Question

Hydrochloric acid is made from hydrogen and chloride as follows:

H2 + CI2 —-> 2HCI

1. Using the bond energy values from textbook (table 7.4.1) calculate the ΔH for above reaction.

2. Is this reaction exothermic or endothermic?

Answer 1

To calculate the ΔH (enthalpy change) for the reaction:

H2 + Cl2 → 2HCl

1. We need to calculate the bond energies of the bonds broken and formed in the reaction. The bond energy values can be found in Table 7.4.1 in the textbook. Let's consider the bond energies involved:

Bond energy of H-H bond (H2) = 436 kJ/mol

Bond energy of Cl-Cl bond (Cl2) = 242 kJ/mol

Bond energy of H-Cl bond (HCl) = 431 kJ/mol

The enthalpy change (ΔH) for the reaction can be calculated using the equation:

ΔH = Σ (bond energies of bonds broken) - Σ (bond energies of bonds formed)

In this case, we have:

ΔH = [2 * (H-H bond energy)] + [1 * (Cl-Cl bond energy)] - [2 * (H-Cl bond energy)]

Plugging in the values:

ΔH = [2 * 436 kJ/mol] + [1 * 242 kJ/mol] - [2 * 431 kJ/mol]

Simplifying:

ΔH = 872 kJ/mol + 242 kJ/mol - 862 kJ/mol

ΔH = 252 kJ/mol

Therefore, the ΔH for the given reaction is 252 kJ/mol.

2. To determine whether the reaction is exothermic or endothermic, we examine the sign of ΔH.

If ΔH is negative (ΔH < 0), it indicates that the reaction is exothermic, releasing heat to the surroundings.

If ΔH is positive (ΔH > 0), it indicates that the reaction is endothermic, absorbing heat from the surroundings.

In this case, since ΔH is positive (ΔH = 252 kJ/mol), the reaction is endothermic. It requires an input of energy to proceed.