Chapter 18 Rates of Reaction

1
Chapter 18Rates of Reaction
2
Collision Theory
The speed of a chemical reactions can vary from
instantaneous (strike a match) to extremely slow
(coal) Speed is measured as a change in distance
in a given interval of time. Rate
distance/time Rate is a measure of the speed of
any change that occurs within an interval of
time. In chemistry, the rate of chemical change
(the reaction rate) is usually expressed as the
amount of reactant changing per unit time.
3
Collision Theory
According to the collision theory, atoms, ions,
and molecules can react to form products when
they collide with one another, provided that the
colliding particles have enough kinetic energy.
Particles lacking the necessary kinetic energy
to react, bounce apart unchanged when they
collide. To illustrate the collision theory, If
soft balls of clay are thrown together with great
force, they will stick tightly together.
(analogous to colliding particles of high energy
that react) Balls of clay thrown together gently,
dont stick to one another. (analogous to
colliding particles of low energy that fail to
react)
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Collision Theory
If you roll clay into a rope and begin to shake
one end more and more vigorously, eventually it
will break. If enough energy is applied to a
molecule, the bonds holding it together can
break. The minimum energy that colliding
particles must have in order to react is called
the activation energy. When two reactant
particles with the necessary activation energy
collide, a new entity called the activated
complex may form. An activated complex is an
unstable arrangement of atoms that forms
momentarily at the peak of the activation energy
barrier.
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Activated Complex
6
Collision Theory
The lifetime of an activated complex is typically
about 10-13 s. The reactants either re-form or
the products form. Both cases are equally likely,
thus the activated complex is sometimes called
the transition state. High activation energies
explain the slow reaction of some natural
substances at room temperature. The collisions
are not great enough to break the bonds, thus the
reaction rate is essentially zero or very slow.
7
Factors Affecting Reaction Rates

• Every chemical reaction proceeds at its own rate.
  Some fast, some slow under the same conditions.
• By varying the conditions of a reaction, you can
  modify the rate of almost any reaction.
• The rate of a chemical reaction depends upon
• temperature
• concentration
• particle size
• the use of a catalyst.

