Problem 5 - Chemistry A Guided Inquiry

Question

How does temperature affect the equilibrium \(\mathrm{H}_{2} \mathrm{O}(\boldsymbol{\ell}) \rightleftarrows \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) ?\) Explain in terms of \(\Delta H \Upsilon\).

Step-by-Step Solution

Answer

Since the conversion of water (liquid) to water vapor (gas) is an endothermic process (absorbs heat), increasing the temperature shifts the equilibrium to the right, favoring the formation of water vapor.

Step 1: Definition of enthalpy

Enthalpy (\(\Delta H\)) is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

Step 2: Understanding equilibrium and reaction direction

In an equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. Depending on the nature of the reaction (endothermic or exothermic), increasing the temperature will shift the equilibrium toward one side or another. If the reaction is exothermic (\(\Delta H\) is negative, heat is released), increasing the temperature will shift the equilibrium towards the reactants. If the reaction is endothermic (\(\Delta H\) is positive, heat is absorbed), increasing the temperature will shift the equilibrium towards the products.

Step 3: Applying concepts to the problem

In the given reaction, water (liquid) is converted into water vapor (gas). This process requires heat, therefore it is an endothermic process, and \(\Delta H\) is positive. So, increasing the temperature shifts the equilibrium to the right, favoring the formation of water vapor.

Key Concepts

Enthalpy

Enthalpy, denoted by the symbol \(\Delta H\), is a fundamental concept in chemistry that relates to the heat content of a system. It represents the total energy within a thermodynamic system, taking into account both the internal energy and the energy required to displace its environment.

• Internal Energy: This is the energy needed to create the system, including vibrations and interactions within the molecules.
• External Components: This consists of energy related to pressure and volume changes in the system.

When considering any chemical reaction, the change in enthalpy tells us whether heat is absorbed or released. This is a key element in predicting how a reaction will behave under varying conditions, particularly temperature changes.

Endothermic Reaction

An endothermic reaction is one that requires the absorption of heat from its surroundings. This means that these reactions have a positive \(\Delta H\). In simpler terms, they soak up energy, which typically results in a cooling effect on the surroundings.

• Energy Requirement: More energy is absorbed than energy given off during the reaction.
• Temperature Influence: Increasing temperature often drives the reaction forward, shifting equilibrium toward the products.

In the reaction \(\mathrm{H}_{2}\mathrm{O}(\ell) \rightleftarrows \mathrm{H}_{2}\mathrm{O}(\mathrm{g})\), transforming liquid water into gaseous water needs heat energy, illustrating its endothermic nature. With an increase in temperature, the process is encouraged further, producing more water vapor.

Le Chatelier's Principle

Le Chatelier's Principle is an essential tool in predicting how a system at equilibrium will respond to external changes such as concentration, pressure, and temperature.This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. For our example reaction, \(\mathrm{H}_{2}\mathrm{O}(\ell) \rightleftarrows \mathrm{H}_{2}\mathrm{O}(\mathrm{g})\), when the temperature is increased:

• Shifts Right: The addition of heat shifts the reaction in the direction that consumes heat, favoring the endothermic path—towards forming more vapor.
• Restoration of Balance: By shifting the equilibrium position, the system attempts to restore the original conditions.

Le Chatelier's Principle not only predicts this shift but also helps us understand the broader implications of changes to equilibrium systems. Introducing heat to an endothermic reaction will always tend to favor the formation of products, due to the systemic drive toward balance.