Jeffrey Mack California State University, Sacramento Chapter 16 Principles of Chemical Reactivity: Equilibria

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<ul><li>Slide 1</li></ul> <p>Jeffrey Mack California State University, Sacramento Chapter 16 Principles of Chemical Reactivity: Equilibria Slide 2 All chemical reactions are reversible, at least in principle. The concept of equilibrium is fundamental to chemistry. The general concept of equilibrium was introduced in Chapter 3 to explain the limited dissociation of weak acids. The goals of this and the following chapter will be to consider chemical equilibria in quantitative terms. The extent to which that equilibrium lies (product favored verses reactant favored) will be discussed. Chemical Equilibrium: A Review Slide 3 At some point in time during the progress of a reaction, if the concentration of the reactants and products remains constant, equilibrium is said to be achieved. The concentrations are NOT equal. Equilibrium Slide 4 Moving towards equilibrium Equilibrium established Equilibrium Slide 5 Equilibrium systems are said to be: Dynamic (in constant motion) Reversible Equilibrium can be approached from either direction. Pink to blue Co(H 2 O) 6 Cl 2 Co(H 2 O) 4 Cl 2 + 2 H 2 O Blue to pink Co(H 2 O) 4 Cl 2 + 2 H 2 O Co(H 2 O) 6 Cl 2 Properties of Chemical Equilibria Slide 6 After a period of time, the concentrations of reactants and products are constant. The forward and reverse reactions continue after equilibrium is attained. They are equal and opposite in rate. Chemical Equilibrium Fe 3+ (aq) + SCN (aq) Fe(SCN) 2+ (aq) Slide 7 Phase changes such as H 2 O(s) vs. H 2 O(liq) Examples of Chemical Equilibria Slide 8 When a general chemical reaction is at equilibrium, the equilibrium constant is given by: If K &gt; 0 then the reaction is said to be product favored. If K &lt; 0 then the reaction is said to be reactant favored. The Equilibrium Constant Slide 9 Product-favored K &gt; 1 Reactant-favored K &lt; 1 The Equilibrium Constant Slide 10 For the formation of HI(g) the equilibrium constant is given by: The Equilibrium Constant Slide 11 11 N 2 O 4 (g) 2NO 2 (g) Start with NO 2 Start with N 2 O 4 Start with NO 2 &amp; N 2 O 4 equilibrium Slide 12 The equilibrium between reactants and products may be disturbed in three ways: (1) by changing the temperature (2) by changing the concentration of a reactant (3) by changing the volume (for systems involving gases) A change in any of these factors will cause a system at equilibrium to shift back towards a state of equilibrium. Le Chateliers principle. This statement is often referred to as Le Chateliers principle. Disturbing a Chemical Equilibrium Slide 13 Effect of the Addition or Removal of a Reactant or Product If the concentration of a reactant or product is changed from its equilibrium value at a given temperature, equilibrium will be reestablished eventually. The new equilibrium concentrations of reactants and products will be different, but the value of the equilibrium constant expression will still equal K Disturbing a Chemical Equilibrium Slide 14 14 If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position. Le Chteliers Principle Changes in Concentration N 2 (g) + 3H 2 (g) 2NH 3 (g) Add NH 3 Equilibrium shifts left to offset stress Slide 15 15 Le Chteliers Principle Changes in Concentration continued ChangeShifts the Equilibrium Increase concentration of product(s)left Decrease concentration of product(s)right Decrease concentration of reactant(s) Increase concentration of reactant(s)right left aA + bB cC + dD Add Remove Slide 16 Effect of Volume Changes on Gas-Phase Equilibria For a reaction that involves gases, what happens to equilibrium concentrations or pressures if the size of the container is changed? (Such a change occurs, for example, when fuel and air are compressed in an automobile engine.) To answer this question, recall that concentrations are in moles per liter. If the volume of a gas changes, its concentration therefore must also change, and the equilibrium composition can change. Disturbing a Chemical Equilibrium Slide 17 17 Le Chteliers Principle Changes in Volume and Pressure A (g) + B (g) C (g) ChangeShifts the Equilibrium Increase pressureSide with fewest moles of gas Decrease pressureSide with most moles of gas Decrease volume Increase volumeSide with most moles of gas Side with fewest moles of gas When the number of gas moles on either side is the same, there is no effect. Slide 18 Effect of Temperatue Changes on Gas-Phase Equilibria Consider the reaction of nitrogen and oxygen to form nitric oxide: As the temperature of the reaction is increased, the equilibrium constant increases. KTemperature (K) 4.5 10 31 298 6.7 10 10 900 1.7 10 3 2300 Why? Disturbing a Chemical Equilibrium Slide 19 Effect of Temperature Changes on Gas-Phase Equilibria Lets write the reaction in this manner: Notice that energy is included as a reactant! Disturbing a Chemical Equilibrium Slide 20 Effect of Temperature Changes on Gas-Phase Equilibria Lets write the reaction in this manner: As temperature (Energy) is increased, equilibrium shifts to the right, favoring products. Disturbing a Chemical Equilibrium Slide 21 Effect of Temperatue Changes on Gas-Phase Equilibria Lets write the reaction in this manner: As the concentration of products increases so does the value of the equilibrium constant. Disturbing a Chemical Equilibrium Slide 22 Effect of Temperatue Changes on Gas-Phase Equilibria This explains the increase in the equilibrium constant with increasing temperature. KTemperature (K) 4.5 10 31 298 6.7 10 10 900 1.7 10 3 2300 Disturbing a Chemical