does not perform work on the surroundings, as typically, decomposition reactions involve no significant volume change.

Work in Methane Combustion

In the combustion of methane \(\mathrm{CH}_4(\mathrm{g})+2 \mathrm{O}_2(\mathrm{g}) \rightarrow \mathrm{CO}_2(\mathrm{g})+2 \mathrm{H}_2\mathrm{O}(\ell)\), the number of gas moles decreases (3 moles of gas reactants form 1 mole of gas product and 2 moles of liquid water), suggesting the system loses gas volume. Hence, the surroundings do work on the system.

Key concepts

Exothermic reaction

Exothermic reactions are fascinating phenomena where a chemical reaction releases energy in the form of heat. A simple way to remember this concept is to associate the prefix 'exo' with 'exit', as energy exits the system and goes into the surroundings.

These reactions are marked by a noticeable increase in temperature, making them easy to spot.

• Common examples include combustion, like burning wood or gas.
• The formation of snowflakes is an exothermic process because heat is released as water vapor turns into ice.

In our exercise, methane combustion is a classic example of an exothermic reaction. As methane reacts with oxygen to form carbon dioxide and water, a great deal of heat is released. This release of energy is what powers many processes in everyday life, such as your car's engine. Understanding exothermic reactions is crucial for grasping how energy transfers in chemical processes and affects our environment and technology.

Endothermic reaction

Endothermic reactions are the chemical processes that absorb energy from their surroundings. The prefix 'endo' signifies 'inside', indicating that these reactions take in heat.

Unlike exothermic reactions, endothermic ones often result in a temperature drop in the surrounding area, since energy is being absorbed.

• A classic example of an endothermic reaction is photosynthesis. Plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.
• The melting of ice is another example, wherein the ice absorbs heat to transform into water.

Even though our current exercise does not focus on endothermic reactions, understanding them helps balance the overall concept of thermochemistry. In these reactions, energy must be supplied for the reaction to proceed, often making them feel energy-consuming.

Combustion

Combustion is a type of chemical reaction where a substance combines with oxygen, releasing energy in the form of heat and light. It is essentially a fast exothermic reaction.

This process is vital to numerous real-world applications, from heating homes to powering vehicles.

• In the combustion reaction of methane, the natural gas burns with oxygen to produce carbon dioxide, water, and plenty of heat.
• Combustion reactions typically have hydrocarbons, like gasoline or wood as fuel, making them omnipresent in both nature and industry.

Typically, combustion is part of a larger subset of reactions, known for being efficient in energy release. These reactions are not only reliable but are often used for their high energy outputs and economic value.

Decomposition reactions

Decomposition reactions involve breaking down a compound into simpler substances. It is a fundamental chemical process where one reactant yields multiple products.

This type of reaction can either absorb or release energy, making them not exclusively exothermic or endothermic.

• An example is the decomposition of ozone, (\( \mathrm{O}_3 \rightarrow \mathrm{O}_2 \)), which releases energy as it occurs.
• In decomposition reactions, the energy difference between bonds in the reactants and products determines if it will be an energy-releasing or absorbing process.

Understanding decomposition helps us appreciate the diverse ways that matter can transform and sustain energy flows. Decomposition processes are crucial in recycling elements within ecosystems, such as breaking down organic matter into simpler molecules.