Rate of Reaction and Chemical Equilibrium

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Rate of Reaction and Chemical Equilibrium
2
Collision Theory

• Molecules must collide to react
• Effective collisions lead to products being
  formed
• Ineffective collisions do not form products
• Reactants will form products if they collide with
  adequate kinetic energy
• Reactants will bounce apart if they do not have
  adequate kinetic energy

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A) Two BrNO molecules approach each other at
high speedB) The collision occurs. C) The
energy of the collision causes Br-N bonds to
break and Br-Br bonds to form.D) The products
one Br2 and two NO molecules.
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Reaction progress of two BrNO molecules.

• Activation Energy
• The minimum amount of energy particles must have
  in order to react
• Activated complex
• Reactants that have attained the activation
  energy level.
• Unstable with short lifetime 10-13s
• Sometimes called transition state

Activation Energy
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Factors Affecting Reaction Rate

• Temperature
• Concentration
• Particle Size
• Catalyst

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Comparison of the activation energies for an
uncatalyzed reaction and for the same reaction
with a catalyst present.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions
The relationship between the rate of a reaction
and the concentration of one reactant is
determined experimentally. A series of
experiments reveals how the concentration of each
reactant affects the reaction rate. An equation
that relates reaction rate and concentrations of
reactants is called the rate law for the
reaction.
It is applicable for a specific reaction at a
given temperature.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued Using the Rate
Law
The general form for the rate law is given by the
following equation where A and B are reactants R
kAnBm The rate law is applicable for a
specific reaction at a given set of conditions
and must be determined from experimental
data. The power to which a reactant
concentration is raised is called the order in
that reactant. The value of n is said to be the
order of the reaction with respect to A, so the
reaction is said to be nth order in A.
The value of k usually increases as the
temperature increases, but the relationship
between reaction rate and concentration almost
always remains unchanged.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued Using the Rate
Law, continued
An order of one for a reactant means that the
reaction rate is directly proportional to the
concentration of that reactant. An order of two
means that the reaction rate is directly
proportional to the square of the reactant. An
order of zero means that the rate does not depend
on the concentration of the reactant, as long as
some of the reactant is present.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued Using the Rate
Law, continued
The sum of all of the reactant orders is called
the order of the reaction, or overall order.
NO2(g) CO(g)
NO(g) CO2(g)

• R kNO22
• second order in NO2
• zero order in CO
• second order overall
• The orders in the rate law may or may not match
  the coefficients in the balanced equation.

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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued Specific Rate
Constant
The specific rate constant (k) is the
proportionality constant relating the rate of the
reaction to the concentrations of
reactants. 1. Once the reaction orders (powers)
are known, the value of k must be determined
from experimental data. 2. The value of k is
for a specific reaction k has a different value
for other reactions, even at the same conditions.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued Specific Rate
Constant, continued
3. The units of k depend on the overall order of
the reaction. 4. The value of k does not change
for different concentrations of reactants or
products. So, the value of k for a reaction
remains the same throughout the reaction and does
not change with time.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued Specific Rate
Constant, continued
5. The value of k is for the reaction at a
specific temperature if we increase the
temperature of the reaction, the value of k
increases. 6. The value of k changes (becomes
larger) if a catalyst is present.
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Section 2 Reaction Rate
Chapter 17

• Sample Problem B
• Three experiments that have identical conditions
  were performed to measure the initial rate of the
  reaction
• 2HI(g) H2(g) I2(g)
• The results for the three experiments in which
  only the HI concentration was varied are as
  follows
• Write the rate law for the reaction. Find the
  value and units of the specific rate constant.

Experiment HI (M) Rate (M/s)
1 0.015 1.1 10-3
2 0.030 4.4 10-3
3 0.045 9.9 10-3
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued

• Sample Problem B Solution

The general rate law for this reaction is R
kHIn
Concentration ratio
rate ratio
rate law R kHI2
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued

• Sample Problem B Solution, continued

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Section 2 Reaction Rate
Chapter 17

• Sample Problem C
• Three experiments were performed to measure the
  initial rate of the reaction
• A B C
• Conditions were identical in the three
  experiments, except that the concentrations of
  reactants varied. The results are as follows
• Write the rate law for the reaction. Find the
  value and units
• of the specific rate constant.

