ASRJC H2 Chem 7. Chemical Equilibria

2024 JC1 H2 Chemical Equilibria - 1 - 2024/Anderson Serangoon JC/Chemistry ANDERSON SERANGOON JUNIOR COLLEGE H2 CHEMISTRY CHEMICAL EQUILIBRIA Content Instructions 1 Introduction 1.1 Reversible and Irreversible Reactions 1.2 Dynamic Equilibrium 1.3 Closed and Open systems 2 Equilibrium Expressions 2.1 Equilibrium constant, Kc 2.2 Equilibrium constant, Kp 2.3 Equilibrium constant of heterogeneous systems 3 Calculations involving Kc 3.1 Calculating Kc given compositions 3.2 Calculating compositions given Kc 4 Calculations involving Kp 4.1 Calculating Kp given initial partial pressure 4.2 Calculating Kp given mole fractions and total pressure at equilibrium REVIEW Characteristics of Kc and Kp 5 Position of Equilibrium 6 Factors affecting the position of equilibrium 6.1 Changing the concentration 6.2 Changing the pressure 6.3 Changing the temperature 6.4 Presence of Catalyst 7 Application of Principles of Reaction Rates and Equilibria to Industrial Processes – Haber Process 8 Relationship between K and spontaneity of reaction 9 Extra reading on relationship between kinetics and equilibrium

2024 JC1 H2 Chemical Equilibria - 2 - 2024/Anderson Serangoon JC/Chemistry Syllabus objectives CONTENT • Reversible reactions and dynamic equilibrium • Equilibrium constants • Factors affecting chemical equilibria • Le Chatelier’s Principle • The Haber process LEARNING OUTCOMES (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 equilibrium 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, Go. [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. References 1. Chemistry for Advanced Level; Peter Cann and Peter Hughes 2. Chemistry The Central Science; Brown, Le May, Bursten 8th edition 3. A–Level Chemistry; E.N. Ramsden; 4th Edition

2024 JC1 H2 Chemical Equilibria - 3 - 2024/Anderson Serangoon JC/Chemistry 1 Introduction The definition of equilibrium in the dictionary is ‘balance or harmony of opposing forces’. The concept of chemical equilibrium is also about balance – between the forward and backward directions of reversible reactions. 1.1 Irreversible vs Reversible Reactions Irreversible Reaction Reversible Reaction Chemical reactions that take place in one direction (forward), indicated by the single arrow line ( ). A + B C + D Chemical reactions that take place in both directions (forward and backward), indicated by the double arrow line ( ). forward reaction A + B C + D backward reaction What happens in the Concentration vs Time graph above for an Irreversible reaction? • At the beginning, only reactants A and B are present, forward reaction takes place to produce C and D. • Thus [A] and [ B] decrease while [ C] and [D] increase. • Eventually at time t, when one (or both) of the reactants is used up, the reaction is complete. • Here, [A] and [ B] equals to zero and [ C] and [D] are constant. What happens in the Concentration vs Time graph above for a Reversible reaction? • At the beginning, only reactants A and B are present, forward reaction takes place to produce C and D. • Thus [ A] and [ B] decrease while [ C] and [ D] increase. • The moment C and D are produced, the reverse reaction occurs to produce A and B. • Eventually at time t, [A], [ B], [ C] and [ D] no longer changes and a mixture of both the reactants and products is obtained . Equilibrium is reached. (for complete reaction) C + D (products ) Concentration Time A + B (reactants used up) t A + B (reactants) C + D (products) t (to reach equilibrium) Time Concentration 0 0

2024 JC1 H2 Chemical Equilibria - 4 - 2024/Anderson Serangoon JC/Chemistry Irreversible Reaction Reversible Reaction What happens in the Rate vs Time graph above for an Irreversible reaction? Rate of reaction decreases as reactants A and B are used up. At time t, rate equals to zero since there is no further reaction. What happens in the Rate vs Time graph above for a Reversible reaction? (i) Rate of the forward reaction decreases as reactants A and B are reacted away. (ii) Simultaneously, rate of reverse reaction increases as products C and D are formed. (iii) Eventually at time t, rate of forward reaction = rate of reverse reaction, [A], [B], [C] and [D] remain constant and the system has reached a state of DYNAMIC EQUILIBRIUM. Example: Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) Example: Haber Process N2(g) + 3H2(g) 2NH3(g) 1.2 Dynamic Equilibrium A state of dynamic equilibrium is one where rate of forward reaction equals to rate of backward reaction for a reversible reaction such that there is no net change in the concentration of the reactants and products in a closed system. Rate Time t t Time Rate DYNAMIC EQUILIBRIUM (i) (ii) (iii) 0 0 Figures (a) and (b) illustrate the difference between static and dynamic equilibrium. t (reaction is completed at this at time = t) (reaction reaches equilibrium at time = t)

2024 JC1 H2 Chemical Equilibria - 5 - 2024/Anderson Serangoon JC/Chemistry Characteristics of Dynamic Equilibrium • Although the rate of forward reaction is equal to the rate of reverse reaction, the [reactants] and [products] are not equal to each other most of the time. • [reactants] and [products] are constant (remain unchanged) . However, the equilibrium mixture is still reacting (both forward and reverse reactions are ongoing). • The same equilibrium position is reached regardless of whether the reversible reaction was started by mixing chemicals on the left or right side of the equation. • Equilibrium can only be achieved in a closed system in which there is no loss or gain of matter to and from surroundings. An open system may allow matter to escape or enter. Thus equilibrium cannot be reached. 1.3 Closed and Open Systems Consider the following example: H2O(g) + C(s) CO(g) + H2(g) • In an open system, - The black carbon is placed in an open jar and heated with steam. - The black carbon will gradually disappear after some time as all of it reacted to form gaseous products which escape from the system. • In a closed system, - The black carbon is placed in a jar with lid and heated with steam. - Some black carbon will remain after some time. - Without any disturbance such as temperature or pressure, the system will remain like this indefinitely as reverse reaction is taking place too. - A state of dynamic equilibrium will be achieved. Exercise 1: Substance A