Question

The atmosphere contains the highly reactive molecule, OH, which acts to remove selected pollutants. Use the values for DeltaHrxn given below to find the DeltaHrxn for the formation of OH and H from the water.

1/2 H₂ (g) + 1/2 O₂ (g) ----> OH (g) DeltaHrxn= 42.1 kJ

H₂ (g) ---> 2H (g) DeltaHrxn= 435.9 kJ

H₂ (g) + 1/2 O₂ ---> H₂O (g) DeltaHrxn= -241.8 kJ

H₂O (g) ---> H (g) + OH (g) DeltaHrxn= ?

Answer 1

1/2 H₂ (g) + 1/2 O₂ (g) OH (g) H₁= 42.1 kJ      ------(1)

H₂ (g) 2H (g)       H₂= 435.9 kJ                           -------(2)

H₂ (g) + 1/2 O₂ H₂O (g)     H₃ = -241.8 kJ           --------(3)

H₂O (g) H (g) + OH (g)    H= ?                         --------(4)

Eqn (4) can be obtained from the first three equations as follows :

Eqn(4) = Eqn(1) + [(1/2)xEqn(2) ] + reverse of Eqn(3)

So H = H₁ + [(1/2) x H₂ ] + (- H₃)

                = 42.1 kJ + [(1/2)x435.9 kJ ] + ( -(-241.8kJ))

                = +501.85 kJ

So H of the reaction H₂O (g) H (g) + OH (g)   is +501.85 kJ