What does X represent in the following equation? H 2(g) + 1/2O2(g) → H2O (l); ∆H = xKJ.

In the given chemical equation, X represents the Enthalpy of Formation. Enthalpy of formation (∆H_f) is defined as the amount of heat energy released or absorbed when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. In the given equation, the reactants are hydrogen gas and oxygen gas in their standard states, and the product is liquid water. The value of ∆H represents the amount of heat energy that is either released or absorbed when one mole of water is formed from its constituent elements. If the value of ∆H is negative, it means that the reaction is exothermic, i.e., heat energy is released during the reaction, while a positive value of ∆H represents an endothermic reaction, i.e., heat energy is absorbed during the reaction.