
Interpretation:
For the given set of reactions in problem 20.51 the free energy ΔGo value have to be calculated using the ΔHof and So values.
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,
ΔH°reactants is the standard enthalpy of the reactants
ΔH°produdcts is the standard enthalpy of the products
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.
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 equation given below helps us to calculate the change in free energy in a system.
ΔG = ΔΗ- TΔS
Where,
ΔG is the change in free energy of the system
ΔΗ is the change in enthalpy of the system
T is the absolute value of the temperature
ΔS is the change in entropy in the system
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

Answer to Problem 20.53P
Reaction-A the standard free energy value is ΔGorxn= -1138 kJ_
Reaction-B the standard free energy value is ΔGorxn= -1379 kJ_
Reaction-C the standard free energy value is ΔGorxn= -226 kJ_
Explanation of Solution
Reaction-A
Given,
BaO(s)+ CO2(g) → BaCO3(s)
The number of particle also decreases, indicating a decrease in entropy.
So, there ΔGof values are zero the solid is less than gas.
Standard enthalpy change is,
The enthalpy change for the reaction is calculated as follows,
ΔH°rxn = ∑m ΔH°f(Products)- ∑n ΔH°f(reactants)
ΔH°rxn = [(1 mol MgO) ( ΔH°fof MgO)] −[(2 mol Mg)(ΔH°f of Mg) + (1 mol O2)(ΔH°f of O2)] ΔH°rxn = [(2 mol MgO)(−601.2 kJ/mol ) ]− [(2 mol Mg)(0 kJ/mol ) +(1 mol O2)(0 kJ/mol )] ΔH°rxn = -1202.4 kJ
The enthalpy change is negative. Hence, the enthalpy (ΔH°rxn) value is -1202.4 kJ_
Entropy change ΔS°system
Standard entropy change equation is,
ΔS°rxn = ∑m S°Products- ∑n S°reactants
Where, (m) and (n) are the stoichiometric co-efficient.
ΔS°rxn = [(1 mol MgO) (So of MgO)]− [ (2 mol Mg)(So of Mg) + (1mol O2) (So of O2)] ΔS°rxn = [(1 mol MgO)(26.9 J/mol×K) ]− [(2 mol N2)(32.69 J/mol×K) + (1 mol O2)(205.0 J/mol×K) ] ΔS°rxn =-216.58 J/K
Therefore, the (ΔS°rxn) of the reaction is -216.58 J/K_
Next calculate the Free enrgy change ΔGorxn
Standared Free energy change equation iss,
ΔGorxn = ΔΗorxn- TΔSorxn
Free enrgy change ΔGof
Calcualted enthalpy and entropy values are
ΔΗorxn=-1202.4 kJ
ΔSorxn=-216.58 J/K
These values are plugging above standard free energy equation,
ΔGorxn= −1202.4 kJ - [(298 K)(−216.58 J/K)(1 kJ/103J)]ΔGorxn= −1137.859 kJ
Therefore, the standard free energy value is ΔGorxn= -1138 kJ_
Reaction-B
Given reaction is,
2CH3OH (g)+ 3O2(g) → 2CO2(g) + 4H2O(g)
The number of particle also decreases, indicating a decrease in entropy.
So, there ΔGof values are zero the solid is less than gas.
Standard enthalpy change is,
The enthalpy change for the reaction is calculated as follows,
ΔH°rxn = ∑m ΔH°f(Products)- ∑n ΔH°f(reactants)
ΔΗorxn= [(2 mol CO2) ( ΔΗorxnof CO2) + (4 mol H2O) ( ΔΗorxnof H2O) ] −[(2 mol CH3OH)(ΔΗorxnof CH3OH) + (3 mol O2)(ΔΗorxn of O2) ] ΔΗorxn = [(2 mol CO2)(−393.5 kJ/mol ) + (4 mol H2O)(−241.826 kJ/mol ) ]− [(2 mol CH3OH)(−201.2 kJ/mol ) +(3 mol O2)(0 kJ/mol )] ΔΗorxn = -1351.904 kJ
The enthalpy change is negative.
