Download Using Hess` Law of additive equations, rearrange and

Using Hess’ Law of additive equations, rearrange and/or multiply the following reactions in order
to find the enthalpy for the radical reaction between hydrochloric acid and isopropyl alcohol
(CH3CHOHCH3) to form water and isopropyl chloride. The dots you see indicate a radical, which
is much different than an ion or an element. Please tell if this process is exothermic or
endothermic.
H2O  H. + .OH
H = +498 kJ/mol
HCl  H. + Cl.
H = +431 kJ/mol
.
CH3CHClCH3  CH3CHCH3 + Cl.
.
CH3CHOHCH3  CH3CHCH3 + .OH
H = +339 kJ/mol
H = +385 kJ/mol
The reaction we’re interested in is:
HCl + CH3CHOHCH3  H2O + CH3CHClCH3
Therefore, we just rearrange the chemical equations above to get the correct chemical species
on the proper side of the equation, as well as eliminating everything else. If we switch a chemical
equation around, then we reverse the sign of the enthalpy term. Sound fair?
H. + .OH  H2O
HCl  H. + Cl.
CH3CHCH3 + Cl.  CH3CHClCH3
CH3CHOHCH3  CH3CHCH3 + .OH
H
H
H
H
=
=
=
=
-498 kJ/mol
+431 kJ/mol
-339 kJ/mol
+385 kJ/mol
Now, we add up the equations, to see what’s left:
HCl + CH3CHOHCH3  H2O + CH3CHClCH3
That’s exactly what we wanted, the enthalpy change for the “radical reaction between
hydrochloric acid and isopropyl alcohol (CH3CHOHCH3) to form water and isopropyl chloride.”
To find the enthalpy change, just add up the enthalpy terms. I get -21 kJ/mol.
Stephen Allen
OTA 104330