18.1
Rates of Reaction
The heat given off by the
corrosion reaction of an ironmagnesium alloy with salt
water can produce a hot meal.
The rate of reaction is
increased by adding salt
water, so heat is produced
rapidly. You will learn some
ways in which the rate of a
reaction can be increased.
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18.1
Rates of Reaction
>
Collision Theory
Collision Theory
How is the rate of a chemical change
expressed?
In chemistry, the rate of chemical
change, or the reaction rate, is usually
expressed as the amount of reactant
changing per unit time.
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18.1
Rates of Reaction
>
Collision Theory
A rate is a measure of the speed of any change
that occurs within an interval of time.
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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.
Factors Affecting Reaction Rates
Explore animations to better understand the factors
that control the speed of a reaction.
Reversible reactions PhET:
http://phet.colorado.edu/en/simulation/reversible-reactions
Video regarding Haber process and effect of catalyst:
http://glencoe.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::600::450::/sites/dl/free/0023654666/339141/catalysisASSEMBLE
D.swf::Catalysis
Virtual lab on kinetics of iodine clock:
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/kin
etics2/iodine_clock.html
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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18.1
Rates of Reaction
>
Factors Affecting Reaction Rates
An inhibitor is a substance that interferes with
the action of a catalyst. Antioxidants and
antimicrobials used in drying fruits and
preserving fruit juices slow the action of microbes
and limit contact with air.
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18.1 Section Quiz.
Assess students’ understanding
of the concepts in Section 18.1.
Continue to:
-or-
Launch:
Section Quiz
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18.1 Section Quiz.
1. The units below that would be appropriate to
measure the rate of a chemical reaction is
a. mol/s.
b. mol/L.
c. kJ/mol.
d. h/mol.
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18.1 Section Quiz.
2. In a chemical reaction, the energy of reactants
is always
a. greater than the energy of the products.
b. more than the activation energy.
c. less than the activation energy.
d. less than the energy of the products.
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18.1 Section Quiz.
4. A catalyst works because it
a. lowers the activation energy.
b. increases the temperature.
c. is permanently changed in a reaction.
d. supplies energy to a reaction.
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Reversible Reactions and Equilibrium
18.2
In the early 1900s, German chemists refined the
process of making ammonia from elemental
nitrogen and hydrogen.This process allows the
manufacture of nitrogen fertilizers. You will learn
how reaction conditions can influence the yield
of a chemical reaction.
Activities:
•POGIL
and Lab on Equilibrium and Le Chatelier’s principle
•Animations
on collision theory and equilibria
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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).
Factors Affecting Equilibrium: Le Châtelier’s Principle 18.2
Concentration
Rapid breathing during and after vigorous
exercise helps reestablish the body’s correct
CO2:H2CO3 equilibrium, keeping the acid
concentration in the blood within a safe range.
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Factors Affecting Equilibrium: Le Châtelier’s Principle
Temperature
Dinitrogen tetroxide is a colorless gas; nitrogen
dioxide is a brown gas. The flask on the left is in
a dish of hot water; the flask on the right is in ice.
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Factors Affecting Equilibrium: Le Châtelier’s Principle
Pressure
Pressure affects a mixture of nitrogen,
hydrogen, and ammonia at equilibrium
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Factors Affecting Reaction Rates - Explore several
factors that control the speed of a reaction.
Reversible reactions PhET:
http://phet.colorado.edu/en/simulation/reversible-reactions
Video regarding Haber process and effect of
catalyst: http://glencoe.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::600::450::/sites/dl/free/0023654666/339141/catalysisA
SSEMBLED.swf::Catalysis
Virtual lab on kinetics of iodine clock:
http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/
kinetics2/iodine_clock.html
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Conceptual Problem
18.1
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Conceptual Problem
18.1
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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
(Missing
cC + dD
equilibrium arrows for this generalized example ; TO Draw)
←
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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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18.1
18.1
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for Sample Problem 18.2
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18.2 Section Quiz.
1. In a reaction at equilibrium, reactants and
products
a. decrease in concentration.
b. form at equal rates.
c. have equal concentrations.
d. have stopped reacting.
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18.2 Section Quiz.
2. In the reaction 2NO2(g)  2NO(g) + O2(g),
increasing the pressure on the reaction would
cause
a. the amount of NO to increase.
b. the amount of NO2 to increase.
c. nothing to happen.
d. the amount of O2 to increase.
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18.2 Section Quiz.
4. For the following reaction, Keq = 1.
A(g) + B(g)  C(g) + D(g)
Therefore, at equilibrium
a. [C] = [A].
b. [A][B] = 0.
c. [AB] = [CD] = 1.
d. [A][B] = [C][D].
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