Of the following, AH°f is not zero for Hint: Standard enthalpy of formation for the most stable form of any element is zero because there is no formation needed when the element is already in its standard state. For example, O2(g) is the most stable form for oxygen, not O atom or O3. Thus AH°¢ for O2lg) is zero but non-zero for O atom or O3. This is also why O2(g) is the elemental source of oxygen. For the same reason, AH°; for C(graphite) is zero but non-zero for C(diamond). For this question, pay attention if the elements are in their most stable physical states. O P4 (s, white) O Si (s) Sc (g) Br2 (1) O Ca (s)
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
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