18.4.3 Free Energy Away from Equilibrium

Question 1

Free Energy Away from Equilibrium

Answer 1

• Chemical equilibrium can be thought of as a balance between the entropy of a reaction and the enthalpy of a reaction.
• Whenever a system is not at equilibrium, the relative amounts of reactants and products in the reaction are expressed by the reaction quotient (Q).
• The free energy of a system is the free energy at standard state adjusted by the value of RTlnQ (ie. ΔG = ΔG° + RTlnQ)

Question 2

note

Answer 2

• Chemical equilibrium can be thought of as a balance between the entropy of a reaction and the enthalpy of a reaction. When a system is at equilibrium, the products and reactants are in a balance. In the system to the left, the reactants are of a higher chemical energy than the products. The system will adjust to a different balance of reactants and products until it reaches equilibrium. It does this by favoring product formation.
• The new equilibrium represents a balance between relative amounts of products and reactants versus the tendency of reactants to go to products.
• Relative concentration within a system is an important factor in chemical equilibrium. Whenever a system is not at equilibrium, the relative amounts of reactants and products in the reaction are represented by the reaction quotient (Q).
• Relative concentration is related to the free energy of the system (ΔG) and the equilibrium (K eq ) by the following equation: ΔG = RTln[Q/K eq ].
• When Q = K eq then ΔG = 0 and the system is at equilibrium. The reaction favors reactants when Q > K eq since ΔG is positive. The reaction proceeds towards the products when Q < K eq since the ΔG is negative.
• The free energy of a system is the free energy at standard state adjusted by the value of RTlnQ (i.e. ΔG = ΔG° + RTlnQ).
• The direction the reaction must go to get to equilibrium, given the starting conditions, can be found by determining the ΔG of the reaction using the equation ΔG = ΔG° + RTlnQ.
• First ΔG° is found using the ΔG° values for products and reactants: ΔG° = ΔG°products – ΔG°reactants. Then Q is calculated from partial pressures using the law of mass action.
• Finally, ΔG° and Q are substituted into the equation to solve for ΔG.
• A negative ΔG indicates that the system will proceed toward products to achieve equilibrium.

Question 3

What is the correct thermodynamic reaction quotient expression for the reaction?
CaCO3(s) <==> CaO(s) + CO2(g)

Answer 3

Q=PCO2

Question 4

Which of the following statements about the expression ΔG = RTln (Q / Keq ) is not correct?

Answer 4

If Q = Keq, then ΔG = 1

Question 5

Look at the expression ΔG = ΔG° + RTlnQ. Which of the following statements about this expression is not correct?

Answer 5

Q does not have the same general appearance as Keq

Question 6

What is ΔG for a reaction if ΔG° = 8.33 kJ / mol, T = 298 K, R = 8.31 × 10−3 kJ / mol • K, and Q = 0.23?

Answer 6

4.69 kJ / mol

Question 7

A reaction analogy using a balance is represented in the diagram for three different points in the reaction. An energy stress, represented by the black rectangle, has been introduced to System A, producing an initial System B and a final System C.

Which statement about this reaction is not correct?

Answer 7

At final equilibrium, the system has shifted toward more products as a result of the induced energy stress represented by the black rectangle.

Question 8

Which of the following statements about ΔG values is not correct?

Answer 8

A ΔG value of 0 means that the system will proceed toward equilibrium by shifting slightly toward the reactants or products.

Question 9

Use the formula for ΔG to find ΔG for a reaction if ΔG° = 7.27 kJ / mol, T = 282 K, R = 8.31 × 10−3 kJ / mol • K, and Q = 0.67. Also, tell whether the reaction will shift toward the products or reactants to reach equilibrium

Answer 9

ΔG = 6.33 kJ / mol; the system will shift toward the reactants to achieve equilibrium.

Question 10

Which of the following statements about the expression ΔG = ΔG° + RTlnQ is not correct?

Answer 10

Q is equal to Keq at all times during the reaction.

Question 11

Look at the new expression for ΔG.

ΔG = ΔG° + RTlnQ

Which of the following statements about this expression is not correct?

Answer 11

ΔG° = −RTlnQ

Question 12

H2(g) + I2(g) <==> 2HI(g)
For this reaction at 298 K, ΔG° = 3.40 kJ/mol, and all the partial pressures are 1 atm. What is the new value for ΔG if we increase the PHI to 1.5 atm and the PH2 to 2.3 atm? In which direction will the system proceed to reach equilibrium?

Answer 12

3.35 kJ / mol; the system will shift toward reactants to achieve equilibrium.