The standard molar enthalpy of formation of diborane, B2H6(g), cannot be determined directly because the compound cannot be prepared by the reaction of boron and hydrogen. It can be calculated from other enthalpy changes, however.The following enthalpy changes can be measured. 4 B(s) + 3O2(g) → 2 B2O3(s) ΔrH∘ = -2543.8 kJ/mol-rxn H2(g) +1/2 O2(g) → H2O(g) ΔrH∘ = -241.8 kl/mol-rxn B2H6(g) + 3O2(g) → B2O3(s) + 3 H2O(g) ΔrH∘ = -2032.9 kJ/mol-rxn (a) Show how these equations can be added together to give the equation for the formation of B2H6(g) from B(s) and H2(g) in their standard Assign enthalpy changes to each reaction. (b) Calculate Δf H∘ for B2H6(g). (c) Draw an energy level diagram that shows how the various enthalpies in this problem are related. (d) Is the formation of B2H6(g) from its elements exo- or endothermic?
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 standard molar enthalpy of formation of diborane, B2H6(g), cannot be determined directly because the compound cannot be prepared by the reaction of boron and hydrogen. It can be calculated from other enthalpy changes, however.The following enthalpy changes can be measured.
4 B(s) + 3O2(g) → 2 B2O3(s)
ΔrH∘ = -2543.8 kJ/mol-rxn
H2(g) +1/2 O2(g) → H2O(g)
ΔrH∘ = -241.8 kl/mol-rxn
B2H6(g) + 3O2(g) → B2O3(s) + 3 H2O(g)
ΔrH∘ = -2032.9 kJ/mol-rxn
(a) Show how these equations can be added together to give the equation for the formation of B2H6(g) from B(s) and H2(g) in their standard Assign enthalpy changes to each reaction.
(b) Calculate Δf H∘ for B2H6(g).
(c) Draw an energy level diagram that shows how the various enthalpies in this problem are related.
(d) Is the formation of B2H6(g) from its elements exo- or endothermic?
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