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EQUILIBRIUM BASICS Chapter 14.1-14.3
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Lesson Objectives Know -Factors that affect/don’t affect a reaction reaching equilibrium -K is equilibrium constant for any reversible reaction -K c and K p are related to each other through K p = K c (RT) ng -Coefficient Rule -Reciprocal Rule -Rule of Multiple Equilibria Understand -Equilibrium is based on when RATES of forward and reverse reactions are equal, not when concentrations of reactants and products are equal -The equilibrium constant K has its roots in the kinetics rate law constant k -The size of the equilibrium constant tells us which side (reactants or products) is favored in an equilibrium reaction -K may only be calculated with concentration or pressure values when the system is at EQUILIBRIUM -Solids and liquids are not used in equilibrium constant expressions Do -Write an equilibrium constant expression for a given reaction -Calculate K based on equilibrium concentrations -Apply the Coefficient Rule, the Reciprocal Rule, and the Rule of Multiple Equilibria to calculate a new K from a known K -Calculate K when given initial concentration or pressure values (ICE Chart)
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Which of the following is NOT true about a reaction at equilibrium? N 2 O 4 2NO 2 A. [N 2 O 4 ] = 2 [NO 2 ] B. The rate of the forward and reverse reactions are the same C. The equilibrium constant will describe whether the process favors reactants or products D. [N 2 O 4 ] and [NO 2 ] remain constant. E. For every 2 molecules of NO 2 reacted, 1 molecule of N 2 O 4 is produced
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AT EQUILIBRIUM AT EQUILIBRIUM Two reactions are occurring (forward and reverse) indicated by double arrow. Equilibrium definition: a reaction has reached equilibrium (balance) when the forward and reverse reactions occur at the same RATE. This means that concentrations of reactants and products stay CONSTANT over time
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An Equilibrium Example See this interactive tutorial herehere
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Equilibrium factors: Equilibrium IS dependent on ◦ TEMPERATURE (affects the equilibrium constant, K) It is NOT dependent on ◦ Original concentrations ◦ Volume of the container ◦ Total pressure of the system Remember – the partial pressure of a gas in a mixture is proportional to the mole fraction of that gas in the mixture!
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The following data are for the system A(g) 2B(g) Time (s)020406080100 P A (atm)1.000.830.720.650.62 P B (atm)0.000.340.560.700.76 How long does it take the system to reach equilibrium?
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K: equilibrium constant K: equilibrium constant Tells us the extent to which the reaction will go until it reaches equilibrium ◦ Large K means the reaction goes mostly to product ◦ Small K means the reaction stays mostly as reactant
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For a reaction 2A(g) + B(g) 2C(g) K = 1 x 10 83 which of the following can be concluded? A. At equilibrium, [A] = ½[B] = [C] B. At equilibrium, the largest concentration is [A] C. At equilibrium, the largest concentration is [C] D. This reaction favors the reactants E. This reaction happens very quickly
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Writing K expression (equilibrium expression) Uses only gaseous or aqueous products and reactants (no solids or liquids; their concentrations do not change during reactions) Coefficients become exponents Products over reactants For aA + bB cC + dD K p = or K c = Relate to each other: K p = K c (RT) Δng ◦ R = gas law constant (0.0821 atm·L/mol·K) ◦ Δng = change in moles of gas (products – reactants) (P C ) c x (P D ) d (P A ) a x (P B ) b [C] c [D] d [A] a [B] b
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Which of the following species should NOT be included in the equilibrium constant expression for the reaction? 2A(s) + B(aq) 2C(l) + D(g) A. A, B, C B. B, C C. A, C D. A, C, D E. B, D
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For a reaction 2A + B 2C When equilibrium is established, the ratio of products to reactants (K) = 0.5. Which of the following initial conditions could be changed to cause a different equilibrium constant value? A. Start with more A B. Dilute the reaction by adding water C. Increase the temperature of the reaction D. Add a catalyst
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To calculate K p from K c for the reaction A(g) + 2B(g) C(g) what would be the exponent to which K c (RT) is raised? A. -2 B. -1 C. 0 D. 1 E. 2
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Write the equilibrium constant (K) expressions for the following reactions: I 2 (g) + 5F 2 (g) 2 IF 5 (g) SnO 2 (s) + 2H 2 (g) Sn (s) + 2H 2 O (l)
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For the reaction 2 NO(g) + O 2 (g) 2 NO 2 (g) determine the K p at 298K if K c is 4.67 x 10 13
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True or False? Given the equation below, N 2 + 3H 2 2 NH 3 if one mole of N 2 is mixed with 3 moles of H 2, 2 moles of NH 3 will form.