8
Temperature
Usually, raising the temperature speeds up
reactions, while lowering the temperature slows
down reactions. At higher temperatures, the
motions of the reactant particles are faster and
more chaotic than they are at lower
temperatures. Increasing the temperature
increases both the frequency of collisions and
the number of particles that have enough KE to
slip over the activation energy barrier to become
products. An increase in temperature causes
products to form faster.
9
Concentration
The number of particles in a given volume affects
the rate at which reactions occur. Cramming more
particles into a fixed volume increases the
frequency of collisions. Increased collision
frequency leads to a higher reaction rate.
10
Particle Size
Surface area plays an important role in
determining the rate of reaction. The smaller
the particle size, the larger the surface area
for a given mass of particles. An increase in
surface area increases the amount of the reactant
exposed for reaction, which increases the
collision frequency and the reaction rate. One
way to increase the surface area of solid
reactants is to dissolve them. In solution,
particles are separated and more accessible to
other reactants. You can also increase the
surface area by grinding it into a fine powder.
11
Catalysts
Increasing the temperature is not always the best
way to speed up a reaction. A catalyst is often
better. A catalyst is a substance that increases
the rate of a reaction without being used up
during the reaction. Catalysts permit reactions
to proceed along a lower energy path. The
activation energy barrier for a catalyzed
reaction is lower than that of a uncatalyzed
reaction. With a lower activation energy barrier,
more reactants have the energy to form products
within a given time. Because a catalyst is not
consumed during a reaction, it does not appear as
a reactant or product in the chemical equation.
12
Catalysts
Enzymes are biological catalysts that increase
the rates of biological reactions. For example,
without catalysts, digesting protein would take
years. An inhibitor is a substance that
interferes with the action of a catalyst. The
inhibitor reduces the amount of functional
catalyst available. Reactions slow or even stop
when a catalyst is poisoned by an inhibitor.
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End of Section 18.1
14
Reversible Reactions
A reversible reaction is one in which the
conversion of reactants to products and the
conversion of products to reactants occur
simultaneously. 2SO2 (g) O2 (g)
2SO3 (g) The double arrow tells you that this
reaction is reversible. 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 amount of SO3 in
the equilibrium mixture is the maximum amount
that can be produced by this reaction under the
conditions of the reaction.
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Reversible Reactions
Chemical equilibrium is a dynamic state. Both the
forward and reverse reactions continue, but
because their rates are equal, no net change
occurs in their concentrations. Even though the
rates are equal at equilibrium, the
concentrations of the components on both side of
the equation are not necessarily the same. The
relative concentrations of the reactants and
products at equilibrium constitute the
equilibrium position of a reaction. The
equilibrium position indicates whether the
reactants or products are favored.
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Reversible Reactions
A B Product is favored. Equilibrium
mixture contains more product than reactant. A
B Reactant is favored. Equilibrium
mixture contains more reactant than product. In
principle, almost all reactions are reversible to
some extent under the right conditions. In
practice, one set of components is often so
favored that the other set cannot be detected. If
one set of components (reactants) is completely
converted to new substance (products), you can
say that the reaction has gone to completion, or
is irreversible.
17
Reversible Reactions
You can mix chemicals expecting to get a reaction
but no products can be detected, you can say that
there is no reaction. Reversible reactions occupy
a middle ground between the theoretical extremes
of irreversibility and no reaction. A catalyst
speeds up both the forward and the reverse
reactions equally. The catalyst lowers the
activation energy of the reaction by the same
amount in both the forward and reverse
directions. Catalysts do not affect the amounts
of reactants and products present, just the time
it takes to get to equilibrium.
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Factors Affecting Equilibrium
Changes of almost any kind can disrupt the
balance of equilibrium When the equilibrium of a
system is disturbed, the system makes adjustment
to restore equilibrium. The equilibrium position
of the restored equilibrium is different from the
original equilibrium position. The amount of
products and reactants may have increased or
decreased. This is called a shift in the
equilibrium system.
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Factors Affecting Equilibrium

• LeChateliers principle states that if a stress
  is applied to a system in equilibrium, the system
  changes in a way that reflects the stress.
• Stresses that upset the equilibrium include
• Changes to the concentration of reactants or
  products
• Changes to temperature
• Changes in pressure

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Change in Concentration
Change the amount of any reactants or product
disturbs the equilibrium. The system adjusts to
minimize the effects of the change. add
CO2 direction of shift H2CO3 (aq)
CO2 (aq) H2O
remove CO2 direction of shift The amount of
products and reactants may have increased or
decreased. This is called a shift in the
equilibrium system.
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Change in Concentration

• Adding a product to a reaction at equilibrium
  pushes a reversible reaction in the direction of
  reactants.
• Removing a product always pushes a reversible
  reaction in the direction of products.
• Farmers use this to increase yield of eggs
• If product are continually removed, the reaction
  will shift equilibrium to produce more product
  until the reactants are all used up. (will never
  reach equilibrium)

22
Changes in Temperature

• Increasing the temperature causes the equilibrium
  position of a reaction to shift in the direction
  that absorbs heat.
• The heat absorption reduces the applied
  temperature stress.
• add heat
• direction
  of shift
• 2SO2 (g) O2 (g)
  2SO3 (g) heat
• remove
  heat
• direction
  of shift
• Heat can be considered a product, just like SO3.
• Cooling, pulls equilibrium to right, and product
  yield increases. Heating pushed equilibrium to
  left and product yield decreases.

