We study a reacting mixture (2F $ F2) in a temperature gradient. We had previously used boundary-driven non-equilibrium molecular dynamics (NEMD) simulations to study this system, and found that the reaction was close to local chemical equilibrium in temperature gradients up to 1012 K/m. Using the condition of local chemical equilibrium, we show that the heat of transfer of the reacting mixture is equal to minus the enthalpy of the reaction. The fact that the sign of the heat of transfer is determined by the type of reaction adds insight to the discussion of the origin of the sign