JPJC 2026 Chemical Equilibrium Annotated Lecture Notes

Jurong Pioneer Junior College H2 Chemistry: Chemical Equilibria 2026 JC1 H2 Chemistry (9476) 1 Topic 7: Chemical Equilibrium Jurong Pioneer Junior College 2026 JC1 H2 Chemistry (9476) Topic 7: Chemical Equilibria Assessment Objectives Candidates 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. (d) deduce expressions for equilibrium constants in terms of concentrations, Kc, and partial pressures, Kp, [treatment of the relationship between Kc and Kp 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) 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 Links: 1. Concept of dynamic equilibrium https://www.youtube.com/watch?v=cHAjhM3y3ds 2. Le Chatelier’s Principle: Effect of Conc, Press & Temp. on Eqm https://www.youtube.com/watch?v=PciV_Wuh9V8 3. CrO42- and Cr2O72- equilibrium http://www.youtube.com/watch?v=RjFW3smI1fY 4. NO2 and N2O4 Equilibrium (Effect of Temperature) https://www.youtube.com/watch?v=oC3klPMRnwo

Jurong Pioneer Junior College H2 Chemistry: Chemical Equilibria 2026 JC1 H2 Chemistry (9476) 2 Topic 7: Chemical Equilibrium Section Content 1 Reversible Reactions 2 Concept of Chemical Equilibrium 2.1 Dynamic Equilibrium 2.2 Characteristics of Dynamic Equilibrium 3 Equilibrium Constants 3.1 Reaction Quotient Q 3.2 Equilibrium Constant Kc 3.3 Equilibrium Constant, Kp (for Gaseous systems only) 3.4 Significance of Equilibrium Constants and Position of Equilibrium 4 Calculations involving Equilibrium Constants 5 Effect of Changing Conditions on the Position of Equilibrium 5.1 Le Chatelier’s Principle 5.2 Change in Concentration or Partial Pressure 5.3 Change in Total Pressure 5.4 Change in Temperature 5.5 Addition of Catalyst 6 Effect of Changing Conditions on Equilibrium Constant 7 Industrial Equilibrium Reaction - The Haber Process 8 Summary Annex A: Irreversible Reactions, Equilibrium Constant, Rate Constants & Temperature Recommended References : 1. Chemistry for Advanced Level by Peter Cann and Peter Hughes (Chapter 9 & 12) 2. Chemistry in Context (6th Edition) by Hill & Holman (Chapter 20 & 21)

Jurong Pioneer Junior College H2 Chemistry: Chemical Equilibria 2026 JC1 H2 Chemistry (9476) 3 Topic 7: Chemical Equilibrium 1 REVERSIBLE REACTION (a) Explain, in terms of rates of the forward and reverse reactions, what is meant by a reversible reaction and dynamic equilibrium. • Reversible reaction is a reaction which can proceed in both the forward and reverse/backward directions. The reaction mixture contains quantities of all reaction species in the system. • Reversible reactions are denoted using a double arrow: ( ) Consider the following reversible reaction: A + B C + D A reversible reaction has the following features: ✓ Reaction can go in both directions. ✓ A and B react to form C and D. This reaction is called the forward reaction. ✓ The products of the forward reaction, C and D, can in turn react to produce the reactants, A and B. This reaction is called the reverse / backward reaction. • Reversible reaction involving gases can only occur in a closed system. A closed system is one in which no substance is either added to the system or lost from it but does allow the free transfer of energy. Example: In a sealed closed container , the gaseous products would not be able to escape in a closed system, thus the reactants and products will be always in contact with each other enabling the forward and reverse reactions to occur simultaneously. (a) Open system: (b) Closed system CaCO3(s) → CaO(s) + CO2(g) CaCO3(s) ⇌ CaO(s) + CO2(g) Figure 1: Thermal decomposition of CaCO3 in (a) open and (b) closed systems. Examples of reversible reactions (in closed system) : (i) 2SO2(g) + O2(g) ⇌ 2SO3(g) (ii) 3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g) CO2(g) CaCO3(s) heat heat CaO(s) CaCO3(s) CO2(g) CaCO3(s) + CaO heat heat Reversible reaction Irreversible reaction forward reverse

Jurong Pioneer Junior College H2 Chemistry: Chemical Equilibria 2026 JC1 H2 Chemistry (9476) 4 Topic 7: Chemical Equilibrium 2 CONCEPT OF DYNAMIC EQUILIBRIUM (a) Explain, in terms of rates of the forward and reverse reactions, what it meant by a reversible reaction and dynamic equilibrium. 2.1 DYNAMIC EQUILIBRIUM Many chemical reactions do not go to completion. They can occur in both directions at comparable rates, and the reactants and products are present. The forward and reverse reactions are taking place simultaneously. • Forward reaction: when the reactants are converted to the products. • Reverse/backward reaction: when the products are converted to the reactants. Consider the reversible reaction (closed system), A + B ⇌ C + D Figure 2.1(a): Concentration – time graph 0 • Initially at t = 0, there is no C and D (products). • At time = teqm, the reaction reaches a state of dynamic equilibrium. • The equilibrium reached is dynamic because the substances, A & B and C & D, are still reacting together although the concentrations of reactants and products remain constant. Concentration teqm time/s Products (C and D) Reactants (A and B) Dynamic equilibrium is reached: [A], [B], [C], [D] remain constant. Not at equilibrium (before teqm)

Jurong Pioneer Junior College H2 Chemistry: Chemical Equilibria 2026 JC1 H2 Chemistry (9476) 5 Topic 7: Chemical Equilibrium Figure 2.1(b): Reaction rate – time graph 0 • Initially at t = 0, there is no C and D  rate of backward reaction is zero. • As the reaction proceeds, [C] and [D] increases  rate of backward reaction increases. • At the same time, [A] and [B] decreases  rate of forward reaction decreases. • After some time, a state of dynamic equilibrium will be reached, i.e. Rate of forward reaction = Rate of backward reaction , and there will be no further changes in [A], [B], [C] and [D]. Note: • Rf is fast when concentration of reactants is high at the start. It gets slower when reactants are using up and eventually equals to Rb when reaction reaches dynamic equilibrium. • Rb is zero at the start when concentration of products is zero. It increases as more product is formed. Eventually, Rb equals to Rf when reaction reaches dynamic equilibrium. Definition: Dynamic equilibrium refers to a state of balance in a reversible reaction in which the rate of the forward reaction equals to the rate of backward reaction ;