2. Reversible Reactions
• Many reactions we have studied this year are “forward reactions”, once the
reactant has changed into products it stays that way.
• In reversible reactions, a reaction that can take place backwards or
forwards, once a product is formed it can turn back into a reactant and a
continuous cycle occurs.
• Reversible reactions are denoted by a double arrow ↔
• How can we predict when these reactions are “finished”– have reached
equilibrium?
4. Equilibrium
• Reversible reactions never come to an end: they just reach equilibrium.
• Equilibrium: the point where there is no further change in concentration of
any species in the reaction.
• Forward and reverse reactions continue to happen, however the moles of
any species in the reaction that are produced or used are equivalent, so
the overall quantities of all reactants and products remain constant.
Talk with a partner: When equilibrium is reached, how do the moles of
reactant lost and product gained in a short period of time compare?
5. Work with your table…
• Complete the data table.
• Respond to the 3 questions following the table.
7. Describing Reaction Rates
•A rate is a measure of how much something changes within a specified
amount of time.
•In chemistry, the rate of a chemical reaction, or the reaction rate, is
usually expressed as the change in the amount of reactant or product per
unit time.
8. Describing Reaction Rates
•The figure below illustrates the progress of a typical
reaction. Over time, the amount of reactant decreases
and the amount of product increases.
9. A model called collision theory is used to
relate the properties of particles to the
rates of chemical reactions.
Collision Theory
•According to collision theory, atoms, ions, and
molecules can react to form products when they
collide if the particles have enough kinetic energy.
•Particles that do not have enough energy to react
bounce apart unchanged when they collide.
10. Describing Reaction Rates
An ineffective collision of oxygen and
hydrogen molecules produces no
reaction; the reactants bounce apart
unchanged.
An effective collision of
oxygen and hydrogen
molecules produces water
molecules.
11. Describing Reaction Rates
•The minimum energy that colliding particles
must have in order to react is called the
activation energy.
•You can think of the activation energy for a
reaction as a barrier that reactants must
cross before products can form.
•Label on your diagram.
12. Describing Reaction Rates
• When two reactant particles collide, they
may form an activated complex.
• An activated complex is an unstable
arrangement of atoms that forms for a
moment at the peak of the activation-
energy barrier and is usually short lived.
• Label on your diagram.
15. Talk with your table…
•What factor determines whether a molecular
collision results in a reaction?
Take notes to share with the class.
16. Le Chatelier’s principle:
Factors Affecting Reaction Rates
•Le Chatelier’s principle: if a stress is applied to a
system in dynamic equilibrium the system changes in a
way that relieves this stress.
•Factors that can affect the rate of a chemical reaction
are:
• temperature
•concentration
•particle size
•use of a catalyst
17. Factors Affecting Reaction Rates:
Temperature
•Usually, raising the temperature speeds up a reaction.
•Lowering the temperature usually slows down a reaction.
•Talk with your partner, why do you think this is true? Take notes to
share with the class.
•At higher temperatures, particles move faster.
• The frequency of collisions increases, therefore more particles have enough kinetic
energy to slip over the activation-energy barrier.
•Thus, an increase in temperature causes products to form faster.
18. Factors Affecting Reaction Rates:
Concentration
•The number of particles in a given volume affects the rate at which
reactions occur.
•Talk with your partner, why do you think this is true? Take notes to
share with the class.
•Cramming more particles into a fixed volume increases the
concentration of reactants, and, thus, the frequency of collision.
•Increased collision frequency leads to a higher reaction rate.
19. Other Factors Affecting Reaction Rates:
Particle Size
• The total surface area of a solid or liquid reactant affects the rate of
a reaction.
• The smaller the particle size, the greater the surface area is for a
given mass of particles.
• The result of an increase in surface area is an increase in the
frequency of collisions and the reaction rate.
20. Other Factors Affecting Reaction Rates:
Catalyst
• 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 the
catalyzed reaction is lower than that of the
uncatalyzed reaction.
• On your diagram draw a reaction with a
catalyst.
21. Question Time!!!!
•Which of the following factors could be increased in order to
decrease a reaction rate?
A.Catalyst concentration
B. Concentration
C.Temperature
D.Particle size
22. Question Time!!!!
•What would happen to the reaction rate if additional reactants
were added to the system?
A.Increase
B. Decrease
C.No Change
23. Question Time!!!!
•What would happen to the reaction rate if the system was cooled?
A.Increase
B. Decrease
C.No Change
24. Question Time!!!!
•What always results in a reaction occurring?
A.Effective Collision
B. Heating the system
C. Adding a catalyst
D. Decreasing the particle size
