Concept of enthalpy at a given temperature
The enthalpy change at a fixed temperature is equal to the amount of heat you need to add in order for the final temperature to be equal to the initial temperature.  If the enthalpy change is negative, this means that the reaction is exothermic, and you need to remove heat for the final temperature to equal the initial temperature.  

If the reaction is carried out adiabatically (no heat addition or removal), the temperature of the products will be either higher or lower than the temperature of the reactants, depending on whether the reaction is exothermic or endothermic, respectively.  To determine the temperature change, you can follow a two-step procedure:  1.  allow the reaction to take place isothermally, by adding or removing the heat of reaction as necessary; then, 2.  reverse the heat addition or removal that you did in step 1, by applying it to the products of the reaction from step 1.  The net effect will be the adiabatic temperature rise (or fall) produced by the chemical reaction.