2 Equilibrium in Chemical Systems Reactions do not always go forward to completion (reactants do not all react)Instead reach equilibrium: concentration of reactants and products becomes constant (but not equal)Use an arrow in the equation to show equilibrium
7 Two Types of Equilibrium Phase change equilibriumSolid in a saturated solution ex) sugar and sugar waterGas above a liquid ex) popVapour above a solid ex) mothballs in a dresser
8 Chemical equilibrium Ex) CaCO(s) CaO(s) + CO2(g) Homogeneous: reactants and products are in the same phase or stateEx) C2H5OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)Heterogeneous: reactants and products are in different phases or statesEx) CaCO(s) CaO(s) + CO2(g)
9 4 Conditions for Equilibrium Forward rate and reverse rate of reaction are the sameMacroscopic properties are constantClosed systemEquilibrium can be approached from either direction
10 Classes of Chemical Reaction Criteria Percent ReactionDescription at EquilibriumPosition at equilibrium<50%Reactants favored>50%Products favored>99%No eq’b
11 Equilibrium ConstantRatio of the product and reactant concentrations is a constant value.This value is called the equilibrium constant Kc
12 Finding Kc For the reaction: aA + bB cC + dD Kc= [C]c[D]d = products [A]a[B]b reactants*Only substances whose concentrations or pressures change can be included in the equilibrium expression. Liquids and solids are not included.
13 If K>1 the reaction favors products If K<1 the reaction favors reactantsIf K=1 there are equal concentrations of reactants and products
14 Calculations with Equilibrium Constants Case 1 – Given all the amounts of species at equilibrium, calculate the equilibrium constant.Case 2 – Given the initial and final conditions, calculate the equilibrium constant.
15 In each case, you will need: The balanced chemical equation for the system at equilibriumThe equilibrium constant expression (remember to include only those that are solutions or gases, not liquids or solids)
16 Case 1 ExamplesCalculate the Kc for the following system at a constant temperature:2NO(g) + O2(g) NO2(g)[NO]: 1.0 mol/L [O2]: 6.0 mol/L [NO2]: 2.0 mol/L
17 For the reaction CO(g) + Cl2(g) COCl2(g) at a particular temperature, Kc = 5.1 x 105. Atequilibrium, there are 0.30 mol of Cl2(g) and 0.072mol of COCl2(g) in a 2.0L container. What is the equilibriumconcentration of CO(g)?
18 Case 2 Examples Use the ICE tables I= initial C=change E= equilibrium Need to use molar ratio as well
19 For the reaction given below, 3. 00 mol of A and 4 For the reaction given below, 3.00 mol of A and mol of B are placed in a 5.00 L container.2A(g) + B(g) C(g)At equilibrium, the concentration of A is 0.40 mol/L. Determine the value of Kc.
20 Initially 2. 0 mol of N2 and 4. 0 mol of H2 were added to a 1 Initially 2.0 mol of N2 and 4.0 mol of H2 were added to a 1.0 L container and the following reaction occurred.3H2(g) + N2(g) NH3(g)The equilibrium concentration of NH3 is 0.68 mol/L. What is the Kc?
21 Le Chatelier’s Principle when a system at equilibrium is disturbed by a change in a property, the system adjusts in a way that opposes this change.Change can occur to:ConcentrationPressure or volumeTemperature (this will also change Kc)Add a Catalyst
22 The application of Le Chatelier’s Principle involves three stages: The initial state of equilibrium in the systemStress exerted on the system by some change in property (temp, conc, pressure) that creates dis- equilibriumShift towards reactants or products, to re-establish a new state of equilibriumLe Chatelier’s Principle can provide a method of predicting the reaction’s response to the change.y/flash/lechv17.swf
23 Concentration Changes The addition of more reactant or the removal of a product will increase the yield of the product, shifting the equilibrium towards the right (products)A + B CIf the opposite occurs, the shift will be towards the left (reactants).
24 Concentration change can occur by adding something that makes a precipitate. Ex) AgNO3(s) Ag+(aq) + NO3-(aq)If you add NaCl, the Cl-(aq) reacts with Ag+ (aq) to form AgNO3(s)Response is to shift to the rightAgNO3(s) Ag+(aq) + NO3-(aq)
25 2. Pressure / Volume Change Gases onlyIf pressure decreases, response is to increase pressure by shifting to the side with more moles of gasEx) Decrease pressure and :A(g) + 2B(g) C (g)
26 In a gaseous system, decreasing the volume of the container increases the pressure. Adding more gas increases the pressurePressure change has no effect on equilibrium if moles are the same on both side.
27 3. Temperature ChangeThe energy in a chemical reaction is treated as if it were a reactant or product.Endothermic: reactants + energy productsExothermic: reactants products + energyEnergy can be added or removed by heating or cooling the system. Equilibrium shifts to minimize the change.Will change Kc
28 Ex) 2 SO3(g) + 97 kJ 2 SO2(g) + O2(g) If you heat it up (the stress), response is to cool down and equilibrium shifts right. Kc increasesIf you cool it down (the stress), response is to heat up and equilibrium shifts left. Kc decreases.
29 4. Catalyst Decreases time to reach equilibrium Lowers activation energy by the same amount in both the forward and reverse directionsDoes not effect final equilibrium position
30 Summary Variables Change Response of the system Concentration Increase Shift to consume added reactant or productDecreaseShift to replenish some of the removed reactant or productTemperatureShifts to consume some of the added heatShifts to replenish some of the removed heatVolume (overall pressure)Increase volume / decrease pressureShifts toward the side with the larger number of gaseous moleculesDecrease volume / increase pressureShifts toward the side with the smaller number of gaseous molecules