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TOPIC 9: KINETICS AND EQUILIBRIUM Part 1 – Kinetics Part 2 – General Equilibrium Part 3 –...
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Transcript of TOPIC 9: KINETICS AND EQUILIBRIUM Part 1 – Kinetics Part 2 – General Equilibrium Part 3 –...
![Page 1: TOPIC 9: KINETICS AND EQUILIBRIUM Part 1 – Kinetics Part 2 – General Equilibrium Part 3 – Solution Equilibrium Part 4 – Thermodynamics.](https://reader035.fdocuments.in/reader035/viewer/2022062409/56649eb15503460f94bb7ac5/html5/thumbnails/1.jpg)
TOPIC 9: KINETICS AND EQUILIBRIUM
Part 1 – KineticsPart 2 – General EquilibriumPart 3 – Solution EquilibriumPart 4 – Thermodynamics
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What is kinetics? What factors affect the rate of chemical
reactions? How can we classify energy in chemical
reactions? How can we interpret Potential Energy
Diagrams?
PART 1 – AIMS
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What is equilibrium? What is the equilibrium expression? How can we calculate K and Kp? What are external factors that affect
equilibrium?
PART 2 – AIMS
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How can we determine and use solubility product constants Ksp?
How can we estimate salt solubility from Ksp? How can we determine the formation of
precipitates? What is the common ion effect?
PART 3 - AIMS
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What is entropy? How can we calculate ΔHformation? How can we calculate bond energies? What is entropy? How can we use Gibbs Free Energy to predict
spontaneity?
PART 4 – AIMS
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Kinetics: the branch of chemistry that deals with the rates of chemical reactions
Collision Theory:1. In order for a reaction to occur, reactants
must collide with each other
2. An effective collision is when reactants come together with the correct amount of energy and in the correct position to form a product
AIM: WHAT IS KINETICS?
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As the amount of effective collisions increases, the faster products are formed (reaction rate increases)
How can we increase the reactions rate (increase the # of effective collisions??
AIM: What Factors Affect the Rate of a Reaction?
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FACTORS THAT AFFECT THE RATE OF CHEMICAL REACTIONS
1. Nature of Reactants 2. Concentration3. Surface Area4. Pressure 5. Temperature 6. Catalyst
AIM: What Factors Affect the Rate of a Reaction?
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NATURE OF REACTANTS:
Reactions involve the breaking of old bonds and the formation of new bonds. In general:
1. Covalent bonds are slower to react than ionic bonds
2. Breaking more bonds requires more energy than making bonds during collisions
(Table I Reactions 1-6)
AIM: What Factors Affect the Rate of a Reaction?
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CONCENTRATION:
Generally: Increase concentration increase rate of
reaction
(especially if volume is decreased)
AIM: What Factors Affect the Rate of a Reaction?
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SURFACE AREA:
Generally: the more surface area that is exposed the
more chances there are for collisions (effective
collisions) and will increase rate of reaction
AIM: What Factors Affect the Rate of a Reaction?
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PRESSURE: No effect on solids and liquids, only gases
Increasing pressure, decreases the volume, increasing the rate
of effective collisions – increases the rate of reaction
AIM: What Factors Affect the Rate of a Reaction?
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TEMPERATURE:
Generally: Increasing temperature increases
kinetic energy of molecules, leading to an
increase in the amount of effective collisions –
increasing the rate of reaction
AIM: What Factors Affect the Rate of a Reaction?
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CATALYST
Addition of a catalyst increases the rate of the
reaction by providing a different and easier pathway
for the reaction
AIM: What Factors Affect the Rate of a Reaction?
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AIM: HOW CAN WE CLASSIFY ENERGY IN CHEMICAL REACTIONS?
3 WAYS TO CLASSIFY ENERGY IN CHEMICAL REACTIONS
1. Look at the reactions (where is E term?)
CH4 + 2O2 CO2 +
2H2O + 890.4kJ
N2 + O2 + 66.4kJ 2NO2
2. Look at the H = heat of the reaction (Table I)
3. Potential Energy Diagrams H = PE products – PE reactants
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Show how the potential energy of reactant particles changes to chemical potential energy stored in bonds
PE diagrams keep track of PE changes during a chemical reaction in stages
AIM: HOW CAN WE INTERPRET POTENTIAL ENERGY DIAGRAMS?
