Problem 9 - General Chemistry

Question

How does the enthalpy change for an endothermic reaction occurring at constant pressure?

Step-by-Step Solution

Answer

The enthalpy change ( \(\Delta H\) ) is positive for an endothermic reaction at constant pressure.

Step 1: Understanding Enthalpy Change

Enthalpy (\(H\)) is a measurement of energy in a thermodynamic system. It is the heat content of a system at constant pressure and is expressed as a change (\(\Delta H\)) during a reaction. In an endothermic reaction, the system absorbs energy from the surroundings, leading to a change in enthalpy.

Step 2: Identifying Endothermic Reactions

An endothermic reaction is one where heat is absorbed. This means the energy needed to break bonds in the reactants is greater than the energy released when new bonds form in the products. Thus, the system gains energy, and this is reflected as a positive enthalpy change (\(\Delta H > 0\)).

Step 3: Applying Constant Pressure Condition

Most chemical reactions in open containers occur at constant pressure (usually atmospheric pressure). Under these conditions, the change in enthalpy (\(\Delta H\)) is equivalent to the heat absorbed or released by the system.

Step 4: Conclusion: Enthalpy Change in Endothermic Reactions

In an endothermic reaction occurring at constant pressure, the system absorbs heat from its surroundings, resulting in an increase in enthalpy. Therefore, the enthalpy change (\(\Delta H\)) is positive, \(\Delta H > 0\).

Key Concepts

Endothermic Reaction

An endothermic reaction is a chemical process where the system absorbs heat from its surroundings. This type of reaction is opposite to an exothermic reaction, which releases heat. In an endothermic reaction, you will often notice that it requires continual input of energy for the reaction to proceed. These reactions are quite common in everyday life.
Some examples include:

• Cooking an egg: Heat energy from the stove is absorbed to cook the egg.
• Photosynthesis: Plants absorb sunlight energy to convert carbon dioxide and water into glucose.

In terms of enthalpy (\(H\)), for an endothermic reaction, the change in enthalpy (\(\Delta H\)) is positive. This is because the system absorbs more energy from the surroundings than it gives off. Hence, you can think of \(\Delta H\) as the energy balance from the perspective of the system, meaning more energy goes into the system than comes out.

Constant Pressure

In many chemical reactions, especially those conducted in open environments like a laboratory, constant pressure is maintained. This is often atmospheric pressure. When a reaction takes place at constant pressure, any change in enthalpy (\(\Delta H\)) can be directly related to the heat absorbed or released by the system.
Imagine you are boiling water on the stove. The atmospheric pressure remains essentially constant regardless of the heat being supplied, and so the enthalpy change is a measure of the energy used to convert water into steam.

• Constant pressure makes it easier to measure energy changes because it aligns closely with our everyday experiences of heating and cooling materials.
• In thermodynamics, the concept of constant pressure is pivotal in calculating changes in a system's energy state.

Thermodynamic System

When we talk about a thermodynamic system, we refer to a particular part of the universe that we study, which is separated from its surroundings by a boundary. This system can exchange energy and matter with its surroundings, depending on the nature of the boundary.

• A closed system can exchange energy (like heat) but not matter, with its surroundings.
• An open system can exchange both energy and matter.
• An isolated system doesn’t exchange energy or matter.

Understanding what constitutes the system and the surroundings is crucial for analyzing thermodynamic processes. For a reaction occurring in a test tube, the **thermodynamic system** could be the chemicals in the tube, whereas the air around would be its surroundings. Enthalpy changes are considered from the perspective of this system. It gives us insights into how energy is absorbed or released during reactions.