ch. 15: chemical equilibrium
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Ch. 15: Chemical Equilibrium. Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry. I. Chapter Outline. Introduction The Equilibrium Constant (K) Le Châtelier’s Principle. I. Introduction. - PowerPoint PPT PresentationTRANSCRIPT
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Ch. 15: Chemical EquilibriumCh. 15: Chemical Equilibrium
Dr. Namphol Sinkaset
Chem 152: Introduction to General Chemistry
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I. Chapter OutlineI. Chapter Outline
I. IntroductionII. The Equilibrium Constant (K)III. Le Châtelier’s Principle
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I. IntroductionI. Introduction• Most reactions are reversible, meaning
they can proceed in both forward and reverse directions.
• This means that as products build up, they will react and reform reactants.
• At equilibrium, the forward and backward reaction rates are equal.
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I. Example EquilibriumI. Example Equilibrium
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II. Equilibrium ConcentrationsII. Equilibrium Concentrations
• Equilibrium does not mean that concentrations are all equal!!
• However, we can quantify concentrations at equilibrium.
• Every equilibrium has its own equilibrium constant.
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II. The Equilibrium ConstantII. The Equilibrium Constant
• equilibrium constant: the ratio at equilibrium of the [ ]’s of products raised to their stoichiometric coefficients divided by the [ ]’s of reactants raised to their stoichiometric coefficients.
• The relationship between a balanced equation and equilibrium constant expression is the law of mass action.
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II. The Equilibrium ConstantII. The Equilibrium Constant
• For a general equilibrium aA + bB cC + dD, the equilibrium expression is:
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II. Example Equilibrium II. Example Equilibrium ConstantConstant
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II. Sample ProblemII. Sample Problem
• Write the equilibrium constant expression for the reaction:
2H2(g) + O2(g) 2H2O(g).
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II. Physical Meaning of KII. Physical Meaning of K
• Large values of K mean that the equilibrium favors products, i.e. there are high [ ]’s of products and low [ ]’s of reactants at equilibrium.
• Small values of K mean that the equilibrium favors reactants, i.e. there are low [ ]’s of products and high [ ]’s of reactants at equilibrium.
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II. Values of KII. Values of K
• Values of K are most easily calculated by allowing a system to come to equilibrium and measuring [ ]’s of the components.
• For the equilibrium H2(g) + I2(g) 2HI(g), let’s say equilibrium [ ]’s at 445 °C were found to be 0.11 M, 0.11 M, and 0.78 M for molecular hydrogen, molecular iodine, and hydrogen iodide, respectively.
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II. KII. Kcc for a H for a H22/I/I22 Mixture Mixture
• Note that units are not included when calculating K’s.
• Thus, equilibrium constants are unitless.
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II. Equilibrium [ ]’s Vs. KII. Equilibrium [ ]’s Vs. K
• For any reaction, the equilibrium [ ]’s will depend on the initial [ ]’s of reactants or products.
• However, no matter how you set up the reaction, the value of the equilibrium constant will be the same if the temperature is the same.
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II. Equilibrium [ ]’s Vs. KII. Equilibrium [ ]’s Vs. K
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III. Predicting Qualitative III. Predicting Qualitative Changes to EquilibriumChanges to Equilibrium
• If a system is at equilibrium, what happens when it is disturbed?
• Le Châtelier’s Principle allow us to make qualitative predictions about changes in chemical equilibria: When a chemical system at equilibrium is
disturbed, the system shifts in the direction that minimizes the disturbance.
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III. Three Ways to Disturb an III. Three Ways to Disturb an EquilibriumEquilibrium
• We look at three ways that disturb a system at equilibrium:
1) Adding/removing a reactant or product.2) Changing the volume/pressure in
gaseous reactions.3) Changing the temperature.
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III. Adding Reactant/ProductIII. Adding Reactant/Product
• If we add or remove a reactant or product, we’re changing the [ ]’s.
• The equilibrium will shift in a direction that will partially consume a reactant or product that is added, or partially replace a reactant or product that has been removed.
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III. Adding ReactantIII. Adding Reactant
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III. Adding ProductIII. Adding Product
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III. Changing VolumeIII. Changing Volume• Via PV = nRT, changing volume is
related to changing pressure and vice versa. For example, decreasing volume is
equivalent to increasing pressure.• Reducing volume of a gaseous reaction
mixture shifts the equilibrium in the direction that will, if possible, decrease the # of moles of gas.
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III. Changing PressureIII. Changing Pressure
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III. Changing TemperatureIII. Changing Temperature• To determine the effect of changing the
temperature, we need to know the heat of reaction, ΔHrxn.
• Once we know if a reaction is exo or endo, we can write “heat” as a product or reactant.
• We then apply Le Châtelier’s Principle in the same manner as when we considered the add product/reactant case.
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III. Changing TemperatureIII. Changing Temperature
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III. Sample ProblemIII. Sample Problem• For the reaction N2(g) + 3H2(g) 2NH3(g),
ΔHrxn = -46.19 kJ/mole. Which way will the equilibrium shift when each of the following occurs?
a) NH3 is removed.
b) The reaction vessel is opened.c) The reaction vessel is cooled by 25 °C.