NOTE: This laboratory activity is based on a xeroxed copy of some lab that one of my former department heads (now retired) had me do back when I first started teaching chemistry. It was probably created by some textbook company or other, but there are no copyright or other annotations anywhere on the photocopies, so I don’t know the ultimate origin of the photocopies I’m basing this lab on.

Presentation init 12/8/2010 by Daniel R. Barnes

Equipment used in Part A:

“BunsenBurner”

forceps

goggles crucible gauze

ringstandcrucible

tongs

evaporating dish

beaker

clay triangle

ringclamp

eyedropper

A. Synthesis“1. Obtain a piece of magnesium about 13 cm long and roll it inot a loose ball.” spiral

“2. Place the Mg in a clean crucible and measure the mass of the crucible and its contents carefully to the nearest 0.01 g. Record the mass.”

ginitial gross mass =

(Please make a little box like this on your lab worksheet just to the right of procedure #2. for part A.)

Make sure to zero the scale properly before placing the crucible with Mg upon it. This probably means hitting the “ON” button before you place anything on the scale.

crucible

A. Synthesis“3. Place the crucible in a clay triangle, and place the clay triangle on a ring stand, as shown in Experiment 4, Figure 4-2, page 26.”

“4. Begin heating, slowly at first. Slowly increase the intensity of heat to the hottest flame of your laboratory burner. CAUTION: The Mg may begin to burn. If it does burn, do not look directly into the flame.”

Magnesium burns with a blinding white light. If you stare at it, you’ll overload your photoreceptors in your retina and you’ll see weird pink (and blue?) patches in your field of vision for a while afterwards. That can’t be too good of a sign.

(This slide is just a white rectangle for having the american flag show up on.)

A. Synthesis“5. When the reaction is complete, stop heating. After the crucible has cooled slightly, remove it from the clay triangle with forceps. Maeasure the mass, then empty the crucible’s contents into the evaporating dish.”

gfinal gross mass =

(Please make a little box like this on your lab worksheet just to the left of procedure #5. for part A.)

As before, make sure the scale is zeroed correctly before measuring the mass of the crucible, especially if there is some kind of protective mesh or foil on top of the scale to protect the scale from the heat of the crucible.

A. Synthesis“6. Examine the contents for a change in composition. Add a few drops of water to the residue and try to detect the odor of ammonia (NH3) gas. What does this suggest? (Hint: Is oxygen the only gas in air which will combine with Mg?”

INSTRUCTOR: pour a little ammonia from a jug onto a paper towel with “NH3” written on it and pass it around so everyone gets a chance to learn what ammonia smells like before the lab.NO HUFFING, KIDS. Just get a whiff, and no more.

Answer the question from step 6 on your sheet of notebook paper once you’ve done step 6. Label your answer as “Question A6”.

A. Synthesis“7. Compare the new mass with the original mass. If a reaction did occur, with what did the magnesium react?

When you reach this point in the lab, please write your answer on your separate sheet of notebook paper entitled “Chemical Changes and Equations Lab Notes”. Label it as “Question A7”.

Dispose of the magnesium ashes as indicated by your instructor.

Please click the mouse and read the next slide before doing part A for real.

When doing any laboratory activity . . .

READ AND FOLLOW THE DIRECTIONS.

DO THE STEPS IN THE CORRECT ORDER.

DON’T SKIP ANY STEPS.

I didn’t give you the lab worksheet just to kill trees.

B. Decomposition“1. From the stock glass tubing (provided by your teacher), use a triangular file to cut a piece of tubing about 15 cm long. Fire polish the ends and bend the glass tubing into a 90o angle. CAUTION: Sharp glass and hot glass. Your teacher may demonstrate the correct techniques for bending and fire polishing glass tubing. See the Laboratory Techniques in the front of this book.”

We’re not going to do this step because we don’t have any glass tubing for you to cut, polish, and bend. Sorry. It would have been fun. Maybe in the future sometime . . .

B. Decomposition“2. Obtain two small spatulas full of CuCO3 and place it in a large, dry test tube.

Mr. Barnes has already done this for you. Just go get the tube with the green powder already in it.

B. Decomposition“3. Insert the glass tube into a one-hole rubber stopper and insert the stopper in the test tube containing the CuCO3. CAUTION: Use a lubricant such as glycerol or water, and towels or some other hand protection. Work slowly. Do not force the tubing.”

Once again, Mr. Barnes (or students from previous years) has already done most of this for you. You will, however, need to put the rubber stopper/bent glass tube assembly into your big test tube that has the green powder in it.