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For Example: A + B AB
The forward reaction is read from left to rightCompare the potential energy of the reactants to the potential energy of the products in the forward reaction (PE diagram #1)
PE REACTANTS: 25 Joules PE PRODUCTS: 75Joules
*Energy must have been absorbed during the reactions PE diagrams with this pattern represent endothermic reactions
AIM: HOW CAN WE INTERPRET POTENTIAL ENERGY DIAGRAMS?
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AIM: HOW CAN WE INTERPRET POTENTIAL ENERGY DIAGRAMS?
FORWARD REACTION!!
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Lets Label the Diagram A. PE of Reactants
B. PE of Products
.C H (heat of reaction) = H = PE products – PE reactants
If H is positive [PE products PE reactants] – endothermic
If H is negative [PE products PE reactants] – exothermic
Table I – shows different chemical reactions and H for each one
AIM: HOW CAN WE INTERPRET POTENTIAL ENERGY DIAGRAMS?
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Label the Diagram D. PE of Activated Complex – intermediate molecule that
forms when reactants have an effective collision. It is unstable and temporary
E. Activation Energy of forward reaction – amount of energy needed to start the reaction in the forward direction
F. Activation Energy (with catalyst) – amount of energy needed to start the reaction if a catalyst is added
* Catalysts – speed the reaction rate by lowering the activation energy needed to start a reactions (gives an alternate pathway)
AIM: HOW CAN WE INTERPRET POTENTIAL ENERGY DIAGRAMS?
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For Example: A + B AB
Compare the potential energy of the reactants to the potential energy of the products in the reverse reaction (PE diagram #2)
PE REACTANTS: 75 Joules PE PRODUCTS: 25Joules
*Energy must have been released during the reactions PE diagrams with this pattern represent exothermic reactions
AIM: HOW CAN WE INTERPRET POTENTIAL ENERGY DIAGRAMS?
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Which diagram is endothermic? Exothermic?
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PRACTICE USING TABLE I:
A negative value means that the reaction is exothermic If we were to rewrite the equation with the heat of
reaction shown it would looks like this
It is added to the right side of the equation because it is an exothermic reaction in which heat is released as a product
TABLE I – Gives us ΔH values for reactions
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PRACTICE USING TABLE I:
A positive value means that the reaction is endothermic
If we were to rewrite the equation with the heat of reaction shown it would looks like this
It is added to the left side of the equation because it is an endothermic reaction in which heat is absorbed as a reactant
TABLE I – Gives us ΔH values for reactions
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Equilibrium: when the forward and reverse reactions occur at the same rate – it is a state of balance
Dynamic Equilibrium: The motion in which the interactions of reacting particles are balanced by the interaction of product particles
Reversible Equilibrium: Many reactions in equilibrium are considered reversible. This is indicated by a double arrow
EQUILIBRIUMAIM: What is equilibrium?
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Physical Equilibrium: the changes that take place in chemical reactions during physical processes such as changes of state or dissolving. - Phase Equilibrium: Equilibrium between
phases 1. Solid and liquid phase – during melting the
rate of dissolving is equal to the rate of crystallization in a closed container (system) (H20 (s) H2O (l))
EQUILIBRIUMAIM: What is equilibrium?
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2. Liquid and Gas phase – during this phase the rate of evaporation is equal to the rate of condensation in a closed container (H20 (l) H2O
(g))
EQUILIBRIUMAIM: What is equilibrium?
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Solution Equilibrium: 1. Solid/Liquid Solution – saturated solutions are examples of solid/liquid solution equilibrium in a closed system (C12H6O11 (s) / C12H22O11 (aq)
2. Gas/Liquid Solution – in a closed system or container, there is equilibrium between the gaseous and dissolved state of the gas
EQUILIBRIUMAIM: How Can We Examine Various Systems at
Equilibrium?
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EQUILIBRIUMAIM: What is equilibrium?
