Problem 63 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Among the following, for which reaction heat of reaction represents bond energy of \(\mathrm{HCl}\) ? (a) \(\mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{H}(\mathrm{g})+\mathrm{Cl}(\mathrm{g})\) (b) \(2 \mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{H}_{2}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})\) (c) \(\mathrm{HCl}(\mathrm{g}) \rightarrow 1 / 2 \mathrm{H}_{2}(\mathrm{~g})+1 / 2 \mathrm{Cl}_{2}(\mathrm{~g})\) (d) \(\mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{H}^{+}(\mathrm{g})+\mathrm{Cl}^{-}(\mathrm{g})\)

Step-by-Step Solution

Answer

The heat of reaction for option (a), \text{HCl(g)} \rightarrow \text{H(g)} + \text{Cl(g)}, represents the bond energy of HCl.

Step 1: Understanding Bond Energy

Bond energy is defined as the average energy required to break one mole of bonds in the gas phase to form products in the gas phase. It is a measure of bond strength in a chemical bond.

Step 2: Analyzing Option (a)

Option (a) describes the dissociation of one mole of gaseous HCl into one mole of hydrogen atom and a chlorine atom, both in the gas phase. The reaction represents the energy required to break the H-Cl bond, which is the bond energy of HCl.

Step 3: Analyzing Option (b)

Option (b) describes the dissociation of two moles of HCl gas to form one mole of H2 gas and one mole of Cl2 gas. This reaction involves breaking two moles of H-Cl bonds and forming a new H-H bond and a Cl-Cl bond, so the heat of reaction does not represent the bond energy of HCl alone.

Step 4: Analyzing Option (c)

Option (c) describes the reaction where one mole of HCl gas is converted to half mole of H2 gas and half mole of Cl2 gas. This is essentially half the process described in option (b) and still does not represent the bond energy of HCl alone because it includes bond formation of H-H and Cl-Cl bonds.

Step 5: Analyzing Option (d)

Option (d) shows the ionization of HCl gas into H+ and Cl- ions. The reaction represents the ionization energy, not the bond energy, because it involves the change of neutral gaseous atoms to gaseous ions.

Key Concepts

Dissociation Energy

Dissociation energy, also known as bond dissociation energy, is the amount of energy required to break a specific chemical bond in one mole of gaseous molecules, leading to the separation of the atoms into individual gaseous atoms. It is a crucial concept when we analyze how chemical reactions occur and how much energy is involved in breaking bonds.

For example, the reaction \( \mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{H}(\mathrm{g})+\mathrm{Cl}(\mathrm{g}) \) illustrates the dissociation of hydrogen chloride into hydrogen and chlorine gas. The dissociation energy in this case would be equivalent to the bond energy of the H-Cl bond since this process directly relates to the breaking of the bond within a single molecule of hydrogen chloride.

Chemical Bonds

Chemical bonds are the attractive forces that hold atoms together in molecules or crystals. There are several types of chemical bonds, including ionic, covalent, and metallic bonds. The strength of chemical bonds is determined by factors such as the presence of electrons and the distance between nuclei.

The stability and properties of substances depend greatly on the types of bonds involved. For example, the covalent bond between hydrogen and chlorine atoms in hydrogen chloride (HCl) is what makes it a distinct compound with unique physical and chemical characteristics. When we talk about bond energy, we refer to the strength of these chemical bonds in a quantitative manner.

Enthalpy Change

Enthalpy change, denoted as \( \Delta H \), is a measure of the total heat content in a chemical reaction. It is the difference in enthalpy between the products and reactants. Enthalpy change plays a pivotal role in determining whether a reaction is endothermic (absorbing heat) or exothermic (releasing heat).

For instance, during the reaction of two moles of HCl gas forming one mole each of H₂ gas and Cl₂ gas, there's an absorption or release of energy involving bond dissociation and formation. The enthalpy change reflects all these interactions, not just the bond energy of a single molecular bond like in HCl.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state, forming a cation. It is often associated with the formation of ions from neutral atoms and is an essential concept in understanding atomic structure and chemical reactivity.

For instance, the ionization of gaseous HCl to produce \( \mathrm{H}^+(\mathrm{g}) \) and \( \mathrm{Cl}^-(\mathrm{g}) \) does not represent the bond energy of the H-Cl bond but instead illustrates the energy needed to remove an electron from HCl. This energy is influenced by the electrostatic forces between the electron and the nucleus, and the ease with which an atom or ion loses an electron.