Chemistry 12 Measuring Reaction Rate Using Volume of Gas ...
Chemistry 12 Measuring Reaction Rate Using Volume of Gas ...
Chemistry 12 Measuring Reaction Rate Using Volume of Gas ...
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Abstract<br />
<strong>Chemistry</strong> <strong>12</strong><br />
<strong>Measuring</strong> <strong>Reaction</strong> <strong>Rate</strong> <strong>Using</strong> <strong>Volume</strong> <strong>of</strong> <strong>Gas</strong><br />
Produced<br />
From previous lectures, the effect <strong>of</strong> various factors on the rate <strong>of</strong> a chemical reaction<br />
was demonstrated. In these demonstrations, the time required to complete the reaction<br />
was measured and a reaction rate was calculated from that quantity. In addition to finding<br />
the time required for a reaction to go to completion, chemists frequently need to know<br />
what the rate is at different times during a reaction, in order to monitor how the reaction<br />
is proceeding. This information is obtainable in a variety <strong>of</strong> ways. For instance, if a gas is<br />
produced in a closed container, then continuous monitoring <strong>of</strong> the pressure indicates the<br />
rate. If a color is produced or used up, monitoring <strong>of</strong> the color intensity with a<br />
spectrophotometer indicates the rate. If a gas is produced and allowed to escape from the<br />
system, the decrease in mass over various time intervals shows how the reaction is<br />
progressing. This latter method can easily be demonstrated with an electronic balance.<br />
When a flask containing hydrochloric acid is placed on the pan <strong>of</strong> an electronic balance<br />
and marble chips are dropped in, the decrease in mass with time gives a measure <strong>of</strong> the<br />
rate at which CO 2 is produced. Yet another method <strong>of</strong> monitoring the rate <strong>of</strong> a reaction<br />
involving gases is to measure the volume <strong>of</strong> gas produced by displacing water from a<br />
eudiometer (gas measuring tube). It is this latter method that will be used in this<br />
experiment.<br />
Ordinary household bleach is an aqueous solution <strong>of</strong> Sodium hypochiorite, NaClO,<br />
containing a little more than 5% NaClO by mass. The bleaching action is caused by the<br />
hypochiorite ion, ClO - . Under normal circumstances the hypochlorite ion breaks down<br />
slowly to give oxygen gas and the chloride ion, Cl - .<br />
2 ClO - (aq) → 2 Cl - (aq) + O 2(g)<br />
To speed this reaction to a measurable rate a catalyst is required. In this experiment, the<br />
catalyst is provided by the addition <strong>of</strong> Cobalt (II) nitrate solution to the bleach. A black<br />
precipitate <strong>of</strong> Cobalt (III) oxide forms and acts as the catalyst for the decomposition <strong>of</strong><br />
ClO - . The volume <strong>of</strong> oxygen produced is measured at 30 second intervals by displacing<br />
water from a eudiometer. From the results, you can plot a graph <strong>of</strong> gas volume produced<br />
versus time and calculate the average rate <strong>of</strong> oxygen evolution in millilitres per minute.<br />
The experiment is repeated at other temperatures and concentrations <strong>of</strong> ClO - and the<br />
effect <strong>of</strong> these changes on the rate is observed.<br />
<strong>Chemistry</strong> <strong>12</strong> - 1 -
Purpose<br />
1. To measure the volume <strong>of</strong> a gas produced from a reaction mixture at regular time<br />
intervals during the reaction.<br />
2. To interpret the results and obtain the overall rate <strong>of</strong> reaction.<br />
3. To observe how the rate changes at different temperatures and concentrations<br />
Materials<br />
Apparatus<br />
250 mL Erlenmeyer flask 10 mL graduated cylinder<br />
One hole rubber stopper (#6) 2 x 600 mL beaker<br />
5 cm <strong>of</strong> glass tubing Thermometer<br />
Rubber tubing<br />
Source <strong>of</strong> hot water<br />
Eudiometer (50 mL)<br />
Ice<br />
Burret stand and clamp<br />
Lab apron<br />
25 mL graduated cylinder Safety goggles<br />
Stopwatch<br />
Hot plate<br />
Reagents<br />
5.25% Sodium hypochlorite<br />
0.10 M Cobalt (II) nitrate, Co(NO 3 ) 2<br />
Procedure<br />
1. Put on your lab apron and safety goggles.<br />
2. Refer to Figure 1 to help with understanding how to set up the apparatus.<br />
Figure 1:<br />
Set-up <strong>of</strong> the equipment<br />
for collection <strong>of</strong> gas.<br />
- 2 -
3. Fill the eudiometer with water and invert it into the 600 mL beaker (half-filled<br />
with water), without letting any water come out. Hold it in the vertical position<br />
with the buret clamp attached to the stand.<br />
4. Join the rubber tubing to the top <strong>of</strong> the glass tube which goes through the stopper<br />
on the flask. Place the other end into the neck <strong>of</strong> the eudiometer.<br />
5. Set up a hot water bath with another 600 mL beaker and your hot plate. The goal<br />
is to achieve a temperature <strong>of</strong> 10 o C above room temperature for use in step 11.<br />
6. Measure 15 mL <strong>of</strong> bleach solution into the 25 mL graduated cylinder and pour it<br />
into the Erlenmeyer flask.<br />
7. Measure 5 mL <strong>of</strong> 0.10 M Cobalt (II) nitrate solution into the 10 mL graduated<br />
cylinder.<br />
8. Pour the Cobalt (II) nitrate solution into the flask and immediately place the<br />
stopper and tube on it. Record the time <strong>of</strong> mixing.<br />
9. Note the formation <strong>of</strong> a black precipitate <strong>of</strong> Cobalt (III) oxide, Co 2 O 3 . From now<br />
