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Standard 9: Chemical EquilibriumStandard 9: Chemical Equilibriumchapter 18chapter 18
Most reactions are ‘reversible’Most reactions are ‘reversible’ Forward reactionForward reaction: reactants make : reactants make
productsproducts
e.x. 3Oe.x. 3O2(g) 2(g) 2O 2O3(g)3(g)
Reverse reactionReverse reaction: products make : products make reactantsreactants
e.x. 2Oe.x. 2O3(g) 3(g) 3O 3O2(g)2(g)
In a reversible reaction the forward and In a reversible reaction the forward and reverse reactions occur at the same timereverse reactions occur at the same time
e.x. 3Oe.x. 3O2(g)2(g) 2 O 2 O3(g)3(g)
Reversible Reversible ReactionsReactions
Standard 9b: equilibrium conditions
Summary 1Summary 1
What is a reversible reaction?What is a reversible reaction?
Reversible reactions reach Reversible reactions reach equilibriumequilibrium: a balance between reactants : a balance between reactants and productsand products
Conditions of EquilibriumConditions of Equilibrium::1.1. rate of forward reaction = rate of reverse reactionrate of forward reaction = rate of reverse reaction2.2. Concentration of reactants and products is Concentration of reactants and products is
constantconstant ( (does not changedoes not change))NOTE:NOTE:RateRate = speed = speedConcentrationConcentration = number of particles or moles = number of particles or moles exampleexample: [HCl]: [HCl] = concentration of HCl = concentration of HCl
6M HCl = 6mole/L HCl 6M HCl = 6mole/L HCl = 6 moles of HCl per liter of solution = 6 moles of HCl per liter of solution
Summary 2Summary 2
What is equilibrium?What is equilibrium?
Reversible reaction: X Reversible reaction: X Y Y
[X]
[Y]
time
X Y
Y X
timera
te
conc
entr
atio
n
Reaction rates are equalConcentrations are constant
equilibrium
Summary 3Summary 3
Describe the part of each graph that Describe the part of each graph that illustrates equilibrium conditions.illustrates equilibrium conditions.
Le Chatelier’s PrincipleLe Chatelier’s Principle A system in equilibrium will react to relieve A system in equilibrium will react to relieve
stress (change) and re-establish equilibriumstress (change) and re-establish equilibrium
StressStress:: Adding reactants or productsAdding reactants or products Removing reactants or productsRemoving reactants or products Changing temperatureChanging temperature Changing pressure (for gases only)Changing pressure (for gases only)
9a: Le Chatelier’s Principle
Summary 4Summary 4
According to Le Chatelier’s Principle, what According to Le Chatelier’s Principle, what will happen to a system at equilibrium if will happen to a system at equilibrium if more reactants or products are added?more reactants or products are added?
Stress relief:Stress relief: Which way does equilibrium shift?Which way does equilibrium shift? What happens to [HWhat happens to [H22]?]?
What happens to [NHWhat happens to [NH33]?]? What happens to heat?What happens to heat?
Adding products or reactantsAdding products or reactants Equilibrium shifts to Equilibrium shifts to removeremove addition addition
Removing products or reactantsRemoving products or reactants Equilibrium shifts to Equilibrium shifts to replacereplace what has been what has been
removedremoved GassesGasses
Equilibrium shifts to produce:Equilibrium shifts to produce:• moremore gas at gas at lowlow pressure pressure• LessLess gas at gas at highhigh pressure pressure
Stress relief.
Summary 6Summary 6
Why would a gas equilibrium system Why would a gas equilibrium system produce more gas at low pressure and produce more gas at low pressure and less gas at high pressure? less gas at high pressure?
Summary 7Summary 7 According to Le Chatelier’s Principle:According to Le Chatelier’s Principle: Increasing reactant concentration will Increasing reactant concentration will
cause:cause: other reactants to __________?other reactants to __________? products to __________?products to __________?
Decreasing reactant concentration will Decreasing reactant concentration will cause:cause: other reactants to __________?other reactants to __________? products to __________?products to __________?
A(g) + B(g) AB(g) + heat
Summary 8Summary 8 Complete the table of Complete the table of
At equilibrium concentrations are constantAt equilibrium concentrations are constant KKeqeq represents concentrations of reactants represents concentrations of reactants
and products at equilibriumand products at equilibrium Example: aA + bB Example: aA + bB cC + dD cC + dD KKeqeq = [C] = [C]cc[D][D]dd
[A][A]aa[B][B]bb
HONORS Standard 9c: equilibrium constant
Summary 9Summary 9
write Keq expression for the Haber-Bosch write Keq expression for the Haber-Bosch Process: NProcess: N2(g)2(g) + 3H + 3H2(g)2(g) 2NH 2NH3(g)3(g)
Concentrations calculated in mol/L (M)Concentrations calculated in mol/L (M) Only solutions(aq) & gases(g) are consideredOnly solutions(aq) & gases(g) are considered
No solids (s)No solids (s) No liquids (l)No liquids (l)
Example: 2HExample: 2H22OO(l)(l) 2H 2H2(g)2(g) + O + O2(g)2(g)