
(a)
Interpretation:
For the following reaction, the ΔSorxn value has to be calculated at 298K, using the given equilibrium K value.
Concept introduction:
Entropy is the measure of randomness in the system. Standard entropy change in a reaction is the difference in entropy of the products and reactants. (ΔS°rxn) can be calculated by the following equation.
ΔS°rxn = ∑m S°Products- ∑n S°reactants
Where,
S°reactants is the standard entropy of the reactants
S°Products is the standard entropy of the products
Standard entropy change in a reaction and entropy change in the system are same.
(a)

Answer to Problem 20.95P
The standard entropy changes of the given reaction value is ΔSorxn =121.5 J/K_.
Explanation of Solution
The given equilibrium reaction is,
2NOBr(g) ⇌ 2NO(g) + Br2(g) K=0.42 at 373 K
Entropy change ΔS°system
ΔSorxn Formations of values are,
NO(g)= 210.65 J/K⋅molBr2(g) = 245.38 J/K⋅molNOBr(g) =272.6 J/K⋅mol
Calculate the change in entropy for this reaction as follows,
ΔS°rxn = ∑m S°Products- ∑n S°reactants
Calculate the change in entropy for this reaction as follows
ΔSorxn = [(2 mol NO) ( So of NO) + (1 mol Br2) ( So of Br2) ] −[ (2 mol of NOBr) ( So of NOBr)]
Substituting the values of standard entropy (So) of individual species in the above equation
ΔSorxn = [(2 mol of NO )(210.65 J/mol⋅K ) + (1 mol of Br2 )(245.38 J/mol⋅K)]− [(2 mol of NOBr)(272.6 J/mol⋅K )] ΔSorxn = 121.48 =121.5 J/K.
The standard entropy of the reaction ΔSorxn =121.5 J/K_ and the entropy change is positive.
(b)
Interpretation:
For the following reaction, the ΔGorxn value has to be calculated at 373 K, using the given data’s.
Concept introduction:
Free energy changeΔG: change in the free energy takes place while reactants convert to product where both are in standard state. It depends on the equilibrium constant K,
ΔG = ΔGo+ RT ln (K) ΔGo=- RT ln (K)
Where,
T is the temperature
ΔG is the free energy
ΔGo is standard free energy change.
(b)

Answer to Problem 20.95P
Given reaction the standard free ΔG°rxn energy value is 2.7×103J/mol.
Explanation of Solution
The given equilibrium reaction is,
2NOBr(g) ⇌ 2NO(g) + Br2(g) K=0.42 at 373 K
Solving for ΔGorxn using following free energy equation,
ΔG= ΔGo+RTln(Q)
Calculate the ΔGorxn of given equilibrium reaction is as follows,
ΔG= 0=ΔGorxn+RTlnKΔGorxn=− RTlnK= −(8.314 J/mol×K)(373 K) In(0.42)=−(8.314 J/mol×K)(373 K) ×(−0.867500)=2690.223 J/molΔGorxn=2.7×103J/mol.
Therefore, the given reaction (ΔGorxn) value is 2.7×103J/mol.
(c)
Interpretation:
For the following reaction, the ΔHorxn value has to be calculated at 373 K, using the given data’s.
Concept introduction:
Free energy change is the term that is used to explain the total energy content in a
ΔGo = ΔΗo- TΔSo
Where,
ΔΗo is the change in enthalpy of the system
ΔGo is the standard change in free energy of the system
T is the absolute value of the temperature
(c)

Answer to Problem 20.95P
Given reaction enthalpy (ΔΗorxn) changes is 4.80×104 J/mol
Explanation of Solution
The given equilibrium reaction is,
2NOBr(g) ⇌ 2NO(g) + Br2(g) K=0.42 at 373 K
Calculation for enthalpy change ΔHorxn
Standared Free energy change equation is,
ΔΗorxn = ΔGorxn- TΔSorxn
Calcualted entropy ΔSorxn and ΔGorxn values are
ΔSorxn=121.48 J/K
ΔGorxn=2690.225 J/mol
These values are plugging following above equation,
ΔΗorxn= 2690.225 J/mol + (373 K) (121.48 J/K)= 4.8002265×104ΔΗorxn=4.80×104 J/mol.
Therefore, the given reaction (ΔΗorxn) value is 4.80×104 J/mol.
(d)
Interpretation:
For the following reaction, the ΔHof value has to be calculated at 298 K.
Concept introduction:
Enthalpy is the amount energy absorbed or released in a process.
The enthalpy change in a system (ΔΗsys) can be calculated by the following equation.
ΔHrxn = ∑ΔH°produdcts-∑ΔH°reactants
Where,
ΔHof(reactants) is the standard enthalpy of the reactants
ΔHof(produdcts) is the standard enthalpy of the products
(d)

