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: CH O 2  CO H 2 O + heat This is an energy-releasing, or exothermic, reaction.

Reaction Energetics Reaction Progress  Energy  E act  H (exothermic) CH 4, O 2 CO 2, H 2 O Transition state CH O 2  CO H 2 O + heat

Reaction Energetics Reaction Progress  Energy  E act  H (exothermic) CH 4, O 2 CO 2, H 2 O Transition state For most reactions, even exothermic ones like this, energy starts out uphill; and there is an activation energy. Why???

Reaction Energetics §For reactants to start changing, some bonding must begin to break up. l Otherwise, the starting material could never exist. l Bond breaking is always endothermic §For the CH 4 + O 2 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=O H O H H O 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 O=OO=O H O H H O H O=C=OO=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 O=OO=O H O H H O H O=C=OO=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  Energy  E act  H (exothermic) CH 4, O 2 CO 2, H 2 O Transition state CH O 2  CO H 2 O + heat

Reaction Energetics, E act Reaction Progress  Energy  E act HH A B Transition state Higher E act normally means slower reaction. ABAB Reaction Progress  Energy  E act HH X Y Transition state XYXY (fast) (slow)

Reaction Energetics, Reaction Progress  Energy  E act  H (exothermic) R P Transition state, pathway 1 Transition state, pathway 2 Pathway 2 occurs with a catalyst and is much faster. Catalysis

Factors Affecting Reaction Rates §Energy of Activation, E act l Large E act  slow reaction Most molecules don’t collide hard enough to react. l Small E act  fast reaction More molecules will collide hard enough to react. With smaller E act, more molecules will have enough kinetic energy at given temperature for reaction.

Reaction Energetics Reaction Progress  Energy  E act HH A B Transition state Reaction Progress  Energy  E act HH X Y Transition state Higher E act normally means slower reaction.

Factors Affecting Reaction Rates §Temperature l Low temperature  slow reaction Most molecules don’t collide hard enough to react. l 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. Molecules  E act KE ave, low T KE ave, high T High T Low T E act

Factors Affecting Reaction Rates §Reactant Concentrations l Low concentration of reactants  slow reaction Low rate of collisions of reacting molecules. l High concentration of reactants  fast reaction Higher rate of collision of reacting molecules. §Catalysts l Accelerate certain reactions l Biological catalysts usually are enzymes.

Factors Affecting Reaction Rates §Energy of Activation, E act §Temperature §Reactant Concentrations §Catalysts/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. l More stable substances will be favored (major). l Less stable substances will be disfavored (minor).

A B Energy  Reaction Progress  E act  H (exothermic) A (reactants) B (products) Transition state Since B is more stable than A, it should predominate at equilibrium.

Disturbing Position of Equilibrium LeChâtelier’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 H 2 O H + + OH - In water and acids In water and bases.

A Real Reaction H 2 O 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, NH 3, be affected by increasing the concentration of H 2 in the system? N H 2 2 NH 3

Your turn… N H 2 2 NH 3 How should the equilibrium concentration of ammonia, NH 3, be affected by increasing the concentration of H 2 in the system? Hint: we’re “pushing “from the left.

Your turn… N H 2 2 NH 3 How should the equilibrium concentration of ammonia, NH 3, be affected by increasing the concentration of H 2 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 + O 2 H + + HbO 2 -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 + O 2 H + + HbO 2 -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 + O 2 H + + HbO 2 -1 Hemoglobin Oxygenated hemoglobin Acid