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Chemical Kinetics and Equilibrium

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Chemical Kinetics and Equilibrium Reaction Rates How fast or slow the reaction occurs Collision Theory 2 conditions must be satisfied for a chemical reaction to occur ... – PowerPoint PPT presentation

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Title: Chemical Kinetics and Equilibrium


1
Chemical Kinetics and Equilibrium
2
Reaction Rates
  • How fast or slow the reaction occurs

3
Collision Theory
  • 2 conditions must be satisfied for a chemical
    reaction to occur
  • Particles of reactants must collide with one
    another and
  • Colliding particles must have sufficient energy

4
Activation Energy
  • Minimum amount of energy needed for a reaction to
    occur
  • Fgt Ea
  • Ea

5
  • F lt Ea
  • Ea

6
Energy Diagram for Exothermic reaction
  • increasing Average energy
  • energy of reactants
  • Energy
    difference bet. Average energy
  • reactants and
    products of products

7
Energy Diagram for Endothermic reaction
  • increasing
  • energy

  • energy of the products
  • energy of
  • reactants
    energy difference

8
Factors Affecting Rate of reaction
  • Surface Area
  • Concentration of reactants
  • Temperature
  • Presence of Catalyst

9
Surface area of reactants
  • Consider the block of wood or wood shavings
  • The smaller the particle size of wood the greater
    the surface area exposed to oxygen thus the rate
    of reaction increases.

10
Concentration of Reactants
  • An increase in concentration of the reactants
    causes an increase in the rate of chemical
    reaction

11
Temperature
  • Higher temperature increases reaction rates
  • Ex ripe fruits are placed in the refrigerator to
    slow down the ripening process.

12
Presence of Catalyst
  • Catalyst
  • Substance that speeds up chemical reaction
    without itself undergoing a chemical change
  • Inhibitor
  • ---gtsubstances that slow down reaction

13
Activation energy of uncatalized reaction
  • catalyzed activation energy
  • Uncatalyzed
  • activation energy
    Note Catalysts increases

  • reaction rates by

  • providing alternative

  • reaction pathways

  • with lower activation

  • energies

14
Lower activation energy
  • Means that a lower amount of energy is needed to
    surpass the energy barrier.

15
Two types of catalyst
  • Homogeneous
  • ? Exists in the same phase as the reactant
  • Heterogeneous
  • Exist as a separate phase from the reactant in a
    reaction mixture
  • Most are solids

16
Heterogeneous and Homogeneous Reactions
  • Homogeneous Reaction
  • ? Reaction involving only one phase
  • Heterogeneous reaction
  • ? Reaction involving reactants in two phases

17
Chemical Equibrium
  • It is always assumed that chemical reaction go to
    completion
  • However, only few chemical reactions proceed to
    completion
  • Reversible reaction
  • N2O4(g) 2NO

18
Do Now
  • Page 547
  • Focus Questions
  • 1 and 2 for 10 pts

19
Reversible Reaction
  • A chemical reaction in which the products can
    regenerate the original reactants
  • N2O4(g) 2NO2
  • colorless brown
  • N2O4(g) -------gt 2NO2(g) ( forward Reaction)
  • 2NO2 (g)-------gt N2O4(g) (backward Reaction)

20
Chemical Equilibrium
  • The rate of forward reaction and backward
    reactions are equal
  • Is dynamic
  • ? The reactants constantly forms the products
    while the products constantly forms the reactants
  • Note
  • In equilibrium, concentration of reactants and
    products do not change

21
Ex dynamic Equilibrium
  • Carbonated Drinks
  • CO2 H20 ---gt H2CO3
  • carbonic acid
  • When the bottle is opened the pressure is
    released, carbon dioxide evolved from the
    decomposition of carbonic acid
  • H2CO3 ------gt CO2 H2O
  • CO2 H20 H2CO3

22
Equilibrium Constants(K)
  • Can be obtained in any equilibrium reaction
  • Equal to the Ratio of the equilibrium
    concentration of the products to the equilibrium
    concentration of the reactants
  • aA bB -----gtcC dD
  • Where
  • a,b,c d -- are the coefficients
  • A,B,C D are the chemical species

23
Equilibrium Constant(K)
  • aA bB -----gtcC dD
  • K Cc Dd products
  • Aa Bb reactants
  • --gt Experimentally at equilibrium, Concentration
    of the products raised to a certain power divided
    the concentration of the reactants also raised to
    a certain power is constant at( fixed
    temperature)
  • Is also called equilibrium expression

