1. Regents Chemistry
Kinetics and Equilibrium

2. What is Kinetics?
Kinetics is the branch of chemistry that deals with rates of chemical reactions
Different factors affect how quickly chemical reactions occur
In order for a reaction to occur, reactant particles must collide
This is called COLLISION THEORY
Reactions occur between collisions of particles that are orientated correctly and have sufficient amounts of energy!

3. What is Kinetics Important?
Kinetics allows chemists to predict how fast a reaction will occur
Important in the synthesis of all kinds of compounds
In manufacturing, it is essential to making products,
timing rates between chemical reactions to get a desired product

4. What Factors Affect Rate of Reaction?
The rate of a chemical reaction depends on a number of factors that affect the number of effective collisions between particles
Nature of Reactants
Concentration
Surface Area
Pressure
Temperature
Presence of a Catalyst

5. Nature of Reactants
Reactions involve the breaking of existing bonds and the formation of new bonds
Generally, covalently bonded substances are slower to react than ionic substances as they have move bonds to be broken
Breaking more bonds requires that the particles must have more energy when they collide

6. Concentration
Most chemical reactions will proceed at a faster rate if the concentration of one or more of the reactants is increased
Ex: Combustion of Paper
Normal air is 20% oxygen, if we use pure oxygen it burns much faster!
Why? KMT says that more collisions between oxygen and paper particles = faster rate of reaction!

7. Surface Area
When more surface area of a substance is exposed, there are more chances for reactant particles to collide
Ex: Lycopodium solid vs. powder phases

8. Pressure
Pressure has little or no effect on rates of reactions between liquids and solids…
It does have an effect on gases!
An increase in pressure has the effect of increasing the concentration of gaseous particles
Therefore, it increases the rate of a reaction that involves only gases

9. Presence of a Catalyst
Catalysts are substances that increase the rate of a reaction by providing a different and easier pathway for a chemical reaction
Catalysts take part in a reaction, but they are unchanged when the reaction is complete
So they are present in the reactants and products and only assist in the rate of the reaction

10. Temperature
By definition, temperature implies that the greater the temperature, the faster the molecules will move (higher kinetic energy)
When particles are moving faster, more collisions occur and increasing the likelihood of a reaction
Higher Temp = particles with more kinetic energy = more effective collisions
VIDEO
worksheet

11. Regents Chemistry
Potential Energy Diagrams

12. What’s Potential Energy?
Chemical bonds are large sources of chemical potential energy
Potential energy has ability to do stuff!
Similar to gravitational potential energy
Gravitational PE can be increased by raising an object higher from the Earth’s surface
And
Reduced by lowering closer to the ground

13. Chemical Bonds Have PE!
Chemical bonds have large supplies of potential energy
Bonds are broken in chemical reactions and new bonds form in products
Energy can be released or gained in these chemical reactions, so…
Potential energy can be increased or decreased depending on the reaction..

14. What’s a PE Diagram?..First look
A PE diagram illustrates the potential energy change that occurs during a chemical reaction
Activated Complex
Reaction Coordinate =
progress of reaction
Reactants
--- Potential energy
Products
Reaction Coordinate

15. Potential Energy Diagrams
In order for a reaction to occur, the reactants must have sufficient energy to collide effectively
As reactant particles approach each other, kinetic energy is converted into potential energy
The molecules must also have proper orientation to come together…this leads to..

16. Activated Complex
When molecules collide with the proper orientation, an intermediate product is formed
Activated Complex – is a temporary, intermediate product that may either break apart and reform the reactants or rearrange the atoms and form new products

17. Example of Collisions
HI molecules collide in a reaction and form H2 and I2 products

18. Reactants and Products on a PE Diagram
Activation
energy
Difference
In PE
H =
Heat of
reaction
A + B  C + D + Heat
Heat is a product because the PE is lower for the
products than in the reactants

