[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 proportion of products to reactants in an equilibrium mixture. LE CHATELIER’S PRINCIPLE (LCP) LCP states that when a system is in equilibrium is subjected to a change in conditions that disturb the equilibrium, the POE shifts in a way to reduce that change to re-establish the equilibrium. How to answer questions using LCP 1) What is the immediate effect due to the disturbance 2) What is the counteract response to the disturbance 3) Direction of the shift of POE in response to the disturbance and explanation (4) Effect of the shift of POE Note: Addition of a catalyst will not have an impact on the POE and Kc as it speed up both the forward and backward reaction by the same extent. ELAINE PEH (COPYRIGHTED) © Le Chatelier’s Principle (LCP)

ELAINE PEH (COPYRIGHTED) © FACTOR 1: Change in concentration or partial pressure of the reactants/products • The concentration/partial pressure of the reactants/products can be changed by a direct addition/removal of the reactants/products at constant volume. • An increase in concentration/partial pressure of the molecule favours the reaction that uses up the molecule • A decrease in concentration/partial pressure of the molecule favours the reaction that produce the molecule Factors Affecting LCP

ELAINE PEH (COPYRIGHTED) © Factors Affecting LCP FACTOR 2: Change in total pressure (only for a gaseous system) • The total pressure of the system can be changed by (1) changing the volume of the container or (2) by adding an inert gas into the container of fixed volume. • An increase in total pressure of the system favours the reaction that decreases the number of gaseous molecule • A decrease in total pressure of the system favours the reaction that increases the number of gaseous molecule Note: This only applies to gaseous system with different number of total moles of reactants and products.