Section 18.1 lecture for Honors & Prep Chemistry

1. EXOTHERMIC REACTIONS CAN HURT!
Which would you rather be near?
ΔH for Rusting -822 kJ/mol
ΔH for H2 explosion -483.6 kJ/mol

2. EXOTHERMIC REACTIONS CAN HURT!
Which would you rather be near?
ΔH for Rusting -822 kJ/mol
ΔH for H2 explosion -483.6 kJ/mol

3. REACTION RATES
HOW FAST?
VERY
ΔH for Rusting -822 kJ/mol slow
ΔH for H2 explosion -483.6 kJ/mol VERY
fast

4. Chemical rates are expressed as
Increase in products/time
or
Decrease in reactants/time

5. A rate measures a change that
occurs over time.

6. Rates of chemical reactions are often
measured as a change in the number
of moles during an interval of time.

7. According to collision theory,
atoms, ions, and molecules can
react to form products when they
collide with one another, IF the
colliding particles have enough
kinetic energy.

8. Effective Collision

9. Ineffective Collision

10. The minimum energy that colliding
particles must have in order to react
is called the activation energy.

11. The activated complex or transition
state is the unstable arrangement of
atoms that forms momentarily at the
peak of the activation-energy barrier.

12. ENDOTHERMIC
EXOTHERMIC
Much bigger
activation
energy!
High activation energy = slower rate
Low activation energy = faster rate

13. A tiny spark will provide
the activation energy
needed to react a few
molecules of gas.
The exothermic
reaction of these few
molecules will
release enough
energy to react the
rest of the gas tank.
BOOM!

14. Activation Energy Video

15. The rate of a chemical reaction
depends upon temperature,
concentration, particle size, and the
use of a catalyst.

16. Temperature –
INCREASE TEMP, INCREASE RATE
Faster particles collide more often and
with more force, so they are more likely to
get over the ACTIVATION ENERGY
BARRIER.

17. Temperature –
INCREASE TEMP, INCREASE RATE
Storing foods in a refrigerator keeps them
fresh longer. Low temperatures slow
microbial action.

18. Concentration-
Increase reactant concentration, increase rate
Crowded reactant particles collide more often.

19. Concentration-
Increase reactant concentration, increase rate
a. In air, a lighted splint glows and soon goes out.
b. When placed in pure oxygen (higher oxygen
concentration), the splint bursts into flame.

20. Particle Size
MORE SURFACE AREA, FASTER RATE
More
particles are
exposed, so
more
collisions are
possible

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

22. Catalysts
ADD CATALYST, INCREASE RATE
A catalyst
lowers the
activation
energy
barrier, so it
is easier to
get over!

23. A catalyst increases reaction rate, but it is
not changed during the reaction.
It is there at the beginning…
… and it is still there at the end.

24. An enzyme is a biological catalyst

25. An inhibitor is a substance that interferes with
the action of a catalyst.
Antioxidants and antimicrobials used in drying
fruits and preserving fruit juices slow the
action of microbes and limit contact with air.

26. 18.1 Section Quiz.
1. The units below that would be appropriate to
measure the rate of a chemical reaction is
a. mmol/s.
b. mol/L.
c. kJ/mol.
d. h/mol.

27. 18.1 Section Quiz.
2. In a chemical reaction, the energy of reactants
is 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.

28. 18.1 Section Quiz.
3. An increase in which one of the following will
NOT increase the reaction rate?
a. temperature
b. concentration of reactants
c. volume of gaseous reactants
d. surface area of reactants

29. 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.