B. Decomposition“4. Pour about 5 mL of lime water, Ca(OH)2(aq), into a small test tube. Place the end of the right angle glass tube in the limewater solution as shown in Figure 5-1.”

NOTE: Regarding the test tube holder used to hold the smaller test tube with the lime water in it: test tube holders let go of the test tube when you squeeze them. This is perverse and counterintuitive, but it’s true.

Figure 5-1. Testing for the presence of CO2.

B. Decomposition“5. Heat the tube containing the CuCO3 while holding the end of the glass tube in the limewater solution. Continue heating until the bubbling has nearly stopped. A cloudy appearance in the Ca(OH)2 indicates the presence of CO2. Observe. Is there a change?

BIG WARNING: Once the bubbling has nearly stopped it is critically important that you take away the limewater from the bent glass tube BEFORE you turn off the flame. If you remove or turn off the flame first, the vacuum created in the larger tube will suck limewater from the smaller tube. When cold water hits hot glass, the glass shatters.Figure 5-1. Testing for the

presence of CO2.

B. Decomposition“6. Let the test tube cool and empty the contents of both tubes.”

Dispose of the chemicals as indicated by your instructor.

C. Displacement“1. Use the apparatus used for part B. Place a small piece of zinc in the a test tube and add 5 mL of 6M HCl. Insert the rubber stopper containing the glass delivery tube. CAUTION: Keep away from open flame.

Mr. Barnes will not let you have your test tube with the acid in it until you return your burner and your sparker to him. If you light a fire anywhere near the gas collection apparatus, there could be a hydrogen gas explosion that blows bits of broken glass and fire everywhere. THIS IS SERIOUS.

Gas collectiontube

FIGURE 5-2: Collecting gas by airdisplacement.

C. Displacement“1. Use the apparatus used for part B. Place a small piece of zinc in the a test tube and add 5 mL of 6M HCl. Insert the rubber stopper containing the glass delivery tube. CAUTION: Keep away from open flame.

Also, be aware that HCl is hydrochloric acid, one of the “strong” acids. Furthermore, a 6M solution of HCl is quite concentrated, so use great care handling the test tube with the acid in it.

Gas collectiontube

FIGURE 5-2: Collecting gas by airdisplacement.

Looks just like water – BUT IT’S ACID!!!

Fun fact: Stomach acid is 0.16M, so 6M HCl is about 36 times stronger than stomach acid.

C. Displacement“2. A reaction should occur and a gas should escape from the tubing. With the glass tubing turned up, collect some of the gas being liberated. Collect the gas by displacement of air by inverting another test tube over the upturned gas delivery tube. See Figure 5-2.”

Make sure to keep the collection test tube upside-down. Hydrogen gas is less dense than air, so it will rise up and out of the test tube if you turn it right-side-up.

Gas collectiontube

FIGURE 5-2: Collecting gas by airdisplacement.

C. Displacement“3. Remove the test tube containing the gas from the glass tubing, keeping it inverted, and bring a burning splint near its mouth. A “pop” or “bark” indicates the presence of hydrogen gas.”

Keep the collection tube upside-down, or you’ll lose the hydrogen.

Gas collectiontube

FIGURE 5-2: Collecting gas by airdisplacement.

NOTE: Make sure all fire is kept far away from the gas generation apparatus. We don’t want that tube full of acid, zinc, and hydrogen to blow up on us.

D. Optional Demonstration“1. Obtain about 2 g of zinc dust and about 1 g of sulfur. Mix thoroughly. CAUTION: Be careful not to grind or crush the material as a violent reaction can occur.”

“2. Under a fume hood place the mixture on an asbestos square and ignite with a laboratory burner. CAUTION: The reaction will be very rapid with a large amount of smoke.

The danger involved in the mixing of the zinc and sulfur powder is one of the reasons why your teacher will be doing this as a demo rather than having you do it yourself.

Results and Conclusions“1. Write the word equation for each reaction.”

Answer questions 1, 2, and 3 on your separate sheet(s) of notebook paper. I think it would acutally be better if you do the word equation for a reaction, then immediately do the balanced chemical equation for that reaction, and identify the type of reaction BEFORE doing the next reaction. 123 123 123 123 . . .

“2. Write a chemical equation for each of the three reactions. Be careful to include correct formulas for all of the reactants and products. Balance the equations.”

“3. Indicate the type of chemical change which has occurred in each reaction.

Questions and Problems“1. How can you tell if a chemical reaction has occurred? What are some distincitve changes that can be observed? How do these changes differ from physical changes?”

“2. When magnesium is burned in air, what products are formed?”

