Intro to Equilibrium

1
Intro to Equilibrium

• Chapter 17 Sections 17.1-17.5

2
Review

• What is the definition of a chemical reaction?
• How does a refrigerator keep food from spoiling?
• What is the difference between the word
  homogeneous and heterogeneous?

3
How do Chemical Reactions occur?

• We are very used to writing chemical reactions.
• Reactants on the left, products on the right.
• But how do the atoms actually break bonds with
  the reactants and form bonds with the products?
• Chemists think the collision model is behind this

4
How do Chemical Reactions occur? Cont

• Chemists believe that molecules react by
  colliding with each other. Some collisions are
  violent enough to break bonds and allowing the
  reactants to form products.
• Example
• 2BrNO n 2NO Br2

(a) Two BrNO molecules approach each other at
high speeds. (b) 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
5
How do Chemical Reactions occur? Cont

• This is the collision model
• The idea that reactions occur through molecular
  collisions
• This model explains many characteristics of
  chemical reactions
• It explains why reactions proceed faster if the
  concentrations of the reactants are higher
• It explains why higher temperatures cause faster
  reactions and cooler temperatures slow down
  reactions

6
Conditions that affect reaction rates

• The minimum amount of energy needed for a
  reaction to occur is called the activation energy
  (Ea)
• If a given reaction (collision) between molecules
  possesses less energy than Ea, then no reaction
  can occur
• If a given reaction (collision) possess more
  energy than Ea, then a reaction can occur
• Reaction progress of
• two BrNO molecules

7
Conditions that affect reaction rates cont

• Greater temperatures means greater energy, so
  reactions can occur faster because Ea is met.
• You can also use a catalyst
• A substance that speeds up a reaction without
  being used or consumed in the reaction
• Examples Enzymes in the body

8
Comparison of the activation energies for an
uncatalyzed reaction and for the same reaction
with a catalyst present.
Conditions that affect reaction rates cont
9
Heterogeneous reactions

• Most reactions we have consider so far this year
  occur in one phase.
• Reactions only involving one phase are called
  homogenous reactions
• Many other reactions involve two or more phases
  these are called heterogeneous reactions.

10
Heterogeneous reactions cont

• Example of a heterogeneous reaction
• Zn 2HCl ? H2 ZnCl2
• First the Zn works very slowly if in a large
  chunk
• But very quickly if ground up
• Because the zinc initially has to attract the H
  to it, and then the H collide and form H2

11
Factors that affect reaction rates

1. Nature of reactants bond strength and structure
2. Concentration or pressure higher concentration
   or pressure more collisions
3. Temperature Increased temperature more speed
   more collisions
4. Surface area more area more places to react
   with

12
The Equilibrium Condition

• Equilibrium means to imply balance
• Chemists define equilibrium as the exact
  balancing of two processes, one of which is the
  opposite of the other

13
The Equilibrium Condition cont

• We first saw this in a closed container with
  evaporation and condensation way back in chapter
  14.
• Net transfer of molecules from the liquid state
  to the vapor state.
• The amount of the substance in the vapor state
  becomes constant.
• The equilibrium state.

14
The Equilibrium Condition cont

• So far in this class we have seen a lot of
  reactions where we have reactants and they turn
  into products until all of the reactants run out.
  
• So all of the reactants are converted into
  products, or at least until the limiting reactant
  is used up
• In reality there are many chemical reactions, in
  which they stop short of completion in a closed
  container
• This is called chemical equilibrium, a state in
  which the concentrations of all reactants and
  products remain constant

15
Chemical Equilibrium

• Example
• 2NO2 n N2O4
• NO2 is brown and N2O4 is colorless, if I place
  this reaction in a closed container, the
  substance never becomes colorless, but merely
  stabilizes at a point of equilibrium
• The amounts of NO2 and N2O4 remain the same amount

16
Chemical Equilibrium A dynamic condition

• Chemical equilibrium is dynamic
• In other words it is not really stable but
  constantly moving, just the amounts are staying
  the same.
• Analogy East of Lake Washington and Seattle
• Same amounts of cars, but different cars

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Chemical Equilibrium A dynamic condition cont

• The same is true for chemical reactions
• CO H2O n CO2 H2
• Equal numbers of moles of H2O and CO are mixed in
  a closed container.
• The reaction begins to occur.
• The reaction continues, and more reactants are
  changed to products.
• No further changes are seen as time continues to
  pass.

18
Chemical Equilibrium A dynamic condition cont

• Changes with time in the rates of reactions.

19
Homework

• Read pages 537-547
• Problems 1-5 on 547