Illustrated Guide to Home Chemistry Experiments

216 DIY Science: Illustrated Guide to Home Chemistry Experiments

LABORATORY 12.2:

dETERmINE THE EffECT of SURfACE AREA oN REACTIoN RATE

If at least one of the reactants is a solid, the

reaction proceeds faster if the solid is finely

divided, because the surface area is larger in

a finely divided solid, exposing more of that

reactant to the other reactant or reactants. For

example, a 50-pound bag of flour is essentially

inert, because the flour, although finely ground,

exposes little of its surface area to the air.

But that same amount of flour dispersed as

airborne dust, if ignited by a spark, explodes

with force sufficient to flatten a large building.

(Military fuel-air explosives, or FAEs, use

this principle by vaporizing a liquid fuel and

detonating it. During Operation Desert Storm,

British troops reportedly sent a flash-priority

report of a nuclear detonation, mistaking the

detonation of an FAE for a tactical nuke.)

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing papers

£ thermometer

£ timer

£ foam cup

£ ruler

£ Alka-Seltzer tablets (3)

£ water

In the last laboratory, we examined reaction rates of Alka-Seltzer
tablets, keeping the initial surface area constant and varying the
temperature. In this laboratory, we’ll explore the effect on reaction
rates of varying the surface area at constant temperature.

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute any brand of fizzy tablets for the
  Alka-Seltzer tablets.


• If you don’t have Alka-Seltzer or similar tablets,
  you may substitute small sticks of chalk or calcium
  carbonate antacid tablets for the Alka-Seltzer tablets
  and a dilute (~3 M) solution of hydrochloric acid for the
  water. If you do that, remember that hydrochloric acid
  is corrosive and hazardous to handle.
  POCEDURER
  1.ou have not already done so, put on your splash If y
  goggles, gloves, and protective clothing.

2. Weigh one of the tablets to 0.01 g and record its mass on
   line A of Table 12-2.


3. Add about 100 mL of cold tap water to the foam cup.
   Again, make sure the combined mass of the foam cup,
   Alka-Seltzer tablet, and 100 mL of water is less than the
   maximum capacity of your balance. If not, reduce the
   quantity of water accordingly.


4. Weigh the foam cup + water and record the mass on
   line B of Table 12-2.


5. Measure and record the temperature of the water on
   line C of Table 12-2.


6. With the cup and water still on the balance, drop the
   tablet into the cup.


7. Note the combined mass of the cup, water, and tablet
   every five seconds and record each mass in Table 12-2.
   (It may be helpful to have one person watching the clock
   while another calls out the mass reading at each 5-
   second milestone.)


8. Continue recording the changing mass until you reach
   one minute or until the reaction completes, as evidenced
   by the cessation of bubbling.


9. When the reaction completes, record the final mass of the
   cup, water, and tablet on line Q of Table 12-2.
   Dispose of the spent solution and rinse out the cup.
   Repeat steps 1 through 9, using a tablet that you have
   split into quarters.
   Repeat steps 1 through 9, using a tablet that you have
   crushed into powder. (This reaction may be too fast to
   time accurately; just do your best.)

10.

11.

12.