Enthalpy problem, thermodynamics

Question

Estimate the change in enthalpy, DH, for the following reaction:

H2(g) Cl2(g) --> 2 HCl(g)

Answer 1

Hi,

H2(g) --> 2 H(g)

Cl2(g) --> 2 Cl(g)



2 H(g) + 2 Cl(g) --> 2 HCl(g)

In the first rxn, H-H bonds and Cl-Cl bonds are broken. In both cases, one mole. you can look up the bond energies for the H-H and Cl-Cl bonds

+436 kJ/mol, + 243 kJ/mol

H1 (this should be delta H, but I dunno how to write in a delta)

= +(436 kJ + 243 kJ) = +679 kJ

When you BREAK bonds, you must put in energy to the system; thus, the positive sign makes sense.

--

In the second step of the reaction, two moles are H-Cl bonds are formed. When bonds are broken, energy is liberated; thus, the change in enthalpy for this rxn:

H2 = -2(431 kJ) = -862 kJ

By applying Hess's Law,

Htotal = H1 + H2

Ht = +679 kJ - 862 kJ

Ht = -183 kJ   <~~Answer.

Hope this helps