Question

For an endothermic reaction, $\Delta H$ is___.
A.$+ ve$
B.$- ve$
C.$0$
D.None

Answer 1

Endothermic reactions are those types of chemical reactions in which the reactants absorb heat energy from the surroundings to produce or get the products. In an endothermic reaction, enthalpy is positive which means $\Delta {\rm H}$ is greater than $0$.

Complete step by step answer: The letter “$H$” is used to represent the enthalpy of the system and it refers to the sum of the internal energy of a system and product of the system’s volume and pressure. While $\Delta {\rm H}$ indicates the change in enthalpy of the system in a reaction. It represents whether the system emits or absorbs heat. For example, when water changes from liquid to solid, $\Delta {\rm H}$ is negative because the water loses heat whereas water changes from liquid to gas, $\Delta {\rm H}$ is positive; it gains heat.
Hence, option A is correct. The SI unit of enthalpy is Joule.

Additional information:
Scientists can not actually measure an enthalpy of a system $\left( H \right)$ because the internal energy contains components that are unknown. So, they measure the change in enthalpy in a system. But enthalpy change is a term used only for reactions that are done at constant pressure. It describes the energy transfer between systems and surroundings.

Note: Do not get confused between heat and temperature as well as an exothermic and endothermic reaction. Temperature is equal to the average kinetic energy of molecules that can change as heat flows from one object to another and this change in heat can be considered as exothermic (heat releasing) or endothermic reaction ( heat absorbing).