The compound phosphine, PH3, is produced by rotting corpses (the human body is a

Question

The compound phosphine, PH3, is produced by rotting corpses (the human body is about 1.0% by mass phosphorus); its spontaneous ignition in air above graveyards gave rise to legends such as the will-othe-wisps: 20 pts. 4 ( ) 8 ( ) ( ) 6 ( ) 3 2 4 10 2 PH g + O g P O s + H O l H f (PH3) = +5.40 kJ/mol; H f (P4O10) = –2984 kJ/mol H f (H2O(l)) = –285.8 kJ/mol ;

Calculate the following quantities:

(1) Hrxn for the balanced stoichiometric reaction (the recipe reaction); Ho = __________ kJ

(2) H when 0.94 kg of phosphine are the limiting reagent; Hº = __________kJ

(3) q for the reaction when 0.94 kg of phosphine are the limiting reagent assuming constant pressure; q = _________ kJ

(4) the work, w, for the reaction when 0.94 kg of phosphine are the limiting reagent; assume T = 290K.

Explanation / Answer

1] 4PH3(g) + 8O2 (g) ==> P4O10(g) + 6H2O (l)

delta H rxn = delta H of products - delta H of reactants

delta Hrxn = [-2984 +6* -285.8 ] - [4*5.4] = - 4720.4 KJ

2]

4 moles of PH3 ---->  - 4720.4 KJ

Moles of PH3 = mass / Molar mass = 0.94*1000 / 34 = 27.647

for 27.647 moles ----> -4720.4*27.647 / 4 = -32626.29 KJ

3]

q = -32636.29 KJ

[At constant pressure, the heat flow for the process is also equal to the change in enthalpy. Heat flow is expressed as "q." The relationship between Delta H and q can be determined based on whether the reaction is exothermic or endothermic. If the reaction absorbs heat from the surroundings, it is endothermic, and q is positive. If q is positive, then Delta H is also positive. If the reaction releases heat to its surroundings, it is exothermic, and q is negative. If q is negative, then Delta H is also negative.]

4]

W = PdV

PdV = delta nRT

delta n = moles of products(gases) - moles of reactants(gases) = 1-12 = -11

W = - 11*8.314*290 = -26521.66 J = -26.521 KJ

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