19. Activation Energy and Heat of Reaction
The amount of energy needed to form the activated complex from the reactants is called the activation energy
The diagram is typically read from left to right and vice versa
So..we specify by saying forward or reverse
HEAT OF REACTION – is the heat required to form products or reactants in this specific reaction
Can be negative or positive depending on viewing the reaction as forward or reverse

20. Regents Chemistry Agenda Finish Potential Energy Diagrams Lecture
YOU NEED HANDOUT FROM WEDNESDAY
Worksheet
HW: Finish remaining problems

21. The Effect of a Catalyst
Catalysts are added to a reaction to lower the activation energy, which in turn speeds up the reaction
The catalyst speed up the reaction by providing a new pathway
In turn, the activation energy of the reverse reaction is also lowered…but
The H remains unaffected..see why

22. Effect of a Catalyst
What does
5 represent?

23. Two Outcomes of PE Diagrams
1. PE of products is lower than the reactants, so energy is lost and released to the environment – Exothermic! (H = -)

24. Two Outcomes of PE Diagrams
PE of the products is greater than the reactants, so energy is absorbed to make the products – Endothermic (H = +)

25. Practice Problem

26. Regents Chemistry
Physical and Chemical Equilibrium

27. What’s Equilibrium?
The potential energy diagrams typically show a forward reaction – Left to Right
Reactants  Activated Complex  Products
But the reverse can also happen…
Products  Activated Complex  Reactants
And they can both happen at the same time!
When they occur at the same rate, the system is said to be in equilibrium

28. Describing Equilibrium
A double arrow is used in the reaction in place of a single arrow
Equilibrium is a state of balance between the rates of two opposite processes that are taking place at the same rate
Exists only in a closed system!
vs.

29. Equilibrium…
Is important because many chemical reactions and physical processes are reversible
We will look at two kinds:
Physical and Chemical Equilibriums
Is an equilibrium between rates, not amounts of reactants and products
For example…

30. Equilibrium in a Closed Container
Open Container
Closed Container
Evaporation continues
in the closed container
but is balanced by
condensing vapor.
This is an equal process
at equilibrium!
H2O(l) H2O(g)
H2O(l) H2O(g)
End

31. Physical Equilibrium (PE)
The evaporation – condensation of water in a closed system is an example of PE
Two examples of PE are
Phase Equilibrium
Solution Equilibrium

32. Phase Equilibrium… H2O(s) H2O(l)
Can exist between the solid and liquid phases of a substance.
This is called the melting point of a solid phase or the freezing point of the liquid phase in a closed container
Example: Water at 0 Celsius (closed container)
Some of the ice is melting and some of the water is freezing
Remember, rates are the same..
not amounts!
H2O(s) H2O(l)

33. Solution Equilibrium You’ve seen this before..saturated solutions!
Solids in liquids exist in equilibrium in a saturated solution
When we add more solute to a saturated solution, the solute may dissolve but some will also recrystallize out of solution
KCl(s) KCl(aq)

34. Solution Equilibrium cont…
Equilibrium may also be attained in a closed system between a gas dissolved in a liquid and the undissolved gas
For example  In a closed Pepsi can, there is an equilibrium between the gaseous and dissolved state of CO2
CO2(g) CO2(aq)

35. Temperature and Physical Equilibrium
Equilibrium is affected by temperature
If the temperature is raised, a solid generally becomes more soluble in a liquid
For a short time the rate of dissolving exceeds the rate of crystallization
However, as more solid is placed into solution, the rate of recrystallization increases until a new equilibrium is reached

36. Temperature and Physical Equilibrium….
Opposite is true for gases in liquids
As the temperature increases, the rate of the gas escaping from the liquid increases while the rate at which the gas particles dissolves decreases
This decreases the solubility of the gas in the liquid
As the temperature rises, the solubility of all gases decreases in a liquid decreases

37. Chemical Equilibrium
When reactants are first mixed and no products are present, only the forward reaction can occur
For example: water vapor and methane
CH4(g) + H2O(g) H2(g) + CO(g)
As time progresses, the concentrations of the reactants
decreases, causing the forward reaction to slow
While the concentrations of the products increases,
causing the rate of the reverse reaction to increases

