Lab 21: Reaction Rate

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Lab 21: Reaction Rate
Introduction
Why is it easier to sweeten hot tea with granular sugar than
cold tea with sugar cubes?
As you begin to stir sugar cubes into cold tea, the sugar particles sluggishly
separate from the cube while following the flow of your spinning stirrer.
Eventually, after much stressful stirring, the little sugar crystals make their
way into solution. Then you notice that your friend has almost finished his
sweet tea and has already returned for more. You complain that you’ve
been stirring forever just to dissolve the sugar in your cold tea. Your friend
responds, “I only had to stir mine for a few seconds, and it was good to
drink!” Why do you think your friend’s sweet tea was finished so much
faster? Well, his sweet tea was made from hot water and granular sugar.
A reaction rate is the time that it takes for the reactants to be changed into
products. The rate is given as the change of the concentration of a reactant
or product in a certain amount of time, and can be described using various
units. Reaction rates are affected by several factors which include the fol‐
lowing: the nature of the reactants, surface area, concentration, tempera‐
ture, pressure, and the presence of a catalyst. Whether a reaction rate will
increase or decrease depends on the rate that the molecules involved effec‐
tively collide to result in a reaction.
Throughout this laboratory exercise, you will use calcium carbonate and cit‐
ric acid to discover how temperature, surface area, and concentration affect
reaction rates. Calcium carbonate is the main compound found in marble.
Marble is often used to make statutes or as decorative rock chips in flower
beds. Citric acid reacts with calcium carbonate to form calcium chloride,
carbon dioxide gas, and water. This is similar to how acid rain degrades
marble statutes.
In this laboratory exercise you will compare how two different surface areas
of calcium carbonate, a powder and a solid rock piece, react with different
concentrations of citric acid at various temperatures. The powder has a large overall surface area due to its many individual
parts. In contrast, the solid, a crystallized rock, has a much smaller surface area. You will record how long it takes for each
reaction to complete, then calculate and compare the reaction rates.
Figure 1: Calcium carbonate is abundant in
nature, and is the primary component of lime‐
stone and marble rock (top). Calcium carbon‐
ate is commonly found in antacids and cal‐
cium supplements.
Concepts to explore:
 Understand how temperature, surface area, and concentration influ‐
ence the rate of a reaction
 Relate reaction rates on a molecular level
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Lab 21: Reaction Rate
Pre‐lab Questions
1. Name five factors that can affect the rate of a reaction.
3. In the opening paragraph example, it took more time to make sweet tea with cold water and sugar cubes than
to sweeten hot tea with granular sugar. Why?
4. What is the primary factor that determines whether a reaction rate increases or decreases?

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Lab 21: Reaction Rate
Experiment: Comparing Reaction Rates
Procedure
1. Label four test tubes 1, 2, 4, and 5 (Reaction 3 takes place in the 50mL beaker, which is why you number the
test tubes this way).
2. Break off a piece of CaCO
3 rock with a mass of approximately 0.2 grams. Record the actual mass in the Data
section. Place the rock in test tube 1.
3. Weigh out three more pieces of approximately 0.2 grams of CaCO
3 rock. These should be as close as possi‐
ble to the mass of the first rock sample. Place the pieces of rock into test tubes
2, 4 and 5. Record each of
their masses Table 1.
4. Into a 50 mL beaker, weigh out approximately 0.2 grams of CaCO
3 powder. This should be as close as possi‐
ble to the amount of the previously weighed pieces of marble rock. Record the mass in Table 1.
Reaction # 1
6. Measure 10 mL of saturated citric acid solution into a 10 mL graduated cylinder. Transfer the acid to test
tube 1 (with the CaCO
3 rock), and place this test tube immediately in an ice bath. Record the start time.
Check on this reaction frequently and record when the reaction no longer produces bubbles (gas). Record all
values in Table 1, along with your observations.
Reaction # 2
6. Measure 5 mL of saturated citric acid solution (60%) into a 10 mL graduated cylinder and dilute to 30% by
adding 5 mL distilled water. Transfer the diluted acid to test tube 2, and place this test tube in the rack. Re‐
cord the start time. Check on this reaction frequently and record the time when the reaction no longer pro‐
duces bubbles (gas). Record all values in Table 1, along with your observations.
Materials
Safety Equipment: Safety goggles, gloves Baking soda
40 mL Saturated citric acid solution (60%) pH Paper
Calcium Carbonate rock (CaCO
3) Permanent marker
Calcium Carbonate powder (CaCO
3) Scale
10 mL graduated cylinder Stopwatch
4 test tubes Stir rod
50 mL beaker Distilled water
*
250 mL beaker Ice*
Test tube holder Boiling water bath*
Test tube rack *You must provide
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Lab 21: Reaction Rate
Reaction # 3
6. Measure 5 mL of saturated citric acid solution (60%) into a 10 mL graduated cylinder and dilute it to 30% by
adding 5 mL distilled water. Transfer this diluted acid to the 50 mL beaker that contains CaCO
3 powder. Use
a stopwatch to time the reaction from when the acid is poured onto the powder and until the reaction no
longer produces bubbles (gas). Record all values in Table 1, along with your observations.
Reaction # 4
6. Measure 10 mL of saturated citric acid solution into a 10 mL graduated cylinder. Transfer the acid to test
tube 4 and place this test tube in the rack
. Record the starting time. Check on the reaction frequently and
record the time when the reaction is no longer fizzing. Record all values in Table 1, along with your observa‐
tions.
Reaction # 5
6. Measure 10 mL of saturated citric acid solution into a 10 mL graduated cylinder. Transfer the acid to test
tube 5, and place this test tube in the previously started hot water bath
. Record the starting time. Check on
this reaction frequently and record the time when the reaction stops (no longer fizzing). Record all values in
Data

Substance,
Reaction #
Variable Mass of the
CaCO
3 (g)
(these should be
close)
Time of the
reaction
(sec)
(start/stop)
% Citric
Acid
Observations
CaCO3 Rock
#1
Saturated acid
solution, iced
CaCO3 Rock
#2
Diluted acid
solution, room
temp.
CaCO3
Powder
#3
Diluted acid
solution, room
temp.
CaCO3 Rock
#4
Saturated acid
solution, room
temp.
CaCO3 Rock
#5
Saturated acid
solution,
heated

Table 1: Reaction rate data and observations
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Lab 21: Reaction Rate
Calculations
Calculate the rate (g/sec) of each of the reactions you observed.
 Reaction 1 (rock, iced, saturated solution):
 Reaction 2 (rock, room temperature, diluted solution):
 Reaction 3 (powder, room temperature, diluted solution):
 Reaction 4 (rock, room temperature, saturated solution):
 Reaction 5 (rock, heated, saturated solution):
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Post‐lab Questions
1. All the reactions that you performed were chemically the same. You just varied several factors. What were the
factors that were varied?
2. Which factor do you think made the biggest influence on the reactions? Why?
3. Out of the five different reactions, which reaction was the slowest? Was this what you expected? Why?
4. Why do you think marble statues require long periods of time to degrade inregions that are affected by acid rain?
Lab 21: Reaction Rate

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