Reaction Energetics

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