“3. In Part C, the test for hydrogen was the sound resulting when some of the gas exploded. Do any other gases have this characterisic? If so, name one.”

“4. Which of the reactions in this experiment [were] exothermic and which [were] endothermic?”

The slides that follow include answers to the “Results and Conclusions” section questions and to the “Questions and Problems” section questions. There are also answers to the questions asked during the procedure of the lab.

Procedure Questions: Part A“6. Examine the contents for a change in composition. Add a few drops of water to the residue and try to detect the odor of ammonia (NH3) gas. What does this suggest? (Hint: Is oxygen the only gas in air which will combine with Mg?”

QA6: Magnesium does combine with oxygen from the air to form magnesium oxide (MgO), but it also combines with nitrogen from the air to form magnesium nitride (Mg3N2).

You can’t make ammonia (NH3) without nitrogen, and neither magnesium oxide nor water has nitrogen in it, so there must have been a nitrogen-containing compound in the ashes.

I suppose it’s also possible that magnesium nitrate (Mg(NO3)2)could also have been formed during the burning of the magnesium, but Barbara Takumi never insinuated this to me.

Procedure Questions: Part A

QA7: The magnesium reacted with both oxygen and nitrogen from the air to form magnesium oxide and magnesium nitride.

Before the reaction, the crucible contained only magnesium. During the reaction, oxygen and nitrogen atoms bonded to the magnesium atoms, thereby increasing the mass of the contents of the crucible.

The opposite of “oxidation” is “reduction”. (Chapter 20) These days, these two words mean “stealing electrons” and “giving electrons”, but when they were coined, they meant “combination with oxygen” and “reduction to pure metal”. With regard to metals, oxidation increases mass and reduction reduces mass.

“7. Compare the new mass with the original mass. If a reaction did occur, with what did the magnesium react?

Procedure Questions: Part A

QA7: The magnesium reacted with both oxygen and nitrogen from the air to form magnesium oxide and magnesium nitride.

Before the reaction, the crucible contained only magnesium. During the reaction, oxygen and nitrogen atoms bonded to the magnesium atoms, thereby increasing the mass of the contents of the crucible.

“7. Compare the new mass with the original mass. If a reaction did occur, with what did the magnesium react?

NOTE: The increase in mass does not indicate that matter was created. It merely means that pre-existing atoms from the air left the air to join the magnesium. No matter is created during a chemical reaction. It just gets rearranged.

Procedure Questions: Part B

QB5: The water turned from relatively clear to white and cloudy. A white solid precipitate formed. After the test tube sat, undisturbed, the white, powdery material settled to the bottom of the test tube to form a layer of white dust.

The white material was calcium carbonate, CaCO3. Calcium carbonate is the chemical that chalk and seashells are made of. It is not particularly soluble in water, so it formed an undissolved solid that slowly sank to the bottom of the test tube. This sinking process is kind of like rain falling down, so that’s why the powder is called a “precipitate”.

“5. Heat the tube containing the CuCO3 while holding the end of the glass tube in the limewater solution. Continue heating until the bubbling has nearly stopped. A cloudy appearance in the Ca(OH)2 indicates the presence of CO2. Observe. Is there a change?

QUESTION FOR THOUGHT on PART D:

Why do we use 2 g of zinc but only 1 g of sulfur?

Think about that.We’ll address it fully in chapter 12.

Results and Conclusions / Questions and Problems

1. Determine the formulas of the following ionic and/or electrolytic compounds. Show your work. (Work not shown yet here . . . )

a. magnesium oxide = MgO

b. magnesium nitride = Mg3N2

c. magnesium hydroxide = Mg(OH)2

d. copper (II) carbonate = CuCO3

e. copper (II) oxide = CuO

f. calcium hydroxide = Ca(OH)2

g. calcium carbonate = CaCO3h. hydrochloric acid = HCl

i. zinc chloride = ZnCl2j. zinc sulfide = ZnS

Reaction SummaryWord Equations for Reactions from Part A:

magnesium + oxygen gas magnesium oxide

magnesium + nitrogen gas magnesium nitride

magnesium nitride + water ammonia + magnesium hydroxide.

Reaction Summary

copper (II) carbonate copper oxide + carbon dioxide

copper (II) hydroxide copper oxide + water

carbon dioxide + calcium hydroxide calcium carbonate + water

Word Equations for Reactions from Part B:

Reaction Summary

Zinc + hydrochloric acid

copper (II) hydroxide copper oxide + water

carbon dioxide + calcium hydroxide calcium carbonate + water

Word Equations for Reactions from Part C: