Transcript Chemical Changes and Equations Lab DRB

Presentation init 12/8/2010 by Daniel R. Barnes
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.
Equipment used in Part A:
goggles
ring
clamp
crucible
gauze
crucible
tongs
ring
stand
forceps
“Bunsen
Burner”
evaporating
dish
beaker
clay
triangle
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.”
initial gross mass =
g
(Please make a little box like this on your lab
crucible
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.
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.”
final gross mass =
g
(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.”
Figure 5-1. Testing for the
presence of CO2.
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.
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
Figure 5-1. Testing for the
hits hot glass, the glass shatters.
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
Gas collection
your test tube with the acid in it
tube
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.
FIGURE 5-2: Collecting gas by air
displacement.
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
Gas collection
hydrochloric acid, one of the
tube
“strong” acids. Furthermore, a
6M solution of HCl is quite
concentrated, so use great care
handling the test tube with the
acid in it.
Looks just like water –
BUT IT’S ACID!!!
FIGURE 5-2: Collecting gas by air
Fun fact: Stomach acid is 0.16M, so 6M HCl
displacement.
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.”
Gas collection
tube
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 rightside-up.
FIGURE 5-2: Collecting gas by air
displacement.
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.”
Gas collection
tube
Keep the collection tube upsidedown, or you’ll lose the
hydrogen.
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.
FIGURE 5-2: Collecting gas by air
displacement.
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.”
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.
“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.
Results and Conclusions
“1. Write the word equation for each reaction.”
“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.
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 . . .
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
“7. Compare the new mass with the original mass. If a reaction
did occur, with what did the magnesium react?
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.
Procedure Questions: Part A
“7. Compare the new mass with the original mass. If a reaction
did occur, with what did the magnesium react?
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.
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
“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?
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”.
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
g. calcium carbonate = CaCO3
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
h. hydrochloric acid = HCl
i. zinc chloride = ZnCl2
j. zinc sulfide = ZnS
Reaction Summary
Word Equations for Reactions from Part A:
magnesium + oxygen gas  magnesium oxide
magnesium + nitrogen gas  magnesium nitride
magnesium nitride + water  ammonia + magnesium hydroxide.
Reaction Summary
Word Equations for Reactions from Part B:
copper (II) carbonate  copper oxide + carbon dioxide
copper (II) hydroxide  copper oxide + water
carbon dioxide + calcium hydroxide  calcium carbonate + water
Reaction Summary
Word Equations for Reactions from Part C:
Zinc + hydrochloric acid 
copper (II) hydroxide  copper oxide + water
carbon dioxide + calcium hydroxide  calcium carbonate + water