Which type of reaction typically involves a positive ΔH?

The correct answer is endothermic reaction. In an endothermic reaction, the system absorbs heat from its surroundings, resulting in a positive change in enthalpy (ΔH). This means that energy is taken in during the reaction, which often leads to a decrease in temperature of the surroundings since heat is being absorbed. Common examples of endothermic processes include photosynthesis and the dissolution of certain salts in water.

In contrast, an exothermic reaction releases heat into the surroundings, leading to a negative ΔH. Combustion reactions, which release energy in the form of heat and light as they break down substances like fuels, are a specific type of exothermic reaction. Redox reactions, while they can be either endothermic or exothermic, do not have a specific sign associated with their ΔH; instead, they are defined by the transfer of electrons between species.