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Reaction Energetics. Factors Affecting Reaction Rates and Equilibrium. Reaction Energetics. Most reactions are accompanied by energy changes, as well as changes of reactants into products. Reactions which give off or produce heat energy are called exo thermic . - PowerPoint PPT Presentation
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Reaction Energetics
Factors Affecting Reaction Rates and Equilibrium
Reaction Energetics
Most reactions are accompanied by energy changes, as well as changes of reactants into products.
Reactions which give off or produce heat energy are called exothermic.
Reactions which taken in or absorb heat energy are endothermic.
Reaction Energetics
Consider the Reaction of methane and oxygen:
CH4 + 2 O2 CO2 + 2 H2O + heat
This is an energy-releasing, or exothermic, reaction.
Reaction Energetics
Reaction Progress
Ener
gy
Eact
H (exothermic)CH4,O2
CO2, H2O
Transition state
CH4 + 2 O2
CO2 + 2 H2O + heat
Reaction Energetics
Eact
CH4,O2
Transition state
For most reactions, even exothermic ones like this, energy starts out uphill; and there is an activation energy. Why???
Reaction EnergeticsFor reactants to start changing, some bonding must
begin to break up. Otherwise, the starting material could never exist. Bond breaking is always endothermic
For the CH4 + O2 reaction, the original molecules must begin to break up before permanent changes can occur.
Reactant molecules usually acquire their needed energy through forceful collisions.
Reaction Energetics
C
H
H
H
H
Reactants Products
O=OO=O
HO
H
HO
H
O=C=O
These bonds must begin to break before reaction can occur
This phase of the reaction has endothermic character.
Reaction Energetics
C
H
H
H
H
Reactants Products
O=OO=O
HO
H
HO
H
O=C=O
As soon as reactant bonds are sufficiently broken, product bonds can begin to form.
Reaction Energetics
C
H
H
H
H
Reactants Products
O=OO=O
HO
H
HO
H
O=C=O
As soon as reactant bonds are sufficiently broken, product bonds can begin to form.
Initial endothermicity is replaced by exothermicity.
Reaction Energetics
Reaction Progress
Ener
gy
Eact
H (exothermic)CH4,O2
CO2, H2O
Transition state
CH4 + 2 O2
CO2 + 2 H2O + heat
Reaction Energetics, Eact
Reaction Progress
Ener
gy
Eact
H A
B
Transition state
Higher Eact normally means slower reaction.
AB
Reaction Progress
Ener
gy
Eact
HX
Y
Transition state
XY(fast) (slow)
Reaction Energetics,
Reaction Progress
Ener
gy
Eact
H (exothermic)R
P
Transition state, pathway 1Transition state, pathway 2
Pathway 2 occurs with a catalyst and is much faster.
Catalysis
Factors Affecting Reaction Rates
Energy of Activation, Eact
Large Eact slow reaction
Most molecules don’t collide hard enough to react. Small Eact fast reaction
More molecules will collide hard enough to react. With smaller Eact, more molecules will have enough kinetic energy at given temperature for reaction.
Reaction Energetics
Reaction Progress
Ener
gy
Eact
H A
B
Transition state
Reaction Progress
Ener
gy
Eact
HX
Y
Transition state
Higher Eact normally means slower reaction.
Factors Affecting Reaction Rates
Temperature Low temperature slow reaction
Most molecules don’t collide hard enough to react.
High temperature fast reaction
More molecules will collide hard enough to react (average K.E. of molecules higher at higher temperatures).
Reaction Energetics
Kinetic Energy
No.
Mol
ecul
es
Eact
KEave, low T
KEave, high T High TLow T
Eact
Factors Affecting Reaction Rates
Reactant Concentrations Low concentration of reactants slow reaction
Low rate of collisions of reacting molecules. High concentration of reactants fast reaction
Higher rate of collision of reacting molecules.Catalysts
Accelerate certain reactions Biological catalysts usually are enzymes.
Factors Affecting Reaction Rates
Energy of Activation, Eact
TemperatureReactant ConcentrationsCatalysts/Enzymes
Summary
Chemical Equilibrium
An equality of rates of opposing processes (forward and reverse reactions).
Does not imply equal amounts or concentrations of reactants and products.
Results in static, unchanging amounts and concentrations of materials.
Especially important when only small energy difference between reactants and products.
Position of Equilibrium
Some chemical systems attain equilibrium with mostly reactants; Some others with mostly products.
Relative energy difference determines what substances are favored. More stable substances will be favored (major). Less stable substances will be disfavored
(minor).
A BEn
ergy
Reaction Progress
Eact
H (exothermic)
A (reactants)
B (products)
Transition stateSince B is more stable than A, it should predominate at equilibrium.
Disturbing Position of EquilibriumLeChâtelier’s Principle
“When a system at equilibrium is disturbed, the system will respond to minimize the disturbance.”
Response to “Disturbances”Disturbance
Add reactant
Add product
Remove reactant
Remove product
Response
Form more product (by using up reactant).
Form more reactant (by using up product).
Use up product (and form more reactant).
Use up reactant (and form more product).
A Real Reaction
H2O H+ + OH-
In water and acids
In water and bases.
A Real Reaction
H2O H+ + OH-
Add H+ (acid)…
H+ decreases.Add OH- (base)…
OH- decreases.
H+ increases.
Remove H+… OH- increases.
Remove OH-…
It’s Like Pushing and Pulling:
A + B C + D
Add some A…
It’s Like Pushing and Pulling:
A + B C + D
Add some A…
A + B C + D
It’s Like Pushing and Pulling:
A + B C + D
It’s Like Pushing and Pulling:
A + B C + D
“Pushing” on the left causes the reaction to shift to the right to re-establish equilibrium.
It’s Like Pushing and Pulling:
A + B C + D
Remove some D...
It’s Like Pushing and Pulling:
A + B C + D
It’s Like Pushing and Pulling:
A + B C + D
“Pulling” on the right causes the reaction to shift to the right to re-establish equilibrium.
Your turn…
How should the equilibrium concentration of ammonia, NH3, be affected by increasing the concentration of H2 in the system?
N2 + 3 H2 2 NH3
Your turn…
N2 + 3 H2 2 NH3
How should the equilibrium concentration of ammonia, NH3, be affected by increasing the concentration of H2 in the system?
Hint: we’re “pushing “from the left.
Your turn…
N2 + 3 H2 2 NH3
How should the equilibrium concentration of ammonia, NH3, be affected by increasing the concentration of H2 in the system?
Good; there’s a rightward shift to minimize the disturbance.
Another…
How should the equilibrium concentration of oxygenated hemoglobin in the blood be affected by increasing the concentration of acid?
HHb + O2 H+ + HbO2-1
Hemoglobin Oxygenated hemoglobin
Acid
Another…
How should the equilibrium concentration of oxygenated hemoglobin in the blood be affected by increasing the concentration of acid?
Hint: we’re “pushing “from the right.
HHb + O2 H+ + HbO2-1
Hemoglobin Oxygenated hemoglobin
Acid
Another…
How should the equilibrium concentration of oxygenated hemoglobin in the blood be affected by increasing the concentration of acid?
Good; “pushing” from the right causes a shift to the left.
HHb + O2 H+ + HbO2-1
Hemoglobin Oxygenated hemoglobin
Acid
