Which would have a higher enthalpy under these conditions?

Question

Considering that the following reaction occurs at room temp and pressure:

2Cl (g) ------> Cl2 (g) Delta H= -243.4kJ

Would 2Cl (g) or Cl2 (g) have a higher enthalpy? Please explain.

Answer 1

Dear Monk,

You should ask this on the (Physical) Chemistry Section where they have greater familiarity!

h = u + pv ----------------with delta u = -244kJ/Mole

It looks to me as if both terms on the right decrease.

The internal energy, u, decreases as an octet electron in both Cl atom pairs up its spin and drops to a lower energy giving out the heat energy 244kJ per mole.

The Accretion Energy term, pv,at constant pressure:
You have half the entities on the RHS so the pv term halves.
If you have 1 Mole of Chlorine molecules at the end, then the volume has dropped from the 2Moles of Chlorine atoms by 22.4litres. You say this is at constant Pressure of 100,000Pascals.
The Work Done is -22400 Joules or 22.4kJ/Mole which is small compared to the bonding energy.
This is the usual situation often allowing deltaU to substitute for deltaH without much error (10% in this case which is a bit high to ignore)


There seems to be no trade off here - with one rising and the other falling. They both go down so the enthalpy decreases by 266.4kJ per mole.
This will drive the reaction to the Right

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