Concept explainers
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) + 3 O2(g) → 2 B2O3(s)
ΔrH° = –2543.8 kJ/mol-rxn
H2(g) +½ O2(g) → H2O(g)
ΔrH° = –241.8 kl/mol-rxn
B2H6(g) + 3 O2(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 states. Assign enthalpy changes to each reaction.
(b) Calculate ΔfH° 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?
(a)
Interpretation:
The equation for the formation of
Concept Introduction:
The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one gram of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
Explanation of Solution
First equation is:
Divide the 1st equation by 2
Multiply the 2nd equation by 3
Reverse the 3rd equation and the obtained equation is:
(b)
Interpretation:
The formation energy for
Concept Introduction:
The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one gram of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
Explanation of Solution
First equation is:
Divide the 1st equation by 2
Second equation is:
Multiply the 2nd equation by 3
And the third equation is:
Adding the 3 equations gives the required equation.
So, the formation constant is
(c)
Interpretation:
The energy level diagram for the various enthalpies in the problem has to be depicted.
Concept Introduction:
The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one gram of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
Explanation of Solution
The energy level diagram is shown below.
Figure 1
(d)
Interpretation:
The nature of the thermodynamic reaction has to be identified.
Concept Introduction:
The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one gram of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.
When heat energy is transferred the directionality of heat transfer between the system and the surrounding is described either as endothermic or exothermic
Energy of the system decrease and energy of the surrounding increases, i.e
Endothermic process, the heat energy is transferred from surroundings to the system. Energy of system increases and that of surroundings decreases i.e,
Explanation of Solution
The formation constant is
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Chapter 5 Solutions
CHEMISTRY+CHEM.REACT. (LL)-W/OWL ACCESS
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