EXOTHERMIC REACTIONS CAN HURT!

Which would you rather be near?

ΔH for Rusting -822 kJ/mol

ΔH for H2 explosion -483.6 kJ/mol

EXOTHERMIC REACTIONS CAN HURT!

Which would you rather be near?

ΔH for Rusting -822 kJ/mol

ΔH for H2 explosion -483.6 kJ/mol

REACTION RATES

HOW FAST?

ΔH for Rusting -822 kJ/mol

ΔH for H2 explosion -483.6 kJ/mol

VERYslow

VERY fast

Chemical rates are expressed as

Increase in products/time

or

Decrease in reactants/time

A rate measures a change thatoccurs over time.

Rates of chemical reactions are oftenmeasured as a change in the numberof moles during an interval of time.

According to collision theory,atoms, ions, and molecules canreact to form products when theycollide with one another, IF thecolliding particles have enoughkinetic energy.

Effective Collision

Ineffective Collision

The minimum energy that collidingparticles must have in order to reactis called the activation energy.

The activated complex or transitionstate is the unstable arrangement ofatoms that forms momentarily at thepeak of the activation-energy barrier.

EXOTHERMIC ENDOTHERMIC Much bigger

activation energy!

High activation energy = slower rateLow activation energy = faster rate

A tiny spark will providethe activation energyneeded to react a fewmolecules of gas.

The exothermicreaction of these fewmolecules willrelease enoughenergy to react therest of the gas tank.BOOM!

Activation Energy Video

The rate of a chemical reactiondepends upon temperature,concentration, particle size, and theuse of a catalyst.

Temperature –

INCREASE TEMP, INCREASE RATE

Faster particles collide more often andwith more force, so they are more likely toget over the ACTIVATION ENERGYBARRIER.

Temperature –

INCREASE TEMP, INCREASE RATE

Storing foods in a refrigerator keeps themfresh longer. Low temperatures slowmicrobial action.

Concentration-

Increase reactant concentration, increase rate

Crowded reactant particles collide more often.

Concentration-

Increase reactant concentration, increase rate

a. In air, a lighted splint glows and soon goes out.

b. When placed in pure oxygen (higher oxygenconcentration), the splint bursts into flame.

Particle Size

MORE SURFACE AREA, FASTER RATE

Moreparticles areexposed, somorecollisions arepossible

The minute size of the reactant particles(grain dust), and the mixture of the graindust with oxygen in the air caused thereaction to be explosive, destroying thegrain elevator.

Catalysts

ADD CATALYST, INCREASE RATE

A catalystlowers theactivationenergybarrier, so itis easier toget over!

A catalyst increases reaction rate, but it isnot changed during the reaction.

It is there at the beginning…

… and it is still there at the end.

An enzyme is a biological catalyst

An inhibitor is a substance that interferes withthe action of a catalyst.

Antioxidants and antimicrobials used in dryingfruits and preserving fruit juices slow theaction of microbes and limit contact with air.

18.1 Section Quiz.

1. The units below that would be appropriate tomeasure the rate of a chemical reaction is

a. mmol/s.

b. mol/L.

c. kJ/mol.

d. h/mol.

18.1 Section Quiz.

2. In a chemical reaction, the energy of reactantsis always

a. greater than the energy of the products.

b. more than the activation energy.

c. less than the activation energy.

d. less than the energy of the products.

18.1 Section Quiz.

3. An increase in which one of the following willNOT increase the reaction rate?

a. temperature

b. concentration of reactants

c. volume of gaseous reactants

d. surface area of reactants

18.1 Section Quiz.

4. A catalyst works because it

a. lowers the activation energy.

b. increases the temperature.

c. is permanently changed in a reaction.

d. supplies energy to a reaction.