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Questions and Answers
Which conditions define the standard state for thermodynamic measurements?
Which conditions define the standard state for thermodynamic measurements?
- 1M concentration of reactants and products, 25°C, and pH = 7 (correct)
- Standard Temperature and Pressure (STP)
- Any temperature and pressure, with concentrations at equilibrium
- 0.1M concentration of reactants, 0°C, and any pH
The Gibbs free energy change (∆G) is calculated using what equation?
The Gibbs free energy change (∆G) is calculated using what equation?
- ∆G = -∆H + T∆S
- ∆G = ∆H + T∆S
- ∆G = ∆H - T∆S (correct)
- ∆G = -∆H - T∆S
What does it mean if a reaction has a negative ∆G?
What does it mean if a reaction has a negative ∆G?
- The reaction is endergonic and requires energy input to proceed.
- The reaction is exergonic and can occur spontaneously. (correct)
- The reaction is exothermic and releases heat.
- The reaction is at equilibrium.
Which statement correctly describes an endergonic reaction?
Which statement correctly describes an endergonic reaction?
For the reaction A <=> B, what is the value of ∆G at equilibrium?
For the reaction A <=> B, what is the value of ∆G at equilibrium?
How does increasing the temperature affect a reaction's spontaneity, assuming ∆H and ∆S are constant?
How does increasing the temperature affect a reaction's spontaneity, assuming ∆H and ∆S are constant?
What is the role of ATP hydrolysis in coupled reactions?
What is the role of ATP hydrolysis in coupled reactions?
Consider a reaction where ∆H is positive and ∆S is negative. Under what conditions, if any, will this reaction be spontaneous?
Consider a reaction where ∆H is positive and ∆S is negative. Under what conditions, if any, will this reaction be spontaneous?
Given the triosephosphate isomerase reaction converts dihydroxyacetone phosphate (DHAP) to glyceraldehyde 3-phosphate (GA3P) with a ∆G = +7.67 kJ/mol, and knowing that $∆G = -RT\ln{K_A}$, how can the reaction proceed spontaneously in the cell?
Given the triosephosphate isomerase reaction converts dihydroxyacetone phosphate (DHAP) to glyceraldehyde 3-phosphate (GA3P) with a ∆G = +7.67 kJ/mol, and knowing that $∆G = -RT\ln{K_A}$, how can the reaction proceed spontaneously in the cell?
If a chemical reaction has a $K_A$ value of 1000 $M^{-1}$, what does this indicate about the relative concentrations of reactants and products at equilibrium, and what would be the sign of $∆G$?
If a chemical reaction has a $K_A$ value of 1000 $M^{-1}$, what does this indicate about the relative concentrations of reactants and products at equilibrium, and what would be the sign of $∆G$?
Flashcards
What is
∆Gº´?
What is ∆Gº´?
The free energy change of a reaction under standard conditions: 1M concentration of reactants, 25°C (298 K), pH = 7 (for biological systems).
What is Gibbs Free Energy (∆G)?
What is Gibbs Free Energy (∆G)?
The portion of a reaction's enthalpy change that is available to do work, accounting for entropy.
What is an Exergonic Reaction?
What is an Exergonic Reaction?
A reaction that can only occur spontaneously if the change in Gibbs free energy (∆G) is negative.
What is an Endergonic Reaction?
What is an Endergonic Reaction?
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What is Equilibrium?
What is Equilibrium?
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What is cellular respiration?
What is cellular respiration?
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What is a Coupled Reaction?
What is a Coupled Reaction?
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What is the Equilibrium Constant (K
A)?
What is the Equilibrium Constant (K A)?
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What makes a reaction spontaneous?
What makes a reaction spontaneous?
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What is the equation for Gibbs Free Energy?
What is the equation for Gibbs Free Energy?
