Please help me with this problem

Question

Given:

 

​2Fe(s) +3Br2 (l) = 2FeBr3(s); H = -249.8 KJ

 

​FeBr3(s) = FeBr2(s) + ½ Br2 (l); H = +18.4 KJ

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​Calculate the heat of formation

 

​2Fe(s) + 2Br2 (l) = FeBr2 (s)

(a) 231.4 KJ (b) 286.6 KJ (c) -213.0KJ (d) 213.0 KJ (e) -286.6 KJ

Answer 1

This is a problems dealing with Hess's Law. Read up on it for more info.

​2Fe(s) + 2Br2 (l) = 2FeBr2 (s) ... TARGET EQUATION

Given:

(1) 2Fe(s) +3Br2 (l) = 2FeBr3(s); ... ∆H = -249.8 KJ

(2) FeBr3(s) = FeBr2(s) + ½ Br2 (l) ...  ∆H = +18.4 KJ

Procedure:

copy eq.(1): 2Fe(s) +3Br2 (l) = 2FeBr3(s); ... ∆H = -249.8 KJ

eq.(2) x 2: 2FeBr3 = 2FeBr2 + Br2 ... ∆H = 2x18.4 = +36.8 kJ

Add these together...

2Fe(s) +3Br2  + 2FeBr3 =  2FeBr3 + 2FeBr2 + Br2

Combine/cancel like terms to get ...

2Fe + 2Br2 = 2FeBr2 ... TARGET EQUATION

∆H = -249.8 kJ + 36.8 kJ = -213 kJ (ANSWER C)

Answer 2

Hi Asked, rearrrange your two given equations until you can add them to give the desired formation equation. Then the (appropriately scaled, i.e. if you had to multiply an equation by a factor, you must multiply the associated .delta.H by the same factor) added .delta.H's give you your answer. It's that simple.

-- Cheers, --Mr. d.