Chemistry 12 Measuring Reaction Rate Using Volume of Gas ...

Abstract
Chemistry 12
Measuring Reaction Rate Using Volume of Gas
Produced
From previous lectures, the effect of various factors on the rate of a chemical reaction
was demonstrated. In these demonstrations, the time required to complete the reaction
was measured and a reaction rate was calculated from that quantity. In addition to finding
the time required for a reaction to go to completion, chemists frequently need to know
what the rate is at different times during a reaction, in order to monitor how the reaction
is proceeding. This information is obtainable in a variety of ways. For instance, if a gas is
produced in a closed container, then continuous monitoring of the pressure indicates the
rate. If a color is produced or used up, monitoring of the color intensity with a
spectrophotometer indicates the rate. If a gas is produced and allowed to escape from the
system, the decrease in mass over various time intervals shows how the reaction is
progressing. This latter method can easily be demonstrated with an electronic balance.
When a flask containing hydrochloric acid is placed on the pan of an electronic balance
and marble chips are dropped in, the decrease in mass with time gives a measure of the
rate at which CO 2 is produced. Yet another method of monitoring the rate of a reaction
involving gases is to measure the volume of gas produced by displacing water from a
eudiometer (gas measuring tube). It is this latter method that will be used in this
experiment.
Ordinary household bleach is an aqueous solution of Sodium hypochiorite, NaClO,
containing a little more than 5% NaClO by mass. The bleaching action is caused by the
hypochiorite ion, ClO - . Under normal circumstances the hypochlorite ion breaks down
slowly to give oxygen gas and the chloride ion, Cl - .
2 ClO - (aq) → 2 Cl - (aq) + O 2(g)
To speed this reaction to a measurable rate a catalyst is required. In this experiment, the
catalyst is provided by the addition of Cobalt (II) nitrate solution to the bleach. A black
precipitate of Cobalt (III) oxide forms and acts as the catalyst for the decomposition of
ClO - . The volume of oxygen produced is measured at 30 second intervals by displacing
water from a eudiometer. From the results, you can plot a graph of gas volume produced
versus time and calculate the average rate of oxygen evolution in millilitres per minute.
The experiment is repeated at other temperatures and concentrations of ClO - and the
effect of these changes on the rate is observed.
Chemistry 12 - 1 -

Purpose
1. To measure the volume of a gas produced from a reaction mixture at regular time
intervals during the reaction.
2. To interpret the results and obtain the overall rate of reaction.
3. To observe how the rate changes at different temperatures and concentrations
Materials
Apparatus
250 mL Erlenmeyer flask 10 mL graduated cylinder
One hole rubber stopper (#6) 2 x 600 mL beaker
5 cm of glass tubing Thermometer
Rubber tubing
Source of hot water
Eudiometer (50 mL)
Ice
Burret stand and clamp
Lab apron
25 mL graduated cylinder Safety goggles
Stopwatch
Hot plate
Reagents
5.25% Sodium hypochlorite
0.10 M Cobalt (II) nitrate, Co(NO 3 ) 2
Procedure
1. Put on your lab apron and safety goggles.
2. Refer to Figure 1 to help with understanding how to set up the apparatus.
Figure 1:
Set-up of the equipment
for collection of gas.
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3. Fill the eudiometer with water and invert it into the 600 mL beaker (half-filled
with water), without letting any water come out. Hold it in the vertical position
with the buret clamp attached to the stand.
4. Join the rubber tubing to the top of the glass tube which goes through the stopper
on the flask. Place the other end into the neck of the eudiometer.
5. Set up a hot water bath with another 600 mL beaker and your hot plate. The goal
is to achieve a temperature of 10 o C above room temperature for use in step 11.
6. Measure 15 mL of bleach solution into the 25 mL graduated cylinder and pour it
into the Erlenmeyer flask.
7. Measure 5 mL of 0.10 M Cobalt (II) nitrate solution into the 10 mL graduated
cylinder.
8. Pour the Cobalt (II) nitrate solution into the flask and immediately place the
stopper and tube on it. Record the time of mixing.
9. Note the formation of a black precipitate of Cobalt (III) oxide, Co 2 O 3 . From now
on you must swirl the flask gently but continually. This is necessary to dislodge
bubbles of oxygen from the surface of the Co 2 O 3 catalyst. If you stop swirling the
rate decreases, so the amount of swirling must be kept uniform throughout this
and subsequent steps of the procedure.