on you must swirl the flask gently but continually. This is necessary to dislodge<br />
bubbles <strong>of</strong> oxygen from the surface <strong>of</strong> the Co 2 O 3 catalyst. If you stop swirling the<br />
rate decreases, so the amount <strong>of</strong> swirling must be kept uniform throughout this<br />
and subsequent steps <strong>of</strong> the procedure.<br />
10. Record the total volume <strong>of</strong> oxygen that has collected in the eudiometer every 30<br />
seconds until a volume <strong>of</strong> 50 mL has been obtained. Also record the actual<br />
elapsed time when the 50 mL mark is reached.<br />
11. Repeat Steps 3 to 9, but have the reactants at a temperature <strong>of</strong> 10°C above room<br />
temperature before mixing them. You can accomplish this by placing both the<br />
flask with bleach and the graduated cylinder with Cobalt (II) nitrate in a water<br />
bath for 10 minutes, then adding the Cobalt (II) nitrate to the flask and putting it<br />
back into the water bath.<br />
<strong>12</strong>. Repeat Step 10, but bring the reactants to a temperature 10°C below room<br />
temperature using an ice water bath.<br />
13. Repeat Steps 3 to 9 at room temperature, but add 20 mL <strong>of</strong> water to the bleach<br />
solution before mixing, so that the overall concentrations are half <strong>of</strong> their original<br />
values.<br />
14. Repeat Steps 3 to 9 at room temperature, but this time add 60 mL <strong>of</strong> water to the<br />
bleach solution before mixing, so that the overall concentrations after mixing are<br />
one quarter <strong>of</strong> their original values.<br />
15. Clean up all your materials, following the instructions for reagent disposal<br />
16. Before you leave the laboratory, wash your hands thoroughly with soap and<br />
water.<br />
- 3 -
Observations<br />
Table 1: Oxygen Production with Full Strength Bleach at Room Temperature<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
(mL)<br />
(mL)<br />
- 4 -
Table 2: Oxygen Production with Full Strength Bleach at 10 o C above Room Temperature<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
(mL)<br />
(mL)<br />
- 5 -
Table 3: Oxygen Production with Full Strength Bleach at 10 o C below Room Temperature<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
(mL)<br />
(mL)<br />
- 6 -
Table 4: Oxygen Production with Half Strength Bleach at Room Temperature<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
(mL)<br />
(mL)<br />
- 7 -
Table 5: Oxygen Production with Quarter Strength Bleach at Room Temperature<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
<strong>Volume</strong> <strong>of</strong> O<br />
Time (s)<br />
2(g)<br />
(mL)<br />
(mL)<br />
- 8 -
Post Lab Considerations<br />
The reaction in which the Co 2+ ion reacts with the ClO - ion is given by the equation:<br />
2 Co 2+ (aq) + ClO - (aq) + 2 H 2 O (l) → Co 2 O 3(s) + 4 H + (aq) + Cl - (aq)<br />
This reaction has to occur before Co 2 O 3 can start catalyzing the bleach, so do not be<br />
surprised if you find that no oxygen is given <strong>of</strong>f during the first 30 or 60 seconds.<br />
When you plot your results, you may find that the rate is not uniform. One cause might<br />
be that large bubbles come through just as you take a measurement. Another is the<br />
decreasing bleach concentration as the reaction proceeds. The trend relating reaction rate<br />
to changing temperature and concentration is nevertheless readily apparent.<br />
Analysis<br />
1. Plot the graphs for each trial on a single sheet <strong>of</strong> graph paper, plotting volume <strong>of</strong><br />
oxygen produced verses time elapsed. Label each graph with the conditions under<br />
which the results graphed were obtained.<br />
2. For each trial, calculate the overall rate <strong>of</strong> production <strong>of</strong> oxygen by dividing the<br />
volume <strong>of</strong> 50 mL by the time taken to produce that amount, in minutes.<br />
3. Compare the calculated values <strong>of</strong> the rates with the temperature used. By what<br />
factor does the rate change with a 10 o C increase in temperature? By what factor<br />
does the rate change with a 10 o C decrease in temperature?<br />
4. Compare the calculated values <strong>of</strong> the rates at different concentrations <strong>of</strong> bleach.<br />
By what factor does the rate change when the concentration <strong>of</strong> the bleach is<br />
halved? By what factor does the rate change when the concentration <strong>of</strong> the bleach<br />
is quartered?<br />
Questions<br />
1. Bleach is made by the action <strong>of</strong> chlorine gas on sodium hydroxide, NaOH:<br />
Cl 2(g) + 2 OH - (aq) → Cl - (aq) + ClO - (aq) + H 2 O (l)<br />
However, if an acid is added to bleach, the process is reversed:<br />
Cl - (aq) + ClO - (aq) + 2 H + (aq) → Cl 2(g) + H 2 O (l)<br />
Why should you never mix bleach with any cleaner or other household product<br />
that may contain an acid?<br />
2. Special cleaning agents such as those used for cleaning mold and mildew <strong>of</strong>f<br />
bathroom tiles may contain 10% Sodium hypochlorite. Predict how the shape <strong>of</strong><br />
the rate curve with this concentration differs from that <strong>of</strong> regular strength bleach.<br />
- 9 -
Conclusion<br />
1. State the overall rate <strong>of</strong> reaction for each temperature and concentration used.<br />
2. <strong>Using</strong> collision theory, describe how the rate changes at different temperatures<br />
and concentrations.<br />
- 10 -