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Reversible Reactions and Equilibrium
Reversible Reactions
Chapter 18
Reversible Reactions and Equilibrium
Reversible reactions are those reactions that occur simultaneously in both
directions.
Ultimately,a chemical equilibrium will be reached. This occurs when the
forward and reverse reactions take place at the same rate.
– There is no net change in the actual amounts of the components of the
system?
– Reversible Reactions and Equilibrium
2SO2(g) + O2(g) ---> 2SO3(g)
2SO2(g) + O2(g) <--- 2SO3(g)
2SO2(g) + O2(g) <---> 2SO3(g)
2SO2(g) + O2(g) <--->> 2SO3(g)
Note: The rate of change reaches an equilibrium. The concentration of
reactants and products is not equal.
Reversible Reactions and Equilibrium
Equilibrium position of a reaction is given as a relative concentration of the
components of the reaction at equilibrium. Huh?
– If, at equilibrium, the reaction favors component B by 99% then A is at
1%
A -----> B
1%<----- 99%
Reversible Reactions and Equilibrium
If all of your reactants have been changed to products, then the reaction is
said to be irreversible.
Catalysts reduce the amount of time needed to establish equilibrium, both
forward and back.
Processes That Reach Equilibrium
Vapor-Liquid Equilibrium
The rate of change from liquid to vapor is equal. A closed system.
H2O(l)   H2O(g)
Processes That Reach Equilibrium
Solutions of solids
This occurs when solutes are
dissolved in a solvent.
O2(g)   O2(aq)
Gas phase equilibrium
N2(g) + 3H2(g)   2NH3(g)
Processes That Reach Equilibrium
Acids and bases
CH3COOH(aq) + H2O   CH3COO- + H3O+
Practice #2, 3, 4
Reversible Reactions and Equilibrium
Le Chatelier’s Principle
– Nature strives for stability.
A system that experiences a change from equilibrium will make
adjustments to restore the equilibrium.
• This new equilibrium may not be the same as the original
equilibrium.
• This is called a shift in the position of equilibrium.
Reversible Reactions and Equilibrium
His principle: If a stress is applied to a system in dynamic equilibrium, the
system changes to relieve the stress.
– Stressors include:
• changes in concentration of reactants or products
• changes in temperature
• changes in pressure.
• Reversible Reactions and Equilibrium
Concentration: changing the concentration of reactants or products at
equilibrium disturbs the equilibrium.
– The system will adjust to minimize the change.
Watch this:
H2CO3(aq) <------>> CO2(aq) + H2O(l)
<1%
>99%
– Reversible Reactions and Equilibrium
H2CO3(aq) <------>> CO2(aq) + H2O(l)
<1%
>99%
At equilibrium H2CO3 is less than 1%
If you add CO2 at equilibrium this happens,
– The ratio of carbonic acid to carbon dioxide changes from 99:1 to
99.5:0.5
– This results in a new ratio of 199:1 wow!
– So now what?
Reversible Reactions and Equilibrium
Well, nature strives for stability so, the reaction will shift in order to regain
the 99:1 ratio thus forming more carbonic acid.
– How does your body maintain the correct balance of CO2 and H2CO3?
• H2CO3(aq) <------>> CO2(aq) + H2O(l)
Moral of the story, if you add more product you will shift the reaction to the
left(reverse.)
Reversible Reactions and Equilibrium
What happens if you remove some product?
– The shift is in the direction of the products.
– Stability must be maintained.
What is the practical application of this process?
– Egg laying hens
– Carbonic acid in your blood.
Reversible Reactions and Equilibrium
Temperature
– Increasing the temperature shifts the reaction in the direction that
absorbs heat.
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Why?
• Because the heat absorption tries to reduce the amount of heat
that was introduced. Heat is the stressor.
• Reversible Reactions and Equilibrium
• Pressure
• Applicable only to gases with unequal quantities of moles of reactants and
products.
• Let’s make ammonia.
• Reversible Reactions and Equilibrium
• Making ammonia
• N2(g) + 3H2(g)<-----> 2NH3(g)
• What conditions of heat and pressure would produce the greatest amount of
ammonia?
• Reversible Reactions and Equilibrium
• Practice time
– 13 page 625
• Reversible Reactions and Equilibrium
• Equilibrium Constants
– is the ratio of the product concentrations to the reactant
concentrations at equilibrium, with each concentration raised to a
power equal to the number of moles of that substance in the balanced
chemical equation. Huh?
– Reversible Reactions and Equilibrium
• Equilibrium Constants
– the reaction: N2O4(g) <----> 2NO2 (g)
– Which is the reactant? Which is the product?
– How many moles of each are there?
– Keq = [ NO2 ]2
[ N2O4 ]
[ XX ] indicate moles/liter
• Reversible Reactions and Equilibrium
• Equilibrium Constants
• Provide information about whether the reaction favors reactants or
products.
– Keq is a ratio of products / reactants.
– If Keq is > 1, products are favored.
– If Keq is < 1, reactants are favored.
– Let’s return to our original reaction.
• Reversible Reactions and Equilibrium
N2O4(g) <----> 2NO2 (g)
If a liter of gas contains 0.0045 mol of N2O4 and 0.030 mol of NO2 what is the Keq ?
– Keq = [ NO2 ]2
[ N2O4 ]
– Keq = [0.030 mol/L NO2 ]2
[0.0045 mol/L N2O4 ]
= .20 mol/L
–
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Reversible Reactions and Equilibrium
– N2O4(g) <----> 2NO2 (g) at 10oC
– Keq = [0.030 mol/L NO2 ]2
[0.0045 mol/L N2O4 ]
= .20 mol/L
Reversible Reactions and Equilibrium
What you need to know:
– Equilibrium is a dynamic process. Huh?
– The only thing “equal” is the rate of change from products to reactants
to products.
– Catalysts lower the activation energy of a reaction. This increases the
rate of reactivity.
– Adding a reactant or removing a product can increase the yield of a
reaction.
– An increase in temperature shifts the reaction towards the direction
that absorbs heat.
– For reactions involving gases with unequal mole ratio amounts, an
increase in pressure shifts the equilibrium toward the fewest
molecules.
Reversible Reactions and Equilibrium
From start to finish. Ozone(O3) screens out about 95% of the ultraviolet rays
from the sun. It is formed from the action of the UV rays on O2 and readily
decays to O2
– Write a balanced equation for the reaction.
3O2(g) <---> 2O3(g)
Identify the reactant, product, and corresponding coefficients.
3O2(g) = 3 mol of reactant
2O3(g) = 2 mol of product
Reversible Reactions and Equilibrium
Write an equillibrium constant for the reaction.
– Keq = [ O3] 2
[O2]3
– Finally, when a reaction reaches equilibrium,the reactants and
products have a fixed numerical relationship which is represented by
the equilibrium constant. If the constant is greater than 1, products
are favored. If the constant is less than 1, reactants are favored.
Review??
Page 607 #’s 1-4
Page 612 #’s 5-8
Page 625 # 13