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.
The methane molecule, CH4, has the geometry shown in
Figure 2.17. Imagine a hypothetical process in which the methane molecule is “expanded,” by simultaneously extending
all four C—H bonds to infinity. We then have the
process
CH4(g)----->C(g) + 4 H(g)
(a) Compare this process with the reverse of the reaction
that represents the standard enthalpy of formation of
CH4(g). (b) Calculate the enthalpy change in each case.
Which is the more endothermic process? What accounts
for the difference in ΔH° values? (c) Suppose that 3.45 g
CH4(g) reacts with 1.22 g F2(g), forming CF4(g) and HF(g)
as sole products. What is the limiting reagent in this reaction?
If the reaction occurs at constant pressure, what
amount of heat is evolved?
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