chapter 14 chemical equilibruim. objectives describe chemical equilibrium write an equilibrium...
TRANSCRIPT
![Page 1: Chapter 14 Chemical Equilibruim. Objectives Describe chemical equilibrium Write an equilibrium constant expression Calculate the equilibrium constant](https://reader035.vdocuments.net/reader035/viewer/2022062221/56649f575503460f94c7c4a5/html5/thumbnails/1.jpg)
Chapter 14
Chemical Equilibruim
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Objectives
• Describe chemical equilibrium
• Write an equilibrium constant expression
• Calculate the equilibrium constant for a reaction
• Relate the equilibrium constant to the position of a reaction
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Chemical Equilibrium
• Point in a chemical reaction where the forward and reverse reactions happen at the same rate.
• There does not need to be equal amounts of reactants and products
– There can be a large amount of product or a large amount of reactant at equilibrium
• The rates just need to be equal
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Chemical Equilibrium
• Equilibrium is indicted by double arrows in reactions– Forward reaction points right
• Products– Reverse reaction points left
• Reactants• The right is still products, the left is still
reactants
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Chemical Equilibrium
• Equilibrium is a dynamic process
– Continually happening
• Reaction will continue even though it will appear to have stopped
• Similar to a person on a treadmill
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Example
• Consider the reaction
A + B C• Initially only A & B are present
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Example
• Consider the reaction
A + B C• A & B begin to disappear, C begins to appear
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Example
• Consider the reaction
A + B C• A & B begin to disappear, C begins to appear
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Example
• Consider the reaction
A + B C• A & B begin to disappear, C begins to appear
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Example
• Consider the reaction
A + B C• A & B begin to disappear, C begins to appear
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Example
• Consider the reaction
A + B C• Eventually equilibrium is reached
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Equilibrium Constant Expressions
• A mathematical equation that relates the concentration of products to the concentration of reactants for a reaction at equilibrium
• The equilibrium constant is Keq• K for Konstant, eq for equilibrium• General form of the expression is
concentration of products over concentration of reactants– Pressure can also be used
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The Equilibrium Constant Expression
• For the reaction
jA + kB lC + mD• Where j, k, l, & m are coefficients and • A, B, C, and D are chemicals• The equilibrium constant expression is
kj
ml
BA
DCKeq
][][
][][
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Equilibrium Constant Expressions
• A species in brackets indicates concentration
• Only solutions and gases are included in equilibrium expressions
• If you have a solid or liquid leave it out
– Just think of it as a 1 in the expression
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Example• Write the equilibrium expression for the
following reaction. HF(aq) H+ + F-
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Example• Write the equilibrium expression for the
following reaction. 2NO(g) N2(g) + O2(g)
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Example• Write the equilibrium expression for the
following reaction. 2C(s) + O2(g) 2CO(g)
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The Equilibrium Constant
• A value that relates the position of a reaction at equilibrium
• Calculated by inserting values into the equilibrium constant expression
• Keq does NOT have units• Keq > 1
– Product favored• Keq <1
– Reactant favored
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The Equilibrium Constant & Temp
• Equilibrium constants are constant for a given temperature
– If the temp. changes so does the constant
– Change depends on whether the reaction is exothermic or endothermic
• More on that later
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Example• Calculate the equilibrium constant for the
following reaction and equilibrium concentrations [HF]= 0.12M [H+]= 9.3x10-3M [F-]= 9.3x10-3M
HF(aq) H+ + F-
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Example• Calculate the equilibrium concentration of
NO given the reaction, Keq, and equilibrium concentrations. [N2] = 0.37M [O2] = 0.45M Keq = 2.3x10-9 2NO(g) N2(g) + O2(g)
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Homework
• p.573 #26,32,35,39,52,55,61
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Objectives
• Explain LeChatliers Principle
• Predict equilibrium shifts
• Describe solubility equilibrium
• Write Ksp expressions
• Perform calculations with Ksp
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My Equilibrium is Stressed
• A reaction at equilibrium can be stressed out by changing reaction conditions– There are three stresses for reactions
• Concentration• Pressure • Temperature
• Q) If you are stressed what do you do?• A) Work to relieve the stress!
– Same goes for reactions
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LeChatlier’s Principle
• When a stress is applied to a reaction at equilibrium the reaction will shift to relieve the stress placed upon it.