The equilibrium position – whether the reaction lies far to the right or to the left depends on three main factors:
1. The initial concentrations (more collisions – faster reaction)2. Relative energies of reactants and products
(nature goes to minimum energy)3. Degree of organization of reactants and products
(nature goes to maximum disorder)4. Significance of K
K>1 means that the reaction favors the products at eq. K<1 means that the reaction favors the reactants at eq.
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Mathematical expression that shows the relationship of reactants and products in a system at equilibrium is called the equilibrium expression
It is a fraction with the concentrations of reactants and products expressed in moles per liter
Each concentration is then raised to the power of its coefficient in a balanced equation.
The expression equals a value called the equilibrium constant Keq, which remains the same for a particular reaction at a specified temperature
AIM: What is the equilibrium expression?
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To write an equilibrium expression follow these steps:
Write a balanced equation for the system. Place the products as factors in the numerator of a
fraction and the reactants as factors in the denominator, Place a square bracket around each formula. The square
bracket means molar concentration. Write the coefficient of each substance as the power of
its concentration. The resulting expression is the equilibrium expression, which should be set equal to the Keq for that reaction.
AIM: What is the equilibrium expression?
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AIM: What is the equilibrium expression?
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Ex) Write the equilibrium expression for the equilibrium system of nitrogen, hydrogen, and ammonia.
___N2 (g) + ___H2 (g) <-> ___NH3 (g)
AIM: What is the equilibrium expression?
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AIM: What is the equilibrium expression?
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The equilibrium constant is a specific numerical value for a given system at a specified temperature
Changes in concentrations will not cause a change in the value of Keq, nor will the addition of a catalyst.
Only temperature will change the value
PURE SOLIDS AND LIQUIDS ARE NOT INCLUDED IN THE EQUILIBRIUM EXPRESSION (g) and (aq) are.
AIM: What is the equilibrium expression?
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AIM: How can we calculate K and Kp?
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AIM: How can we calculate K and Kp?
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AIM: How can we calculate K and Kp?
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Can you… … write an equilibrium constant expression? … tell how to find K for a summary equation? … explain what K is telling you about a reaction?
AIM: How can we calculate K and Kp?
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If a stress is applied to a system at equilibrium the equilibrium will shift to release the effects of the stress
Stressors: 1. Temperature 2. Concentration3. Pressure
AIM: What are external factors that affect a reaction at equilibrium?
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Increase in temperature favors the endothermic reaction
Decrease in temperature will favor the exothermic reaction
* *The side you shift towards will increase and the side you shift away from will decrease**
Le Chateliers PrincipleTEMPERATURE
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Increase conc. of reactants
Favors FORWARD reactionSpeeds up the forward direction
SHIFTS RIGHT
Decrease conc. of reactants
Favors REVERSE reactionSpeeds up the REVERSE direction
SHIFTS LEFT
Increase conc. of products
Favors REVERSE reactionsSpeeds up the REVERSE direction
SHIFTS LEFT
Decrease conc. of the products
Favors FORWARD reactionSpeeds up the FORWARD direction
SHIFTS RIGHT
Le Chateliers PrincipleCONCENTRATION
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ADD AWAY (increase)
TAKE TOWARDS (decrease)
Pen/pencil will go up on which ever side you shift towards
CONCENTRATION
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Need to know how many gas molecules are on the reactant side and on the product side
Equal # of gas molecules – pressure will have no effect!!
Increase pressure – shift from more gas molecules towards less gas molecules
Decrease pressure – shift from less toward more
PRESSURE
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Ex)
4NH (g) + 5O2(g) 4NO(g) + 6H2O(g)
4 + 5 = 9 gas molecules 4+6 = 10 gas molecules
Increase pressure: shift to the left (more to less) 9 10Decrease pressure: shift to the right (less to more) 9 10
PRESSURE
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At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction
Concentrations are constant not equal
Adding a catalyst would speed up the forward and reverse reactions to the same extent
AIM: What are the conditions at equilibrium?