24
Note
  • K products
  • reactants
  • is the concentration of products over that of the
    reactants
  • If the K value is small, it means that the
    equilibrium concentration of the products is
    small while the reactants are large. This
    indicates that at equilibrium, the system consist
    mostly of reactants
  • If the K value is great , it means that the
    equilibrium system consist mostly of the products

25
Writing equilibrium expression
  • If a pure liquid or solid is involved in a
    reaction, its concentration is omitted in the
    equilibrium expression because it has constant
    concentration in mol/L at constant Temperature

26
Ex
  • Write the equilibrium expression for the
    reaction below
  • H2(g) F2(g) 2HF(g)
  • K HF2
  • H2 F2

27
Ex2
  • Write the equilibrium expression for the
    reaction below
  • N2(g) 3H2(g) 2NH3(g)
  • K NH32
  • N2H23

28
Ex3
  • Write the equilibrium expression for the
    reaction below
  • CaCO3(s) CaO(s) CO2(g)
  • K CO2

29
Ex3
  • Write the equilibrium expression for the
    reaction below
  • 2Hg(l) O2(g) 2HgO(s)
  • K 1 since solid and liquid is omitted
  • O2

30
Ex3
  • Write the expression of the following
  • Reactions
  • 2H2S(g) 3O2(g) 2H2O(g) 2SO2(g)
  • CO2(g) H2(g) CO(g) H2O(l)
  • FeO(s) H2(g) Fe(s)
    H2O(g)

31
Exercises
  • Write the expression of the following
  • Reactions
  • 2H2S(g) 3O2(g) 2H2O(g) 2SO2(g)
  • CO2(g) H2(g) ---gt CO(g) H2O(l)
  • FeO(s) H2(g) ---gt

32
Calculating Equilibrium Constant (K)
  • Ex
  • For the reaction 2NBr(g) N2(g) 3Br2(g)
  • The system at equilibrium at a particular
    temperature is analyzed and the following
    concentrations are found
  • NBr3 2.07 x 10-3M
  • N2 4.11x10-2M
  • Br2 1.06x 10-3M
  • Calculate the value of K for the reaction
  • Solution
  • K 4.11x10-2M1.06 x 10-3M3
  • 2.07x 10-3M2
  • K 1.14 x 10-5
  • Note the coefficients in the balanced equation
    are the exponents you see in the solution

33
What is Ksp?
  • Ksp is the sollubility product constant
  • It refers to a reaction in which a solid is
    dissolving in water
  • Ex AB(s)---gt A(aq) B-(aq)

34
How are Ksp and K different? Similar?
  • Solubility product constant(Ksp) is a specific
    type of chemical equilibrium (K ). It is used
    when the reactant is a solid and the products are
    ions. In both cases the rules for setting up the
    equation are the same and all terms have units of
    mol/L

35
Writing Solubility Product Expression
  • When CaF2 dissolves in water it dissociates and
    forms ions
  • CaF2(s) ------gt Ca2 F- unbalanced
  • CaF2(s) ------gt Ca2(aq) 2 F-(aq)
    balanced
  • In writing the Ksp expression in the above
    reaction, the solid is being omitted so
  • Ksp Ca2 F-2

36
Write the balanced equation when PbCl2 solid is
dissolving in water and write the Ksp expression
  • PbCl2(s) ----gt Pb2(aq) Cl-(aq)
  • PbCl2(s) ----gt Pb2(aq) 2Cl-(aq) balanced
  • Ksp Pb2 Cl-2

37
Exercises
  • Write the balanced equation for the reaction
    describing the dissolving of each of the
    following solids in water. Also write the Ksp
    expression for each solid
  • BaSO4(s)
  • Fe(OH)3(s)
  • Ag3PO4(s)

38
Le Chateliers Principle
  • States that if a change in conditions is imposed
    on a system at equilibrium, the equilibrium
    position will shift in the direction that tends
    to reduce the effects of that change
  • --gtThe effect of change in concentration
  • ? The effect of change in Volume
  • ?The effect of change in temperature

