Yes, it is possible for an exothermic reaction to have a positive value of ΔH, and for an endothermic reaction to have a negative value of ΔH.
The enthalpy change (ΔH) is a measure of the heat energy exchanged between a system and its surroundings during a chemical reaction. It indicates whether the reaction releases heat to the surroundings (exothermic) or absorbs heat from the surroundings (endothermic).
In an exothermic reaction, the products have lower energy than the reactants, and heat is released to the surroundings. The value of ΔH for an exothermic reaction is typically negative because the system loses heat. However, under certain circumstances, if the heat gained by the surroundings is greater than the heat lost by the system, the overall change in enthalpy can be positive. This can happen, for example, if the system absorbs some additional heat from the surroundings during the reaction, resulting in a positive ΔH value.
Conversely, in an endothermic reaction, the products have higher energy than the reactants, and heat is absorbed from the surroundings. The value of ΔH for an endothermic reaction is typically positive because the system gains heat. However, if the heat lost by the surroundings is greater than the heat gained by the system, the overall change in enthalpy can be negative. This can occur, for instance, if the system loses heat to the surroundings during the reaction, resulting in a negative ΔH value.
It's important to note that the sign of ΔH depends on the reference point and the conventions used for the system and surroundings. The sign convention is defined to be consistent with energy conservation principles, but the absolute value of ΔH does not determine whether a reaction is exothermic or endothermic. The direction of heat flow (into or out of the system) determines the classification of a reaction as exothermic or endothermic.