[H2 CHEM] Chapter 9 - Chemical Equilibria

Chapter 9 ELAINE PEH (COPYRIGHTED) © Chemical Equilibria

CHAPTER ANALYSIS FOCUS EXAM WEIGHTAGE ELAINE PEH (COPYRIGHTED) ©

Dynamic Equilibrium & Equilibrium Constants Calculations involving Equilibrium Constant (I.C.E Table) Le Chatelier’sPrinciple (LCP) Applications of LCP – Haber Process KEY CONCEPT ELAINE PEH (COPYRIGHTED) ©

DYNAMIC EQUILIBRIUM • It occurs when the forward and backward reactions are occurring at the same rate. • Only in a closed system • Concentrations of reactants and products are constant under a given conditions of temperature, pressure and initial amount of substance. • Rate ≠ 0 ELAINE PEH (COPYRIGHTED) © Dynamic Equilibrium & Equilibrium Constants Concentration against Time Rate against Time (1) : At the start when time = 0 (1) : At the start there is only [SO2] and [O2] hence only the forward reaction occurs. Ratef = kf[SO2][O2] Note: There is no [SO3] hence Rateb = 0 Rateb = kb[SO3] (2) : As reaction proceed, [SO2] and [O2] starts to decrease as reaction takes place. [SO3] increases as products are formed. Note: As [SO3] is formed, backward reaction is also occurring at this point. (2) : As reaction proceed, [SO2] and [O2] starts to decrease hence Rf decreases. [SO3] increases as products are formed and thus Rb increases Note: Rate ∝ [reactants/products] (3) : At teqm, dynamic equilibrium is reached and [SO2], [O2] and [SO3] remain constant. (3) : At teqm, dynamic equilibrium is reached, Rf = Rb ≠ 0

ELAINE PEH (COPYRIGHTED) © Dynamic Equilibrium & Equilibrium Constants EQUILIBRIUM CONSTANTS, Kc • Units = (mol dm-3)c+d-a-b • Concentration of Water/Solvent, Excess Liquids, Solids and Immiscible Liquids are excluded from the equilibrium expression • Note: Kc is only affected by temperature changes EQUILIBRIUM CONSTANTS, Kp • Units = (Pa or atm)c+d-a-b • It only takes into account the partial pressure of gases • Note: Kp is only affected by temperature changes

Linking Kc with rate constant (Chapter 8: Reaction Kinetics) N2O4 (g) ⇌ 2NO2 (g) Rf = kf[N2O4] ; Rb = kb[NO2]2 At equilibrium, Rf = Rb = kf[N2O4] = kb[NO2]2 Using Kc Linking Kc with ΔG (Chapter 7: Chemical Energetics) • ΔG = - RT ln(Kc) • Kc indicates the extent of reaction at equilibrium • If Kc is large, [products] >> [reactants], and the forward reaction is almost complete, hence indicating the forward reaction is more favoured and is more spontaneous (ΔG<0) ELAINE PEH (COPYRIGHTED) © Dynamic Equilibrium & Equilibrium Constants

Step 1: Write out the equation to get the mol ratio and draw out the I.C.E Table Step 2: Fill in the information you have form the question in the I.C.E Table Step 3: Based on the mol ratio, calculate the change for the other reactants Step 4: Fill up the remaining information in the I.C.E Table ELAINE PEH (COPYRIGHTED) © 2A B 2C D I Initial no. of mols C Change (loss/gain) E Eqm no. of mols Calculations involving Equilibrium Constant (I.C.E Table)

POSITION OF EQUILIBRIUM (POE) POE refers to the relative