Final answer:
The force involved when Cl2 and H2 combine is called chemical bond energy, with the reaction being exothermic due to the enthalpy change (ΔH). Bond breaking and forming cause absorption and release of energy, respectively.
Step-by-step explanation:
The name of the force when Cl2 (chlorine gas) and H2 (hydrogen gas) are combined is chemical bond energy. The reaction can be represented by the equation H2(g) + Cl2(g) → 2HCl(g), where energy is required to break the existing bonds in H2 and Cl2, and energy is released when new H-Cl bonds are formed in the product hydrogen chloride (HCl). The difference in energy between the broken bonds and the formed bonds is known as the enthalpy change (ΔH), which in this case is negative, indicating that the reaction is exothermic.
To obtain two moles of HCl, one mole each of H-H bonds and Cl-Cl bonds must be broken, requiring energy inputs of 436 kJ/mol and 243 kJ/mol, respectively. Conversely, the formation of two moles of H-Cl bonds releases 864 kJ in total. Because the total energy released from bond formation exceeds the energy required to break the initial bonds, the reaction is exothermic, leading to a net release of energy.