Anyone have ideas on how to incorporate heat of reaction release into the 2d heat equation?

My system is a semi-solid (assume solid) body which starts out at T0 (temperature). Three sides of the solid are open to ambient conditions while one is insulated. I have a reaction which proceeds within the mass (neglect mass transfer) which gives off a pretty good exotherm. I want to be able to couple the time-dependant reaction with the 2d time-dependant heat equation. Any thoughts?