Equilibrium Slide 23 Effect of Temperature Changes on Gas-Phase Equilibria Conclusion Conclusion: Increasing rightIncreasing the temperature of an endothermic reaction favors the products, equilibrium shifts to the right. Increasing leftIncreasing the temperature of an exothermic reaction favors the reactants, equilibrium shifts to the left. Lowering temperature results in the reverse effects. Disturbing a Chemical Equilibrium Slide 24 K c (273 K) = 0.00077 K c (298 K) = 0.0059 Temperature Effects on Equilibrium Slide 25 25 Catalyst lowers E a for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium. Adding a Catalyst does not change K does not shift the position of an equilibrium system system will reach equilibrium sooner Le Chteliers Principle Slide 26 ChangeReaction Shift Adding more N 2 (g)Right Removing H 2 (g)Left Decreasing the container volumeRight Increasing the container temperatureLeft Increasing the container volumeLeft Decreasing the container temperatureRight adding a catalystno effect adding argon to the containerno effect Le Chateliers Principle Practice Slide 27 27 Le Chteliers Principle - Summary ChangeShift Equilibrium Change Equilibrium Constant Concentrationyesno Pressureyes*no Volumeyes*no Temperatureyes Catalystno *Dependent on relative moles of gaseous reactants and products Slide 28 In an equilibrium constant expression, all concentrations are reported as equilibrium values. Product concentrations appear in the numerator, and reactant concentrations appear in the denominator. Each concentration is raised to the power of its stoichiometric balancing coefficient. Values of K are dimensionless. The value of the constant K is particular to the given reaction at a specific temperature. Writing Equilibrium Constant Expressions Slide 29 Reactions Involving Solids So long as a solid is present in the course of a reaction, its concentration is not included in the equilibrium constant expression. Equilibrium constant: Writing Equilibrium Constant Expressions Slide 30 Reactions in Solution If water is a participant in the chemical reaction, its concentration based on magnitude is considered to remain constant throughout. Equilibrium constant: Writing Equilibrium Constant Expressions Slide 31 Reactions Involving Gases: K c and K p Concentration data can be used to calculate equilibrium constants for both aqueous and gaseous systems. In these cases, the symbol K is sometimes given the subscript c for concentration, as in K c. For gases, however, equilibrium constant expressions can be written in another way: in terms of partial pressures of reactants and products. Writing Equilibrium Constant Expressions Slide 32 Reactions Involving Gases: K p Notice that the basic form of the equilibrium constant expression is the same as for K c. In some cases, the numerical values of K c and K p are the same. They are different when the numbers of moles of gaseous reactants and products are different. Writing Equilibrium Constant Expressions Slide 33 Reactions Involving Gases: K p &amp; K c The general relationship between K p and K c is derived in chapter 26, pa726. When the number of gas mole is equivalent on either side of the chemical equation, the two equilibrium constants are the same value. R = the gas constant T = the absolute temperature n = (mols gas product) mols gas reactant) Writing Equilibrium Constant Expressions Slide 34 REACTION QUOTIENT, Q In general, ALL reacting chemical systems are characterized by their REACTION QUOTIENT, Q. If Q = K, then system is at equilibrium. The Reaction Quotient, Q Slide 35 If Q &lt; K then the system is heading towards equilibrium: There are more reactants than products as expected at equilibrium. The reaction is said to be headed to the right. If Q &gt; K the system has gone past equilibrium. There are more products than reactants as expected at equilibrium. The reaction is said to be headed to the left. If Q = K then the system is at equilibrium. The Reaction Quotient, Q Slide 36 36 K &gt;&gt; 1 K Effect of Volume Changes on Gas-Phase Equilibria What would happen if the total volume of the system was suddenly doubled? Q = 342 &gt; K Q = 342 &gt; K: Therefore some products must shift to reactants (left) to reestablish equilibrium. Disturbing a Chemical Equilibrium Slide 92 Effect of Volume Changes on Gas-Phase Equilibria Conclusion: Increasing the volume of a container favors the side of equilibrium with the greatest number of gas moles. Decreasing the volume favors the side with the least number of moles. When the number of gas moles on either side is the same, there is no effect. Disturbing a Chemical Equilibrium Slide 93 Slide 94 94 Chemical Kinetics and Chemical Equilibrium A + 2B AB 2 kfkf krkr rate f = k f [A][B] 2 rate r = k r [AB 2 ] Equilibrium rate f = rate r k f [A][B] 2 = k r [AB 2 ] kfkf krkr [AB 2 ] [A][B] 2 =K c = Slide 95 95 Modifying the Chemical Equation (contd) What will be the equilibrium constant K" c for the new reaction? Consider the reaction:2 NO(g) + O 2 (g) 2 NO 2 (g) [NO 2 ] 2 K c = = 4.67 x 10 13 (at 298 K) [NO] 2 [O 2 ] Now consider the reaction: NO 2 (g) NO(g) + O 2 (g) [NO] [O 2 ] 1/2 1 1/2 K" c = = [NO 2 ] 2 K c = 2.14 x 10 14 = 1.46 x 10 7 Slide 96 96 If the coef in the reaction is:Then K is: DoubledSquared HalvedSquare root Reversed in signInverted Multiplied by a constant nRaised to the n th power Calculate K of reversed reaction, of a reaction or doubled Example:2SO 2(g) + O 2(g) 2SO 3(g) 4SO 2(g) + 2O 2(g) 4SO 3(g) SO 2(g) + O 2(g) SO 3(g) 2SO 3(g) O 2(g) + 2SO 2(g) Example: 2CO (g) + O 2(g) 2CO 2(g) K = 2.75 x 10 20 @1000K CO 2(g) CO (g) + O 2(g) K = 6.03 x 10 -11 @1000K Summarize: </p>