Subject: Chemistry for Bioscientists I (CHEM10021) from the University of Manchester
Section: C9 - Equilibrium constant
Focus: Understanding the concept of the equilibrium constant in chemical reactions.
Equilibrium Condition:
ArG: Maximum useful work, relates to chemical equilibrium.
Equilibrium Expression: At equilibrium, Q (reaction quotient) equals K (equilibrium constant).
Reaction Direction:
If Q > K: The reaction shifts to the left (favoring reactants).
If Q < K: The reaction shifts to the right (favoring products).
If Q = K: The system is at equilibrium (ArG = 0).
Equilibrium Dynamics:
At equilibrium, the reaction quotient equals the equilibrium constant, K.
Gibbs Free Energy Relation:
ArG = G° + RT ln(Q)
At equilibrium, ArG = 0, so: 0 = G° + RT ln(K).
Rearranging results in: ArG° = -RT ln(K).
Interpreting K:
If K > 1: Products dominate at equilibrium.
If K >> 1: Reaction essentially favors products.
If K < 1: Reactants dominate at equilibrium.
If K = 0: Reaction is not thermodynamically feasible.
Gibbs Energy Relation with Temperature:
G° = -RT ln(K) and G° = H° - T S°.
Resulting equation: ln(K) = H°/R - S°/R T.
Key Formula: ln(K1) - ln(K2) = R(T2 - T1).
Effect of Temperature:
Increase in T affects 1/T.
Exothermic Reactions:
Effect: ln(K) change is negative; shifts to left/reactants.
Endothermic Reactions:
Effect: ln(K) change is positive; shifts to right/products.
Forward and Backward Rates:
Forward Rate: R -> P with rate constant kf.
Backward Rate: P -> R with rate constant kb.
Rate Definitions:
Rate of formation of P: Rate = kf[R].
Rate of decomposition of P: Rate = kb[P].
Reaction Representation:
Coiled DNA (R) vs. Uncoiled DNA (P).
Net rate of formation of P: d[P]/dt = kf[R] - kb[P].
At Equilibrium:
[R] and [P] concentrations at equilibrium are [R]eq and [P]eq.
Hence, at equilibrium: d[P]/dt = 0.
This leads to: kf[R]eq = kb[P]eq.
Definition of K: K = [P]eq / [R]eq = kf / kb.
General Reaction: A + B ↔ C + D
Equilibrium Constant: K = [C][D]/[A][B].
Forward and Backward Rates:
Forward rate = kf[A][B]; Backward rate = kb[C][D].
At equilibrium, kf[A][B] = kb[C][D].
Effect of Catalysts:
They alter the activation energy for forward and backward reactions.
Catalysts do not affect the position of equilibrium or the equilibrium constant, but they accelerate attaining equilibrium.
Summary of Equilibrium Constants and their importance in understanding chemical reactions in biosciences.