Experiment A (M) B (M) Rate (M/s)
1 1.2 2.4 8.0 108
2 1.2 1.2 4.0 10-8
3 3.6 2.4 7.2 10-7
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued

• Sample Problem C Solution
• The general rate law for this reaction is R
  kAnBm.
• To find m, compare Experiments 1 and 2, which
  have the same A.

Concentration ratio
rate ratio
m is 1, and the reaction is first order in B
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued

• Sample Problem C Solution, continued
• To find n, compare Experiments 1 and 3, which
  have the same B.

Concentration ratio
rate ratio
n is 2, and the reaction is second order in
A The rate law is R kA2B.
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Section 2 Reaction Rate
Chapter 17
Rate Laws for Reactions, continued

• Sample Problem C Solution, continued

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Catalysts Inhibitors

• Catalysts increase the rate of a chemical
  reaction without being used up in the reaction
• Permit the reaction to occur with lower
  activation energy than normal
• Pt
• 2H2(g) O2(g) ? 2H2O(l)
• Your body makes extensive use of catalysts or
  enzymes to allow bodily functions to progress
  rapidly
• Inhibitors are substances that interfere with the
  action of a catalyst
• Poisons the catalyst

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The amount of the substance in the vapor state
becomes constant and reaches equilibrium How
would we reverse this process?
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Reversible Reactions

• 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.
• At chemical equilibrium, no net change occurs in
  the actual amounts of the components of the
  system.
• The relative concentrations of the reactants and
  products at equilibrium constitute the
  equilibrium position of a reaction.

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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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Changes in Concentrationof Reactants and Products
Initially SO2 Initially SO3 Present
O2 Present
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Le Chateliers Principle1884
Henri Le Chatelier

• When a system at chemical equilibrium is
  disturbed, the system will adjust itself to
  minimize the disturbance
• Excellent guide for estimating how a chemical
  reaction will shift in response to changes

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Factors Affecting Equilibrium Le Châteliers
Principle

• Concentration
• Rapid breathing during and after vigorous
  exercise helps reestablish the bodys correct
  CO2H2CO3 equilibrium, keeping the acid
  concentration in the blood within a safe range.

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Factors Affecting Equilibrium Le Châteliers
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.

Warm Cool
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Factors Affecting Equilibrium Le Châteliers
Principle

• Pressure
• Pressure affects a mixture of nitrogen, hydrogen,
  and ammonia at equilibrium

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Equilibrium Concentration

• aA bB ? cC dD
• Keq CcDd
• AaBb
• Keq gt 1 products favored at equilibrium
• Keq lt 1 reactants favored at equilibrium

Square brackets indicate concentration or
molarity
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The Solubility Product Constant

• Ksp, solubility product constant
• equals the product of the concentrations of the
  ions, each raised to a power equal to the
  coefficient of the ion in the dissociation
  equation.
• The smaller the numerical value of the solubility
  product constant, the lower the solubility of the
  compound.
• AgCl (s) ? Ag (aq) Cl- (aq)
• Ksp Ag1 Cl-1

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The Solubility Product Constant
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The Solubility Product Constant
18.3
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System initially at equilibrium.CaCO3 ? CO2 CaO
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a) A mixture of NH3(g), N2(g), and H2(g) at
equilibrium.b) The volume is suddenly
decreased.c) The new equilibrium position.
N2 3H2 lt--gt 2NH3
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Le Chateliers Principle and Temperature

• Exothermic Reaction
• H2 I2 ? 2HI heat
• Heating drives reaction to the left
• Cooling drives reaction to the right
• Endothermic Reaction
• heat NH4Cl ? NH3 HCl
• Heating drives reaction to the right
• Cooling drives reaction to the left

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Rules for assigning oxidation states and
catalyzed reactions.
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Some Reaction are Reversible
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Initial equilibrium mixture(b) Addition of N2.
(c) New equilibrium position.
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(a) Equal numbers of moles of H2O and CO are
mixed in a closed container.(b) The reaction
begins to occur.(c) The reaction continues, and
more reactants are changed to products.(d) No
further changes are seen as time continues to
pass.