Chemical Equilibrium and Le Chatelier's Principle PDF

CHEMICAL EQUILIBRIUM and LE CHATELIER’S PRINCIPLE  OBJECTIVES Explain chemical equilibrium in terms of the reaction rates of the forward and the reverse reaction Calculate equilibrium constant and the pressure or concentration of reactants or products in...

CHEMICAL EQUILIBRIUM and LE CHATELIER’S PRINCIPLE  OBJECTIVES Explain chemical equilibrium in terms of the reaction rates of the forward and the reverse reaction Calculate equilibrium constant and the pressure or concentration of reactants or products in an equilibrium mixture State the Le Chatelier’s principle and apply it qualitatively to describe the effect of changes in pressure, concentration and temperature on a system at equilibrium. Chemical Equilibrium  CHEMICAL EQUILIBRIUM Chemical equilibrium (dynamic equilibrium) – refers to the state in a chemical reaction wherein the concentrations of the reactants and the products remain constant, in which the rates of the forward and reverse reactions are the same. Reactants ↔ Products  REVERSIBLE REACTIONS Reversible reactions – chemical reactions that can proceed in either direction (to the right or to the left) For example: CH4(g) + Cl2(g) ↔ CH3Cl(g) + HCl(g) ✓ The double-headed arrow signifies that the previous reaction is a reversible one. Both forward (toward product formation) and backward reactions (toward the reactants) take place at the same time, wherein reactants combine to form the products, and products react to yield the reactants.  STATE OF BALANCE A state of balance is also referred to as a state of equilibrium. In a reversible reaction, when the reactants start to form the products, the products would then start to reform the reactants. The two opposing processes happen at different rates but a certain point in the reaction will be reached where the rates of the forward and backward reactions are the same. This is the state of chemical equilibrium. STATE OF BALANCE In a state of chemical equilibrium, since the rate of product formation is equal to the rate of the reformation of the reactants, then the concentrations of the reactants and products remain becomes constant.  STATE OF BALANCE The state of chemical equilibrium is a highly dynamic state. This means that though there is no change in the composition of the reaction mixture and no visible changes taking place, the particles are continuously reacting. Also, a system at chemical equilibrium can be easily disturbed by changes in the reaction conditions.  Types of Equilibrium Homogeneous Heterogeneous equilibrium equilibrium when the products and reactants when the products and reactants of reaction are all the same state of are not the same state of matter. matter. Ex. CaCO3(s) ↔ CaO(s) + CO2(g) Ex. 2NH3(g) ↔ N2(g) + 3H2(g)  HABER PROCESS The state of chemical equilibrium is a highly dynamic state. This means that though there is no change in the composition of the reaction mixture and no visible changes taking place, the particles are continuously reacting. Also, a system at chemical equilibrium can be easily disturbed by changes in the reaction conditions.  Importance of Haber Process N2(g) + 3H2(g) ↔ 2NH3(g) On the molecular level, the reaction has not really It enables the In the production stopped. The reactants commercial of ammonia, the decrease in concentration due production of said reaction is to their consumption to form ammonia from slow; however, the products. However, the combining nitrogen rate of change in the under appropriate concentration of the reactants gas with hydrogen conditions, the and the products is so small gas. system eventually that it can be considered achieves negligible; therefore, they appear to be constant.actually equilibrium. in a cold place. It’s full of iron oxide dust Equilibrium Constant, (Kc or Keq) The Law of Mass Action Expression/Equilibrium Constant Expression The relationship between the concentrations of the reactants and products may be expressed using the law of mass action expression/equilibrium constant expression.  EQUILIBRIUM CONSTANT (KC) For the general equilibrium reaction: aA + bB ↔ cC + dD the law of mass action expression is written as: [𝐶]𝑐 [𝐷]𝑑 𝐾𝑒𝑞 = [𝐴]𝑎 [𝐵]𝑏 where the [ ] is the concentration expressed in molarity and Keq is the equilibrium constant. If molar concentrations are used, Keq may also be referred to as Kc.The law of mass action is basically the ratio of the concentrations of the products raised to their respective stoichiometric coefficients to that of the reactants. Equilibrium constants for homogeneous gaseous equilibria may also be expressed in terms of partial pressures. The expression is written in much the same way as described, only that the partial pressure is raised to the coefficient instead of the molar concentration. 1. 2 O3(g) ↔ 3 O2(g) 2. 2 NO(g) + Cl2(g) ↔ 2 NOCl(g) 3. HF(aq) + C2O42–(aq) ↔ 2 F–(aq) + H2C2O4(aq) For equilibrium reactions where the reactants and products are in different phases (heterogeneous equilibria), pure solids and pure liquids are excluded in writing equilibrium constant expressions. For example, consider the reaction: 1. PbCl2(s) ↔ Pb2+(aq) + 2Cl–(aq) 2. CO2(g) + H2(g) ↔ CO(g) + H2O(l)  SAMPLE PROBLEM Gaseous hydrogen iodide is placed in a closed container at 425°C, where it partially decomposes to hydrogen and iodine: 2 HI(g) ↔ H2(g) + I2(g). At equilibrium, it is found that [HI] = 3.53 × 10–3 M, [H2] = 4.79 × 10–4 M and [I2] =4.79 × 10–4 M. What is the value of Kc at this temperature? In general, If K >> 1 (large K If K K, the products dominate the reaction mixture so the products must react to form the reactants; reaction proceeds in the backward direction until equilibrium is attained; If Q < K, the reactants dominate the reaction mixture so the reactants must react to form the products; reaction proceeds in the forward direction until equilibrium is attained.  SAMPLE PROBLEM The Kc value for the reaction N2(g) + 3 H2(g) ↔ 2NH3(g) at 472°C is 0.105. Suppose a mixture of 2.00 mol of H2, 1.00 mol of N2 and 2.00 mol of NH3 is placed on a sealed 1.00-L container. How would we know if the reaction is already at equilibrium? In what direction will the reaction proceed in order to attain the equilibrium? Le Chatelier’s Principle  Henri Louis Le Châtelier (1850-1936) Highlights – 1884 Le Chatelier's Principle: A system in equilibrium that is subjected to a stress reacts in a way that counteracts the stress – If a chemical system at equilibrium experiences a change in concentration, temperature or total pressure the equilibrium will shift in order to minimize that change. – Industrial chemist involved with industrial efficiency and labor-management relations Moments in a Life – Le Chatelier was named "chevalier" (knight) of the Légion d'honneur in 1887, decoration established by Napoléon Bonaparte in 1802.  STRESS Adding or Change in Removing Concentration Reactants or Products Change in Change in Temperature Pressure  Effects of Adding or Removing Reactants or Products PCl5(g) ↔ PCl3(g) + Cl2(g) K = 11.5 @ 300oC = Q add extra PCl5(g) add extra PCl3(g) remove some PCl5(g) remove some PCl3(g)  Effects of Changing the Volume/Pressure of the System Effects of Changing the Temperature “Stress acts as an accelerator: it will push you either forward or backward, but you choose which direction.” —Chelsea Erieau Thanks! 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