For the reaction: \(\mathrm{H}_{2(g)}+\mathrm{Cl}_{2(g)} \rightarrow 2 \mathrm{HCl} ;\) \(\Delta H=-44 \mathrm{kcal}\) What is the enthalpy of decomposition of HCl? (a) \(+44 \mathrm{kcal} / \mathrm{mol}\) (b) - \(44 \mathrm{kcal} / \mathrm{mol}\) (c) \(-22 \mathrm{kcal} / \mathrm{mol}\) (d) \(+22 \mathrm{kcal} / \mathrm{mol}\)

Enthalpy change is a measure of the heat energy that is absorbed or released during a chemical reaction at constant pressure. It is denoted by the symbol eeedddd (ΔH). Understanding enthalpy change is crucial because it helps us predict whether a reaction is endothermic (absorbs heat) or exothermic (releases heat). For instance, when hydrogen gas (H_2(g)) reacts with chlorine gas (Cl_2(g)) to form hydrochloric acid (HCl), enthalpy change tells us the amount of energy involved per mole of reactants.
In our exercise, the enthalpy change is -44 kcal, indicating an exothermic reaction. This value is critical as we delve into calculations involving the enthalpy of a reaction, particularly when studying the decomposition of compounds. To ensure clarity, remember that a negative enthalpy change signifies the release of energy, which is common in the formation of stable compounds like HCl.

A chemical reaction is the process by which substances (reactants) convert into different substances (products) with altered chemical properties. Each reaction is governed by its stoichiometry, which is the quantitative relationship between reactants and products.

In the provided exercise, we analyze a reaction where hydrogen gas combines with chlorine gas to yield hydrochloric acid gas. The stoichiometry is expressed by the balanced equation: H_2(g) + Cl_2(g) → 2HCl(g). This implies that one mole of H_2 and one mole of Cl_2 will produce two moles of HCl. By thoroughly understanding chemical equations, students can interpret and predict the results of chemical reactions, such as the energy change associated with them.

Thermochemistry is the study of the energy and heat associated with chemical reactions and physical transformations. It is a subsection of thermodynamics that deals with heat changes, particularly enthalpy change in reactions.

Applying thermochemistry concepts to our example allows us to comprehend the energy dynamics in the formation and decomposition of substances. For the decomposition of HCl, which is the reverse reaction of HCl formation, the same amount of energy absorbed or released must be accounted for but in the opposite direction. Thus, if forming 2 moles of HCl releases 44 kcal of energy, breaking down those 2 moles back to the original gases under the same conditions would require 44 kcal of energy. Thermochemistry equips us with the knowledge to determine these energy requirements, also known as the enthalpy of decomposition, assisting us in many practical applications such as industrial synthesis and energy production.