38. CH4(g) + H2O(g) 3H2(g) + CO(g)
This process continues until equilibrium is reached
This can be represented in a graph summary – p. 115
Remember..the system must be closed!
No product or reactant can leave the system
If a precipitate or a gas is formed in a system that is not closed, equilibrium will not be reached
The effect of any change on equilibrium is explained by Le Chatelier’s principle
End

39. Regents Chemistry
Le Chatelier’s Principle

40. Le Chatelier’s Principle
Any change in temperature, concentration or pressure on an equilibrium system is called a stress
Le Chatelier’s principle explains how a system at equilibrium responds to relieve any stress on the system
We will look at these separately…

41. Concentration Changes
CH4(g) + H2O(g) H2(g) + CO(g)
If the stress is the addition of more methane (CH4), the rate
of the forward reaction will increase and more products
will form
As more product forms, the reverse reaction will also
increase until reactions are equal again
Overall, if the concentration of one substance is increased,
the reaction that reduces the amount of the added
substance is favored

42. Concentration Changes cont..
CH4(g) + H2O(g) H2(g) + CO(g)
If the concentration of methane is reduced, the rate of the
forward reaction decreases
When the concentration of a substance decreases, the reaction
that produces that substance is favored
Initially the reverse reaction will take place faster than the
forward reaction, and the system is said to be shifting
to the left, or toward the reactant side

43. Concentration Changes - Ex #2
Stress: More NH3
4NH3(g) + 5O2(g) NO(g) + 6H2O(g) + heat + - + + +
Stress: Less NH3
4NH3(g) + 5O2(g) NO(g) + 6H2O(g) + heat - - - - +

44. Temperature Changes
Increasing or decreasing the temperature changes the amount of heat going into / leaving the system
We can consider heat as a reactant or product,
Increasing the amount of heat drives the reaction in the opposite direction of the location of the heat
Decreasing the heat drives the reaction towards the location of the heat

45. Temperature - Example 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) + heat
Stress: More Heat
4NH3(g) + 5O2(g) NO(g) + 6H2O(g) + heat + + - - +
Stress: Less Heat
4NH3(g) + 5O2(g) NO(g) + 6H2O(g) + heat - - - + +

46. Pressure Changes
Pressure does not affect the rate of reactions of solids and liquids…however
Gases are affected!
We will look at how pressure affects
CO2 gas in aqueous solution (contains 1 gaseous molecule)
Nitrogen and hydrogen gases in the production of ammonia gas (contains more than 1 gaseous molecule)

47. CO2(g) CO2(aq) Pressure increases Pressure decreases
1 gas
molecule
Pressure increases
System shifts to the right forming more CO2(aq)
Pressure decreases
System shifts to the left forming more CO2(g)
We see this when we open a can of pop
Pressure decreases, so CO2(g) increases and it comes out of solution
An increase in pressure favors the side of the reaction
that contains the gas

48. Systems with More than 1 Gaseous Substance
An increase in pressure will increase the increase the concentration of gaseous molecules on both reactant and product sides of the reaction, but the effects will be unequal
An increase in pressure will favor the reaction toward the side with fewer gas molecules

49. N2(g) + 3H2(g) 2NH3(g) Reactants have 4 gas molecules
Products have 2 gas molecules
An increase in pressure will favor the reaction towards the products
A decrease in pressure will have the opposite effect
So a decrease favors more N2 and H2 and less NH3

50. H2(g) + Cl2(g) 2HCl(g) Both sides have the same # of gas molecules
Equal gas
molecules
Both sides have the same # of gas molecules
In this case, pressure changes have no effect on the system
Also, don’t forget catalysts!
End

51. Regents Chemistry
Entropy and Enthalpy

52. Entropy and Enthalpy
Many factors cause and chemical and physical changes to occur..
For example: Pressure, temperature, concentration changes
We see how these changes occur, but why exactly do they occur?