PROBLEM SOLVING NUMBER 2 only

Chemistry: The Molecular Science
5th Edition
ISBN:9781285199047
Author:John W. Moore, Conrad L. Stanitski
Publisher:John W. Moore, Conrad L. Stanitski
Chapter16: Thermodynamics: Directionality Of Chemical Reactions
Section16.3: Measuring Dispersal Of Energy: Entropy
Problem 16.3CE
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PROBLEM SOLVING NUMBER 2 only

Therefore, at 25°C (298.15 K),
-AH
ASsurr
rxn
-(-1648400 D
5528.8 J/K
%3D
%3D
T
298.15 K
To get the total entropy of the process (ASuniv),
[ASsys +ASurr 1
=-549.5 J/K + 5528.8 J/K
= 4979.3 J/K
- Notice that the change in entropy of the reaction is negative, but the entropy change in the surroundings
is large enough such that the total entropy of the process is positive. This indicates that the oxidation of
iron is a spontaneous process at standard conditions.
Activity 2 Problem Solving
Directions: Calculate the total entropy change ASuniv and determine if it is spontaneous or not.
1. Calculate the total entropy change for the decomposition of hydrogen peroxide (H,O2). Is the reaction
spontaneous or not? Hint: 1kj = 1000J
AUm
(1)
Page | 4
Transcribed Image Text:Therefore, at 25°C (298.15 K), -AH ASsurr rxn -(-1648400 D 5528.8 J/K %3D %3D T 298.15 K To get the total entropy of the process (ASuniv), [ASsys +ASurr 1 =-549.5 J/K + 5528.8 J/K = 4979.3 J/K - Notice that the change in entropy of the reaction is negative, but the entropy change in the surroundings is large enough such that the total entropy of the process is positive. This indicates that the oxidation of iron is a spontaneous process at standard conditions. Activity 2 Problem Solving Directions: Calculate the total entropy change ASuniv and determine if it is spontaneous or not. 1. Calculate the total entropy change for the decomposition of hydrogen peroxide (H,O2). Is the reaction spontaneous or not? Hint: 1kj = 1000J AUm (1) Page | 4
H,O20
109.6
-187.8
Substance
205.0
69.9
(kJ/mol)
S (J/mol-K)
AH°,
-285.8
Solutions:
Lesson 4 Gibbs Free Energy and Spontaneity
V Gibbs free energy also determines the spontaneity of a reaction
The spontaneity of a process depends on two thermodynamic quantities: enthalpy and entropy. Combining these
two into a single equation introduces a third thermodynamic quantity that ultimately determines spontaneity. This quantity
is referred to as the Gibbs free energy (G), named after Josiah Willard Gibbs. Free energy is a portion of the total energy
of a system that is available to do useful work. The three thermodynamic quantities are related by the equation
a.
G=H-TS
Like enthalpy and entropy, Gibbs free energy is also a state function. As such, considering the final and initial
states invol in a process, the equation above can be modified to give the Gibbs-Helmholtz equation expressed as
AG= AH- TAS
reaction is spontaneous if AG is negative; it is non-spontaneous if AG is positive. This follows that for a reaction to be
spontaneous, either of these two conditions must be met:
AH must be negative, while AS is positive.
If AH is positive, AS must be large or T must be high enough so that AG will be negative.
Under standard conditions of 1 atm and 25°C, the standard free energy change (AG") of a chemical reaction can
be obtaine from the standard enthalpy change (AH) and standard entropy change (AS). The Gibbs-Helmholtz equation
then becomes
AG =AH°-TAS
he standard free energy change of a reaction can also be calculated from the standard free energy of formation
(AG°) of the reactants and products.
EAG, (products) - EAG"; (reactants)
%3D
Similar to enthalpy, the AG of elements is zero.
Sample Probl 1. Consider the dissolution of silver chloride (AgCl)
AgCl ()
→
(be)
(be) D +
The AG of the reaction can be calculated from the standard molar free energies of formation of the substances.
