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Given the following data, what is the enthalpy associated with the reaction? #(i)# #3H_2(g) + O_3(g) rarr 3H_2O(g)#; #DeltaH_"rxn"=??#

#(ii)# #2H_2(g) + O_2(g) rarr 2H_2O(g)#; #DeltaH_1=-483.6kJmol^-1#

#(iii)# #3O_2(g) rarr 2O_3(g)#; #DeltaH_2=284..6kJmol^-1#

Answer 1

Mateo Aguilar

Approx. #-100*kJ*mol^-1#

#(i)# #3H_2(g) + O_3(g) rarr 3H_2O(g)#; #DeltaH_?=??#

#(ii)# #2H_2(g) + O_2(g) rarr 2H_2O(g)#; #DeltaH_1=-483.6*kJ*mol^-1#

#(iii)# #3O_2(g) rarr 2O_3(g)#; #DeltaH_2=284..6*kJ*mol^-1#

Algebraically handling these:

#1/2xx(iii)+3/2xx(ii)# #=#

#3H_2(g) +3O_2(g) rarr3H_2O(g) + O_3(g)# #=# #(i)# as required.

And thus #DeltaH_?=3/2DeltaH_2+1/2DeltaH_1#

#(3/2(284.6)+1/2(-483.6))*kJ*mol^-1# #=# #???#

All I have done here is treated each reaction algebraically. In effect, this is a problem with 2 linear equations and 2 unknowns. If I had to reverse the equations, I would have had to reverse the sign on #DeltaH#. Capisce?

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Answer 2

Mateo Aguilar

DeltaH_rxn = 3 * DeltaH_f(H2O) - 3 * DeltaH_f(H2) - DeltaH_f(O3)

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.