10. Record the total volume of oxygen that has collected in the eudiometer every 30
seconds until a volume of 50 mL has been obtained. Also record the actual
elapsed time when the 50 mL mark is reached.
11. Repeat Steps 3 to 9, but have the reactants at a temperature of 10°C above room
temperature before mixing them. You can accomplish this by placing both the
flask with bleach and the graduated cylinder with Cobalt (II) nitrate in a water
bath for 10 minutes, then adding the Cobalt (II) nitrate to the flask and putting it
back into the water bath.
12. Repeat Step 10, but bring the reactants to a temperature 10°C below room
temperature using an ice water bath.
13. Repeat Steps 3 to 9 at room temperature, but add 20 mL of water to the bleach
solution before mixing, so that the overall concentrations are half of their original
values.
14. Repeat Steps 3 to 9 at room temperature, but this time add 60 mL of water to the
bleach solution before mixing, so that the overall concentrations after mixing are
one quarter of their original values.
15. Clean up all your materials, following the instructions for reagent disposal
16. Before you leave the laboratory, wash your hands thoroughly with soap and
water.
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Observations
Table 1: Oxygen Production with Full Strength Bleach at Room Temperature
Volume of O
Time (s)
2(g)
Volume of O
Time (s)
2(g)
(mL)
(mL)
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Table 2: Oxygen Production with Full Strength Bleach at 10 o C above Room Temperature
Volume of O
Time (s)
2(g)
Volume of O
Time (s)
2(g)
(mL)
(mL)
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Table 3: Oxygen Production with Full Strength Bleach at 10 o C below Room Temperature
Volume of O
Time (s)
2(g)
Volume of O
Time (s)
2(g)
(mL)
(mL)
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Table 4: Oxygen Production with Half Strength Bleach at Room Temperature
Volume of O
Time (s)
2(g)
Volume of O
Time (s)
2(g)
(mL)
(mL)
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Table 5: Oxygen Production with Quarter Strength Bleach at Room Temperature
Volume of O
Time (s)
2(g)
Volume of O
Time (s)
2(g)
(mL)
(mL)
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Post Lab Considerations
The reaction in which the Co 2+ ion reacts with the ClO - ion is given by the equation:
2 Co 2+ (aq) + ClO - (aq) + 2 H 2 O (l) → Co 2 O 3(s) + 4 H + (aq) + Cl - (aq)
This reaction has to occur before Co 2 O 3 can start catalyzing the bleach, so do not be
surprised if you find that no oxygen is given off during the first 30 or 60 seconds.
When you plot your results, you may find that the rate is not uniform. One cause might
be that large bubbles come through just as you take a measurement. Another is the
decreasing bleach concentration as the reaction proceeds. The trend relating reaction rate
to changing temperature and concentration is nevertheless readily apparent.
Analysis
1. Plot the graphs for each trial on a single sheet of graph paper, plotting volume of
oxygen produced verses time elapsed. Label each graph with the conditions under
which the results graphed were obtained.
2. For each trial, calculate the overall rate of production of oxygen by dividing the
volume of 50 mL by the time taken to produce that amount, in minutes.
3. Compare the calculated values of the rates with the temperature used. By what
factor does the rate change with a 10 o C increase in temperature? By what factor
does the rate change with a 10 o C decrease in temperature?
4. Compare the calculated values of the rates at different concentrations of bleach.
By what factor does the rate change when the concentration of the bleach is
halved? By what factor does the rate change when the concentration of the bleach
is quartered?
Questions
1. Bleach is made by the action of chlorine gas on sodium hydroxide, NaOH:
Cl 2(g) + 2 OH - (aq) → Cl - (aq) + ClO - (aq) + H 2 O (l)
However, if an acid is added to bleach, the process is reversed:
Cl - (aq) + ClO - (aq) + 2 H + (aq) → Cl 2(g) + H 2 O (l)
Why should you never mix bleach with any cleaner or other household product
that may contain an acid?
2. Special cleaning agents such as those used for cleaning mold and mildew off
bathroom tiles may contain 10% Sodium hypochlorite. Predict how the shape of
the rate curve with this concentration differs from that of regular strength bleach.
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Conclusion
1. State the overall rate of reaction for each temperature and concentration used.
2. Using collision theory, describe how the rate changes at different temperatures
and concentrations.
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