Note: Before the day of this lab, you MUST have the purpose, procedure, pre-lab questions, and data chart prepared for Part I and Part II. If you don't have it, you will not be allowed to do the lab.
Safety:
Hydrogen peroxide is harmful to body tissues. Goggles must be worn at all times to protect your eyes, not your forehead or neck. Avoid contact with your skin.
Hydrogen peroxide (H2O2) is an unstable liquid that spontaneously (automatically) breaks down into water (H2O) and oxygen (O2) through a decomposition reaction. The reaction is relatively slow, but can be sped up by the use of a catalyst. In this case, the catalyst will be potassium iodide (KI). The catalyst assists the reaction without being involved chemically. The reaction is written
2H2O2 2H2O + O2.
Part I- Preparation of O2 gas
Procedure:
In this procedure, approximately 50 mL of O2 will be produced. To perform this procedure, weigh out .10 grams of KI and place it in the vial cap. Float the cap on the water in an overfilled inverted syringe with your finger over the hole. Slowly release the water allowing the vial cap to lower to the bottom without tipping. Put the syringe plunger back into the syringe and press it down until it just contacts the vial cap. Now slowly draw exactly 4 mL of 6% H2O2 into the syringe. Now place the plastic cap on the opening in the syringe and shake to mix the reactants. The reaction will be slow, but will actually produce more than 60 mL, so you must be ready to point the syringe upward and remove the syringe cap when 60 mL of O2 have been produced. Then point the syringe downward, flush out the excess liquid reagents into the sink, and recap the syringe. Now the inside of the syringe must be washed. Draw approximately 5 mL of distilled water into the syringe and shake it up to clean the inside walls of the syringe. Now discharge the water, but not the gas. Repeat this washing at least one more time.
To test your gas, you are going to inject it into the intake of a Bunsen burner and make some observations about the results. Take your syringe over to the lit Bunsen burner. First, draw a sketch of what the flame looks like noting the colors and any cones inside of the large cone. Then use the thermocouple (high temperature thermometer) to measure the temperature of the flame. Note that the small cone at the mouth of the burner is the coolest part of the flame because the air cannot get to it. If this part gets larger, that means that the flame is getting cooler; if it gets smaller, the flame is getting hotter. The best chemistry teacher in the world (no we're not having a guest speaker, that's me!) will tell you when to inject your gas. One partner should watch the flame as the gas is being injected and the other should watch the thermocouple. Everyone in the group should have a sketch of the flame and the temperature before and after the injection in their lab notebooks.
Pre-Lab Questions-
1) What do you think will happen to the inner cone and the temperature of the Bunsen burner if oxygen is injected into it?
2) If 80 mL of oxygen is produced at 25oC and 730 mmHg, how many mL is that at STP?
3) In number 2, how many moles of O2 is that? How many moles of H2O2 did it take to produce it?
Post-Lab Questions-
1) What happened to the structure, shape and color of the flame as the gas was injected? What did this suggest about the temperature of the flame?
2) What happened to the temperature of the flame according to the thermocouple? Did this agree with your answer to question number 1?
3) The balanced equation for the burning of methane (CH4) follows:
CH4 + 2 O2 CO2 + 2 H2O
If the methane in the Bunsen burner is flowing at a rate of 1 mole per minute, how fast should the oxygen flow to make the stoichiometrically correct mixture of CH4 and O2 according to the balanced equation?
Part II- Preparation of Cl2 gas
In this procedure, chlorine gas will be produced by reacting hydrochloric acid with household bleach (sodium hypochlorite, NaOCl). The reaction is
2 HCl(aq) + NaOCl(aq) Cl2(g) + NaCl(aq) + H2O(l)
For this reaction to proceed, it must be acidic. If extra NaOCl (a base) were added, the products would actually decrease.
Safety note- 6 M HCl is extremely dangerous to you skin and eyes. Use extreme care in handling it. Wear goggles at all times and rinse with plenty of water if contact is made.
Procedure- Use a calibrated micropipette (the plastic eyedropper with the lines on it) to put exactly 1.0 mL of 6 M HCl into the vial cap. From the usual procedure, get the cap into the syringe, insert the plunger, and draw 3.0 mL of bleach into the syringe. Cap the syringe and shake it to activate the reaction. You may need to help the plunger move out. Pull it gently every 15-20 seconds. When the reaction is complete (it stops bubbling or the syringe is full), point the syringe up and remove the cap. Then point it down and release the excess liquid into the neutralization/waste container. Rinse the chlorine gas twice with distilled water. You may notice the color change when you rinse the gas. This is normal.
Now you will do two experiments with the Cl2 gas today and a couple of more in the future. First, put a small amount of one of the colored fruit juices into a plastic weighing boat and bubble about 20 mL of the Cl2 gas through it. Record in your lab notebooks what happened as you did. Don't use too much gas or you won't have enough for the second part. Now rinse the weighing boat and fill it with distilled water. Put two drops of Universal Indicator into the dH2O. In the fume hood, bubble 5 mL of the gas at a time through the liquid and record the color of the solution at each 5 mL.
Pre-Lab Questions-
1. Besides the Cl2 gas, will you be able to see any of the other products from this reaction?
2. 1.0 mL of 6.0 M HCl contains .006 moles of HCl. Theoretically, how many mL of Cl2 gas would be formed at STP if all of the HCl were used up?
3. Do you think that you will get more or less than the theoretical value? Why?
Post-Lab Questions-
1. Which fruit juice did you choose? What happened to the color of the fruit juice?
2. What happened to the color of the universal indicator solution? What does that mean about the pH of the solution as you bubbled the gas through?
Questions? Comments??