Chemistry Help?

Question

How much heat will be transferred when 25.1 g of biborane B2H6 reacts with 57 g of oxygen according ot the following equation
B2H6(g)+3O2(g)--->B2O3(s)+3H2O enthalpy cahnge is -1940 kJ

How much heat is transferred when 8 g of Nitrogen N2 reacts with 15 g of hydrogen H2 according to teh following equation
N2(g)+3H2(g)--->2NH3(g) enthalpy change is 46.2 kJ

I am really stuck on these two problems so could you please help me

Answer 1

First you change all the mass into moles.
Then you find the limiting reactant.
Then you multiply by the enthalpy of change
Viola, there's your answer.

1 mole B2H6 = 27.6 g
1 mole O2 = 32 g
so you have 0.90942 moles of B2H6 and 1.78 moles of O2 but you need 3 moles of O2 for every mole of B2H6 so your limiting reactant is O2 so you yield 0.594 moles of B2O3 which gives out 1152 KJ

N2 = 28g, H2 = 2
N2 = 0.286 moles, H2 = 7.5 moles
Limiting is N2 so 0.286 moles yields 13.2 KJ

Answer 2

http://www.public.asu.edu/~jpbirk/CHM-115_BLB/Chpt22/sld001.htm