Chapter 19, part II Notes

Equilibrium ReactionsEquilibrium Reactions

LeChatelier’s PrincipleLeChatelier’s Principle

Equilibrium ConstantsEquilibrium Constants

Reactions so far…

• Up to now, we have Up to now, we have assumed that reactions assumed that reactions go one way and stop go one way and stop when all reactants have when all reactants have become products.become products.

• This is true sometimes, This is true sometimes, but not always. There are but not always. There are also…also…

Reversible Reactions

• In a In a reversible reactionreversible reaction, , the reaction occurs the reaction occurs simultaneously in both simultaneously in both directions.directions.

• Products are being formed Products are being formed and are also being used to and are also being used to form reactants.form reactants.

2SO2 + O2 2SO3

Equilibrium

• So how much product is So how much product is formed? It depends on the formed? It depends on the reaction. reaction.

• Eventually, the reaction will Eventually, the reaction will reach reach chemical equilibriumchemical equilibrium, , where forward and reverse where forward and reverse reactions take place at the reactions take place at the same rate.same rate.

Equilibrium

• This does NOT mean that the This does NOT mean that the amounts of product and amounts of product and reactant are the same.reactant are the same.

• The The equilibrium positionequilibrium position is is the relative percent of the relative percent of products and reactants in a products and reactants in a system at equilibrium.system at equilibrium.

LeChatelier’s Principle

• Henri LeChatelier studied Henri LeChatelier studied shifts in the equilibrium of shifts in the equilibrium of reactions and came up with reactions and came up with his principle. He said:his principle. He said:

• If a stress is applied to a If a stress is applied to a system in dynamic system in dynamic equilibrium, the system equilibrium, the system changes to relieve the stress.changes to relieve the stress.

LeChatelier’s Principle

• Let’s look at how several factors will shift the equilibrium point.

Concentration

• Increasing the concentration Increasing the concentration of a product or reactant in of a product or reactant in equilibrium will shift the equilibrium will shift the equilibrium point away from equilibrium point away from that substance.that substance.

• Decreasing the concentration Decreasing the concentration would have the opposite would have the opposite effect.effect.

H2CO3 H2O + CO2

• Which way will equilibrium Which way will equilibrium shift if adding:shift if adding:– COCO22

– HH22COCO33

– HH22OOWhich way will equilibrium Which way will equilibrium

shift if removing:shift if removing:– COCO22

– HH22COCO33

Temperature

• Adding heat to a reaction at Adding heat to a reaction at equilibrium will shift the equilibrium will shift the equilibrium point towards the equilibrium point towards the side that absorbs heat.side that absorbs heat.

• Removing heat will shift the Removing heat will shift the equilibrium point towards the equilibrium point towards the side that produces heat.side that produces heat.

2SO2 + O2 2SO3 + heat

• Which way will equilibrium shift if heat is added?

• Which way will it shift if heat is removed?

Pressure

• Adding pressure to a system Adding pressure to a system at equilibrium will shift the at equilibrium will shift the equilibrium point towards the equilibrium point towards the side that has the least moles side that has the least moles of gas.of gas.

• Decreasing pressure shifts Decreasing pressure shifts equilibrium towards the side equilibrium towards the side with the most moles of gas.with the most moles of gas.

N2 + 3H2 2NH3

• Assuming all the products Assuming all the products and reactants are gases, and reactants are gases, which way does equilibrium which way does equilibrium shift when pressure is shift when pressure is added?added?

• Which way does it shift when Which way does it shift when pressure is relieved?pressure is relieved?

Equilibrium Constants

• Generally, the equilibrium point is expressed as a numerical value, called the equilibrium constant, for any reversible reaction at a given temperature.

Given the equation of a general reaction Given the equation of a general reaction to be:to be:

aA + bB aA + bB cC + cC + dDdD

coefficients

The equilibrium constant would be:

KKeqeq = =[C]c[D]d

[A]a[B]b

The brackets around the substances The brackets around the substances means it is their concentration means it is their concentration (molarity).(molarity).

Equilibrium constant

• If the equilibrium constant If the equilibrium constant (K(Keqeq) is bigger than one, it ) is bigger than one, it means the products are means the products are favored at equilibrium.favored at equilibrium.

• A KA Keqeq less than one means less than one means the formation of reactants is the formation of reactants is favored at equilibrium.favored at equilibrium.

N2O4 2NO2

• In the above reaction, at the In the above reaction, at the equilibrium point, dinitrogen equilibrium point, dinitrogen tetroxide has a concentration tetroxide has a concentration of 0.0045M and nitrogen of 0.0045M and nitrogen dioxide has a concentration dioxide has a concentration of 0.030M. What is the of 0.030M. What is the equilibrium constant for the equilibrium constant for the reaction?reaction?

N2 + 3H2 2NH3

• Calculate KCalculate Keqeq for the for the reaction if at equilibrium in reaction if at equilibrium in 1.0L there is 0.15mol H1.0L there is 0.15mol H22, , 0.25mol N0.25mol N22 and 0.10mol and 0.10mol NHNH33..

2HI H2 + I2

• KKeqeq for the above reaction is for the above reaction is 0.020; if the concentration 0.020; if the concentration of hydrogen and iodine are of hydrogen and iodine are both 0.50M, what is the both 0.50M, what is the concentration of HI?concentration of HI?