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Concentration
• A reaction is stressed when the concentration of a reactant or product is increased OR decreased
– Add a chemical to increase
– Remove a chemical to decrease
• Reactions shift away from increased concentrations
• Reactions shift toward decreased concentrations
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Example
• Consider the reaction
2SO2(g) + O2(g) 2SO3(g)
• How will the reaction shift with the following– Increase the amount of SO2
• Right– Increase the amount of SO3
• Left– Decrease the amount of O2
• Left
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Pressure
• Pressure can increased
• By decreasing volume
• Pressure can be decreased
• By increasing volume
• Increased pressure shifts reactions away from side with more gas molecules
• Decreased pressure shifts reactions toward side with more gas molecules
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Example
• Consider the reaction
2SO2(g) + O2(g) 2SO3(g)
• How will the reaction shift with the following
– Increase the pressure in the container
• Right
– Decrease the pressure in the container
• Left
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Temperature
• Direction of shift depends of whether the reaction is exothermic or endothermic
– How do you know
• Exothermic reactions release heat
–It is a product
• Endothermic reactions absorb heat
–It is a reactant
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What Type of Thermic Is It?
• Exothermic
A + B C + 49kJ
A + B C ΔH = -49kJ
• ΔH means enthalpy (energy change)
• Endothermic
A + B + 49kJ C
A + B C ΔH = 49kJ
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Temperature Change
• Think of the temp as a concentration of heat
– Increasing the temp. will shift the reaction away from the heat
– Decreasing the temp. will shift the reaction toward the heat.
• Same as concentration shift
• Change in temp. is the only stress that changes the equilibrium constant.
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Example
• Consider the reaction
2SO2(g) + O2(g) 2SO3(g) + 197kJ
• How will the reaction shift with the following
– Increase the temperature
• Left
– Decrease the temperature
• Right
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Common Ion Effect
• The addition of an ion that is present in the equilibrium
HF H+ + F-
Adding a H+ or F- shifts the reaction left
• How do you add them?
– H+ can come from any acid
– F- could be from NaF, KF, CaF2
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Addition of Other Stuff
• Adding species not related to the equilibrium reaction have no effect on the reaction
– NO SHIFT
• For Example
–Catalysts
–Inert Gases
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Solubility Equilibrium
• Dissolving is an equilibrium process
• Chemicals that we say are insoluble are actually very slightly soluble
– Meaning only a small amount of the solute dissolves
– Concentrations are very small
• 10-5M to 10-20M are common
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Solubility Equilibrium
• Iron (II) Sulfide is “insoluble” by solubility rules
• However, it still dissolved to a small extent
• Consider the reaction
FeS(s) Fe+2 + S-2
• Since dissolving is an equilibrium process we can write an equilibrium expression
• Keq = [Fe+2] [S-2]
• FeS is omitted because it is a solid
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Solubility Product Constant
• The previous equilibriums expression is a “special” type of equilibrium expression
• Ksp – solubility product constant
• Ksp = [Fe+2] [S-2]
– Only used for slightly soluble salts
– Never includes the reactants
– Ksp’s are very small
– Check out the table 14.1 page 568
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Ksp Values
• Lead (II) Chloride 2.4X10-4
• Strontium Carbonate 3.8X10-9
• Nickel (II) Hydroxide 5.5X10-16
• Copper (II) Hydroxide 2.2X10-20
• Cadmium Sulfide 8.0X10-28
• Silver Sulfide 6.0X10-51
• And my personal favorite
• Bismuth Sulfide 1.6x10-72
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Ksp Values
• Q) What does a small Ksp value mean?
• A) Low concentration of ions
• Small solubility
• Q) Is the compound with the smallest Ksp the least soluble?
• A) Not necessarily
– There are different numbers of ions that changes the expression
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Solubility of Salts
• We can easily compare solubility for salts that have the same # of ions
• When there are the same # of ions the salt with the smallest Ksp is least soluble
• Which salt is least soluble, most soluble AgCl, FeS,
– FeS 6.0x10-19 Least soluble
– AgCl 1.6x10-10 Most soluble
– BaSO4 1.1x10-10 Middle
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Calculating Solubility
• We can calculate solubility of salts and the concentration of the ions in solutions from Ksp
• Deal with saturated solutions
– The salt has dissolved as much as it can
• Ksp has been reached
• In saturated solutions the concentrations of the ions are related to the mole ratio
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Example• Calculate the solubility of silver chloride
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Example• Calculate the solubility of
mercury (II) sulfide
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Example• Calculate the solubility of calcium phosphate
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Example• A saturated solution of Iron (III) Hydroxide
has a concentration of Fe+3 of 1.8x10-8. What is the Ksp of Iron (III) Hydroxide?
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Common Ion Revisited
• Anyone ever have a barium shake for a scan?
• Contains barium sulfate.
– Barium ions are very toxic to the body
– Treated as calcium
• How can you have fewer barium ions in solution?
• Add Sodium Sulfate
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Homework
• p.574 #71,77,89,95,97,102