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# 1.1 the equilibrium constant .pdf Premium

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Department
Chemistry
Course
Chemistry 1027A/B
Professor
Felix Lee
Semester
Winter

Description
Chapter 1: Chemical Equilibrium Topic 1.1: The Equilibrium Constant What is Equilibrium? • Equilibrium is achieved when: Rate of forward reaction = Rate of reverse reaction • Asystem that is at equilibrium is dynamic and not a static For any equilibrium, there is a mathematical relationship between the pressures or • concentrations of the reactants and products; symbol used for this relationship is K eq ), which is known as the equilibrium constant • May be expressed in terms of pressures, K punits of atmosphere) or concentrations, K p (units of molarity; molL ) Effects of Temperature • Equilibrium constants are temperature dependent If the temperature of the system changes, the numerical value of the equilibrium constant • changes • The value of K increases or decreases depending on whether temperature increases/ decreases or whether the reaction is endothermic/exothermic Homogeneous and Heterogeneous Equilibria • If all the components are in the same phase, the system is a homogeneous equilibrium • If all the components are in different phases, the system is a heterogeneous equilibrium • Concentrations of pure solids and pure liquids are not included in the equilibrium constant expression The Equilibrium Constant Expression Ageneral equilibrium reaction is written as: • aA+ bB cC + dD reactants products • The general form of the equilibrium constant expression for this reaction using concentrations is: [C] [D] d K = a b [A] [B] • The concentration of any solid, or of any solvent, is considered to be constant and is therefore omitted from the equilibrium constant expression Reversing an Equilibrium Reaction • If the chemical equation for an equilibrium reaction is reversed, the expression for the equilibrium constant must be inverted • Thus, when reversing an equilibrium reaction, the new K value is 1/K original • Reverse an equilibrium; K’= 1/K Multiplying or Dividing an Equilibrium Reaction • If the chemical equation for an equilibrium is multiplied by two, the K expression is squared Thus, when multiplying this reaction, the new K value is K 2p • • Conversely, if the chemical equation is divided by two (multiplied by 1/2), then the K expression is the square root of the original expression, K’= K1/2 • So, the general rule is that when an equilibrium reaction is multiplied by some coefficient, #, the new K value is K# original x • Equilibrium is multiplied by some coefficient, x; K’= K Combining Equilibria • If two or more equilibria are combined to produce an overall equilibrium, the equilibrium constant for the overall equilibrium can be determined • Thus, when adding reactions, multiply the K values: K new= K1× K 2 Add equilibrium reactions; K’= K x K • 1 2 Significance of the Magnitude of K The magnitude of the equilibrium constant is an indication of the relative amount of • product or reactant present at equilibrium • If K is greater than 1, the equilibrium lies towards the right of the equation; indicating that there are more products than reactants If K is less than 1, the equilibrium lies towards the left of the equation; indicating that • there are more reactants than products The Reaction Quotient, Q • If the amounts of products and reactants can be determined, a quantity known as the reaction quotient may be calculated • This number indicates whether or not the system is at equilibrium, and the direction in which the reaction must proceed to attain equilibrium • The general reaction is: aA+ bB cC + dD • The expression for the reaction quotient, Q, is: c d Q = [C] [D] [A] [B] b The reaction quotient expression is the same as the equilibrium constant expression except • that the concentrations (or pressures) used to calculate Q are not necessarily the equilibrium concentrations (or pressures) • Comparing the numerical value of Q to that of K will indicate if the reaction is at equilibrium and, if not, whether more products or more reactants must be made to attain equilibrium • Asystem is at equilibrium only when Q = K • If the numerical value of Q is less than K, (Qeq K ) the system must proceed in the forward (→ ) direction to attain equilibrium because there is either too much reactants or not enough products • If the numerical value of Q is greater than K, (eq>K ) the system must proceed in the reverse ( ) direction to attain equilibrium because there is either too much product or not enough reactant Le Châtelier’s Principle • Equilibrium can be disturbed in several ways: ✦ Adding or removing one of the reactants or products ✦ Changing the pressure of a gaseous system ✦ Changing the temperature • System will move in one direction or the other to return to equilibrium; amounts of species will be different after the shift The numerical value of the equilibrium constant will not change, unless the temperature • changes Concentration Changes • In an equilibrium reaction, if the concentrations of the reactant is increased, the reaction will proceed to the right • If the concentration of the reactant is decreased, the reaction will procee
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