Hence, the enthalpy (ΔH°rxn) value is -1351.904 kJ_
Entropy change ΔS°system
Standard entropy change equation is,
ΔS°rxn = ∑m S°Products- ∑n S°reactants
Where, (m) and (n) are the stoichiometric co-efficient.
ΔSorxn= [(2 mol CO2) ( Soof CO2) + (4 mol H2O) ( Soof H2O) ] −[(2 mol CH3OH)( Soof CH3OH) + (3 mol O2)( So of O2) ] ΔSorxn = [(2 mol CO2)(213.7 J/mol⋅K ) + (4 mol H2O)(188.72 J/mol⋅K ) ]− [(2 mol CH3OH)(238 J/mol⋅K ) +(3 mol O2)(205.0 J/mol⋅K )] ΔSorxn = -91.28 J/K
Therefore, the (ΔS°rxn) of the reaction is -91.28 J/K_
Next calculate the Free energy change ΔGorxn
Standared Free energy change equation iss,
ΔGorxn = ΔΗorxn- TΔSorxn
Free enrgy change ΔGof
Calcualted enthalpy and entropy values are
ΔΗorxn=-1351.904 kJ
ΔSorxn=-91.28 J/K
These values are plugging above standard free energy equation,
ΔGorxn= −1351.904 kJ - [(298 K)(−91.28 J/K)(1 kJ/103J)]ΔGorxn= −1379.105 kJ
Therefore, the standard free energy value is ΔGorxn= -1379 kJ_
Reaction-C
Given reaction is,
BaO(s)+ CO2(g) → BaCO3(s)
The number of particle also decreases, indicating a decrease in entropy.
So, there ΔGof values are zero the solid is less than gas.
Standard enthalpy change is,
The enthalpy change for the reaction is calculated as follows,
ΔH°rxn = ∑m ΔH°f(Products)- ∑n ΔH°f(reactants)
ΔΗorxn= [(1 mol BaCO3) ( ΔΗorxnof BaCO3)] −[(1 mol BaO)(ΔΗorxnof BaO) + (1 mol CO2)(ΔΗorxn of CO2)] ΔΗorxn = [(1 mol BaCO3)(−1219 kJ/mol )]− [(1 mol BaO)(−548.1 kJ/mol ) +(1 mol CO2)(−393.5 kJ/mol )] ΔΗorxn = -277.4 kJ
The enthalpy change is negative. Hence, the enthalpy (ΔH°rxn) value is -277.4 kJ_
Entropy change ΔS°system
Standard entropy change equation is,
ΔS°rxn = ∑m S°Products- ∑n S°reactants
Where, (m) and (n) are the stoichiometric co-efficient.
ΔSorxn= [(2 mol BaCO3) ( Soof BaCO3)] −[(1 mol BaO)( Soof BaO) + (1 mol CO2)( So of CO2) ] ΔSorxn = [(1 mol )(112 J/mol⋅K )]− [(1 mol )(72.07 J/mol⋅K ) +(1 mol)(213.7 J/mol⋅K )] ΔSorxn = -173.77 J/K
Therefore, the (ΔS°rxn) of the reaction is -173.77 J/K_
Finally calculate the Free enrgy change ΔGorxn
Standared Free energy change equation iss,
ΔGorxn = ΔΗorxn- TΔSorxn
Free enrgy change ΔGof
Calcualted enthalpy and entropy values are
ΔΗorxn=-277.4 kJ
ΔSorxn=-173.77 J/K
These values are plugging above standard free energy equation,
ΔGorxn= −277.4 kJ - [(298 K)(−173.77 J/K)(1 kJ/103J)]ΔGorxn= −225.6265 kJ
Therefore, the standard free energy value is ΔGorxn= -226 kJ_
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