39
Effects of Change in Concentration on reaction
Equilibrium
  • Consider the equation below
  • N2 3H2 2NH3(g)
  • This equation shows equilibrium because of the
    two arrows of the same length that is rate of
    forward reaction is equal to the rate of backward
    reaction.
  • What will happen if you increase the
    concentration of N2?
  • ?increasing the concentration of the reactants
    means more collision between them that increases
    the rate of forward reaction as shown below
  • N2 3H2 2NH3(g)
  • Notice that the arrow pointing to the right has a
    greater length which indicates forward reaction
    increases meaning more production of NH3

40
What happens if you increase the concentration of
NH3 in the previous example?
  • N2 3H2 2NH3(g)
  • Increasing the concentration of NH3 will shift
    the reaction to the left.
  • Note
  • ?If a reactant or product is added to the system,
    the system shifts away from the added component
  • ?If a reactant or product is removed, the system
    shifts toward the removed component

41
Exercise
  • Suppose the reaction system
  • 2SO2(g) O2(g) 2SO3 (g)
  • Has reached equilibrium. Predict the effect of
    each of the following changes on the position of
    the equilibrium
  • SO2(g) is added to the system
  • The SO3(g) present is liquefied and removed from
    the system
  • Some of the O2 gas is removed from the system

42
The effect of Change in Volume and pressure
  • Recall
  • Boyles law Pressure is inversely
    proportional to Volume of gases
  • The smaller the volume, the higher the
    pressure between molecules
  • Avogadros Law V is directly
    proportional to the of moles at constant T
  • Consider the equation below suppose the gases
    below are mixed in a vessel
  • at equilibrium
  • 2 NOCl(g) 2NO(g)
    Cl2(g)
  • What will happen to the equilibrium position if
    we reduce the volume?
  • Reducing volume means increasing the pressure
    between the molecules of the gases, the system
    moves in the direction that lowers its pressure.
    In the equation above, the direction will shift
    to the left because the products has more
    molecules (3 molecules) than the reactant
    (2molecules).
  • In other words, the equilibrium position will
    shift toward the side of the reaction that
    involves the smaller number of gaseous molecules
    in the balanced equation
  • 2 NOCl(g)
    2NO(g) Cl2(g)

43
What happens when volume is increased in the
previous example?
  • 2 NOCl(g) 2NO(g) Cl2(g)
  • When the volume is increased, it lowers the
    pressure of the system, thus the direction will
    shift to the right to increase the total number
    of gaseous molecules present

44
Predict the shift in equilibrium of the reaction
below when volume is reduced
  • a) P4(s) 6Cl2(g) 4PCl3(l)
  • 6 gaseous molecules 0
    gaseous molecules
  • The direction will shift toward the right since
    it has 0 gaseous molecules
  • b) PCl3(g) Cl2(g)
    PCl5(g)
  • 2 gaseous molecules 1 gaseous
    molecule
  • The direction will shift to the right since it
    has lesser number of gaseous molecules
  • c) PCl3(g) 3NH3(g) P(NH2)3(g)
    3HCl(g)
  • 4 molecules 4
    molecules
  • Since the of gaseous molecules in both sides
    are equal, a change in volume will have no effect
    on the equilibrium position

45
Practice
  • For each of the following reaction predict the
    direction in which the equilibrium will shift
    when volume in the container is increased
  • a) 4NH3(g) 5O2(g) 4NO(g)
    6H2O(g)
  • b) FeO(s) H2(g) Fe(s)
    H2O(g)

46
The effect of change in Temperature
  • The change in concentration and volume in the
    system alter the equilibrium position but not the
    equilibrium constant(K)
  • The effect of temperature on equilibrium is
    different because the value of equilibrium
    constant (K) is change
  • If the reaction is exothermic that means it
    releases heat the direction will shift to the
    left since heat is is one of the products
  • If the reaction is endothermic, energy is one of
    the reactants so the direction will shift to the
    right

47
Ex
  • A) C(s) O2(g) CO2(g) heat
  • B) N2(g) O2(g) 2NO(g) endothermic
  • Answer
  • A. since heat is one of the products it is
    exothermic thus the direction of the shift is to
    the right
  • B. since it is endothermic, heat is one of the
    reactants so the direction of the shift is toward
    the left

48
Exercise
  • Predict the shift would each of the changes below
    have on the reaction
  • O2(g) 2CO(g) 2CO2(g) heat
  • a)Increasing CO
  • b)Increasing CO2
  • c) Increasing Temperature
  • d) Decreasing volume
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