Chemical equilibrium By/ BATAA EL GAFAARY

Equilibrium: Equilibrium is a state in which there are no observable changes as time goes by. Chemical equilibrium is achieved when: • the rates of the forward and reverse reactions are equal and • the concentrations of the reactants and products remain constant Physical equilibrium H2O (l) H2O (g) Chemical equilibrium N2O4 (g) 2NO2 (g) BATAA EL GAFAARY

N2O4 (g) 2NO2 (g) BATAA EL GAFAARY

BATAA EL GAFAARY

Homogenous equilibrium applies to reactions in which all reacting species are in the same phase. BATAA EL GAFAARY

Ex. BATAA EL GAFAARY

The equilibrium concentrations for the reaction between carbon monoxide and molecular chlorine to form COCl2 (g) at 740°C are [CO] = 0.012 M, [Cl2] = 0.054 M, and [COCl2] = 0.14 M. Calculate the equilibrium constants Kc and Kp. BATAA EL GAFAARY

The equilibrium constant Kp for the reaction 2NO2 (g) 2NO(g) + O2(g) is 158 at 1000K. What is the equilibrium pressure of O2 if the PNO2 = 0.400 atm and PNO = 0.270 atm? BATAA EL GAFAARY

Heterogenous equilibrium Heterogenous equilibrium applies to reactions in which reactants and products are in different phases. BATAA EL GAFAARY BATAA EL GAFAARY 1438 H

Consider the following equilibrium at 295 K: NH4HS(S) NH3(g) + H2S(g) The partial pressure of each gas is 0.265 atm. Calculate Kp and Kc for the reaction? BATAA EL GAFAARY

BATAA EL GAFAARY

Writing Equilibrium Constant Expressions 1. The concentrations of the reacting species in the condensed phase are expressed in M. In the gaseous phase, the concentrations can be expressed in M or in atm. 2. The concentrations of pure solids, pure liquids and solvents do not appear in the equilibrium constant expressions. 3. The equilibrium constant is a dimensionless quantity. 4. In quoting a value for the equilibrium constant, you must specify the balanced equation and the temperature. 5. If a reaction can be expressed as a sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions. BATAA EL GAFAARY

Chemical Kinetics and Chemical Equilibrium BATAA EL GAFAARY

The reaction quotient (Qc) is calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (Kc) expression. BATAA EL GAFAARY

Calculating Equilibrium Concentrations 1. Express the equilibrium concentrations of all species in terms of the initial concentrations and a single unknown x, which represents the change in concentration. 2. Write the equilibrium constant expression in terms of the equilibrium concentrations. Knowing the value of the equilibrium constant, solve for x. 3. Having solved for x, calculate the equilibrium concentrations of all species. BATAA EL GAFAARY

Le Chatelier’s Principle If a stress is applied to a system at equilibrium, the system will change to relieve that stress and re –establish equilibrium. It is like the “undo” button on your computer! Factors that Affect Equilibrium : Concentration Temperature Pressure For gaseous systems only! The presence of a catalyst BATAA EL GAFAARY

Concentration Changes Add more reactant Shift to products Remove reactants Shift to reactants Add more product Shift to reactants Remove products Shift to products Reaction Quotient The reaction quotient for an equilibrium system is the same as the equilibrium expression, but the concentrations are NOT at equilibrium! BATAA EL GAFAARY

Changes in concentration are best understood in terms of what would happen to “Q” if the concentrations were changed. •Q = Keq at equilibrium •If Q< K then there are too many reactants, the reaction will shift in the forward direction (the products) •If Q>K then there are too many products, the reaction will shift to the reactants BATAA EL GAFAARY

Temperature Changes Consider heat as a product in exothermic reactions – Add heat Shift to reactants – Remove heat Shift to products A + B = AB + Heat • Consider heat as a reactant in endothermic reactions A + B + heat = AB – Add heat Shift to products – Remove heat Shift to reactants BATAA EL GAFAARY

Pressure Changes Only affects equilibrium systems with unequal moles of gaseous reactants and products. Increase Pressure * Stress of pressure is reduced by reducing the number of gas molecules in the container . . . . . . • There are 4 molecules of reactants vs. 2 molecules of products. – Thus, the reaction shifts to the product( ammonia ). BATAA EL GAFAARY

There are two product gas molecules vs. one reactant gas molecule. • Decrease Pressure Stress of decreased pressure is reduced by increasing the number of gas molecules in the container. There are two product gas molecules vs. one reactant gas molecule. Thus, the reaction shifts to the products BATAA EL GAFAARY

Presence of a Catalyst • A Catalyst lowers the activation energy and increases the reaction rate. • It will lower the forward and reverse reaction rates, • Therefore, a catalyst has NO EFFECT on a system at equilibrium! • It just gets you to equilibrium faster! BATAA EL GAFAARY

Presence of an Inert Substance An inert substance is a substance that is not- reactive with any species in the equilibrium system. These will not affect the equilibrium system. If the substance does react with a species at equilibrium, then there will be a shift! BATAA EL GAFAARY