Reaction that absorbs heat (ΔH positive)?

• A. Endothermic reaction
• B. Exothermic reaction
• C. Isothermal reaction
• D. Adiabatic reaction

Answer: (A)

Heat flow into the system corresponds to a positive enthalpy change. When ΔH is positive, the reaction requires heat from the surroundings to proceed, so it is endothermic. This happens because breaking bonds and reorganizing atoms often needs more energy than is released when new bonds form, leaving the products at higher enthalpy than the reactants and drawing heat from the surroundings.

In contrast, an exothermic reaction releases heat, giving ΔH a negative value. An isothermal process maintains a constant temperature, which can involve heat transfer but isn’t defined by a positive ΔH. An adiabatic process involves no heat exchange with the surroundings at all, so it does not describe heat absorption from the surroundings.