Question

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=-483.6*kJ*mol^-1`

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

Answer 1

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

Explanation:

`(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`

Treating these algebraically:

`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?