Problem 12 - Chemistry The Molecular Nature of Matter

Question

Suggest a reason why the value of \(\Delta H_{\text {soln }}\) for a gas such as \(\mathrm{CO}_{2},\) dissolving in water, is negative.

Step-by-Step Solution

Answer

The value of Delta H_{soln} for CO_2 dissolving in water is negative because the process is exothermic, releasing more energy through the formation of interactions between CO_2 and water molecules than is consumed in breaking the initial intermolecular forces.

Step 1: Understanding Delta H_{soln}

The value Delta H_{soln} stands for the enthalpy change of a solution when a solute dissolves in a solvent. A negative Delta H_{soln} indicates that the process is exothermic, meaning that energy is released to the surroundings during the dissolution process.

Step 2: Considering Intermolecular Forces

The dissolution of a gas in water includes the breaking of intermolecular forces in the gas and the formation of new interactions between the gas molecules and water molecules. When CO_2 dissolves in water, the energy released by the formation of new interactions between CO_2 molecules and water is greater than the energy required to break the intermolecular forces in the gas and in the water.

Step 3: Rationalizing the Negative Delta H_{soln}

Since the overall process of dissolving CO_2 in water releases more energy than is consumed, the enthalpy change for the solution process is negative. This implies that the dissolution of CO_2 in water is an exothermic reaction.

Key Concepts

Exothermic Process

An exothermic process is a chemical reaction or physical transformation that releases heat energy into its surrounding environment. This is because the total energy of the products is less than that of the reactants, leading to a negative enthalpy change (Delta H), which indicates that the process is exothermic. This type of reaction can be felt as warmth or seen as a rise in temperature in the environment where the reaction occurs. For example, when carbon dioxide (CO_2) dissolves in water, it is an exothermic process since it releases energy to the surroundings, often observed by a slight increase in water temperature.

During this process, the system loses heat, leading to a negative Delta H value. The release of energy occurs because the new bonds formed in the solution are stronger than the original bonds that were broken when the solute and solvent mixed. Hence, energy is released as excess to the surrounding medium.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles. They are responsible for the behavior of a substance in various states of matter: solid, liquid, or gas. When a solute dissolves in a solvent, intermolecular forces between the solute and solvent are under significant change.

As gases like CO_2 dissolve in water, two main actions involving intermolecular forces occur. The original forces within the CO_2 gas must be overcome, and new forces between the CO_2 molecules and water molecules are formed. If the new interactions are stronger, more energy is released than is used to separate the molecules, contributing to a negative Delta H(soln). This concept is crucial in understanding why the process of dissolution for gases like CO_2 in water is typically exothermic.

Dissolution of Gases in Water

Dissolution of gases in water is a process that involves the interaction between water molecules and gas molecules, leading to the dispersal of the gas throughout the water. This process is influenced by temperature, pressure, and the nature of the gas and the solvent.

For instance, when CO_2 gas dissolves in water, it forms weak acids adding to the overall solubility of the gas in the liquid. This process is spontaneous and is driven by the entropy increase as the ordered gas molecules disperse in the solvent, as well as the energy change associated with the interaction between the gas molecules and solvent. The dissolution of most gases in water is generally exothermic, as heat is released due to the interactions between water molecules and the gas, creating a negative enthalpy change (Delta H(soln)).

Delta H(soln)

Delta H(soln), or the enthalpy change of solution, is a thermodynamic quantity that indicates the amount of heat absorbed or released when a solute dissolves in a solvent to form a homogeneous mixture, known as a solution.

When the enthalpy change of solution is negative, as in the dissolution of CO_2 in water, the process is exothermic. This negative Delta H(soln) suggests that the energy needed to break apart the solute and solvent molecules is less than the energy released when the new solute-solvent interactions are formed. The degree of the enthalpy change depends on the specific intermolecular interactions between the solute and solvent, resonating with the importance of considering both the strength and nature of these interactions when predicting or understanding the enthalpy change associated with dissolving a gas in water.