the equilibrium constant
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The Equilibrium Constant. 7.3. Opposing Rates and the Law of Chemical Equilibrium. The Law of Chemical Equilibrium : At equilibrium, there is a constant ratio between the concentrations of the products and reactants in any change. Example. - PowerPoint PPT PresentationTRANSCRIPT
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The Equilibrium Constant
7.3
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Opposing Rates and the Law of Chemical Equilibrium
• The Law of Chemical Equilibrium: At equilibrium, there is a
constant ratio between the concentrations of the products and reactants in any change.
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Example
Consider the decomposition reaction of: dinitrogen tetroxide (N2O4(g)) into nitrogen dioxide (NO2(g)).
- Initial concentration of N2O4(g) is 0.0200 mol/L. - Initial concentration of No2(g) is ______. - What do you think will happen?
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Example
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- Initial rate of the forward reaction, kf , is _________________.
- Initial rate of reverse reaction, kr , is _____. - As the reaction proceeds, kf _____________
because _____________________________.- At the same time, kr _____________ because
_____________________.
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What do you think?
Can you come up with a SIMPLE equation relating kf to kr at equilibrium?
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The Equilibrium Constant, Keq
The equilibrium constant, Keq, is the ratio of the forward rate constant and reverse rate constant:
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Question: if Kf is 3.4 and Kr is 1.7, what is Keq?
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- However, usually we don’t know kf or kr. - We can use the equilibrium equation and the
molar concentrations of the reactants and products to find Keq.
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Concentrations of products are in the NUMERATOR. Concentrations of the reactants are in the DENOMINATOR.
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Sample Problem
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Work together to solve the following problem:
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Work on the next 5 problems:
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EQUILIBRIUM CONCENTRATIONS
Consider the following reaction:
Suppose the initial concentration of Fe3+ is 0.0064 mol/L and the initial concentration of SCN- is 0.0010 mol/L.
By measuring the intensity of the colour, we determine that the concentration of Fe(SCN)2+ is 4.5 x 10-4 mol/L.
WHAT CAN WE INFER ABOUT THE EQUILIBRIUM CONCENTRATIONS OF THE REACTANTS?
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ICE TABLES
Suppose the initial concentration of Fe3+ is 0.0064 mol/L and the initial concentration of SCN- is 0.0010 mol/L.
By measuring the intensity of the colour, we determine that the concentration of Fe(SCN)2+ is 4.5 x 10-4 mol/L.
LET’S SET UP AN ICE TABLE FOR THIS REACTION!!!
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TIPS FOR SOLVING EQUILIBRIUM CONSTANT QUESTIONS
1) If given the initial concentrations of all parts and the change in concentration of 1+ parts (reactants or products), you can use stoichiometry to find the change in the other parts.
2) If not given, let x be the change in concentration of the reactant with the smallest coefficient (to avoid fractions)
3) You will usually need to do one of these steps in order to solve:
1) Take the square root of the equation2) Use the quadratic formula.
4) Don’t forget the +/- !5) Use logic to determine which value for x to use!
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Qualitatively Interpreting the Equilibrium Constant
The Kc for
Is
What can we say about a large Kc? Small? Value of 0? Value of 1?
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When Kc > 1, __________________ are favoured, and the equilibrium lies far to the ___________. ________________________
When Kc = 1, ____________________________When Kc < 1, ___________________ are
favoured, and the equilibrium lies far to the ___________. ________________________.
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Using the Approximation Method
Sometimes the change in concentrations is so small that it can be ignored.
Eg:
If Kc is super small, the term “[initial] – x” is just [initial].
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Ignoring ‘x’
- Values of Kc are not measured with accuracy better than 5%.
- Divide the initial concentration by the value of Kc. If:More than 500, can be ignored. between100-500, may be ignored (will not see in
this chapter)Less than 100, can not be ignored (carry out full
calculation!)
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7.4Predicting the Direction of a Reaction
• Any problems with the homework?
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The Reaction Quotient
• Reaction quotient, Qc:– expression that is identical to the equilibrium
constant expression– Concentrations not necessarily at equilibrium.
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• If Qc = Kc, ________________________.• If Qc > Kc, ________________________.
– _______________ must be large.– _______________ must be larger than concentrations at
equilibrium. – System will attain equilibrium by moving to the _____________.
• Qc < Kc, _____________________________.– _________________ must be large.– _________________ must be larger than concentrations at
equilibrium.– System will attain equilibrium by moving to the _____________.
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Work on PPs 26 – 28 on Page 356.
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Le Chatelier’s Principle
• Predicts the way that an equilibrium system responds to change.
• A dynamic equilibrium tends to respond as to relieve the effect ofany change in the conditions thataffect the equilibrium.
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The Common Ion Effect
• A fancy way of saying “the effect of adding substances already present”
• A “concentration” effect. Ex//
Effect of adding phosphorus pentachloride gas?___________________________________
Effect of removing chlorine gas?_______________________________
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The Effect of Temperature
• Reaction will shift to relieve the temperature changes on a system.
Recall: ∆H > 0 : _____________________________________________________∆H < 0 : _____________________________________________________
Example:
Left to right: energy is ______________ and converted to chemical potential energy.
Adding heat: ________________________. Kc will ___________. Removing heat: _____________________. Kc will ____________.
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The Effect of Temperature(2)Effect of temperature on the position of equilibrium can be summarized as
follows:
If ∆H > 0 (Endothermic): Increase in temperature shifts equilibrium to the _________________: More
__________. Kc ____________________. Decrease in temperature shifts equilibrium to the ________________: More
___________. Kc ______________________.
If ∆H < 0 (Exothermic):Increase in temperature shifts the equilibrium to the ______________:
More____________. Kc ____________________.Decrease in temperature shifts equilibrium to the ________________: More
____________. Kc ____________________.
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The Effect of Volume/Pressure
• Recall that :
As the volume of gas __________, the pressure ___________.
• Ex//
If the volume of the system were to decrease, how would the system relieve this pressure? (Hint: look at the number of moles!)
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The Effect of Volume/Pressure(2)
Number of molecules on the left side: __________.Number of molecules on the right side: __________.
The pressure of the system would decrease if it shifted to the ________________.
Why?
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Adding Inert Gases
Ex//
• If oxygen were to be injected into a reaction vessel containing this reaction, what would happen? ___________________________
• If helium gas were to be added to the system, what would happen? ___________________ – This is the same as increasing _______________.
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Adding a Catalyst
• A catalyst speeds up the RATE of a reaction by lowering the ACTIVATION ENERGY.
• Activation energy is lowered the same amount for the forward and reverse reactions.
• Therefore, a catalyst does not affect the position of equilibrium, Kc.
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PPs 29-33, pg 366.
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