8 Chemical Equilibrium

Chemical Equilibrium Victoria Junior College 1 VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Mdm Toh Chui Hoon (toh.chuihoon@vjc.sg) Chemical Equilibrium LECTURE OUTLINE 1 Introduction 1.1 Concept of Dynamic Equilibrium 1.2 Irreversible Reactions 1.3 Reversible Reactions 2 Position of Equilibrium and Equilibrium Constants - Kc and Kp 2.1 Definition of Equilibrium constant, Kc 2.2 Types of reaction mixtures 2.2 Rules for writing Kc and Kp expressions 2.3 Equilibrium constant in terms of partial pressure, Kp 2.4 Significance of the Equilibrium Constants, Kc and Kp 2.5 Use of Le Chatelier’s Principle 3 Factors affecting Position of Equilibrium 3.1 Effect of Concentration changes 3.2 Effect of Pressure changes 3.3 Effect of adding Inert gases 3.4 Effect of Temperature changes 3.5 Effect of Catalyst 4 Calculations involving equilibrium constants Kc and Kp 5 Industrial Application of LCP: The Haber Process 6 Standard Gibbs Free Energy and Position of Equilibrium LECTURE SCHEDULE Lecture no. Lecture contents Tutorial Qs 1 1.1 to 1.3; 2.1 to 2.3 1 & 2 2 2.4 to 2.5; 3.1 to 3.5 3 to 6 3 4, 5 & 6 7 to 10 REFERENCE TEXTS 1. A-level Chemistry by E.N. Ramsden 2. Chemistry in Action by Michael Freemantle

Chemical Equilibrium Victoria Junior College 2 ASSESSMENT OBJECTIVES H2 Candidates should be able to: 1. Explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium 2. State Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in concentration, pressure or temperature, on a system at equilibrium 3. Deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction 4. Deduce expressions for equilibrium constants in terms of concentrations, Kc, and partial pressures, Kp [Treatment of the relationship between Kp and Kc is not required] 5. Calculate the values of equilibrium constants in terms of concentrations or partial pressures from appropriate data 6. Calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations) 7. Show understanding that the position of equilibrium is dependent on the standard Gibbs free energy change of reaction, ΔG [Quantitative treatment is not required] 8. Describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of chemical equilibrium in the chemical industry H1 Candidates should be able to: 1. Explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium 2. State Le Chatelier’s Principle and apply it to deduce qualitatively (from appropriate information) the effects of changes in concentration, pressure or temperature, on a system at equilibrium 3. Deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction 4. Deduce expressions for equilibrium constants in terms of concentrations, Kc 5. Calculate the value of Kc in terms of concentrations from appropriate data 6. Calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations) 7. Describe and explain the conditions used in the Haber process, as an example of the importance of an understanding of chemical equilibrium in the chemical industry

Chemical Equilibrium Victoria Junior College 3 1 Introduction 1.1 Concept of Dynamic Equilibrium Static Equilibrium vs Dynamic Equilibrium 1. Static equilibrium is a state in which there is no change in both macroscopic and microscopic (molecular scale) properties in a system. E.g. two objects of equal mass on opposite sides of a balance. static dynamic 2. On the other hand, for dynamic equilibrium, things are still happening. For example, the reactants are still continually forming products while the products are still continually forming reactants in a reversible reaction. This is why we say that the situation is dynamic. However, this cannot be deduced from observation as all macroscopic properties for a system at dynamic equilibrium are constant. 3. Dynamic equilibrium can only be achieved when the following 2 conditions are fulfilled: o The system is closed, i.e. one which does not allow matter to enter or leave but allows free transfer of energy. o The reaction is reversible. LO: Explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium

Chemical Equilibrium Victoria Junior College 4 SLS QUIZ 1: True or false? Questions T or F (1) When dynamic equilibrium is reached, both the forward and backward reactions stop and the composition of the equilibrium mixture remain unchanged. (2) When dynamic equilibrium is achieved, the concentrations of reactants are equal to the concentrations of products. (3) Dynamic equilibrium can be attained in both open and closed systems. (4) Dynamic equilibrium can only be attained if the reaction can take place in both directions (reversible). (5) Dynamic equilibrium is achieved when rate of forward reaction is the same as rate of backward reaction whereby there are no more changes to the concentration of both reactants and products. 1.2 Irreversible Reactions  Irreversible reactions are chemical reactions that proceed to completion. These chemical reactions take place in one direction almost exclusively and denoted by a single–headed arrow (). The limiting reagent is completely used up in an irreversible reaction. E.g. Burning of magnesium in air: Mg(s) + O2(g)  2MgO(s)

Chemical Equilibrium Victoria Junior College 5  Consider the irreversible reaction: aA + bB  cC + dD - tc is time taken for the reaction to be completed. - If A is the limiting reagent, A will be consumed at the end of the reaction, tc - [B], [C] and [D] remain constant after time tc. Graph of Concentration against time 1.3 Reversible Reactions  Reversible reactions are reactions that proceed in both the forward and backward directions and denoted by a double–headed arrow ( ). Reversible reactions are incomplete and reach a state of dynamic equilibrium containing a mixture of both reactants and products. E.g. Contact Process: 2SO2(g) + O2(g) 2SO3(g)  What is dynamic equilibrium? A state of dynamic equilibrium is said to have reached when the rate of the forward reaction is equal to the rate of the backward reaction in a reversible reaction and the concentrations of both reactants and products are constant (does not change). [A] [C] [D] concentration / mol dm–3 tc t [B] 0 0 Rate / mol dm-3 s-1 tc t 0 SLS QUIZ 2: Fill in the blanks 1. Why does the curve plateau after tc? Reaction has …………….… because no more …...… left. 2. Which reactant is present in excess? ………..…….. 3. How does the rate of reaction change with time? Reaction rate ...................with time until becoming …………….

Chemical Equilibrium Victoria Junior College 6  How is dynamic equilibrium achieved? Consider the reversible reaction: A + B C + D o At time = 0, the reactants A and B are allowed to mix in a closed container. The forward r