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Unit 9 Kinetics and Equilibrium
Ms. Randall
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Lesson 2: Reaction Rates Objective: To compare and contrast factors that affect the rate of a chemical reaction
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Kinetics Study of the RATE or SPEED at which REACTIONS occur
A Reaction is the BREAKING and REFORMING of BONDS to make entirely new compounds as products. Demo water to milk to juice
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Reaction Mechanism STEP BY STEP PROCESS needed to make a product; how you get from “a” to “b” (like a recipe).
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REACTANTS PRODUCTS Just like when we bake a cake we must follow directions CAN’T OMIT any STEPS! CAN’T CHANGE THE ORDER of the steps! CAN’T OMIT any REACTANTS (ingredients)
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Collision Theory In order for a reaction to occur, reactant PARTICLES MUST COLLIDE Proper amount of ENERGY Proper ALIGNMENT/ DIRECTION/ ORIENTATION
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Only when particles collide and these two conditions are met will there be an EFFECTIVE COLLISON, resulting in a reaction Model
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What Determines The Rate of a Reaction?
NUMBER OF STEPS= more steps cam mean a slower reaction RATE DETERMINING STEP= the SLOWEST STEP of the reaction; most important factor influencing reaction rate.
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Factors Affecting the Rate of Reaction
SIX FACTORS that affect the rate of reaction by changing the number of effective collisions that take place between particles. The MORE EFFECTIVE COLLISIONS, THE FASTER THE REACTION!
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Type of substance: Ionic substances react faster: bonds require less energy to break AgNO3 (aq)+NaCl(aq)AgCl(s)+NaNO3 (aq) In solution ionic solids dissociate into ions: Ag+ NO Na+ Cl-
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Covalent react more slowly: bonds require more energy to break
H2 (g)+I2 (g)2 HI (g) Bonds must be broken then be reformed. (takes more time)
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IONIC substances react FASTER
COVALENT substances react SLOWER
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Concentration Increases rate due to the fact that more particles are in a given volume, which creates more collisions. Low Concentration High Concentration
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Pressure Increases the rate of reactions involving gases only
As pressure Volume so: spaces between molecules frequency of effective collisions Low Pressure High Pressure
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Temperature INCREASE temperature, INCREASE Average Kinetic Energy. More particles colliding! High Temp Low Temp
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Surface Area Increases rate due to increased reactant interaction or collisions (powder vs. lump)
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Catalyst SPEEDS UP THE REACTION WITHOUT CHANGING THE NATURE OF THE REACTANTS/PRODUCTS
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Provides a SHORTCUT or ALTERNATIVE PATHWAY for the mechanism
**NOTE: catalysts DO NOT change the nature of the reaction (the same reaction is still occurring), the reaction is just completed in a shorter amount of time! Activation energy: amount of energy required to “start” a reaction
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“Real World Example” Catalytic Converter in vehicles
The NO molecules Bind to the Catalyst Surface which Weakens their Bonds. The job of the catalytic converter is to convert harmful pollutants into less harmful emissions before they ever leave the car's exhaust system.
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Quick Review – Factors that affect reactions
Ionic solutions have faster reactions than molecule compounds. (bonding) Temp. Rate conc. rate surface area rate Pressure rate, P rate Catalysts speed up reactions.
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Check your understanding and practice
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Lesson 3: Energy in reactions Objective: To apply changes during a reaction in energy to a PE diagram
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A reaction is the breaking and reforming of bonds
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Heat of Reaction (ΔH) The amount of HEAT ENERGY LOST or GAINED throughout a REACTION (ΔH = entHalpy) PE OF THE PRODUCTS - PE OF THE REACTANTS Enthalpy (ΔH = Hproducts – Hreactants)
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Two types of reactions Reactions that release energy EXOTHERMIC
Δ H = negative value (-) energy released (on right) A + B C + D + ENERGY
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2. Reactions that absorb/gain energy ENDOTHERMIC Δ H = positive value (+) energy absorbed (on left) A + B + ENERGY C + D
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Table I Includes heats of reaction for combustion, synthesis (formation) and solution reactions. You must remember equation stoichiometry (balanced equations). Endothermic: heat is a reactant Exothermic: heat is a product
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Forward Reaction Reading LEFT TO RIGHT in a reaction; reaction moves toward the right
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Reverse Reaction Reading RIGHT TO LEFT in a reaction; reaction moves toward the left
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Table I Tells us if particular reactions are exothermic or endothermic based on sign of the Δ H value + 66.4 ENDOthermic -91.8 EXOthermic ENDOthermic + 91.8
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Potential Energy Diagrams
Chemists use diagrams called Potential Energy Diagrams to illustrate the potential (or stored) energy changes that occur during specific chemical reactions.
