20. The ?Hfº of MgO is –602 kJ•mol?1. When 20.15 g MgO is decomposed at constant

Question

20. The ?Hfº of MgO is –602 kJ•mol?1. When 20.15 g MgO is decomposed at constant pressure according to the equation below, how much heat will be transferred?

2 MgO(s) -> 2 Mg(s) + O2(g)

(A) 1.20*10^3 kJ of heat is released
(B) 6.02*10^2 kJ of heat is absorbed
(C) 6.02*10^2 kJ of heat is released
(D) 3.01*10^2 kJ of heat is absorbed

22.
Using the bond dissociation enthalpies (BDE) in the table, estimate ?Hº for the disproportionation of hydrazine described in the equation below.
3 N2H4(g) -> 4 NH3(g) + N2(g)

Explanation / Answer

20. mol MgO = mass/MW = 20.15/40.3044 = 0.49994

Q = n*HRxn = 0.49994*-602/2 = -150.48194 kJ

it is negative so it is released

choose A since it is the nearest value

22

.HRxn = Hproducts - Hreactants = 4*(3*388) + (944) - (3*163 + 3*4*388) = 455 kJ/mol

nearest is D, it is negative since it is th einverse reaction

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