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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 concentrations because more reactant molecules are available and therefore collisions between reactant molecules are more likely. In the two preceding labs, we used Alka-Seltzer tablets and water to demonstrate the effects of temperature and surface area on reaction rates. Obviously, we’ll have to use some other method to demonstrate the effect of concentration on reaction rates, because the amounts of the citric acid and sodium bicarbonate reactants in a fizzy tablet are fixed and cannot be changed.

n this lab, we’ll use another over the counter medicine with varying concentrations of hydrochloric acid to demonstrate the effect of concentration on reaction rates. Some (but not all) antacid tablets contain primarily calcium carbonate, along with flavoring, binders, and other inactive ingredients. These tablets neutralize excess stomach acid, which is actually dilute hydrochloric acid, according to the following equation:

CaCO3(s) + 2HCI(aq) → CaCI2(aq) + CO2(g) + H2O(I)

We’ll use exactly this reaction to observe and quantify the effect of concentration of reaction rates.

Required Equipment and Supplies

  • goggles, gloves, and protective clothing
  • balance and weighing paper
  • graduated cylinder, 100 mL
  • thermometer
  • timer
  • foam cup (3)
  • marking pen
  • calcium carbonate antacid tablets (3)
  • hydrochloric acid, 4 M (100 mL)
  • hydrochloric acid, 2 M (100 mL)
  • hydrochloric acid, 1 M (100 mL)
  • 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

Hydrochloric acid is corrosive and emits strong fumes. Wear splash goggles, gloves, and protective clothing at all times.

Substitutions and Modifications

  • You may substitute small (~ 1 cm) pieces of chalk (calcium carbonate) for the calcium carbonate antacid tablets.
  • In making up the dilute hydrochloric acid solutions, you may substitute muriatic acid from the hardware store, which is typically 31.45% (10.3 M), for reagent grade hydrochloric acid, which is typically 37% (12 M). Adjust quantities accordingly.

Procedure

  1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
  2. Label three foam cups 4 M, 2 M, and 1 M.
  3. In the first foam cup, make up 100 mL of 4 M hydrochloric acid by adding 33.3 mL of 12 M HCl to 66.7 mL of water. Again, make sure that the combined mass of the foam cup, antacid tablet, and 100 mL of hydrochloric acid is less than the maximum capacity of your balance. If not, reduce the quantity of hydrochloric acid accordingly.)
  4. In the second foam cup, make up 100 mL of 2 M hydrochloric acid by adding 16.7 mL of 12 M HCl to 83.3 mL of water.
  5. In the third foam cup, make up 100 mL of 1 M hydrochloric acid by adding 8.3 mL of 12 M HCl to 91.7 mL of water.
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CAUTION: Add Acid to Water and Wait

Always add acid to water. Adding water to a concentrated acid can cause splattering. Diluting acid produces heat. Because we want concentration to be the only variable, it’s important to ensure that all three acid solutions are at the same initial temperature. Make your dilute acid solutions ahead of time and allow all three cups to cool to room temperature before proceeding.

  1. Weigh one of the antacid tablets to 0.01 g and record its mass on Line A of Table 12-3.
  2. Place the first foam cup with 4 M hydrochloric acid on the balance and record the mass to 0.01 g on Line B of Table 12-3.
  3. Measure and record the temperature of the HCl solution on Line C of Table 12-3.
  4. With the cup and its contents still on the balance, drop the antacid tablet into the cup.
  5. Note the combined mass of the cup, acid, and tablet every five seconds and record each mass in Table 12-3. (It may be helpful to have one person watching the clock while another calls out the mass reading at each 5-second milestone.)
  6. Continue recording the changing mass until you reach one minute or until the reaction completes, as evidenced by the cessation of bubbling.
  7. When the reaction completes, record the final mass of the cup, water, and tablet on Line Q of Table 12-3.
  8. Dispose of the spent solution and rinse out the cup.
  9. Repeat steps 6 through 9, using 2 M hydrochloric acid.
  10. Repeat steps 6 through 9, using 1 M hydrochloric acid.

Table 12-3. Effect of concentration on reaction rate – observed and calculated data

Item Trial A (4 M HCI) Trial B (2 M HCI) Trial C (1 M HCI)
A. mass of tablet
___.__g
___.__g
___.__g
B. mass of cup + hydrochloric acid
___.__g
___.__g
___.__g
C. temperature of hydrochloric acid
___.__°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

Neutralize the spent acid solutions with sodium bicarbonate or another base and flush the neutralized solutions 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 4 M, 2 M, and 1 M HCl. Determine if the reaction rate is linear over time and whether the concentration of the HCl affects linearity.
  • In the first lab, we mentioned that a rule of thumb says that an increase of 10 °C in temperature should approximately double reaction rate. (Did your experimental data verify or disprove this speculation?) Repeat this experiment, using 100 mL of 1 M HCl at a temperature 10 °C higher than the temperature of the acid in your original run. Compare the results to your data for the 1 M and 2 M HCl at the lower temperature.

Review Questions

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

Q2: Based on the data you recorded in Table 12-3, is the effect of concentration on reaction rate linear?

Q3: Based on the data you recorded in Table 12-3, at any particular concentration 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.

One Response to Laboratory 12.3: Determine the Effect of Concentration on Reaction Rate

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