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Activation Energy amount of ENERGY NEEDED TO GET A REACTION STARTED OR FORM THE ACTIVATED COMPLEX of a reaction (you must get over the “bump” in order for a reaction to be successful!)
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The ACTIVATED COMPLEX is lowered which also lowers the ACTIVATION ENERGY needed for the reaction to get started (gets reaction started easier and makes the reaction complete itself faster)
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HOW EXACTLY DOES A CATALYST SHORTEN THE REACTION TIME NEEDED FOR A REACTION TO COMPLETE?
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ENDOTHERMIC Potential Energy Diagrams POSITIVE ΔH
Product side (to the R) always HIGHER than the reactant side (to the L) meaning that ENERGY is ABSORBED
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Label the following: A = Potential Energy of the Reactants
B = Potential Energy of the Products C = Potential Energy of the Activated Complex D = Activation energy of the Forward rxn E = Activation Energy of the Reverse rxn F = Heat of the Reaction (ΔH = Hp – Hr)
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D E F C B A
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EXOTHERMIC Potential Energy Diagrams NEGATIVE ΔH
Product side (to the R) always LOWER than the reactant side (to the L) meaning that ENERGY is RELEASED
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Label the following: A = Potential Energy of the Reactants
B = Potential Energy of the Products C = Potential Energy of the Activated Complex D = Activation energy of the Forward rxn E = Activation Energy of the Reverse rxn F = Heat of the Reaction (ΔH = Hp – Hr)
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D C E F A B
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Check your understanding and practice
Check your understanding and practice
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Lesson 4:Equilibrium Objective: To compare and contrast the different types of system equilibriums
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Reversible Reactions Most chemical reactions are able to proceed in both directions under the appropriate conditions. Example: Fe3O4 (s) H2 (g) ↔ 3 Fe(s) + 4 H2O(g)
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Reversible Reactions cont.
In a closed system, as products are produced they will react in the reverse reaction until the rates of the forward and reverse reactions are equal. Ratefwd = Raterev This is called chemical equilibrium.
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EQUILIBRIUM WHEN THE RATE OF THE FORWARD REACTION EQUALS THE RATE OF THE REVERSE REACTION in a closed system. When equilibrium is reached, IT DOES NOT MEAN that the reactants and products are of equal QUANTITIES. So…
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Equilibrium is represented by DOUBLE ARROWS instead of a single arrow.
Equilibrium is DYNAMIC which means that it is constantly CHANGING or FLUCTUATING Equilibrium means that reactant and product CONCENTRATIONS are CONSTANT. *Equilibrium does NOT mean that reactant and product concentrations are equal.
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TYPES OF EQUILIBRIUM Phase equilibrium Solution Equilibrium
All occur in CLOSED SYSTEMS *ITS ALL ABOUT THE EQUAL RATES! Phase equilibrium Solution Equilibrium Chemical Equilibrium
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Equilibrium Equilibrium is dynamic condition where rates of opposing processes are equal. Types of Equilibrium: Phase equilibrium Solution Equilibrium Chemical Equilibrium
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Phase Equilibrium Rate of one phase change is equal to the rate of the opposing phase change. Occurs when two phases exist at the same temperature.
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Solution Equilibrium Rate of dissolving = rate of crystallization
Occurs in saturated solutions
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Chemical Equilibrium Rateforward reaction = Ratereverse reaction
Concentration of reactants and products are constant NOT necessarily equal. [reactants] and [products] is constant.
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The Concept of Equilibrium
As a system approaches equilibrium, both the forward and reverse reactions are occurring. At equilibrium, the forward and reverse reactions are proceeding at the same rate.
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Check your understanding and practice
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Lesson 5: Le Chatelier’s Principle Objective: To apply Le Chatelier’s principle to a system at equilibrium and predict the changes on the system
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LE CHATELIER’s PRINCIPLE
Le Chatelier’s principle explains HOW A SYSTEM AT EQUILIBRIUM WILL RESPOND TO STRESS.
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Le Chatelier’s Principle
Whenever stress is applied to a reaction at equilibrium, the reaction will shift its point of equilibrium to offset the stress. Stresses include: Temperature, pressure, changes in reactant or product concentrations
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SHIFT An increase in the RATE of EITHER the forward OR the reverse reaction.
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Demo of LeChatelier’s Principle
NO2 N2O4 Cooling will cause a shift that produces heat (in this case the forward reaction). Thus, the forward reaction is Exothermic.
