A scientist measures the standard enthalpy change for the following reaction to

Question

A scientist measures the standard enthalpy change for the following reaction to be 4.6 kJ :
Fe(s) + 2 HCl(aq)---> FeCl2(s) + H2(g)
Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of HCl(aq) is ___ kJ/mol.

A scientist measures the standard enthalpy change for the following reaction to be -761.0 kJ :
2NO(g) + 2 H2---> (g)N2(g) + 2 H2O(l)
Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of NO(g) is __ kJ/mol.

Explanation / Answer

1) For the given reaction,

Fe(s) + 2HCl(aq) --> FeCl2(s) + H2(g)

enthalpies for H2(g) and Fe(s) is zero

So reaction enthalpy becomes,

dHo(rxn)(kJ) = dHo(FeCl2(s)) - 2 x dHo(HCl(aq))

4.6 kJ = -341.79 - 2 x dHo(HCl(aq))

dHo(HCl(aq)) = -173.2 kJ/mol

2) for the given reaction,

2NO(g) + 2H2(g) --> N2(g) + 2H2O(l)

dHo(rxn) = -761 kJ

dHo for N2 and H2 are zero

dHo(rxn) = 2 x dHo(H2O(l)) - 2 x dHo(NO(g))

-761 = (2 x -241.82) - 2 x dHo(NO(g))

dHo(NO(g)) = 138.68 kJ/mol

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