Question

Use Hess's law and the measured mean enthalpy changes for the NaOH-HCl(36.2) and NH3-HCl(31.9) reactions to calculate the enthalpy change to be expected from the reaction: NaOH + NH4Cl----NaCl + NH3 +H2O

Answer 1

Answer- We are given the reaction and it enthalpy change and from using the Hess's law we need to calculate the enthalpy change from the given third reaction NaOH + NH₄Cl----> NaCl + NH₃ +H₂O.

We know,

NaOH + HCl -----> NaCl + H₂O ∆H = 36.2 kJ ….1

NH₃ + HCl -----> NH₄Cl   ∆H = 31.9 kJ ……2

We know Hess's law and it is state that the total enthalpy change for the reaction is the sum of all changes of the multiple steps. So we need to rearrange given equation number 1 and 2 such way that when we added them we will get the equation which we need to calculate the enthalpy change.

We need to reverse the reaction number 2 and then added in to the equation number 1. We need to reverse the equation number 2, since we need NH₄Cl at the reactant side. When reaction reverse then sign of the enthalpy changes also reverse.

       NaOH + HCl -----> NaCl + H₂O,   ∆H = 36.2 kJ

       NH₄Cl --------> NH₃ + HCl , ∆H = - 31.9 kJ

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NaOH + NH₄Cl----> NaCl + NH₃ +H₂O , ∆H = 4.3 kJ