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DIFFERENT TYPES OF STRESSES
1) Concentration as initial stress: Equilibrium changes (or shifts) when a reactant or product is added (introduced) or decreased (taken away) in a reaction that is at equilibrium
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When the concentration of a reactant or product is INCREASED:
The reaction will SHIFT AWAY from the increase (use up the excess) LEFT INCREASE RIGHT INCREASE LEFT DECREASE [NO2], [H2O]
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When the concentration of a reactant or product is DECREASED:
the reaction will SHIFT TOWARD the side that has experienced the decrease in concentration (replaces what was taken) LEFT [C6H12O6] DECREASE RIGHT [C6H12O6] INCREASE [CO2], [H2O] [CO2], [H2O]
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2) Temperature As initial stress: (involves increasing or decreasing the “HEAT” component of a reaction) NOTE: HEAT/ENERGY/J/KJ will either be a reactant or a product
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When temperature (or HEAT) is increased:
The reaction will SHIFT AWAY from the rxn side containing “HEAT” (in the ENDOTHERMIC direction)
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When temperature (or HEAT) is decreased:
The reaction will SHIFT TOWARD the rxn side containing “HEAT” (in the EXOTHERMIC direction)
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Example #1: [NO2], [H2O] [NH3], [O2]
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Example #2: [CH4], [O2] [CO2], [H2O]
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3) Pressure As initial stress: Recall, pressure affects GASES ONLY!!! So every other state (s,l,aq) in the reaction is UNAFFECTED for this type of stress.
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INCREASE PRESSURE: rxn shifts to side with LEAST # GAS MOLECULES (or least # of moles of gas)
DECREASE PRESSURE: rexn shifts to side with GREATEST # GAS MOLECULES (or greatest # moles of gas)
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NOTE: If the rexn contains NO GAS MOLECULES or if the rxn has the SAME # GAS MOLECULES on each side, there is NO EFFECT and NO SHIFT results from an increase or decrease in pressure
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Example 1: CO2 (aq) CO2 (g) CO2 (g) CO2 (aq)
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The Haber Process Application of LeChatelier’s Principle
N2 (g) + 3 H2 (g) 2 NH3 (g) + 92 kJ increase pressure Shift decrease Temp remove NH3 add N2 and H2 ****Maximum yields of NH3 occurs under high pressures, low temperatures and by constantly removing NH3 and adding N2 & H2
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Equilibrium shifts due to stresses:
Concentration increase shift away from increase Concentration decrease shift toward decrease pressure shifts in direction of fewer gas molecules. pressure shifts in direction of more gas molecules temperature favors endothermic reaction Shift away from heat temperature favors exothermic reaction Shift towards heat
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Effect of Catalyst: Addition of catalysts changes the rate of both the forward and reverse reactions. There is no change in concentrations but equilibrium is reached more rapidly.
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Reactions that go to completion:
Equilibrium is not reached if one of the products is withdrawn as quickly as it is produced and no new reactants are added. Reaction continues until reactants are used up. Products are removed if: Gases in liquid solution Insoluble products (precipitate)
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Check your understanding and practice!
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Lesson 6: Entropy/ Enthalpy Objective: To define the conditions necessary for a spontaneous reaction to occur
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Entropy (ΔS) Randomness, disorder in a sample of matter
Nature tends to proceed to a state of GREATER entropy, or disorder. Gases have high entropy Solids have low entropy
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ENTROPY (ΔS): Degree of RANDOMNESS or CHAOS or DISORDER or “MESSINESS” in a system
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Increasing ΔS Phase change from s l g Mixing gases
Dissolving a substance
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What physical state of matter has the highest entropy?
GAS aq g s l
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Entropy INCREASES when a compound is broken down
*Entropy INCREASES when a compound is broken down. *Entropy DECREASES when a compound is created and bonds are formed.
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For the following determine if there is an increase, decrease, or no change in entropy:
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NONE NOTE: If all states are the same in reaction, count up the # of molecules on each side
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ENTHALPY (ΔH) the ENERGY in a system; nature tends to proceed to a state of LOWER enthalpy, or energy Exothermic reactions RELEASE energy & move to LOWER energy state
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** Most common types of reactions because less energy has to be put in to get the reaction started (LOWER activation energy)
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Endothermic reactions ABSORB energy & move to a HIGHER energy state (HIGHER ACTIVATION ENERGY NEEDED)
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** Not as common because more energy must be put in to get the reaction started (HIGHER activation energy)
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Spontaneous Reactions
Nature favors low energy (more stable) and high entropy Reactions are spontaneous when heat (ΔH) decreases and entropy (ΔS) increases ΔH = (-) ΔS= (+)
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Stability of Products and H
Help determine if a reaction is spontaneous Products tend toward Lower energy (-ΔH) Products tend toward more randomness (+ΔS) Products of exothermic reactions are usually more stable. Result in lower amounts of heat. The more negative the H, the more stable the product is. Gas products result in increased Entropy.
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Analogy: Your Bedroom You like to have low enthalpy (low energy) when it comes to household chores. As a result, your room tends to have high entropy (very messy, disorderly). This is what nature prefers: low enthalpy and high entropy.
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Check your understanding and practice
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