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True or False? Given the equation below, N 2 + 3H 2 2 NH 3 for every 1 mole of N 2 that reacts, 3 moles of H 2 will also react and 2 moles of NH 3 will form.
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Calculate K for the reaction NH 4 CO 2 NH 2 (s) 2 NH 3 (g) + CO 2 (g) if at equilibrium there are 0.159 g of NH 4 CO 2 NH 2 and pressures of 0.0451 atm of CO 2 and 0.0961 atm NH 3
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K depends on the form (coefficients) of the balanced equation! This means that K changes when: ◦ The coefficients are multiplied ◦ Reactions are reversed ◦ Reactions are added together (mechanism)
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Coefficient rule: If coefficients are multiplied by a factor (n), then K is RAISED TO THE (n) EXPONENT A 2 (g) + 2B (g) 2AB (g) K = 3A 2 (g) + 6B (g) 6AB (g) K = P AB 2 P A x P B 2 P AB 6 P A 3 x P B 6
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Calculate K for the formation of 2 moles of ICl(g) ICl (g) ½I 2 (g) + ½Cl 2 (g) K = 2.2x10 -3
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Reciprocal rule: If the reaction is reversed, K of the forward and K of the reverse are reciprocals of each other (new K = 1/old K) A 2 (g) + 2B (g) 2AB (g) K = 2AB (g) A 2 (g) + 2B (g) K = P AB 2 P A x P B 2 P AB 2 P A x P B 2
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Rule of multiple equilibria If multiple reactions are added together to get an overall reaction, the overall K is the PRODUCT of the individual K’s A 2 (g) + 2B (g) 2AB (g) 2AB (g) + C (g) A 2 B 2 C (g) A 2 (g) + 2B (g) + C (g) A 2 B 2 C (g) Find overall K it by multiplying individual K’s:
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Given the reactions below and their constants, calculate K for the rxn: Fe(s) + H 2 O(g) FeO(s) + H 2 (g) H 2 O(g) + CO(g) H 2 (g) + CO 2 (g) K = 1.6 FeO(s) + CO(g) Fe(s) + CO 2 (g) K = 0.67
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Determining K K must use values AT EQUILIBRIUM! When given initial concentrations or pressures in a problem, you must determine what the values will be at equilibrium before you can calculate K Use an ICE Chart to determine how the initial values will increase or decrease to get to equilibrium, then solve for K. ◦ Solids and liquids have no effect on equilibrium, so they don’t need values in your ICE chart.
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The ICE Chart Initial/Change/Equilibrium Given initial pressures, find equilibrium constant A 2 = 0.1 atm B = 0.2 atm A 2 (g) + 2B(g) 2AB(g) I C E 0.1 atm 0.2 atm 0.0 atm -x-2x +2x 0.1 - x 0.2 - 2x 2x
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For the decomposition reaction NH 4 HS(s) NH 3 (g) + H 2 S(g) In a sealed flask are 10.0 g of NH 4 HS, NH 3 with a partial pressure of 0.692 atm and H 2 S with a partial pressure of 0.0532 atm. When equilibrium is established, it is found that the partial pressure of NH 3 has increased by 12.4%. Calculate K for this reaction.
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Lesson Objectives Know -Factors that affect/don’t affect a reaction reaching equilibrium -K is equilibrium constant for any reversible reaction -K c and K p are related to each other through K p = K c (RT) ng -Coefficient Rule -Reciprocal Rule -Rule of Multiple Equilibria Understand -Equilibrium is based on when RATES of forward and reverse reactions are equal, not when concentrations of reactants and products are equal -The equilibrium constant K has its roots in the kinetics rate law constant k -The size of the equilibrium constant tells us which side (reactants or products) is favored in an equilibrium reaction -K may only be calculated with concentration or pressure values when the system is at EQUILIBRIUM -Solids and liquids are not used in equilibrium constant expressions Do -Write an equilibrium constant expression for a given reaction -Calculate K based on equilibrium concentrations -Apply the Coefficient Rule, the Reciprocal Rule, and the Rule of Multiple Equilibria to calculate a new K from a known K -Calculate K when given initial concentration or pressure values (ICE Chart)
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