Chemical reactions, reaction rate, equilibrium

1. CHAPTER 7
REACTION RATES
AND
CHEMICAL EQUILIBRIUM
CHEM 103K Dunlap

2. Chemical kinetics:Chemical kinetics: the study of the rates of chemical
reactions. To determine the rate of this reaction, we
measure changes in the concentration of reagents or
products over time.
Blue dye is oxidized with bleach.Blue dye is oxidized with bleach.
Its concentration decreases with time.Its concentration decreases with time.
The rate can be determined from the plot.The rate can be determined from the plot.
Blue dye is oxidized with bleach.Blue dye is oxidized with bleach.
Its concentration decreases with time.Its concentration decreases with time.
The rate can be determined from the plot.The rate can be determined from the plot.

3. A B
At the beginning most
reactions are linear

4. What is the rate of reaction if the
concentration of the product increases from
0.5 M to 1.5 M in 5 minutes?
1. -0.3 M/min
2. -0.2 M/min
3. 0.2 M/min
4. 0.3 M/min

5. The rates of chemical reactions are affected by:
a) molecular collisions
b) activation energy
c) nature of the reactants
d) concentration of the reactants
e) temperature
f) presence of a catalyst

6. a) Molecular collisions:a) Molecular collisions: In order for two species to react they must
first collide. The rate of collisions is far greater than the rate at which
they react. Most collisions do not result in a reaction.
Effective collisions require enough energy (to pass activation energy)
and the right orientation during collision.
PRODUCTSREAGENTS

7. b) Activation energy:b) Activation energy: minimum energy required for a reaction to take
place. In most chemical reactions, one or more covalent bonds must be
broken and energy is required for this to happen. If the collision energy is
large, there is sufficient energy to break the necessary bonds, and
reaction takes place.
Transition stateReactants Products

8. Exothermic vs endothermic energy diagrams
Energy of reaction
(exothermic)
Energy of products
Breaking a bond requires energy, forming a bond releases energy.
If the energy released in making the new bonds is greater than the
energy required to break the original bonds the reaction is
exothermic. The opposite applies to endothermic reactions.
The lower the activation reaction the faster the reaction and the
higher the activation reaction the slower the reaction.

9. 1. endothermic; the activation
energy is greater than the
energy of reactants
2. exothermic; the activation
energy is greater than the
energy of reactants
3. endothermic; the energy of the
products is greater than the
energy of the reactants
4. exothermic; the energy of the
reactants is greater than the
energy of the products
This chemical reaction is best classified as _______
because _______.

10. Br2(g) + H2(g) 2HBr(g)
The overall rate of the reaction in a several step reaction is controlled
by the slowest of the three steps, just as the slowest-moving car
controls the flow of traffic on a street.
Highest activation energy

11. c) Nature of reactants:c) Nature of reactants: In general, reaction between
ions in aqueous solution are very fast (activation energies
are very low). In general, reaction between covalent
compounds, whether in water or another solvent, are
slower (their activation energies are higher).

12. d) Concentration:d) Concentration: In most cases, reaction rate increases when the
concentration of either or both reactants increases. For many
reactions, there is a direct relationship between concentration and
reaction rate; when concentration doubles the rate doubles.
Mg(s) + 2 HCl(aq) ---> MgClMg(s) + 2 HCl(aq) ---> MgCl22(aq) + H(aq) + H22(g)(g)

13. e) Temperature:e) Temperature: In general, reaction rate increases as temperature
increases.
Thumb rule : A 10°C increase in temperature doubles the reaction rate.
When temperature increases, molecules move faster, collide more
frequently and more frequent collisions mean higher reaction rates.
Not only do molecules move
faster at higher temperatures,
but the fraction of molecules
with energy equal to or greater
than the activation energy also
increases.

14. The reaction rate will increase by a factor of _____
if the temperature of the reactants is increased by
20°C.
1. 2
2. 4
3. 6
4. 8

15. f) Catalyst:f) Catalyst: a substance that increases the rate of a chemical
reaction without itself being used up.
Uncatalyzed reactionUncatalyzed reaction
Catalyzed reactionCatalyzed reaction
MnOMnO22 catalyzes decomposition of Hcatalyzes decomposition of H22OO22
2 H2 H22OO22 ---> 2 H---> 2 H22O + OO + O22

16. Many catalysts provide a surface on which reactants can meet.
When they are mixed and shaken with a finely divided transition metal
catalyst, such as Pd, Pt, or Ni, the reaction takes place readily at room
temperature.
In auto exhaust systems — Pt, NiOIn auto exhaust systems — Pt, NiO
2 CO + O2 CO + O22 ---> 2 CO---> 2 CO22
2 NO ---> N2 NO ---> N22 + O+ O22

17. Reversible Reactions

20. Equilibrium:Equilibrium: dynamic state in which the rate of the forward reaction
is equal to the rate of the reverse reaction.
At equilibrium there is no change in concentration of either reactants
or products. Reaction, however, is still taking place.
Equilibrium constant, K:Equilibrium constant, K: How to find it
Concentration of C
Coefficient in the balanced reaction
Note: Knowing the equilibrium constant allows us to predict the concentration of the reagents
and products at equilibrium knowing their original concentrations

21. Write the equilibrium constant expression for this reversible reaction:
Note: By convention the exponent “1” is understood but not written. K has no units because molarities
cancel.
When H2 and I2 react at 427°C, the following equilibrium is reached:
The equilibrium concentrations are [I2] = 0.42 mol/L, [H2] = 0.025 mol/L,
and [HI] = 0.76 mol/L. Using these values, calculate the value of K.

