A 0.500-g sample of KCl is added to 50.0 g of water in a calorimeter (Figure 9.12). If the temperature decreases by \(1.05^{\circ} \mathrm{C},\) what is the approximate amount of heat involved in the dissolution of the \(\mathrm{KCl}\), assuming the specific heat of the resulting solution is \(4.18 \mathrm{J} / \mathrm{g}^{\circ} \mathrm{C}\) ? Is the reaction exothermic or endothermic?

Calorimetry is a technique used in the field of chemistry to measure the amount of heat exchanged in chemical reactions or physical changes. This method involves using a device known as a calorimeter which is designed to minimize energy exchange with its surroundings.

In the context of our exercise, we'd use calorimetry to determine how much heat is involved when KCl dissolves in water. This is measured by observing the temperature change of the water, given that it's a well-mixed solution within the calorimeter. A temperature decrease implies that heat is absorbed by the KCl, pointing to an endothermic process.

To yield accurate results, it's important to consider factors like the total mass of both KCl and water, as well as the specific heat capacity of the resulting solution. Calorimetry hinges on an understanding that in a closed system, any heat lost or gained by the substance within the calorimeter must have been gained or lost by its surroundings.

The specific heat capacity, often denoted as 'c', is a property of a material that describes how much heat energy, in joules, is required to raise the temperature of one gram of the substance by one degree Celsius. This value varies from substance to substance, indicating the different capacities materials have for storing thermal energy.

In our exercise, the specific heat capacity of the KCl solution is given as 4.18 J/g°C, which is actually very close to the specific heat capacity of pure water. Using this information, combined with the mass of the solution and the observed temperature change, we could apply the formula for heat exchanged in a process, represented as Q = mcΔT, where Q is heat, m is mass, and ΔT is temperature change, to calculate the heat involved during dissolution of KCl.

An endothermic reaction is a chemical reaction that absorbs heat from its surroundings, often causing the temperature of the reaction vessel to decrease. Conversely, an exothermic reaction would release heat, increasing the temperature.

In our KCl example, the decrease in temperature as KCl dissolves suggests an endothermic process, since the system (in this case, the KCl solution) is absorbing heat from the water that surrounds it, effectively cooling the water. Endothermic reactions often require additional energy to proceed, breaking bonds within the reactants and forming new ones. Understanding endothermic and exothermic processes is essential to grasping energy changes in chemical reactions.

Thermochemistry is the study of the energy and heat associated with chemical reactions and physical transformations. It is a branch of thermodynamics that can be applied to reactions such as the dissolution of KCl in water.

One of the fundamental principles of thermochemistry is the law of conservation of energy, which states that in an isolated system, energy cannot be created or destroyed, only converted from one form to another. Thus, in our exercise, the heat lost by the water is equal to the heat gained by the KCl during dissolution. Understanding thermochemistry is crucial for predicting and controlling energy changes, which play a large role in the feasibility and spontaneity of chemical reactions.