Answer to Problem 20.95P
Given reaction standard enthalpy changes is ΔH°rxn =81.7 kJ/mol_.
Explanation of Solution
Consider the equilibrium reaction is,
2NOBr(g) ⇌ 2NO(g) + Br2(g) K=0.42 at 373 K
Standard enthalpy change is,
The equation for the standard enthalpy of the above reaction is,
ΔH°rxn = ∑m ΔH°f(Products)- ∑n ΔH°f(reactants)
Calculate the change in entropy for this reaction as follows
ΔHorxn = [(2 mol NO) ( ΔHof of NO) + (1 mol Br2) ( ΔHof of Br2)] −[ (2 mol of NOBr) ( ΔHof of NOBr)]
Substituting the values of standard enthalpies of individual species in the above equation
ΔHorxn = [(2 mol of NO )(90.29 kJ/mol ) + (1 mol of Br2 )(30.91 kJ/mol )]− [(2 mol of NOBr)(4.8002265×104 J )(1 kJ/103 J)] ΔHorxnof NOBr = 81.7438675= 81.7 kJ/mol.
Hence, the standard enthalpy of the reaction ΔH°rxn =81.7 kJ/mol_.
(e)
Interpretation:
For the following reaction, the ΔGorxn value has to be calculated at 298 K.
Concept introduction:
Free energy (or) entropy change is the term that is used to explain the total energy content in a thermodynamic system that can be converted into work. The free energy is represented by the letter G. All spontaneous process is associated with the decrease of free energy in the system. The equation given below helps us to calculate the change in free energy in a system.
ΔGo = ΔΗo- TΔSo
Where,
ΔGo is the standard change in free energy of the system
ΔΗo is the standard change in enthalpy of the system
T is the absolute value of the temperature
ΔSo is the change in entropy in the system
(e)

Answer to Problem 20.95P
The ΔGorxn value of the reaction is 1.18×104 J/mol_.
Explanation of Solution
Consider the equilibrium reaction is,
2NOBr(g) ⇌ 2NO(g) + Br2(g) K=0.42 at 373 K
Calculate the Free energy change ΔGorxn
Standared Free energy change equation is,
ΔGorxn = ΔΗorxn- TΔSorxn
Calcualted enthalpy and entropy values are
ΔΗorxn=4.8002265×104 J/mol
ΔSorxn=121.48 kJ/K
These values are plugging following standard free energy equation,
ΔGorxn =4.8002265×104 J/mol-(298 K)(121.48 J/mol⋅K)= 1.1801225×105 ΔGorxn= 1.18×104 J/mol.
Therefore, the ΔGorxn value of the reaction is 1.18×104 J/mol_.
(f)
Interpretation:
For the following reaction, the NOBr ΔGof value has to be calculated at 298 K.
Concept introduction:
Free energy (Gibbs free energy) is the term that is used to explain the total energy content in a thermodynamic system that can be converted into work. The free energy is represented by the letter G. All spontaneous process is associated with the decrease of free energy in the system. The standard free energy change (ΔG°rxn) is the difference in free energy of the reactants and products in their standard state.
ΔG°rxn=∑mΔGf°(Products)-∑nΔGf°(Reactants)
Where,
nΔGf°(Reactants) is the standard entropy of the reactants
mΔGf°(products) is the standard free energy of the products
(f)

Answer to Problem 20.95P
The standard free energy value is ΔGof= 82.3 kJ/mol.
Explanation of Solution
Given equilibrium reaction is,
2NOBr(g) ⇌ 2NO(g) + Br2(g) K=0.42 at 373 K
Free energy changes ΔG°rxn
Gibbs free energy equation is,
ΔG°rxn=∑mΔGf°(Products)-∑nΔGf°(Reactants)
Free energy change for the reaction is calculated as follows,
ΔG°rxn = [(2 mol of NO) ( ΔGofof NO) + (1 mol of Br2) ( ΔGofof Br2)] −[(2 mol of NOBr)(ΔGof of NOBr)] ΔG°rxn = [(2 mol of NO)(86.60 kJ/mol ) + (1 mol of Br2)(3.13 kJ/mol )]− [(2 mol of NOBr)(1.1801225×104 J )(1 kJ/103 J)] ΔGof of NOBr = 82.264=82.3 kJ/mol.
Hence, the standard free energy value is ΔGof= 82.3 kJ/mol.
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Chemistry: The Molecular Nature of Matter and Change - Standalone book
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