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SOLUBILITY PRODUCT CONSTANT A special case of equilibrium
involving dissolving
Because the constant is a product of a solubility, we call it the solubility product constant Ksp
AIM: How can we determine and use solubility product constants? Ksp
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SOLUBILITY PRODUCT PROBLEMS: Given Ksp, find solubility Given solubility, find Ksp Predicting precipitation Find solubility in a solution with
a common ion
AIM: How can we determine and use solubility product constants? Ksp
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AIM: How can we determine and use solubility product constants? Ksp
Mg(NO3)2 Mg2+ + 2NO3-
Solid Positive Ion + Negative Ion
Keq = [Mg2+ ] [NO3
-]2
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EX) Write out the equilibrium law expression:
AIM: How can we determine and use solubility product constants? Ksp
BaF2(S) Ba+2(aq) + 2F-
(aq)
Ksp = [Ba+2][F-]2
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SOLUBILITY GENERALIZATIONS (TABLE F) All nitrates are soluble All compounds of the alkali metals are
soluble (Li, Na, K, etc.) All compounds of the ammonium
(NH4+) are soluble
AIM: How can we determine and use solubility product constants? Ksp
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Need to set up RICE tables
R- Reaction I – initial concentration C – Change in concentration E – Concentration at equilibrium
AIM: How can we estimate salt solubility from Ksp?
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Ex) What is the solubility of silver bromide (Ksp = 5.2 x 10-13)
AIM: How can we estimate salt solubility from Ksp?
AgBr Ag+ + Br-
(x)(x) = 5.2 x 10-
13X2 = 5.2 x 10-13 X = 7.2 x 10-
7\
Let x = the solubility
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Ex) What is the solubility of PbI2
(Ksp = 7.1 x 10-9)
AIM: How can we determine and use solubility product constants? Ksp
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Step 1: Write out the dissolving equations
Step 2: Determine the most likely precipitate and write out its equation
Step 3: Determine the molar concentrations and
calculate the reaction quotient (Q) The reaction quotient (Q) - The product of the Ksp
equation using the ion concentration before any reaction interaction
If Q > Ksp then a precipitate will form
AIM: How can we determine the formation of precipitates?
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Ex) A student mixes 0.010 mole Ca(NO3)2 in 2 liters of 0.10M Na2CO3 solution. Will a precipitate form?
AIM: How can we determine the formation of precipitates?
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Ex) 0.15 moles of AgNO3 is mixed with 5 liters of .02M NaCl solution. What is the most likely precipitate and will it form?
AIM: How can we determine the formation of precipitates?
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The common ion effect is used to describe the effect on an equilibrium involving a substance that adds an ion that is a part of the equilibrium.
The common ion effect is responsible for the reduction in the solubility of an ionic precipitate when a soluble compound is combining one of the ions of the precipitate is added to the solution in equilibrium with the precipitate.
AIM: What is the common ion effect?
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When AgNO3 is added to a salt solution of AgCl it is described as a source of a common ion, Ag+ ion.
Common ion – ion that enters the solution from 2 different sources
Common ion effect can be used to make an “insoluble” salt even less soluble
AIM: What is the common ion effect?
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Ex) Calculate solubility of CaF2 in a 0.0050M solution of NaF
AIM: What is the common ion effect?
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Enthalpy (H) – flow of energy (heat exchange) at constant pressure when two systems are in contact
Measure only the change in enthalpy ΔH (the difference between the potential energies of the products and reactants)
Exothermic reactions are favored ΔH= -
Δ H = q at constant pressure - open container
AIM: What is enthalpy?
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Δ H can be calculated from several sources including: Stoichiometry Calorimetry q=mcΔT From tables of standard values Hess’s Law Bond Energies
AIM: How can we calculate ΔH ?
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STOICHIOMETRY:
AIM: How can we calculate ΔH ?
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CALORIMETRY:
AIM: How can we calculate ΔH ?
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CALORIMETRY:
AIM: How can we calculate ΔH ?
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TABLES:
AIM: How can we calculate ΔH ?
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HESS’S LAW
AIM: How can we calculate ΔH ?
ΔHrxn = ΣΔHf(products) – ΣΔHf(reactants)
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HESS’S LAW
AIM: How can we calculate ΔH ?
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HESS’S LAW
AIM: How can we calculate ΔH ?
