CHAPTER 6 Energy General, Organic, & Biological Chemistry Janice Gorzynski Smith
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Transcript of CHAPTER 6 Energy General, Organic, & Biological Chemistry Janice Gorzynski Smith
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CHAPTER 6Energy
General, Organic, & Biological Chemistry
Janice Gorzynski Smith
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CHAPTER 6: Energy
Smith. General Organic & Biolocial Chemistry 2nd Ed.
Learning Objectives: Definition of Energy, Kinetic Energy, Potential Energy
Heat transfer in reactions
Enthalpy
Exothermic and endothermic
Energy unit conversions and calculations
Bond Strength
Energy diagrams
How to change the rate of a reaction
Catalysts
Equilibrium: definition and calculations
Re-establishing equilibrium and Le Chatlier’s principle
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Energy Definition of Energy
Smith. General Organic & Biolocial Chemistry 2nd Ed.
Total Energy
Potential Energy
Kinetic Energy
= +
Energy is the capacity to do work.
Potential energy is stored energy.
The law of conservation of energy states that the total energy in a system does not change. Energy cannot be created or destroyed.
Kinetic energy is the energy of motion.
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Energy Kinetic Energy
Smith. General Organic & Biolocial Chemistry 2nd Ed.http://www.petervaldivia.com/technology/energy/
http://scienceisntscary.wordpress.com/tag/kinetic-energy/
Kinetic Energy (KE)
Energy of motion
KE = ½mv 2
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Energy Potential Energy
Smith. General Organic & Biolocial Chemistry 2nd Ed.
Potential Energy
= Stored energy
Exists in natural attractions and repulsions
Chemical Energy• PE possessed by chemicals
• Stored in chemical bonds• Breaking bonds requires energy• Forming bonds releases energy
PE
Reactants Products
Lower PEFavorable
Stable
Higher PEUnfavorable
UnstableR
P
P
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Energy Units of Energy
Smith. General Organic & Biolocial Chemistry 2nd Ed.
•A calorie (cal) is the amount of energy needed to raise the temperature of 1 g of water by 1 oC.
•A joule (J) is another unit of energy.
1 cal = 4.184 J
•Both joules and calories can be reported in the larger units kilojoules (kJ) and kilocalories (kcal).
1,000 J = 1 kJ 1,000 cal = 1 kcal
1 kcal = 4.184 kJ
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Energy Example: Energy in a Gummy Bear
Smith. General Organic & Biolocial Chemistry 2nd Ed.
A gummy bear is 9.000 Calories (nutritional calories). How much energy is stored in a gummy bear in units of Joules?
9.000 Cal = 9.000 kcal x 1000 cal = 9000. cal
9000. cal x 4.184 J = 37660. J = 37.66 kJ
1 kcal
1 cal
Gummy Bear Video: https://www.youtube.com/watch?v=6YWGnfnEmgM&src_vid=Jzoi7dJAiSc&feature=iv&annotation_id=annotation_713078
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Energy Breaking and Forming Bonds
Smith. General Organic & Biological Chemistry 2nd Ed.
ClCl
To cleave this bond, 58 kcal/mol must be added.
H = +58 kcal/molEndothermic To form this bond, 58
kcal/mol is released.H = −58 kcal/mol
Exothermic
Breaking bonds requires energyForming bonds releases energy
H is the energy absorbed or released in a reaction; it is called the heat of reaction orthe enthalpy change.
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Energy Enthalpy Change & Bond Dissociation Energy
Smith. General Organic & Biological Chemistry 2nd Ed.
The bond dissociation energy is the H for breakinga covalent bond by equally dividing the e− between the two atoms.
Bond dissociation energies are positive values, because bond breaking is endothermic (H > 0).
Bond formation always has negative values, because bond formation is exothermic (H < 0).
H H H + H
H + H H H
H = +104 kcal/mol
H = −104 kcal/mol
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Energy Bond Strength
Smith. General Organic & Biological Chemistry 2nd Ed.
The stronger the bond, the higher its bond dissociation E.
H indicates the relative strength of the bonds broken and formed in a reaction:
• H negative: Exothermic reaction: more energy required to break products then reactant bonds: products have stronger bonds.
• H positive: Endothermic reaction: less energy required to break products then reactant bonds: products have weaker bonds.
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Energy Endothermic & Exothermic
Smith. General Organic & Biological Chemistry 2nd Ed.
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Reactions Energy Diagrams
Smith. General Organic & Biological Chemistry 2nd Ed.
