Answer :
The heat absorbed during the reaction between bromine and hydrogen is determined by the enthalpy change, calculated using bond energies. Breaking bonds in the reactants requires 629 kJ, whereas formulating new product bonds releases 732 kJ. The enthalpy change (H) indicates the reaction is exothermic, releasing 103 kJ.
The heat absorbed when bromine reacts with hydrogen can be determined by calculating the enthalpy change of the reaction using bond energies. Breaking one mole of H2 requires 436 kJ/mol, and breaking one mole of Br2 requires 193 kJ/mol. Forming two moles of HBr releases 366 kJ/mol for each mole of HBr formed.
Consequently, the total energy to break the bonds is 436+193=629 kJ, and the energy released in forming the bonds is 2×366=732 kJ.
To calculate the enthalpy change (H) of the reaction H2(g) + Br2(g) ⇒ 2HBr(g), you subtract the energy needed to break the bonds (629 kJ) from the energy released when new bonds are formed (732 kJ). This results in
H = 732 kJ - 629 kJ = 103 kJ. Since energy is released, the reaction is exothermic.