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This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.

 

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.)

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

Required Equipment and Supplies

  • goggles, gloves, and protective clothing
  • balance and weighing paper
  • thermometer
  • timer
  • foam cup
  • ruler
  • Alka-Seltzer tablets (3)
  • water

All of the specialty lab equipment and chemicals needed for this and other
lab sessions are available individually from Maker Shed or other laboratory
supplies vendors. Maker Shed also offers customized laboratory kits at special
prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the
Supplemental Chemicals Kit.

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CAUTION

Although the chemicals used in this laboratory are not hazardous, it is good practice to wear splash goggles, gloves, and protective clothing at all times.

Substitutions and Modifications

  • You may substitute any brand of fizzy tablets for the Alka-Seltzer.
  • If you don’t have Alka-Seltzer, or similar tablets, you may substitute small sticks of chalk or calcium carbonate antacid 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.

Procedure

  1. If you have not already done so, put on your splash 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 the 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.
  10. Dispose of the spent solution and rinse out the cup.
  11. Repeat steps 1 through 9, using a tablet that you have split into quarters.
  12. 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.)

Table 12-2. Effect of surface area on reaction rate – observed and calculated data

Item Trial A (solid) Trial B (chunks) Trial C (powder)
A. mass of tablet
___.__g
___.__g
___.__g
B. mass of cup + water
___.__g
___.__g
___.__g
C. temperature of water
___.__°C
___.__°C
___.__°C
D. mass at 0:00 (A + B)
___.__g
___.__g
___.__g
E. mass at 0:05
___.__g
___.__g
___.__g
F: mass at 0:10
___.__g
___.__g
___.__g
G: mass at 0:15
___.__g
___.__g
___.__g
H: mass at 0:20
___.__g
___.__g
___.__g
I: mass at 0:25
___.__g
___.__g
___.__g
J: mass at 0:30
___.__g
___.__g
___.__g
K: mass at 0:35
___.__g
___.__g
___.__g
L: mass at 0:40
___.__g
___.__g
___.__g
M: mass at 0:45
___.__g
___.__g
___.__g
N: mass at 0:50
___.__g
___.__g
___.__g
O: mass at 0:55
___.__g
___.__g
___.__g
P: mass at 1:00
___.__g
___.__g
___.__g
Q: mass at completion of reaction
___.__g
___.__g
___.__g
R: mass loss (D – Q)
___.__g
___.__g
___.__g
S: mass loss percentage [(R/A)·100]
___.__%
___.__%
___.__%

Disposal

All of the solutions from this laboratory can be flushed down the drain with plenty of water.

Review Questions

Q1: What effect did you observe surface area to have on reaction rate?

Q2: Use the ruler to measure the diameter and thickness of a tablet. Calculate its approximate surface area. Make the same calculation for surface area of the quartered tablet. Does the increase in reaction rate you observed with the quartered tablet approximately correspond to the increase in surface area? If not, propose an explanation.

Q3: Most fizzy tablet remedies direct the user to dissolve two tablets in a glass of water. Why don’t they simply make the tablets larger and recommend dissolving only one of the larger tablets?

6 Responses to Laboratory 12.2: Determine the Effect of Surface Area on Reaction Rate

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