unit 8 kinetics and equilibrium. i. kinetics what does “kinetics” mean? what do you think of...
TRANSCRIPT
Unit 8
Kinetics and Equilibrium
I. Kinetics
What does “kinetics” mean?What do you think of when you hear
kinetics?A branch of chemistry that concerns itself
with the speed of chemical reactionsRates of reaction!
A. Collision Theory
Collision Theory states:
In order for reactants to change into products, the reactants must collide with the proper orientation and sufficient kinetic energy
You must have an effective collision!
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/collis11.swf
B. Factors Affecting the Rate of a Reaction
1- Temperature:
2- Concentration:
3- Surface Area:
4- Bond Types:
5- Catalyst:
How do these factors increase or decrease the reaction rates???
Reaction Rates
1- Increases in Temperature cause an increase in the number of effective collisions, and this increases the rate of the reaction
2- Increases in concentration of reactants increase number of effective collisions, which increases the rate of reaction
Factors, (cont’)
3- Increase in surface area, essentially increases the concentration, gives more exposure to reactants, and increases effective collisions
4- Compatible bond types react faster than those without similar bond types/IMF’s
Last factor…
5- Addition of a catalyst will decrease the activation energy, therefore making the reaction proceed faster…
Anytime the Ea is decreased, the reactants form the activated complex faster, and the reaction proceeds faster!
C. Potential Energy Diagrams
Drawings of how a reactant changes into a product
Show the energy changes associated with a chemical reaction
Terms to Define:
1. Reactants
2. Products
3. Activated complex
4. Activation energy
5. Heat of reaction
6. Exothermic
7. Endothermic
8. Catalyst
Parts of the Potential Energy Curve
Potential Energy Diagram
Labeling a PE Diagram
Effects of a Catalyst
A catalyst will lower the activation energy of a reaction
Definition… [see review book!]This will cause the reaction to proceed faster…
– Why?
Online demo and references:
http://www.kentchemistry.com/links/Kinetics/PEDiagrams.htm
Catalyst Effects
Catalysts give reactants an alternative pathway to the intermediate, or activated, complex
This path requires less energy, therefore it becomes products faster!
Endothermic Reactions
Description of an Endothermic Reaction
PE of Products is GREATER than the PE or the reactants
(+) ΔHrxnNeeds heat energy to
proceed!
Exothermic Reactions
Description of Exothermic Reactions
PE of the Products is LESS than PE of the reactants
(-)ΔHrxnReleases energy!
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf
D. Heat of Reaction [ΔHreaction]
Table I!Table I!
Based on MOLE quantities!!![look at coefficients within equation!]
Negative ΔHrxn means…?Positive ΔHrxn means…?
II. Equilibrium
Equilibrium = a system that contains BOTH the reactants and products
The rate of the forward reaction is the same as the rate of the reverse reaction
A. Equilibrium Constants, Kc
Equilibrium constants are numerical values that compare the ratios of concentrations of products to the concentrations of the reactants
Contains gases and aqueous substances
KC =[C]c [D]d
[A]a [B]b
Kc Expression
C
For the reaction: aA + bB cC + dD
where A, B, C and D are the chemical species and a, b, c and d are the stoichiometric coefficients,
[C]c[D]d
K1 = --------- [A]a[B]b
Descriptions of Kc
LARGE Kc value… suggests that the concentration of products present is much, much greater than the concentration of the reactants
Reaction went forward!!!!
SMALL Kc value… suggests that the concentration of reactants is very high and few products are made
Reaction did not go forward!
B. LeChatlier’s Principle
1. Definition:
When a system is at equilibrium and it is disturbed, it will “shift” to correct the disturbance and reform a new equilibrium
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swf
LeChatlier’s Disturbances
1-Temperature Changes:
An increase in Temperature will ALWAYS favor the endothermic reaction
Ex] A + B C + energy
Increases in Temperature similar to increasing ‘energy’; shifts to REVERSE direction!
2-Concentration Changes
An increase in concentration for a reactant causes the reaction to shift to the forward direction to use the extra reactant
An decrease in the concentration of a reactant causes the reaction to shift to the reverse direction to replace the missing reactant
3. Common Ion Effect
Adding a substance that generates the same ion already in the equilibrium system…
Increases the concentration of that ion!
Same effect as concentration!
Adding a substance that will bond with one of the ions in the system…
Decreases the concentration of that ion!
Same effect as concentration!
4. Pressure [for gases only]
An increase in the pressures of a gas will decrease the volume, therefore the system will shift to the side with the fewest number of moles of gas
A decrease in pressure causes the system to shift to the side with more moles of gas
Pressure effects cont’.
If a system has
1] the same number of moles of gas on each side
OR
2] has no gases present
Changes in pressure HAVE NO EFFECT!!
5. Catalysts
If the system is AT equilibrium, the catalyst speeds BOTH the forward and reverse reaction rates
If the system has not yet REACHED equilibrium, the catalyst will increase the rate, causing it to reach equilibrium FASTER!
C. Spontaneity
A spontaneous reaction = one that occurs without the application of external work
Happens on its own! Spontaneous in one direction, nonspontaneous in the opposite direction
– Ex.] phase changes occur without help when the conditions are favorable!
Spontaneous Processes: [Enthalpy and Entropy]
Conditions favoring a spontaneous process:
– These help explain why a process may proceed more easily…
– Incorporates entropy changes from reactants to products
Formation of a gas
Formation of a precipitate
Formation of water
