AH CHa(g) C(g) +4H(g) AH l(graphite)] + 4AH GH2(g)] AH (CHA(9) C(graphite) + 2H2(g) Figure 6.13: An enthalpy cycle to find the average bond nergy of the C-H bond. The dashed line shows the wO-step route. he enthalpy cycle for calculating the average C-H ond energy is shown in Figure 6.13. Using the enthalpy ycle shown in Figure 6.13, the average C-H bond nergy can be found by dividing the value of AH on the iagram by four (because there are four C-H bonds in molecule of methane).
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.
![AH
CH4(g)
C(g) + 4H(g)
AH ((graphite))
+ 4AH H2(g)]
AH (CHa(g)
C(graphite) + 2H2(g)
Figure 6.13: An enthalpy cycle to find the average bond
energy of the C-H bond. The dashed line shows the
two-step route.
The enthalpy cycle for calculating the average C-H
bond energy is shown in Figure 6.13. Using the enthalpy
cycle shown in Figure 6.13, the average C-H bond
energy can be found by dividing the value of AH on the
diagram by four (because there are four C-H bonds in
a molecule of methane).
Question
11 Use the information in Figure 6.13 and the
information below to demonstrate that the
average bond energy of the C-H bond is
415.9 kJ mol-.
AH [CH,]=-74.8 kJ mol
AH H] = +218 kJ mol-
AH [C(graphite)] = +716.7 kJ mol-
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