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Questions and Answers
What condition must be met for a system to be considered at equilibrium?
What condition must be met for a system to be considered at equilibrium?
What does the equation ΔG° = -RTlnKeq indicate?
What does the equation ΔG° = -RTlnKeq indicate?
Which statement accurately describes the implications of ΔG° being greater than zero?
Which statement accurately describes the implications of ΔG° being greater than zero?
Which of the following equations reflects the relationship necessary for a spontaneous process?
Which of the following equations reflects the relationship necessary for a spontaneous process?
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What is the condition for a process to be spontaneous according to Gibbs free energy?
What is the condition for a process to be spontaneous according to Gibbs free energy?
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What must the relationship between ΔG and Keq be at equilibrium?
What must the relationship between ΔG and Keq be at equilibrium?
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In a process controlled by enthalpy, what would be the characteristics of ΔH and ΔG?
In a process controlled by enthalpy, what would be the characteristics of ΔH and ΔG?
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For a reaction where the change in Gibbs free energy is zero, which of the following statements is true?
For a reaction where the change in Gibbs free energy is zero, which of the following statements is true?
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What happens to the Gibbs free energy (ΔG) at chemical equilibrium?
What happens to the Gibbs free energy (ΔG) at chemical equilibrium?
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Given that ΔS represents entropy, which condition must be satisfied for a process to favor an increase in disorder?
Given that ΔS represents entropy, which condition must be satisfied for a process to favor an increase in disorder?
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What is the significance of a large positive ΔS in a process?
What is the significance of a large positive ΔS in a process?
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If isomerization of glucose-6-phosphate results in ΔG° of 2.1 kJ mol−1, what does this suggest about the reaction favorability under standard conditions?
If isomerization of glucose-6-phosphate results in ΔG° of 2.1 kJ mol−1, what does this suggest about the reaction favorability under standard conditions?
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For the reaction ATP → ADP + Pi with ΔG° = -30.5 kJ mol-1, what does this indicate about the reaction?
For the reaction ATP → ADP + Pi with ΔG° = -30.5 kJ mol-1, what does this indicate about the reaction?
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What is the significant condition for a spontaneous process at constant temperature and pressure?
What is the significant condition for a spontaneous process at constant temperature and pressure?
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What does the term 'irreversible change' in a spontaneous process imply?
What does the term 'irreversible change' in a spontaneous process imply?
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In the unfolding of a protein with ΔH° = 250.8 kJ mol−1 and ΔS° = 752 J K−1 mol−1, above what temperature will the process be spontaneous?
In the unfolding of a protein with ΔH° = 250.8 kJ mol−1 and ΔS° = 752 J K−1 mol−1, above what temperature will the process be spontaneous?
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What is the standard state of free energies denoted by G°?
What is the standard state of free energies denoted by G°?
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If ΔH is small and positive, and ΔS is significant and positive, what inference can be made about the overall process?
If ΔH is small and positive, and ΔS is significant and positive, what inference can be made about the overall process?
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How does temperature (T) influence the balance between ΔH and ΔS in determining ΔG?
How does temperature (T) influence the balance between ΔH and ΔS in determining ΔG?
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Study Notes
Equilibrium
- Equilibrium is defined by equilibrium constants (Keq)
- Equilibrium constants predict the structure of systems
- Systems are stable at equilibrium
- Systems are unstable when not at equilibrium
Energy
- Energy is defined by the first law ( enthalpy, H ) and second law (entropy, S) of thermodynamics
- The change in Gibbs Free Energy (G) = H – TS
- A system is at equilibrium when G = 0
- A system undergoes spontaneous change when G < 0
Relationship between ΔG and Keq
- At equilibrium, G° = −RTlnKeq
- This relationship holds true at equilibrium
- When not at equilibrium, use G − G° = RTlnKeq
- G is the free energy of the system
- G° is the free energy of the process at equilibrium
Tutorial: ΔG, Keq problem
- The concentration of a solution of G3P was initially 0.05 M.
- Isomerase was added.
- After the mixture reached equilibrium at 25 °C, the concentration of G3P was 0.002 M.
- ΔG° for the reaction can be calculated using the given data
Energy and Equilibrium of Systems
- The first law of thermodynamics states that enthalpy (H) is equal to the heat (qp) transferred at constant pressure, or:
- H = qp= U + PV
- Enthalpy concerns energy changes to the system during processes
- Processes that lower enthalpy are favored
- The second law of thermodynamics states that entropy (S) is equal to the heat (q) transferred divided by the temperature (T), or:
- S = q
- Entropy concerns changes in the disorder of the system during processes
- Processes that increase entropy are favored
- We need a thermodynamic concept that captures both of these concepts
Criterion for the Direction of Spontaneous Change
- For any spontaneous process at constant T and P:
- S > 𝑞𝑖𝑟𝑟𝑒𝑣 𝑇 or qirrev < TS
- A spontaneous process implies irreversible change
- q = H (const.T, P)
- Therefore, H < TS or H − TS < 0
- The process will continue as long as H − TS < 0
Gibbs Free Energy
- We need a relationship such that H − TS < 0
- Define Gibbs Free Energy as G = H − TS
- The process will proceed as long as G < 0
- G is known as the Gibbs free energy
- G is a thermodynamic state function
ΔG = ΔH − TΔS
- If G < 0, the process is favored in that direction
- Equilibrium is reached when G = 0
- If the H term dominates, have an enthalpy controlled process ( H large and –ve)
- If the TS term dominates, have an entropy controlled process
Enthalpy vs. Entropy
- The analysis of Mg2+ using complexation by EDTA is a good example
- EDTA tetra sodium salt is widely used for the analysis of Mg ions in aqueous solution
- Mg2+ ion is highly solvated by water
- The method is used pharmaceutically, in pharmacology, biochemistry, etc.
- Note, that the process goes essentially to completion i.e., fully to the right-hand side as drawn above
Enthalpy vs. Entropy
- Can determine the relative contributions of enthalpy and entropy to the process.
- We can determine the thermodynamic driving forces for the process and…
- …infer the molecule-level processes
- We find that H is small and +ve
- But that S is significant and +ve
- The release of five H2O molecules and two NaCl increases disorder, leading to more components generated
- Overall G is –ve, making the process spontaneous from left to right as drawn
Standard States of Free Energies
- G°: is the value of G for a pure substance under 1 atmosphere of pressure, at a specified temperature
- G°: is the change in free energy when 1 mole of products in their standard states are converted to 1 mole of products in their standard states
Using ΔG Values, Example
- ATP, ADP, and AMP are key molecules in biochemical energy transfer
- We can use standard free energy values to calculate the change in free energy for a reaction
Tutorial: ΔG = ΔH − TΔS
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- ATP → ADP + Pi G° = −30.5 kJ mol−1
- (from thermal analysis, H° = −20.1 kJ mol−1)
- Calculate S° at 37 °C
- Comment on the significance of G, H and S values.
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- H° and S° for the unfolding of a protein are 250.8 kJ mol−1 and 752 J K−1 mol −1 respectively.
- Above what temperature (in °C) will unfolding of the protein be spontaneous?
- Comment on the values.
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