Chemistry for Biology Students (CHEM0010) Notes - Ions and Chemical Forces

D1: Introduction to Equilibrium, Reaction Quotients and Gibbs Energy
Ions and Forces
 Aims and objectives
o Relate equilibrium constants, quotient with Gibbs free energy of reactions
o Distinguish between acid and conjugate acid and base and conjugate base
o Design a buffer and calculate the buffer pH using the Henderson-Hasselbalch equation
o Split redox reactions into oxidation and reduction reactions
o Calculate standard electrochemical potentials for electrochemical cells
o Calculate the chemical change at an electrode surface from the current passed through an
electrolyte
 What makes cell processes tick?
o Biological systems
 Highly organised
 Systems described in individual chemical reactions
 In reality – the chemical reactions are closely linked
o Cells
 Organelles
 Have particular roles
 Can be extracted and studied chemically

Equilibrium
 Fundamental concepts in chemistry
o Kinetics
 How quickly or slowly a reaction occurs
o Thermodynamics
 Changes in the form of energy when a reaction occurs – e.g. converting chemical energy to
heat
o Equilibrium
 Reactions in which the reactants and products coexist
o Reversibility of chemical change
 Reactions occurs until no change in chemical composition
 E.g. ATP  ADP + Pi
o A – Start with ADP and phosphate + no ATP
 Concentrations of ADP and Pi will deplete
 Concentrations of ATP increases
 Until equilibrium concentrations are
achieved
o B – Start with ATP only + no ADP and phosphate
 Concentrations of ATP will deplete
 Concentrations of ADP and Pi increases
 Until equilibrium concentrations are
achieved
o No matter where you start from – the reaction
occurs
 Equilibrium
o aA + bB  cC + dD
 Reactants = A, B
 Products = C, D
o Equilibrium constant K
 Kc = ([C]c[D]d) /([A]a[B]b) = [products] / [reactants]

,D1: Introduction to Equilibrium, Reaction Quotients and Gibbs Energy


 Kp = ((PC)c(PD)d)/ ((PA)a(PB)b) = (Pproducts) / (Preactants)
 K = constant for a given reaction at a particular temperature and pressure
 Units
o Kc – solutions
 Concentration – moldm-3 or moldkg-1
o Kp – gases
 Pressure – Pa or bar





 Large K
o Equilibrium favours product formation
 Small K
o Equilibrium favours reactant formation
o ∆rG
∆G = ∆H - T∆S
 Energy associated with a chemical reaction that can be used to do work
o Negative ∆rG
 Equilibrium favours product formation
o Positive ∆rG
 Equilibrium favours reactant formation
 G – total Gibbs energy of the system
 Chemical potential = G / n = Gibbs energy / number of moles
o Reaction quotient Q
 Measures relative amounts of products and reactants present during a reaction at a
particular point in time
 Qc = ([C]c[D]d)/ ([A]a[B]b)



o
 Describes the progress of the reaction
 Solids and pure liquids are in their standard state and always equal to 1

, D1: Introduction to Equilibrium, Reaction Quotients and Gibbs Energy




o
 More on Equilibrium and Gibbs Energy
o Equilibrium
 Dynamic process in which the forward and backward reactions occur in balance
 ∆rG = ∆rG0 + RTlnQ
o Q = reaction quotient
o ∆rG0 = Gibbs energy at standard conditions
o R = gas constant
o T = temperature





o K is at the bottom of the curve
 Right of K = positive gradient
 Left of K = negative gradient
 Q<K
o Reaction will favour products
o ∆rG = negative
o A+BC+D
 Q>K
o Reaction will favour reactants
o ∆rG = positive
o A+BC+D
 Q=K
o System is at equilibrium
o ∆rG = 0
o A+BC+D
o ∆rG = - RTlnK
 pH
o Protons – H3O+
 Do not exist as H+ - exist as H3O+
 Controls:
 Enzyme activity
 Synthesis of ATP
 Oxygen transport
 Charge on proteins and lipids