Transcript Regents Chemistry

Regents Chemistry
 Kinetics and Equilibrium
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!
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
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
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
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!
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
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
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
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
Regents Chemistry
 Potential Energy Diagrams
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
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..
What’s a PE Diagram?..First
look
 A PE diagram illustrates the potential energy
--- Potential energy
change that occurs during a chemical reaction
Activated Complex
Reaction Coordinate =
progress of reaction
Reactants
Products
Reaction Coordinate
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..
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
Example of Collisions
HI molecules
collide in a
reaction and
form H2 and I2
products
Reactants and Products on a PE Diagram
Activation
energy
Difference
In PE
A + B  C + D + Heat
Heat is a product because the PE is lower for the
products than in the reactants
H =
Heat of
reaction
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
Regents Chemistry
Agenda
Finish Potential Energy Diagrams Lecture
YOU NEED HANDOUT FROM WEDNESDAY
Worksheet
HW: Finish remaining problems
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
Effect of a Catalyst
What does
5 represent?
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 = -)
Two Outcomes of PE
Diagrams
PE of the products is greater than the
reactants, so energy is absorbed to make
the products – Endothermic (H = +)
Practice Problem
Regents Chemistry
 Physical and Chemical Equilibrium
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
Describing Equilibrium
A double arrow is used in the reaction in
place of a single arrow
vs.
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!
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…
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
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
Phase Equilibrium…
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
H2O(s)
H2O(l)
Remember, rates are the same..
not amounts!
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)
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)
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
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
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)
3H2(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
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
Regents Chemistry
•Le Chatelier’s Principle
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…
Concentration Changes
CH4(g) + H2O(g)
3H2(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
Concentration Changes cont..
CH4(g) + H2O(g)
3H2(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
Concentration Changes - Ex #2
Stress: More NH3
4NH3(g) + 5O2(g)
+
4NO(g) + 6H2O(g) + heat
-
+
+
+
Stress: Less NH3
4NH3(g) + 5O2(g)
-
+
4NO(g) + 6H2O(g) + heat
-
-
-
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
Temperature - Example
Stress: More Heat
4NH3(g) + 5O2(g)
+
+
4NO(g) + 6H2O(g) + heat
-
-
+
Stress: Less Heat
4NH3(g) + 5O2(g)
-
-
4NO(g) + 6H2O(g) + heat
+
+
-
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)
CO2(g)
CO2(aq)
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
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
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
H2(g) + Cl2(g)
2HCl(g)
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
Regents Chemistry
 Entropy and Enthalpy
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?