EJC 2023 Chemical Eqm Lecture Notes Teacher Copy

2023 JC1 H2 CHEMISTRY (9729) CORE IDEA 3 - TRANSFORMATION Topic 7: CHEMICAL EQUILIBRIA Name: ___________________________________________ Civics Group: _________ Learning Outcomes Students should be able to: (a) explain, in terms of rates of the forward and reverse reactions , what is meant by a reversible reaction and dynamic equilibrium (b) 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 (c) deduce whether changes in concentration, pressure or temperature or the presence of a catalyst affect the value of the equilibrium constant for a reaction (d) deduce expressions for equilibri um constants in terms of concentrations , Kc, and partial pressures, Kp [treatment of the relationship between Kp and Kc is not required] (e) calculate the values of equilibrium constants in terms of concentrations or partial pressures from appropriate data (f) calculate the quantities present at equilibrium, given appropriate data (such calculations will not require the solving of quadratic equations) (g) show understanding that the position of equilibrium is dependent on the standard Gibbs free energy change of reaction, G. [Quantitative treatment is not required] (h) 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 Recommended References: 1. Chemistry (for CIE AS & A Level) by Peter Cann & Peter Hughes 540 CAN 2. A-Level Chemistry by E.N. Ramsden 540 RAM Suggested Videos: (via URLs and QR codes provided or search using the titles) 1. What is Dynamic Equilibrium? The Chemistry Journey (Introductory video for chapter) 3. NO2 and N2O4 Equilibrium (Effect of Temperature) URL: https://youtu.be/wlD_ImYQAgQ URL: http://www.youtube.com/watch?v=tlGrBcgANSY 2. What is chemical equilibrium? George Zaidan and Charles Morton (Introductory video for chapter) 4. Chemistry: Demo: Shifting Equilibrium of [Fe(SCN)]2+ (Effect of Concentration) URL: https://youtu.be/dUMmoPdwBy4 URL: https://www.youtube.com/watch?v=ZOYyCTvLa9E TEACHER’S COPY

E u n o i a J u n i o r C o l l e g e 2 | P a g e Table of Contents Homogeneous vs. Heterogeneous Equilibrium ................................ ...................... 7 Calculation of Kc from Concentrations ................................ ................................ ... 9 Calculation of Concentrations from Kc ................................ ................................ . 10 Calculation of Kp from Partial Pressures ................................ .............................. 13 Calculation of Partial Pressures from Kp ................................ .............................. 14 G

3 G i b b s F r e e E n e r g y a n d E q u i l i b r i u m C o n s t a n t E u n o i a J u n i o r C o l l e g e 3 | P a g e INTRODUCTION Reversible Reaction Notation: A reversible reaction is denoted by using the  sign in the equation. 1. A reversible reaction is a reaction where the reactants form products that can react with each other to re-form the reactants. In other words, the reaction proceeds in either direction. E.g. Haber Process: N2(g) + 3H2(g)  2NH3(g) • Both forward and backward reactions occur. • As soon as NH3 molecules are formed (forward reaction), some of the NH3 molecules formed will decompose to form N2 and H2 (backward reaction). • Thus, in a reversible reaction, the reactants are never completely converted to the products. Instead, a mixture of products and reactants will be obtained. 2. Convention: A + B  C + D H = endothermic or exothermic • A & B are referred to as reactants and C & D as products of the forward reaction. • A + B → C + D is called the forward reaction. • C + D → A + B is called the backward reaction. • H indicates the enthalpy change of the forward reaction. ‒ If H is positive, the forward reaction is endothermic. ‒ If H is negative, the forward reaction is exothermic. 3. Some processes proceed in a single direction or are irreversible because they take place in an open system (see Fig. 1(a)) , where some of the products can escape, making the backward reaction impossible. E.g. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Chemical reactions, in principle, are all reversible in a closed system, though some may appear to go to completion (see Fig. 1(b)). Fig. 1 Thermal decomposition of calcium carbonate in (a) open and (b) closed systems. LO (a) explain, in terms of rates of the forward and reverse reactions , what is meant by a reversible reaction and dynamic equilibrium

3 G i b b s F r e e E n e r g y a n d E q u i l i b r i u m C o n s t a n t E u n o i a J u n i o r C o l l e g e 4 | P a g e Making Thinking Visible Q: If a reaction is reversible and does not go to completion, wouldn’t the reaction be useless in the chemical industry? A: Not necessarily! We can still optimise the product yield of a reversible reaction by changing conditions such as concentration, pressure, temperature, and usage of a catalyst. This is what this chapter is all about. Q: Does it matter whether the reaction is written as A + B  C + D same as C + D  A + B? A: No, they are not the same. This is because for C + D  A + B, the forward reaction is C + D → A + B while the backward reaction is A + B → C + D. In addition, the enthalpy change of reaction for C + D  A + B is the reversed of that for A + B  C + D. Dynamic Equilibrium Consider the following reversible reaction: A + B  C + D After some time, the reaction reaches a state of equilibrium in which both reactants and products are present, and there are no further changes in the amounts of reactants and products. Macroscopically, the reaction will appear to have stopped. However, at the microscopic level, A and B are still reacting to form C and D (i.e. forward reaction), and at the same time, C and D are still reacting to form A and B (i.e. backward reaction) at the same rate. forward reaction backward reaction at the beginning of the reaction (time = 0) • [A] and [ B] are high, rate of forward reaction is high • C and D are not present, rate of backward reaction is zero as reaction proceeds, (time = 0 to t) • [A] and [B] decrease, hence rate of forward reaction decreases • [C] and [D] increase, hence rate of backward reaction increases time when equilibrium is reached (time = t) • Forward and backward reactions are both taking place, where the rate of forward reaction = rate of backward reaction Note: rates of forward and backward reactions are NOT zero. The forward and backward reactions are still taking place, even though no observable changes can be seen. • Concentrations of reactants and products remain constant (but are not necessarily equal) • [A] and [B] ≠ 0; [C] and [D] ≠ 0 The system is in a state of dynamic equilibrium.

3 G i b b s F r e e E n e r g y a n d E q u i l i b r i u m C o n s t a n t E u n o i a J u n i o r C o l l e g e 5 | P a g e When dynamic equilibrium is established, ratef = rateb For an elementary reversible reaction, this would hence imply:      A B C Dfbkk = By re-arrangement,      c CD AB constant f b kK k== = These ratios give us a sense of the extent of the reaction at equilibrium. Note: The term “dynamic” is opposite of static and is explained by the fact that the forward and backward reactions are still occurring. While the term “equilibrium” is explained by the fact that the concentrations of the reactants and products do not change. Characteristics of a State of Dynamic Equilibrium 1. When a system is at a state of equilibrium, the concentration of all reactants and products remain const