📗AK LECTURES BIOCHEMISTRY: First and Second Law of Thermodynamics
Knowt Notes
Q: What is the First Law of Thermodynamics?
A: Energy cannot be created or destroyed; it can only be converted from one form to another. The total energy of the universe is constant.
Q: What is internal energy (U)?
A: The total energy contained within a system, including kinetic and potential energy of all particles. Its change is given by ΔU = Q − W, where Q is heat added and W is work done by the system.
Q: What is enthalpy (H)?
A: A thermodynamic quantity defined as H = U + PV. At constant pressure, the change in enthalpy (ΔH) equals the heat transferred. ΔH < 0 = exothermic; ΔH > 0 = endothermic.
Q: What is the Second Law of Thermodynamics?
A: For any spontaneous process, the total entropy of the universe (system + surroundings) increases. Entropy (S) measures disorder or the number of accessible microstates.
Q: What is entropy (S)?
A: A measure of the randomness, disorder, or number of possible microscopic arrangements of a system. Higher entropy means greater disorder.
Q: What is Gibbs free energy (G)?
A: The thermodynamic potential that predicts spontaneity at constant temperature and pressure. Defined as G = H − TS; its change is ΔG = ΔH − TΔS.
Q: What does a negative ΔG indicate?
A: The process is spontaneous (exergonic) — it can occur without external energy input under the given conditions.
Q: What does a positive ΔG indicate?
A: The process is nonspontaneous (endergonic) — it requires an input of energy to proceed.
Q: What does ΔG = 0 indicate?
A: The system is at equilibrium; no net change occurs, and the forward and reverse processes occur at equal rates.
Q: What is the equation for Gibbs free energy change?
A: ΔG = ΔH − TΔS, where ΔH is enthalpy change, T is absolute temperature in Kelvin, and ΔS is entropy change.
Q: When is a reaction always spontaneous regardless of temperature?
A: When ΔH is negative (exothermic) and ΔS is positive (increase in entropy), ΔG will always be negative.
Q: When is a reaction never spontaneous?
A: When ΔH is positive (endothermic) and ΔS is negative (decrease in entropy), ΔG will always be positive.
Q: When is a reaction spontaneous only at low temperature?
A: When both ΔH and ΔS are negative. The favorable exothermic term dominates at low T; at high T, the unfavorable −TΔS term dominates.
Q: When is a reaction spontaneous only at high temperature?
A: When both ΔH and ΔS are positive. The favorable entropy term (−TΔS) dominates at high T; at low T, the unfavorable ΔH dominates.
Q: What is ATP hydrolysis and why is it exergonic?
A: ATP + H₂O → ADP + Pi; ΔG ≈ −30.5 kJ/mol. It is exergonic due to relief of charge repulsion between phosphate groups, resonance stabilization of products, and increased entropy.
Q: How do cells drive endergonic reactions?
A: By coupling them to exergonic reactions such as ATP hydrolysis. The overall ΔG of the combined reactions must be negative for the process to occur.
Q: Why does protein folding have a negative ΔG despite decreased protein entropy?
A: Although the protein itself becomes more ordered (ΔS_protein < 0), the hydrophobic effect releases ordered water molecules from clathrate cages, greatly increasing water entropy (ΔS_water >> 0). Combined with favorable ΔH from new hydrogen bonds and van der Waals interactions, the total ΔG is negative.
Q: How does the hydrophobic effect make membrane formation spontaneous?
A: Nonpolar tails aggregate to minimize contact with water, releasing low-entropy clathrate cage water molecules. This creates a large positive ΔS for water, so ΔG becomes negative and the bilayer forms spontaneously.
Q: What is the relationship between ΔG°′ and equilibrium constant (Keq)?
A: ΔG°′ = −RT ln Keq. A more negative standard free energy change corresponds to a larger Keq and a more product-favored reaction.
Q: What is the difference between thermodynamics and kinetics?
A: Thermodynamics (ΔG) tells whether a reaction can occur spontaneously. Kinetics (activation energy, Ea) tells how fast it will proceed. Enzymes lower activation energy but do not change ΔG or Keq.
Q: What is standard free energy change (ΔG°′)?
A: The free energy change under standard biochemical conditions: 25°C, pH 7, 1 M concentrations of reactants/products, and 1 atm pressure.
Q: What equation relates ΔG, ΔG°′, and reaction quotient (Q)?
A: ΔG = ΔG°′ + RT ln Q. At equilibrium, Q = Keq and ΔG = 0.