Problem 39 - Chemistry: The Central Science

Question

Consider the following reaction, which occurs at room temperature and pressure: $$ 2 \mathrm{Cl}(g) \longrightarrow \mathrm{Cl}_{2}(g) \quad \Delta H=-243.4 \mathrm{~kJ} $$ Which has the higher enthalpy under these conditions, \(2 \mathrm{Cl}(g)\) or \(\mathrm{Cl}_{2}(g) ?\)

Step-by-Step Solution

Answer

In conclusion, under the given conditions of room temperature and pressure, the 2 Cl atoms (reactants) have higher enthalpy compared to 1 mole of Cl2 molecules (products), since the reaction has a negative enthalpy change of \(-243.4\, \mathrm{kJ}\), indicating an exothermic reaction.

Step 1: Identify what a negative enthalpy change means

A negative enthalpy change, ΔH, means that the reaction is exothermic. In other words, the system releases energy to the surroundings as the reaction proceeds. This also implies that the enthalpy is lower for the products compared to the reactants.

Step 2: Compare the enthalpy of the reactants and products

We have the reaction: \(2\, \mathrm{Cl}(g) \longrightarrow \mathrm{Cl}_{2}(g) \quad \Delta H = -243.4\, \mathrm{kJ} \) Since the enthalpy change is negative, it means the products release heat and have a lower enthalpy compared to the reactants. Therefore, the reactants (2 moles of Cl atoms) have a higher enthalpy than the products (1 mole of Cl2 molecules). In conclusion, under the given conditions of room temperature and pressure, the 2 Cl atoms (reactants) have higher enthalpy compared to 1 mole of Cl2 molecules (products).

Key Concepts

Exothermic Reaction

An exothermic reaction is a fundamental concept in chemistry that describes a process where energy is released into the surroundings. In these reactions, the energy required to break the bonds of the reactants is less than the energy released when new bonds are formed in the products. As a result, heat is given off, and you might actually feel the surroundings getting warmer.

Let's take a closer look at the reaction provided: \(2 \mathrm{Cl}(g) \longrightarrow \mathrm{Cl}_{2}(g)\). This reaction comes with a negative enthalpy change, \(\Delta H = -243.4 \mathrm{~kJ}\). What this tells us is that the reaction is exothermic, meaning it actively releases 243.4 kJ of energy while forming \(\mathrm{Cl}_{2}(g)\) from \(2\ \mathrm{Cl}(g)\).

• Energy is transferred from the system (reaction) to the surroundings.
• The system’s enthalpy decreases.
• The surrounding environment's temperature might increase due to released heat.

Understanding exothermic reactions helps us predict behavior in chemical processes and also provides insights into designing processes where energy release is beneficial, like in combustion engines.

Enthalpy Change

Enthalpy change is a key parameter in chemistry used to quantify the heat change in reactions at constant pressure. It's typically represented by \(\Delta H\). In an exothermic reaction like the conversion of \(2 \mathrm{Cl}(g)\) to \(\mathrm{Cl}_{2}(g)\), the enthalpy change has a negative value indicating that energy is leaving the system.

To tell who has higher enthalpy, you look at the value of \(\Delta H\). For our reaction, since \(\Delta H = -243.4 \mathrm{~kJ}\), this implies the products (\(\mathrm{Cl}_{2}(g)\)) have less enthalpy than the reactants (\(2 \mathrm{Cl}(g)\)). This is because the system loses enthalpy when energy is released:

• If \(\Delta H\) is negative: products have lower enthalpy.
• If \(\Delta H\) is positive: products have higher enthalpy.

In general, when evaluating chemical reactions' enthalpy change, it helps predict whether energy is absorbed or released, aiding in understanding the energetic requirements or outputs of chemical processes.

Thermodynamics

Thermodynamics is the study of energy, heat, and their transformations. It's an essential area in chemistry that provides us principles to understand how energy transfer can cause reactions to occur spontaneously. In any chemical reaction, thermodynamics helps in understanding why reactions have particular energy flows and how these relate to the changes in temperature and pressure.

For the reaction \(2 \mathrm{Cl}(g) \longrightarrow \mathrm{Cl}_{2}(g)\), thermodynamics explains that because the process is exothermic, it is favorable under standard conditions. This is because systems naturally tend to proceed to a state with lower energy or enthalpy, releasing energy as heat:

• The first law of thermodynamics: Energy cannot be created or destroyed, only transformed.
• Entropy change (disorder) and enthalpy change guide the spontaneity of reactions.

By applying principles of thermodynamics, chemists can not only predict the direction and extent of chemical reactions but also design conditions under which reactions will proceed to maximize efficiency and product yields.