Question

What will be the signs of \(\Delta H\) and \(\Delta S\) when \(\mathrm{NaOH}\) is dissolved in water? \(\begin{array}{ll}\Delta H & \Delta S \\ \text(a) - & \- \\ \text(c) - & \+ & \end{array}\) \(\begin{array}{ll}\Delta H & \Delta S \\ \text(b) + & \- \\ \text(d) + & +\end{array}\)

Short answer

(H) is negative and (S) is positive.

Step-by-step solution

Understanding (H)

Enthalpy change (H) indicates whether a process is exothermic (-) or endothermic (+). When NaOH dissolves in water, it releases heat, making the process exothermic. Therefore, (H) is negative.

Understanding (S)

Entropy change (S) represents the degree of disorder or randomness. Dissolving NaOH in water increases the disorder as the solid NaOH separates into ions, thus the entropy increases: (S) is positive.

Identifying Correct Option

Based on the exothermic nature and increase in disorder, we can conclude that (H) is negative and (S) is positive. Therefore, we can choose the correct option which matches these signs.

Key concepts

Understanding Enthalpy Change (\begin{array}{ll}\text(b) + & - \ \text(d) + &+\text(

Enthalpy change (\text(Delta H\text()) is a fundamental concept in thermodynamics that describes the heat absorbed or released during a chemical reaction or physical process at constant pressure. A negative enthalpy change (+-) is indicative of an exothermic process—this means that the system gives off heat to its surroundings, leading to a decrease in the enthalpy of the system. Conversely, a positive enthalphy change (+) occurs in an endothermic process, which absorbs heat from the surroundings, thus increasing the system's enthalpy.

Considering our specific example of dissolving \(\mathrm{NaOH}\) in water, it's known that this dissolution is accompanied by the release of heat. Therefore, the \(\text(Delta H\text())\) value for this reaction would be negative (-), confirming the exothermic nature of the process.

Entropy Change and Disorder in Chemical Systems

Entropy (\text(Delta S\text()) captures the concept of disorder within a system. A positive entropy change (+) signifies that the disorder has increased, while a negative change (-) indicates a more ordered system. When a solid substance like \(\mathrm{NaOH}\) dissolves, the ions that compose the solid become dispersed in the solvent, increasing the system's overall randomness. This transformation leads to a rise in entropy.

Consequently, we can deduce that the entropy change for \(\mathrm{NaOH}\) dissolving in water is positive. The greater spread of particles that were once part of the ordered crystal structure of \(\mathrm{NaOH}\) into the more chaotic state of ion dispersion in water illustrates this principle. This scenario exemplifies how chemical processes can increase the level of disorder within a system.

Exothermic Reactions in Thermodynamics

Exothermic reactions are chemical processes that release energy in the form of heat to the environment. This release makes the temperature of the surroundings increase. Chemical bonds within the reactants contain potential energy, and when they are re-arranged to form the products, some of this energy is released if the products are more stable than the reactants.

In the context of our exercise, the dissolution of \(\mathrm{NaOH}\) in water is an exothermic reaction. As the ionic bonds in the solid \(\mathrm{NaOH}\) break and new interactions form between the \(\mathrm{Na^+}\), \(\mathrm{OH^-}\) ions and water, energy is released. This makes the water warmer, a common observation that serves as a practical indication of the exothermic nature of this process.

Disorder in Chemical Systems and Entropy

Disorder in chemical systems relates directly to the concept of entropy. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. In chemistry, this law is observed when substances transition from a state of order (solid crystals, for instance) to disorder (as in dissolved ions).

When \(\mathrm{NaOH}\) dissolves and its ions are spread out in solution, the system's disorder increases significantly. Each ion encounters water molecules in a random manner, increasing the system's overall entropy. Such a spread of particles brings about a more disordered state compared to the original crystalline structure of \(\mathrm{NaOH}\). This concept plays a critical role in understanding the behavior of chemical systems and predicting the direction of chemical reactions.