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Changes in Pressure

• A change in pressure affects only gaseous
  equilibria that have an unequal number of moles
  of reactants and products.
• add
  Pressure
• direction
  of shift
• N2 (g) 3H2 (g) 2NH3
  (g)
• reduce pressure
• direction
  of shift
• When pressure is increased for gases at
  equilibrium, the pressure momentarily increases
  because the same number of molecules is contained
  in a smaller volume.
• System immediately relieves some of the pressure
  by reducing the number of gas molecules.

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Change in Concentration

• N2 (g) 3H2 (g) ?
  2NH3 (g)
• Removing a product always pushes a reversible
  reaction in the direction of products.
• Farmers use this to increase yield of eggs
• If product are continually removed, the reaction
  will shift equilibrium to produce more product
  until the reactants are all used up. (will never
  reach equilibrium)

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Le Châteliers Principle
If an external stress is applied to a system at
equilibrium, the system adjusts in such a way
that the stress is partially offset as the system
reaches a new equilibrium position.

• Changes in Concentration

14.5
26
Le Châteliers Principle

• Changes in Concentration continued

Change
Shifts the Equilibrium
Increase concentration of product(s)
left
Decrease concentration of product(s)
right
Increase concentration of reactant(s)
right
Decrease concentration of reactant(s)
left
14.5
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Le Châteliers Principle

• Changes in Volume and Pressure

Change
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas
14.5
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End of Section 18.2
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The Concept of Equilibrium
Equilibrium is a state in which there are no
observable changes as time goes by.

• Chemical equilibrium is achieved when
• the rates of the forward and reverse reactions
  are equal
• the concentrations of the reactants and products
  remain constant

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The Concept of Equilibrium

• As the reaction progresses
• A decreases to a constant,
• B increases from zero to a constant.
• When A and B are constant, equilibrium is
  achieved.

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The Equilibrium Constant

• No matter the starting composition of reactants
  and products, the same ratio of concentrations is
  achieved at equilibrium.
• For a general reaction
• the equilibrium constant expression is
• where Keq is the equilibrium constant. The
  square brackets indicate the concentrations of
  the species.

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The Equilibrium Constant Expression

• For the general reaction
• aA bB ? gG hH
• The equilibrium expression is

Each concentration is simply raised to the power
of its coefficient
Products in numerator.
Reactants in denominator.
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4.63 x 10-3
Law of Mass Action
Equilibrium Will
K gtgt 1
Lie to the right
Favor products
K ltlt 1
Lie to the left
Favor reactants
14.1
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Le Châteliers Principle

• Adding a Catalyst
• does not change K
• does not shift the position of an equilibrium
  system
• system will reach equilibrium sooner

Catalyst lowers Ea for both forward and reverse
reactions.
Catalyst does not change equilibrium constant or
shift equilibrium.
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Le Châteliers Principle
Change
Shift Equilibrium
no
Concentration
yes
Pressure
yes
no
Volume
yes
no
Temperature
yes
yes
Catalyst
no
no
14.5
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Write the equilibrium expression for Keq for the
following reactions
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Calculation of the Equilibrium Constant
At 454 K, the following reaction takes place 3
Al2Cl6(g) 2 Al3Cl9(g) At this temperature,
the equilibrium concentration of Al2Cl6(g) is
1.00 M and the equilibrium concentration of
Al3Cl9(g) is 1.02 x 10-2 M. Compute the
equilibrium constant at 454 K.
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Ionic Compounds Solubility Rules
39
Ksp Solubility Product Constant

• Ksp is the equilibrium constant between an ionic
  solute and its ions in a saturated solution.
• A very small Ksp indicates that only a small
  amount of solid will dissolve in water.
• Ksp is equal to the product of the concentration
  of the ions in the equilibrium, each raised to
  the power of its coefficient in the equation.
• The smaller Ksp the lower the solubility of the
  compound.

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Solubility Products

• Ksp is not the same as solubility.
• Solubility is generally expressed as the mass of
  solute dissolved in 1 L (g/L) or 100 mL (g/mL) of
  solution, or in mol/L (M).

