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

 

Reactions proceed faster at higher temperatures because increasing the temperature also increases the average kinetic energy of the reactant molecules, making it more likely that a collision between two reactant molecules will have sufficient energy to initiate the reaction. An old rule of thumb states that increasing the temperature by 10 °C doubles the rate of reaction. (So, of course, reducing the temperature by 10 °C halves the reaction rate.)

We’ll test that rule in this lab by reacting Alka-Seltzer tablets with water at different temperatures. When an Alka-Seltzer tablet is dropped into water, it emits carbon dioxide as it dissolves and its components react. We could get basic data about the reaction rate simply by determining how long it takes a tablet to dissolve completely in water at various temperatures. But using that method gives us data only about the starting and ending points. It doesn’t tell us, other than in the most general terms, whether the reaction rate is linear or if it changes over time.

We know that this reaction evolves gaseous carbon dioxide, which means that the mass of the reaction vessel and contents decreases as the reaction proceeds. By recording the mass of the reaction vessel and its contents periodically as the reaction progresses, we can gather the data needed to determine if the reaction rate is linear and whether reaction temperature affects the linearity of the reaction rate.

Required Equipment and Supplies

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

sciRoomCAUTION2.gif 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 cold and warm dilute (~ 3 M) solutions 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 12-1.
  3. Add about 100 mL of cold tap water to the foam cup. (Use a foam cup rather than a beaker to keep the mass of the reaction vessel and its contents under the maximum capacity of your balance. If the combined mass of the foam cup, Alka-Seltzer tablet, and 100 mL of water is greater than the maximum capacity of your balance, reduce the quantity of water accordingly.)
  4. Weigh the foam cup + water and record the mass on Line B of Table 12-1. (Make sure the combined mass of the cup and water
  5. Measure and record the temperature of the water on Line C of Table 12-1.
  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 12-1. (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 12-1.
  10. Dispose of the spent solution and rinse out the cup.
  11. Repeat steps 1 through 9, using hot tap water.
  12. Repeat steps 1 through 9, using a mixture of half hot and half cold tap water.

figure12-01.jpg

Figure 12-1. Observing the mass change as the Alka-Seltzer tablet reacts and carbon dioxide is evolved

Item Trial A Trial B Trial C
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 lab can be flushed down the drain with plenty of water.

Optional Activities

If you have time and the required materials, consider performing these optional activities:

  • Graph your results for the trials using cold, hot, and warm water. Determine if the reaction rate is linear over time and whether the reaction temperature affects linearity.
  • The main component of fizzy tablets is sodium bicarbonate, usually mixed with citric acid, aspirin, binders, and other inactive components. When it comes into contact with water, the citric acid and sodium bicarbonate react to form, among other products, carbon dioxide. The carbon dioxide is evolved as a gas, which accounts for the mass loss. Write a balanced equation for this reaction, and use the data from Table 12-1 to calculate the approximate percentage of sodium bicarbonate by weight in the tablet.

Review Questions

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

Q2: Based on the data you recorded in Table 12-1, does the 10 °C rule of thumb provide a reasonably close approximation of the observed reaction rates in this laboratory? How far do your data depart from the expected values based on the rule of thumb?

Q3: Based on the data you recorded in Table 12-1, does reaction rate appear to be approximately linear over time? If you noticed an increase or decrease in reaction rate over time, propose at least one possible explanation.

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