18.1
Kinetics and Equilibrium review (Items 114-132 of 200 ways ..)
Kinetics deals with the rates of chemical
reactions. In chemistry, the rate of chemical
change, or the reaction rate, is usually
expressed as the amount of reactant
changing per unit time. Equilibrium refers to
the condition where forward and reverse
reactions are occurring at the same rates.
18.1
Rates of Reaction
>
Collision Theory
Rates of chemical reactions are often measured
as a change in the number of moles during an
interval of time.
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18.1
Rates of Reaction
>
Collision Theory
Effective Collision
According to collision theory, atoms, ions, and molecules
can react to form products when they collide with one
another, provided that the colliding particles have enough
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kinetic energy and have the proper orientation.
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18.1
Rates of Reaction
>
Collision Theory
Ineffective Collision
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18.1
The minimum energy that colliding particles must
have in order to react is called the activation
energy.
An activated complex
is an unstable
arrangement of atoms
that forms momentarily
at the peak of the
activation-energy
barrier.
The activated complex
is sometimes called the
transition state.
18.1
Factors affecting The rate of a chemical
reaction depends upon
Reaction Rates
What four factors
influence the rate
of a chemical
reaction?
• temperature,
• concentration,
• particle size,
• the use of a catalyst.
18.1
Rates of Reaction
>
Factors Affecting Reaction Rates
Temperature
Storing foods in a refrigerator keeps them fresh
longer. Low temperatures slow microbial action.
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18.1
Rates of Reaction
>
Factors Affecting Reaction Rates
Concentration
a. In air, a lighted splint glows and soon goes out.
b. When placed in pure oxygen (higher oxygen
concentration), the splint bursts into flame.
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18.1
Rates of Reaction
>
Factors Affecting Reaction Rates
Particle Size
The minute size of the reactant particles (grain
dust), and the mixture of the grain dust with oxygen
in the air caused the reaction to be explosive,
destroying the grain elevator.
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18.1
Rates of Reaction
>
Factors Affecting Reaction Rates
Catalysts
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A reversible reaction is one in which the conversion
of reactants to products and the conversion of products
to reactants occur simultaneously.
When the rates of the forward and reverse reactions are equal,
the reaction has reached a state of balance called chemical
equilibrium. The relative concentrations of the reactants and
products at equilibrium constitute the equilibrium position of a
reaction. At chemical equilibrium, no net change occurs in the
actual concentration of the reactants and products – i.e.
concentrations are constant.
(Note this does not mean that the concentration of reactants and products
are equal – avoid this common mistake).
Reversible Reactions
How do the amounts of
reactants and products
change in a chemical
system at equilibrium?
If the rate of the shoppers
going up the escalator is
equal to the rate of the
shoppers going down,
then the number of
shoppers on each floor
remains constant, and
there is an equilibrium.
At chemical equilibrium,
no net change occurs in
the actual amounts or
concentration of the
reactants and products.
Reversible Reactions
SO3
decomposes
to SO2 and O2
SO2 and O2
react to give
SO3
At equilibrium, all three types of molecules are present.
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Reversible Reactions
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18.2
The French chemist Le Châtelier proposed what has
come to be called Le Châtelier’s principle: If a stress
is applied to a system in dynamic equilibrium, the
system changes in a way that relieves the stress.
Three stresses can cause a change in the equilibrium
position of a chemical system?
•
changes in the concentration of reactants or
products,
•
changes in temperature,
•
changes in pressure (for equilibria involving gases).
for Conceptual Problem 18.1
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Equilibrium Constants
What does the value of the equilbrium constant, Keq , indicate
about the equilibrium position of a reaction?
The equilibrium constant (Keq) is the ratio of product
concentrations to 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.
aA + bB
cC + dD
←
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Equilibrium Constants
A value of Keq greater than 1 means that products
are favored over reactants; a value of Keq less than
1 means that reactants are favored over products.
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Kinetics and Equilibrium
Physical
Equilibria
Types of
Equilibria
Chemical
Equilibria
Phase
Changes
Saturated
Solutions
Reversible
Chemical
Reactions
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Physical Equilibria – Phases Equilibria
H2O (s)
H2O (l)
At 0 oC (273 K) there exists a phase equilibrium
between solid and liquid H2O.
At 100 oC (373 K) there exists a phase equilibrium
between liquid and gaseous H2O.
H2O (l)
H2O (g)
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for Sample Problem 17.4
How many joules of heat are required to melt a
10.0 grams popsicle at 0 oC. Assume that the
popsicle has the same heat of fusion as water.
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Heats of Vaporization and Condensation
The quantity of heat absorbed by a melting a
solid is exactly the same as the quantity of
heat released when the liquid freezes; that
is,
∆Hfusion = –∆Hsolidification
The quantity of heat absorbed by a vaporizing
liquid is exactly the same as the quantity of
heat released when the vapor condenses;
that is, ∆Hvaporization = –∆Hcondensation
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17.3
Heats of Vaporization
and Condensation
a. Enthalpy changes accompany changes in
state.
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17.3
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Heating (Endothermic) and Cooling (Exothermic)
Curves
Heating versus Cooling Curves
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Physical Equilibria – Saturated Solutions
Saturated solutions are another example involving
physical equilibrium. The term “saturated
solutions” refers to a solution containing the
maximum amount of solute that will dissolve at a
given temperature. The rate at which a
substance crystallizes out of solution is equal to
the rate at which it dissolves.
pHET simulation for saturated solutions
http://phet.colorado.edu/en/simulation/soluble-salts
Saturated Solution:
= contains the maximum amount of solute for
a given quantity of solvent at a constant
temperature & pressure
if additional solute is added, it will not
dissolve; rather it will precipitate out
Unsaturated Solution:
= a solution that contains less solute than a
saturated solution at a given temperature &
pressure
if additional solute is added, it will dissolve
Supersaturated Solution:
= contains more solute than it can
theoretically hold at a given temperature
crystallization will be initiate if a very small
“seed crystal” of solute is added
**very unstable**
Sample Individual Solubility
Curve- Potassium nitrite
Saturated Solution:
= contains the maximum amount of
solute for a given quantity of solvent at a
constant temperature & pressure
if additional solute is added, it will not
dissolve; rather it will precipitate out
Unsaturated Solution:
= a solution that contains less solute
than a saturated solution at a given
temperature & pressure
if additional solute is added, it will
dissolve
Supersaturated Solution:
= contains more solute than it can
theoretically hold at a given temperature
http://www.stevespanglerscience.com/expe
riment/00000078
crystallization will be initiate if a very
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small “seed crystal” of solute is added
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**very unstable**
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