CHEMICAL
CHANGES
AND
EQUATTONS
There are many ways to detec,tor "observe' a chemical change. Some highly precise
instrumentsenableus to measurenot only the degreeof changein composition,but alsothe
change in energywhich takes placewhen substancesreactchemically.For many reactions,the
changesthat occur in energy and composition during these reactionsfollow similar patterns.
A reaction that releasesenergy is exothermic.A reactionthat absorbsenergy is endothermic.
After performing many reactions,scientistsare able to classifythe reactionstrends they have
observed. From these trends, they can develop generalizationsabout the nature of matter.
Such generalizationis the heart of chemistry.
A chemical equation represeintsa chemical reaction and indicatesthe substancesthat
interact,the reactants,and the new substances,or products,that are formed. For the equation
to be completely correct it must be balanced.A balancedequation shows the same number
of each kind of atom (representedby a symbol) on both sides of the equation. For example,
when iron rusts it combines with oxygen to form a new compound, which is mostly FO2O3.
The chemicalequationfor this reactionwould be
4Fe+3O2-2Fe2O3
Reactantsyield Product
In this experimgnt,you will examinefour classesof chemicalreactions-synthesis,
decomposition, singledisplacement,and double displacement.You will be askedto identifysome
of the products formed in these reactionsand write balancedchemicalequationsfor each.
Objectives
ln this experiment,you will
o perform four types of chemical reactions,
o identify some of the products of the reactions,and
o write and balanceequationsfor the reactionsobserved.
EQUIPMENT
gogglesand apron
crucible
burner
ring standand ring
clay triangle
forceps
evaporatingdish
triangularfile
flame spreader
glasstubing(15cm)
spatula
2 largetest tubes
smalltesttube
one hole stopper
testtube holder
utilityclamp
wood splint
PROCEDURE
Preparea datatableasdirectedin the Analysis.Safety
gogglesand lab apron must be worn for this experiment.
A. Synthesis
1. Obtain a pieceof magnesiumabout 13 cm long
and roll it into a looseball. Placethe Mg in a
clean,dry crucibleand measurethe massof the
crucible and its contentscarefullyto the nearest
0.01 g. Recordthe mass.
2. Placethe crucible in a clay triangle,and place
the claytriangleon a ring,asshownin Experiment
3.
4.
5.
6.
4, Figure4-1.Beginheating,slowlyatfirst.Cradually increasethe intensityof heat to the hottest
flame of your laboratoryburner.CAUTION: Mg
is flammable and may begin to burn. lf it does
burn, do not look directly into the flame and remove heat.
When the Mg beginsto glow brightly,stop heating. Afterthe cruciblehascooledslightly,remove
it from the claytrianglewith forceps.Measurethe
masswhen completelycool,thenemptythecruc.
ible'scontentsinto an evaporatingdish.
Examinethecontents
for a changein composition.
Add a few dropsof water to the residueand try
to detect the odor of ammoniagas,NH3.
Comparethe new masswith the originalmass.
Discardthe solidcontentsintothewastecontainer
designatedby your teacher.
B . Decomposition
1 . Usea triangularfile to cut a pieceof glasstubing
about 15 cm longfrom the stockglasstubing(provided by your teacher).Firepolishthe endsand
bendthe glasstubingintoa 90oangle.CAUTION:
Sharp glass and hot g/ass.Your teachershould
demonstratethe correct techniquesfor bending
and fire polishingglasstubing.Seethe Laboratory
Techniquesin the front of this book.
.,
Obtain two small spatulasfull of coppe(ll)
carbonate,
CuCOr,and placethem in i large,dry
testtube. CAUTION: CuCOj is poisonous;avoid
skin contact.
3 . Insertthe glasstube into a one.holerubberstop
per.CAUTTON:Use a lubricant such as glycerol
or water,and towels or some other hand protection when inserting the glass tubing into the
)stopper. Work slowly. Do not force the tubing.
Use only a fire-polished tube and point away
from you when inserting the tube. Insert the
stopperin the testtube containingthe CuCQr.
spillswith plenty of water.Insertthe rubberstopper,containing
the glassdeliverytubeand clamp
testtube in placeas shownin Figure9-2.
2 . A reactionshouldoccurand a gasshouldescape
from the tubing.With the glasstubingturnedup,
collect some of the gas being liberated.Collect
of air by invertinganother
the gasby displacement
testtubeoverthe upturnedgasdeliverytube.(See
Figure9-2.)
into
4 . Pourabout5 cm3of limewater,
Ca(OH)r(aq),
a small test tube. CAUTION: Ca(OH)z causes
burns; avoid skin contact. Placethe end of the
right angle glasstube in the limewatersolution
as shown in Figure9-1.
FIGURE9-2. Air displacementmethodfor collectinga
product.
