Final answer:
A negative ΔH denotes an exothermic reaction, where energy is released as heat. Reaction (a) is exothermic as it releases energy, while reaction (b) is endothermic as it requires energy input. The Gibbs free energy equation represents enthalpy by ΔH.
Step-by-step explanation:
When discussing a negative ΔH, it signifies an exothermic reaction, in which heat is released into the surroundings and the enthalpy of the system decreases. This can be understood through the concept that breaking bonds requires energy (endothermic process), while forming new bonds releases energy (exothermic process). For reaction (a) 2H₂(g) + O₂(g) → 2H₂O(l) + 135 kcal, the presence of energy being released with the products indicates it is an exothermic reaction. Conversely, for reaction (b) N₂(g) + O₂(g) + 45 kcal → 2NO(g), the energy is required to drive the reaction, signifying it is an endothermic reaction.
As per the Gibbs free energy equation, enthalpy is denoted by ΔH. Comparatively, an exergonic reaction is more likely to occur spontaneously as it releases energy, while an endothermic reaction or an endergonic reaction requires energy input to proceed.