The equilibrium constants of the two reactions C
2
H
4
+ H
X
= C
2
H
5
X
, where
X
= Cl or Br, have been measured for
X
= Cl from 449 to 491° K, and for
X
= Br from 515 to 573° K, The methods of preparing and purifying the substances used, of carrying out the analyses and of determining the equilibrium constants have been described. The results for the ethyl chloride equilibrium were combined with calculations of the entropy change using Gordon & Giauque’s barrier height in ethyl chloride of 3700 cal/mole to obtain a value for the heat content change. The value for this, corrected to 298° K, is 17·1 kcal/mole. This leads to a heat of formation of ethyl chloride of – 26·7 and a heat of dissociation of the C—Cl bond in ethyl chloride of 80·9 kcal/mole. For the ethyl bromide equilibrium, the entropy change was calculated using barrier heights in ethyl bromide of 3000, 4000 and 5000 cal/mole. Using the entropy changes calculated it was concluded that the heat of reaction, corrected to 298° K, is within 0·3 kcal/mole of 19·1. This leads to a heat of formation of ethyl bromide of – 15·3 and a heat of dissociation of the C—Br bond in ethyl bromide of 67·2 kcal/mole. The two bond dissociation energies have been incorporated in the recent tables of Mortimer, Pritchard & Skinner listing such energies. The significance of the values for the bond dissociation energies in the series of
RX
molecules, where
R
=
Me, Et, n-Pr, n-Bu, iso-Pr
and
tert.-Bu
, and
X
= H, Cl and Br have been discussed.