CI (ag)
Substance
AG (kJ/mol)
-131.3
-109. 8
AG = [AG'; (Ag') + AGr(CI)] –[AG, (AgCl) ]
=[(1 mol)(77.1 kJ/mol) + (1 mol)(- 131.3 kJ/mol)]-[(1 mol)(- 109.8 kJ/mol) ]
The positive value of AG° indicates that the reaction is non-spontaneous at 25°C. This means that the reverse
reaction is more spontaneous.
= 55.7kJ 55700 J
Activity 3 Problem Solving
Directions: Calculate the AH, AS", AG°. Show your complete solution.
1. Determine the spontaneity of the dissociation of ammonium nitrate (NH,NO3), the compound used in cold packs.
Interpret the values obtained from the calculations. The temperature is 298.15 K.
4.
+ NO3 (aq)
NH,NO3 ()
+.
(be)
NO3 (ag)
Substance
AH (kJ/mol)
S (J/mol-K)
NH,NO3 (9)
-365.6
151.1
-207.4
-132.5
113.0
146.4
a.
Calculate the AH°,
Transcribed Image Text:H,O20 109.6 -187.8 Substance 205.0 69.9 (kJ/mol) S (J/mol-K) AH°, -285.8 Solutions: Lesson 4 Gibbs Free Energy and Spontaneity V Gibbs free energy also determines the spontaneity of a reaction The spontaneity of a process depends on two thermodynamic quantities: enthalpy and entropy. Combining these two into a single equation introduces a third thermodynamic quantity that ultimately determines spontaneity. This quantity is referred to as the Gibbs free energy (G), named after Josiah Willard Gibbs. Free energy is a portion of the total energy of a system that is available to do useful work. The three thermodynamic quantities are related by the equation a. G=H-TS Like enthalpy and entropy, Gibbs free energy is also a state function. As such, considering the final and initial states invol in a process, the equation above can be modified to give the Gibbs-Helmholtz equation expressed as AG= AH- TAS reaction is spontaneous if AG is negative; it is non-spontaneous if AG is positive. This follows that for a reaction to be spontaneous, either of these two conditions must be met: AH must be negative, while AS is positive. If AH is positive, AS must be large or T must be high enough so that AG will be negative. Under standard conditions of 1 atm and 25°C, the standard free energy change (AG") of a chemical reaction can be obtaine from the standard enthalpy change (AH) and standard entropy change (AS). The Gibbs-Helmholtz equation then becomes AG =AH°-TAS he standard free energy change of a reaction can also be calculated from the standard free energy of formation (AG°) of the reactants and products. EAG, (products) - EAG"; (reactants) %3D Similar to enthalpy, the AG of elements is zero. Sample Probl 1. Consider the dissolution of silver chloride (AgCl) AgCl () → (be) (be) D + The AG of the reaction can be calculated from the standard molar free energies of formation of the substances. CI (ag) Substance AG (kJ/mol) -131.3 -109. 8 AG = [AG'; (Ag') + AGr(CI)] –[AG, (AgCl) ] =[(1 mol)(77.1 kJ/mol) + (1 mol)(- 131.3 kJ/mol)]-[(1 mol)(- 109.8 kJ/mol) ] The positive value of AG° indicates that the reaction is non-spontaneous at 25°C. This means that the reverse reaction is more spontaneous. = 55.7kJ 55700 J Activity 3 Problem Solving Directions: Calculate the AH, AS", AG°. Show your complete solution. 1. Determine the spontaneity of the dissociation of ammonium nitrate (NH,NO3), the compound used in cold packs. Interpret the values obtained from the calculations. The temperature is 298.15 K. 4. + NO3 (aq) NH,NO3 () +. (be) NO3 (ag) Substance AH (kJ/mol) S (J/mol-K) NH,NO3 (9) -365.6 151.1 -207.4 -132.5 113.0 146.4 a. Calculate the AH°,
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