The orientation of the two molecules must be correct as well.
For a reaction to occur, two molecules must collidewith enough kinetic energy to break bonds.
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Reactions Energy Diagrams
Smith. General Organic & Biological Chemistry 2nd Ed.
Ea, the energy of activation, is the difference in energy between the reactants and the transition state. It can be thought of as the energy barrier that must be overcome for the reaction to occur.
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Reactions Energy Diagrams
Smith. General Organic & Biological Chemistry 2nd Ed.
H is positive, the reaction is endothermic:
H is negative, the reaction is exothermic:
When the Ea is high, few molecules have enough energy to cross the energy barrier, and the reaction is slow.
When the Ea is low, many molecules have enough energy to cross the energy barrier, and the reaction is fast.
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E
Reactants
ProductsEXOTHERMIC
ENDOTHERMIC
Heat released
Heat absorbed
ENDOTHERMICHeat + Reactants Products
Products have weaker bonds and a higher energy then Reactants.
Heat is absorbed by the system.ΔE + ΔH +
EXOTHERMICReactants Products + heat
Products have stronger bonds and a lower energy then Reactants.
Heat is released by the system.
ΔE - ΔH -
PE increasesas bonds
break
PE decreasesas bonds
form
Summary Energy & Reactions
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Reactions Ex: Splitting Water
Smith. General Organic & Biological Chemistry 2nd Ed.
Requirments: Very Endothermico Need a minimum of 1.23 V to split water
o Kinetically infrared light could do this, but the reaction is very slow
o The potential really needs to be at least 3.0 V to utilize the full spectrum of light
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Reactions Rates of Reactions
Smith. General Organic & Biological Chemistry 2nd Ed.
Increasing the concentration of the reactants:•Increases the number of collisions•Increases the reaction rate
Increasing the temperature of the reaction:•Increases the kinetic energy of the molecules•Increases the reaction rate
A catalyst is a substance that speeds up the rate of a reaction and can be recovered unchanged.
•Catalysts lower Activation Energy, Ea.
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Reactions Catalysts
Smith. General Organic & Biological Chemistry 2nd Ed.
•The uncatalyzed reaction (higher Ea) is slower.
•The catalyzed reaction (lower Ea) is faster.
H is the same for both reactions.
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Reactions Catalysts: Photosystem II
Smith. General Organic & Biological Chemistry 2nd Ed.
PQ + H2O --> PQH2 + O2 (g)
The overall reaction of Photosystem II is the oxidation of water and the reduction of plastoquinone.
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Reactions Equilibrium
Smith. General Organic & Biological Chemistry 2nd Ed.
A reversible reaction can occur in either direction.
CO(g) + H2O(g) CO2(g) + H2(g)
The forward reaction proceeds to the right.
The reverse reactionproceeds to the left.
•The system is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.
•The net concentrations of reactants and products do not change at equilibrium.
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Reactions Equilibrium
Smith. General Organic & Biological Chemistry 2nd Ed.
a A + b B c C + d D
equilibriumconstant = K =
[C]c [D]d
[A]a [B]b=
[products][reactants]
The relationship between the concentration of the products and the concentration of the reactants is the equilibrium constant, K.
*Brackets, [ ], are used to symbolize concentration in moles per liter (mol/L).
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Reactions Equilibrium
Smith. General Organic & Biological Chemistry 2nd Ed.
N2(g) + O2(g) 2 NO(g)
equilibriumconstant
= K =[N2] [O2]
[NO]2
*The coefficient becomes the exponent.
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Reactions Equilibrium
Smith. General Organic & Biological Chemistry 2nd Ed.
HOW TO Calculate the Equilibrium Constant for a Reaction
A2 + B2 2 AB
Step [1]
Write the expression for the equilibriumconstant from the balanced equation.
[AB]2
[A2][B2]K =
Step [2]
Substitute the given concentrations inthe equilibrium expression and calculate K.
[AB]2
[A2][B2]K = =
[0.50]2
[0.25][0.25] =0.25
0.0625= 4.0
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Reactions Le Châtelier’s Principle
Smith. General Organic & Biological Chemistry 2nd Ed.
If a chemical system at equilibrium is disturbed orstressed, the system will react in a direction thatcounteracts the disturbance or relieves the stress.
Some of the possible disturbances:
1) Concentration changes
2) Temperature changes
3) Pressure changes
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Reactions Le Châtelier’s Principle
Smith. General Organic & Biological Chemistry 2nd Ed.