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Chapter 18: Section 1
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Rate is a measure of speed of any change that occurs within an interval of time
In chemistry, the reaction rate (rate of a chemical change) = amount of reactant per unit time
o For example: 0.2 mol/1 month
Collision theory- atoms, molecules, and ions can react to form products when they collide with
one another ONLY IF they have enough kinetic energy
o If 2 particles collide and they do NOT have enough chemical energy, then they bounce
off each other and remain unchanged
Activation energy = the minimum amount of energy that colliding particles must have in order
to react
o activation energy = barrier
Activated complex = an unstable arrangement of atoms that forms momentarily at the peak of
the activation energy barrier
o When 2 reactants combine with the necessary activation energy, a new product is
formed, called the activated complex
o Only forms if there is enough activation energy
o Transition state = activated complex
 It’s the top of the curve on the chart on page 543
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Factors affecting Reaction Rates
 Temperature increases, reaction rate increases
-more particles  greater force in collisions  faster rate
 Concentration increases, reaction rate increases
-more particles  greater number of collisions  faster rate
 Small particles, reaction rate increases
 More surface area, reaction rate increases
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Catalysts increase reaction rates
o Catalysts speed up both the forward and reverse reactions equally because the reverse
reaction is exactly the opposite of the forward reaction
o The catalyst lowers the activation energy of the reaction
o Catalysts remain unchanged and don’t affect the amount of reactants at products, only
the rate that it takes them to achieve equilibrium
o Inhibitors interfere with catalysts
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Chapter 18: Section 2
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Reversible reaction- reaction in which conversion of reactants to products and products to
reactants occur simultaneously
o In principle all reactions are reversible
o Reactants go to products in the forward direction, and products go to reactants in the
reverse direction
Chemical equilibrium- state of balance in which forward and reverse reactions take place at the
same rate
o At chemical equilibrium, no net change occurs in the actual amounts of the components
of the system
o But chemical equilibrium is still a dynamic state and both the forward and reverse
reactions continue
Equilibrium position- relative concentrations of reactants and products of a reaction that has
reached equilibrium
o When rate of conversion of reactants = rate of conversion of products
o Even though the rates are equal, the concentrations don’t have to be the same
Le Chateliers Principle: when stress is applied to a system at equilibrium, the system changes to
relieve the stress
o This predicts the direction of change in the position of equilibrium
o Theses stresses include:
 Changes in concentration of reactants and products
-If you add something to the left, then the system shifts to the right
-Vice versa: If you add something to the right, then it shifts left
 Changes in temperature
-If you increase the heat on the left, then the system shifts right
-vice versa: if you add heat on the right, it shifts left
-If you decrease the heat on the left, then the system shifts left
-vice versa: if you lower heat on the right, it shifts right
 Changes in pressure
-If you increase pressure, it shifts in the direction that favors the
products
-if you decrease the pressure, it shifts in the direction that favors the
reactants
Equilibrium constant- It’s essentially the measure of the ratio of products to reactants at
equilibrium
o Keq
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o
[C]c X [D]d
[A]a X [B]b
If Keq > 1, products favored at equilibrium
If Keq < 1, reactants favored at equilibrium
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Chapter 18: Section 4
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Free energy- energy in a reaction that is available to do work
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Spontaneous reaction- reactions that favor formation of products under the specified
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conditions
o Reactions that occur as written are called spontaneous reactions
o All spontaneous reactions release free energy
o All spontaneous reactions produce lots of product
o Spontaneous reactions go on and on
Nonspontaneous reactions- reactions that don’t give products under the specified conditions
o Equations for the reactions may be written, but the reactions are nonspontaneous
o All nonspontaneous reactions require free energy
o All nonspontaneous reactions produce lots of reactant
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Entropy- measure of the disorder in the system
o Entropy increases if:
 Number of particles increases
 Shaking increases
 Temperature increases
 Change from a solid to liquid
 Change from a liquid to gas
 Things are divided into parts
 Things are mixed with dissolving
o Entropy increases if:
 Change from a liquid to solid
 Change from a gas to liquid
 Number of particles decreases
 Shaking decreases
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Law of disorder (2nd Law of Thermodynamics) – it is the natural tendency of systems to move in
the direction of maximum disorder
o All things tend to go to lower enthalpy and toward increased entropy
 Enthalpy = change in heat
 Entropy = disorder
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2 factors that affect spontaneity:
1) Entropy
2) Enthalpy
During equilibrium, low temperature favors enthalpy and a high temperature favors entropy
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ΔH
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+
–
+
ΔS
+
–
–
+
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ΔG
–
+
– (if low temperature)
– (if high temperature)
Spontaneous?
Yes
No
Yes (if low temperature)
Yes (if high temperature)
Gibbs free-energy change: maximum amount of energy that can be coupled to another process
to do work
ΔG = ΔH – TΔS
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ΔH = enthalpy
T = temperature in kelvins
ΔS = entropy
ΔG = gibbs free energy-change
Spontaneous reactions are exothermic = they release energy = ΔH is usually negative
Nonspontaneous reactions are endothermic = they absorb energy = ΔH is usually positive
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