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Study Notes
- The value of ΔG depends on reactant and product concentrations
- Textbook values are for standard free energy change, denoted as ΔG°' (delta G nought prime)
- ΔG°' is the free energy change in standard conditions: 1M concentration of reactants/products, 25°C (298 K), pH = 7 (biological systems)
Gibbs Free Energy Change
- Both enthalpy and entropy changes determine if a reaction occurs
- The equation ΔG = ΔH - TΔS defines changes in Gibbs free energy, representing the available energy from a reaction
- T is temperature in Kelvin, and ΔG is measured in J mol⁻¹ (or kJ mol⁻¹)
Exergonic and Endergonic Reactions
- Reactions can only occur spontaneously if ΔG is negative, indicating an exergonic reaction
- Reactions cannot occur spontaneously if ΔG is positive, indicating an endergonic reaction.
- Exergonic processes decrease the system's free energy (ΔG < 0), while endergonic processes increase it (ΔG > 0)
- Exothermic processes decrease the system's enthalpy (ΔH < 0), while endothermic processes increase it (ΔH > 0)
- Reactions can be exergonic and endothermic, meaning they are energetically favorable but enthalpically unfavorable
Spontaneous Reactions
- Reactions occur spontaneously if ΔG is negative, but this does not indicate reaction rate
Equilibrium and ΔG
- As reactions proceed towards equilibrium, the value of ΔG changes
- At equilibrium, ΔG = 0
Triosephosphate Isomerase Equilibrium
- For the reaction catalyzed by triosephosphate isomerase
- Dihydroxyacetone phosphate ↔ Glyceraldehyde 3-phosphate,
- The standard free energy change (ΔG) is +7.67 kJ mol⁻¹
- The forward reaction is unfavorable
Chemical Reaction Equilibrium Thermodynamics
- The combustion of hydrogen to form water: 2H₂ + O₂ = 2H₂O
- ΔG = -237 kJ mol⁻¹, ΔH = -286 kJ mol⁻¹, TΔS = -49 kJ mol⁻¹
- The combustion of glucose in oxygen: C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O
- ΔG = -2883 kJ mol⁻¹, ΔH = -2813 kJ mol⁻¹, TΔS = +70 kJ mol⁻¹
ATP and Coupled Reactions
- ATP hydrolysis releases free energy, producing ADP (adenosine diphosphate) and inorganic phosphate (Pi): ATP + H₂O → ADP + Pi + H⁺
- ΔG = -30.5 kJ mol⁻¹, making it a very favorable (exergonic) reaction
- In cells, the actual free energy change is closer to -50 kJ mol⁻¹ due to non-unity concentrations
- Hydrolyzed ADP is rapidly re-phosphorylated to regenerate ATP, maintaining constant ATP levels
Spontaneity in Cells
- Cellular conditions are not standard (concentrations are typically 10⁻³ to 10⁻⁶ M), ΔG°' indicates reaction spontaneity.
Non-Spontaneous Reactions
- Glutamate + NH₄⁺ → glutamine + H₂O (ΔG = +15 kJ mol⁻¹) is not spontaneous alone
- ATP + H₂O → ADP + Pi + H⁺ (ΔG = -30 kJ mol⁻¹) is spontaneous
Coupled Reactions
- Favorable processes that generate energy can drive unfavorable systems to allow that system
- Free energy change = ΔG₁ - ΔG₂ (coupled reaction)
- Unfavorable reactions are frequently linked to ATP hydrolysis:
- Glutamate + NH₄⁺ → glutamine + H₂O; ΔG = +15 kJ mol⁻¹; (No)
- ATP + H₂O → ADP + Pi + H⁺; ΔG = -30 kJ mol⁻¹; (Yes)
Chemical Equilibrium
- For A + B ↔ AB, the equilibrium constant is KA = [AB]/([A]x[B])
- ΔG = -RT ln KA; where ΔG is Gibbs free energy change, R is the gas constant, and T is temperature
- ΔG = ΔH - TΔS; where ΔH is enthalpy change and ΔS is entropy change
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