22. What is the equilibrium constant
expression for the following chemical
equation?
1.
2.
3.
4.
2 NH3 N2 (g) + 3 H2 (g)

23. There is no relationship between a reaction rate and the value of K.
Reaction rate depends on the activation energy.
It is possible to have a large K and a slow rate at which equilibrium is reached.
It is possible to have a small K and a fast rate at which equilibrium is reached.
It is possible to have any combination of K and rate in between these two
extremes.
There is a relationship between K and the direction of the reaction.
K values > 1 indicate that the formation of products is favored
K values < 1 indicate that the reverse reaction (formation of reagents) is favored

25. What is the typical relationship between the
rate of reaction and the value of the
equilibrium constant?
1. It is a direct proportionality.
2. It is an exponential proportionality.
3. It is an inverse proportionality.
4. It is random; that is, there is no
relationship between these factors.

26. LeChatelier’s Principle
When a stress is applied to a chemical system at equilibrium,
the position of the equilibrium shifts in the direction to relieve
the applied stress.
Types of stress that can be applied to a chemical equilibrium:
a) addition of a reaction component
b) removal of a reaction component
c) change in temperature
d) change in pressure

27. Addition and removal of reaction components
Reagents
Amount of water in
this compartment
represents the
reagents
Products
Amount of water in this
compartment
represents the
products
Add water
Remove water
Add water
Remove water

28. a)a) Addition of a reaction componentAddition of a reaction component shifts the position of
equilibrium to the side that reduces the component that has been
added.
The rate of the forward reaction increases and the concentrations of ethyl acetate and
water increase. As this happens, the rate of the reverse reaction also increases.
In time, the two rates will again become equal and a new equilibrium will be established.
– At the new equilibrium, the concentrations of reactants and products again become constant, but with
different values than before the addition of acetic acid.
– The concentrations of products are now higher, and the concentration of ethanol is lower.
– The concentration of acetic acid is also higher, but not as high as it was after we added acetic acid
The system has relieved the stress by increasing the components on the other side of the equilibrium.

29. b) Removal of a reaction componentb) Removal of a reaction component shifts the position of
equilibrium to the side that produces more of the component that has
been removed.
If ethyl acetate is removed, the position of equilibrium shifts to the right to produce more ethyl acetate
and restore equilibrium.
The effect of removing a component is the opposite of adding one.

30. 1. to the left
2. to the right
3. no shift
4. Not enough information is given.
If more NH3 is added to the following system at equilibrium,
how will the equilibrium shift?
2 NH3 N2 (g) + 3 H2 (g)

31. c) Change in pressure:c) Change in pressure: Pressure affects equilibrium only if one or
more component in the reaction mixture are gases.
An increase in pressure shifts the reaction toward the side with fewer
moles of gas.
A decrease in pressure shifts the reaction toward the side with more
moles of gas.
1 mol of gas 2 mol of gas

32. Acid rain (H2SO4(aq))attacks marble (calcium carbonate), the following
equilibrium can be written:
How does the fact that CO2 is a gas
influence the equilibrium?

33. 1. to the left
2. to the right
3. no shift
4. Not enough information is given.
If the pressure is increased on the following system
at equilibrium, how will the equilibrium shift?
2 NH3 (g) N2 (g) + 3 H2 (g)

34. d) Change in temperatured) Change in temperature The effect of a change in temperature
on a reversible reaction depends on whether the forward reaction is
exothermic or endothermic.
Reaction type
exothermic
endothermic
Change in
temperature
increase
decrease
increase
decrease
D irection equilibrium
is driven
to the left; toward reactants
to the right; toward products
to the right; toward products
to the left; toward reactants

35. Hot water 50° C Cold water 0° C

36. Effect of a catalyst in a reversible reactionEffect of a catalyst in a reversible reaction
In a reversible reaction a catalyst always increases the rates of both
the forward and reverse reactions to the same extent. However,
addition of the catalyst before the equilibrium is reached speeds up
reaching it.

37. 1. to the left
2. to the right
3. no shift
4. Not enough
information is given.
If a catalyst is added to the following system at
equilibrium, how will the equilibrium shift?
2 NH3 N2 (g) + 3 H2 (g)