Given the standard enthalpy of reaction for the formation of hydrogen chloride: H ₂( g ) + Cl ₂( g ) 2 HCl ( g ) _ r H ^ = -184.6 ~kJ Calculate the enthalpy change when 18.25 ~g of HCl ( g ) decomposes completely into its constituent elements. (Molar mass of HCl = 36.5 ~g ~mol ⁻¹ )

Step-by-step solution

The given reaction is for the formation of 2 moles of HCl with _ r H ^ = -184.6 ~kJ . The decomposition of 2 moles of HCl is the reverse reaction, so its enthalpy change is +184.6 ~kJ . First, calculate the number of moles in 18.25 ~g of HCl : Number of moles = Given mass Molar mass = 18.25 36.5 = 0.5 ~mol . Since the decomposition of 2 moles of HCl requires +184.6 ~kJ , the enthalpy change for the decomposition of 0.5 moles is: H = +184.6 ~kJ 2 ~mol 0.5 ~mol = +46.15 ~kJ . Answer: +46.15 ~kJ