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Energy must be added/absorbed to BREAK bonds (endothermic) in order to overcome the attractive forces between each nuclei and the shared electrons
Energy is released when bonds are FORMED (exothermic) because resulting attractive forced between the bonded atoms lowers potential energy causing a release.
AIM: How can we calculate bond energies ?
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**BARF**
ΔH = Σ Bond Energies (broken) – Σ Bond Energies (formed)
AIM: How can we calculate bond energies ?
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Ex)
AIM: How can we calculate bond energies ?
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What does ΔH tell you about the changes in energy regarding a chemical reaction? ΔH = + reaction is endothermic and heat energy is added into the system
ΔH = - reaction is exothermic and heat energy is lost from the system
(Nature tends to favor the lowest possible energy state!)
SUMMARY FOR ENTHALPY
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ENTROPY ΔS: disorder or randomness of the matter and energy of a system (more disordered/dispersal is favored)
Nature favors CHAOS (high entropy low energy)
AIM: What is entropy?
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Thermodynamically favored processes or reactions are those that involve a decrease in internal energy of the components (ΔH<0) and increase in entropy (ΔS >0)
These are spontaneous or thermodynamically favored
AIM: What is entropy?
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ΔS is + when dispersal/disorder increases (favored)
ΔS is – when dispersal/disorder decreases
NOTE: Units are usually J/(molrxn • K) (not kJ!)
ΔSrxn = ΣΔSf(products) – ΣΔSf(reactants)
AIM: What is entropy?
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Ex)Predict which has the largest increase in entropy:
CO2(s) CO2(g) H2(g) + Cl2(g) 2HCl(g) KNO3(s) KNO3(l) C(diamond) C(graphite)
AIM: What is entropy?
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Ex)
AIM: What is entropy?
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The calculation of Gibbs free energy, ΔG is what ultimately decides whether a reaction is thermodynamically favored or not
A NEGATIVE sign on ΔG indicates that a
reaction is thermodynamically favored (spontaneous)
Several ways to calculate ΔG that links thermochemistry, entropy, equilibrium, and electrochemistry together!
AIM: How can we use Gibbs free energy to predict spontaneity?
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ΔG = ΔH – TΔS
AIM: How can we use Gibbs free energy to predict spontaneity?
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Ex)
AIM: How can we use Gibbs free energy to predict spontaneity?
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Ex) If ΔG is NEGATIVE, the reaction is thermodynamically favorable
If ΔG is ZERO, the reaction is at
equilibrium and If ΔG is POSITIVE, the reaction is NOT
thermodynamically favorable
SUMMARY
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Ex)
SUMMARY
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Spontaneous reactions: DOESN’T REQUIRE EFFORT (proceed on their own without intervention)
Ex) ice melting, nuclear reactions- natural transmutation
AIM: How can we determine if a reaction is spontaneous?
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2 Conditions for a reaction to be considered spontaneous1. Tendency toward lower energy (PE) exothermic
ΔH = (-) Table I * products have lower energy and more stable
AIM: How can we determine if a reaction is spontaneous?
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2. Tendency toward randomness ΔS = (+)Entropy measure of randomness or disorder
Physical changed and entropy :
Low Entropy High Entropy
Intermediate Entropy
Solid Gas Liquid
Table I (stability) greater exothermic reaction the more stable the products are ( lower PE)
AIM: How can we determine if a reaction is spontaneous?
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AIM: How can we determine if a reaction is spontaneous?
Chemical changes and Entropy:
- Free elements (ex. O2, Na, Fe) high entropy
- Compounds (ex. H2O, NH3) lower entropy
- ***NATURE FAVORS REACTIONS THAT HAVE LOW ENERGY AND HIGH ENTROPY***
Free Energy ΔG = Lets us know if a reaction is spontaneous ΔH (negative, exothermic) ΔS (positive, high entropy) **always have ΔG = (-) SPONTANEOUS
Gibbs Free Energy Equation ΔG = ΔH - TΔS
where
ΔG = Gibbs Free Energy, in kJΔH = heat of reaction
T = temperature, in KelvinΔS = entropy change (in kJ · K-
1)