Regents Chemistry

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Regents Chemistry

• Kinetics and Equilibrium

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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!

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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

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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

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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

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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!

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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

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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

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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

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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

worksheet
VIDEO
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Regents Chemistry

• Potential Energy Diagrams

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Whats 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 Earths surface
• And
• Reduced by lowering closer to the ground

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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..

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Whats 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
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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 togetherthis leads to..

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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

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Example of Collisions

• HI molecules collide in a reaction and form H2
  and I2 products

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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
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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

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Regents Chemistry

• Agenda
• Finish Potential Energy Diagrams Lecture
• YOU NEED HANDOUT FROM WEDNESDAY
• Worksheet
• HW Finish remaining problems

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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 loweredbut
• The ?H remains unaffected..see why

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Effect of a Catalyst
What does 5 represent?
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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 -)

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Two Outcomes of PE Diagrams

• PE of the products is greater than the reactants,
  so energy is absorbed to make the products
  Endothermic (?H )

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Practice Problem
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Regents Chemistry

• Physical and Chemical Equilibrium

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Whats 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

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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.
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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

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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
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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

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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

Remember, rates are the same.. not amounts!
H2O(s) H2O(l)
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Solution Equilibrium

• Youve 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)
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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)
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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

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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

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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

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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 Chateliers principle

End
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Regents Chemistry

• Le Chateliers Principle

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Le Chateliers Principle

• Any change in temperature, concentration or
  pressure on an equilibrium system is called a
  stress
• Le Chateliers principle explains how a system
  at equilibrium responds to relieve any stress on
  the system
• We will look at these separately

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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
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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

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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
-
-
-

-
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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

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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
-
-
-


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Pressure Changes

• Pressure does not affect the rate of reactions of
  solids and liquidshowever
• 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)

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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
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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

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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

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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, dont forget catalysts!

End
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Regents Chemistry

• Entropy and Enthalpy

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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?