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The Solubility Product Constant Ksp
Example AgBr(s) ? Ag(aq) Br-(aq)
Ksp AgBr-
Example Ca(OH)2(s) ? Ca2(aq) 2OH-(aq)
Ksp Ca2OH-2
Example Ag2CrO4(s) ? 2Ag(aq) CrO42-(aq)
Ksp Ag2CrO42 -
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• What is the concentration of lead ions and
  chromate ions in a saturated lead chromate
  solution at 25ºC (Ksp 1.8 x 10-14)
• PbCrO4 (s) ? Pb2 (aq) CrO42- (aq)
• Ksp Pb2 x CrO42- 1.8 x 10-14
• At equilibrium Pb2 CrO42-
• Ksp Pb2 x Pb2 1.8 x 10-14
• Pb22 1.8 x 10-14
• Pb2 CrO42- 1.8 x 10-14
• Pb2 CrO42- 1.3 x 10-7 M

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Ksp and Solubility

• 1. NaCl has a solubility of 35.7 g/100 mL. What
  is the molar solubility and Ksp? (Ans 6.10 M,
  37.2)
• 2. CaCl2 has a solubility of 74.5 g/100 mL. What
  is the molar solubility and Ksp? (Ans 6.71 M,
  1.21 X 103)

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Ksp and Solubility

• 1. A saturated soln of AgCl is found to have an
  eq. concentration of Ag 1.35 X 10-5 M.
  Calculate Ksp. (Ans 1.82 X 10-10.)
• A saturated soln of MgF2 is prepared. At eq.
  the concentration of Mg2 is measured to be
  0.0012 M. Calculate Ksp.
• (Ans 7.0 X 10-9)

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Explaining the Common Ion Effect

• The presence of a common ion in a solution will
  lower the solubility of a salt.
• LeChateliers Principle
• The addition of the common ion will shift the
  solubility equilibrium backwards. This means
  that there is more solid salt in the solution and
  therefore the solubility is lower
• CaF2(s) ? Ca2(aq) 2F-(aq)
• a) Add Ca2 (shifts to reactants)
• b) Add F- (shifts to reactants)

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• The Ksp of silver bromide is 5.0 x 10-13. What is
  the bromide ion concentration of a 1.00L
  saturated solution of AgBr to which 0.020 mol of
  AgNO2 is added?
• AgBr (s) ? Ag (aq) Br- (aq)
• Ksp Ag x Br- 5.0 x 10-13
• Ag x X 5.0 x 10-13
• X 5.0 x 10-13 / Ag
• X 5.0 x 10-13 / 0.020
• X 2.5 x 10-11

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• What is the concentration of sulfide ion in a 1.0
  L solution of iron (II) sulfide to which 0.04 mol
  of iron (II) nitrate is added. The Ksp of FeS is
  8 x 10-19
• FeS (s) ? Fe2 (aq) S2- (aq)
• Ksp Fe2 x Sr2- 8 x 10-19
• Fe2 x X 8 x 10-19
• X 8 x 10-19 / Fe2
• X 8 x 10-19 / 0.04
• X 2.0 x 10-17

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Will a ppt form?

• Q Reaction Quotient product of a
  concentration of the ions
• Q lt Ksp Shifts to prod (no ppt)
• Q Ksp Eq. (ppt)
• Q gt Ksp Shifts to reac.(ppt)

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Will a ppt form?

• Will a ppt form if a solution is made from 0.50L
  of 0.002M Ba(NO3)2 and mix it with 0.50L of
  0.008M Na2SO4? Ksp of BaSO4 is 1.1 x 10-10
• Q Ba(NO3)2 x Na2SO4 (0.002/2)(0.008/2)
• Q 4 x 10-6
• A precipitate will form because 4 x 10-6 is
  larger than 1.1 x 10-10

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End of Section 18.3
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End of Chapter 17