FIGURE 9-1. Apparatus 6et-up for limewater test.
Heatthe tube containingthe CuCO,while holding the end of the glasstube in the limewater
solution. CAUTION: Do not point test tube at
yourselfor any other person Continue heating
until bubblinghas nearlystopped.Removethe
limewater tube before removing the heat from
the CuCOr.A cloudy appearancein the Ca(OH),
indicatesthe presenceof COr. Observe.
6 . Discardthe solid leftoverfrom heatingtheCuCO3
into the waste container designatedby your
teacher.The limewater solution may be rinsed
down the drain using plenty of water.
5.
c. SingleDisplacement
1 . Cleanand dry the apparatususedfor PartB. Place
a smallpieceof zinc in a testtubeand add 5 cm3
of 6M HCl. CAUTION: Keep away from open
flame. HClcausesburns:avoid skin contact Rrnse
54
prod3. Removethetesttubecontaining
thegaseous
glass
ucts from the
tubing. Keep it invertedand
bringa burningsplintnearits mouth.A "popoor
"batk" indicatesthe presenceof hydrogengas.
4. Removethe rubber stopperfrom the test tube.
Carefullyadd severalcm3of tap waterto the contentsof the testtube, then decantthe liquid part
into the sink; rinsewith plentyof water.The solid
may be discardedinto a waste containeror returned to the reagenttable as directed by your
teacher.
D. DoubleDisplacement
1. Add 5 cm' of sodiumsulfatesolution,Na2SOa,
to a largetesttube.Add about10dropsof calcium
nitratesolution,Ca(NO:)2,to the Na2SO4.
CAUT|ON: Nitrates can be corrosive;avoid skincontact. Rinse spills with plenty of water.
2. After observingthe formation of a precipitate,
emptythe liquid and solidcontentsintothe waste
containerdesignatedby your teacher.
ANATYSIS
Prepare
a datatablefor yourobservations,
using
Table9-1asa guide.Thefirstentryshowsanexample
of how to enteryourdata.
Table9-1
Reactants
sodiumandwater
Observatlons
Products
sodiummovesrapidlyacrossthesurfaceof thewateras
pH
it reacts;a gasis released;
solutioninbsakerturns
paperblueindicating
thepresence
ofa base.
coNcrusroNs
1. Write a chemical equationfor each of the reactions. lndicatethe type of chemical reactionfor
each. NOTE: ln writing and balancingan equation, the following stepsshouldbe considered.
First, determinethe reactantsand the products.
Second,write the formulas for each substance
usingthe oxidationnumbersto balancethe formulas.Finallybalancethe equationusingcoefficientsto equalizethe numberof atomson each
sideof the equation.
2. Which of the reactionsin this experimentwere
' exothermic and which were endothermic?Provide evidenceto supportyour decision.
3. How canyou tell if a chemicalreactionhasoccurred?What are somedistinctivechangesthat can
be observed?How do thesechangesdiffer from
physicalchanges?
4. ln Part A, Step 4, were you able to detect the
presenceof ammonia gas?lf so, what does this
(Hint: ls Or the only gas in air
resultsuggest?
which will combinewith Mg?)
FURTHERI NVESTI.GATIONS
1. In Part C, the test for hydrogenwas the sound
resultingwhen someof the gasexploded.Do any
lf so, name
other gaseshave this characteristic?
one.
sodiumhydroxide
gas
andhydrogen
2 . What is meantby a reversiblereaction?How do
chemistsindicatea reversiblereactionwhen writing a chemicalequation?
3 . Preparea table of reactiontypes,usingTable9-2
as a guide. List the four types of reactions,the
generalform for each, and selectfrom the list
below three reactionsthat fit each classification.
Rewritethe equationsas balancedchemicalequations.
a. sulfurplus oxygenyield sulfurdioxide
b. magnesiumchlorideplus sulfuricacid yield
. magnesiurn
sulfateplus hydrogenchloride
c. calciumcarbonateyieldscalciumoxide plus
carbondioxide
d . nitrogenplus hydrogenyield ammonia
yield
acidplussodiumhydroxide
e. hydrochloric
sodiumchloridepluswater
f. sodium plus water yield sodium hydroxide
plus hydrogengas
a
lead(ll)oxideyieldsleadplusoxygen
o'
h . copperplussilvernitrateyieldsilverpluscopper (ll) nitrate
potassium
chlorateyieldspotassium
chloride
plusoxygen
calciumchlorideplussilvernitrateyieldsilver
chloridepluscalciumnitrate
k. oxygenplus hydrogenyield water
t . sodiumbromideplus chlorineyield sodium
chlorideplus bromine
Table9-2
Tlpeof Reactlon
singledisplacement
GeneralForm
slement+ compoundr
element+ compound
Examples
1.2Na+2H,O*2NaOH+H.
2.
3.
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