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Limiting Reactants

PRE-LABASSIGNMENTS:

To be assigned by your lab instructor.

STUDENTLEARNINGOUTCOMES:

Making calculations based upon the stoichiometry of a reaction

Performing an acid- base titration

Endpoint

Limiting reactant

Indicators

Equivalence point

EXPERIMENTALGOALS:

The limiting reactant in an acid-base reaction ill be identified e!perimentally" and the amount

of e!cess reactant remaining after the reaction ill be determined by titration. These data ill beused to e!plore the limiting reagent concept.

INTRODUCTION:

#hen e are given a reaction beteen to or more reactants" one may be completely

consumed before the other$s%. The reaction stopsat this point, and no further product is made.The reactant that e run out of first is knon as the limiting reactant" because it limits theamount of product$s% that can form. The amount of product$s% that are made based on the

limiting reactant is the te!retical "iel#. The other reactants are present in excess" because some

amount of those reactants ill still be present hen the reaction stops.

&or e!ample" suppose e ere making standard '-door cars" and e had the folloing

veryincomplete list of (ingredients.) *o many cars could e make+

' engines ' steering heels ' rear-vie mirrors

, headlights , license plate holders , indshield ipers

' drivers seats / doors heels

&rom the ' engines" , headlights $one on each side%" ' drivers seats" ' steering heels" , license

plate holders $one in front and one in back%" ' rear-vie mirrors" and , indshield ipers $oneon each side%" e can make ' cars. *oever" if e have / doors" and e need ' per car" e can

only make 0 complete cars. The (yield) of cars is even further reduced hen e consider the

heels1 if e have heels" and need ' per car" e ill only be able to make 2 complete cars.Thus" in this e!ample" the heels are the limiting reactant" and the theoretical yield is 2 cars.

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4o" lets consider a chemical e!ample. 5mmonia" 4*0" is synthesi6ed from nitrogen

gas" 42" and hydrogen gas" *2" in the *aber process by the folloing reaction1

42$g% 7 0*2$g% 24*0$g%

Example 1: 8uppose e mi! .99 moles of 42and /.99 moles of *2. #hat is the

ma!imum amount of 4*0that can be produced+ *o much *2ill be left over+

In this e!ample" figuring out the limiting reactant and theoretical yield is straight-

forard1 according to the stoichiometry of the reaction" mole of 42requires 0

moles of *2. 8ince e have /.99 moles of *2" only 0.99 moles of the *2can react

ith the .99 moles of 42" hich ill produce 2.99 moles of 4*0" and there illbe 2.99 moles of *2left over.

Example 2: 8uppose e mi! 2./ moles of 42and :./ moles of *2. #hat is the

theoretical yield of 4*0+ *o much of the e!cess reactant ill be left over+

In this case" ere not dealing ith hole numbers" so e need to do some math.To figure out the limiting reactant" e ill consider each reactant" and see ho

much of the product is formed

Assuming the N2reacts completely, how much N!can "e made#

0

2

0

2 4*mol'.094mol

4*mol24mol2./ =

Assuming the 2reacts completely, how much N!can "e made#

0

2

0

2 4*mol'.9*mol0

4*mol2*mol:./ =

8ince the *2produces the smallest amount of 4*0" *2is the limiting reactant" and

the theoretical yield of 4*0is '.9 moles.

The 42ill be present in e!cess. To figure out ho much is left over" first" eneed to figure out ho much ill react1

2

2

22 4mol2.9/

*mol0

4mol*mol:./ =

8ince 2.9/ moles of 42react" and e have 2./ moles of 4 2available" the amount

left over ill be1

2./ mol 42initially ; 2.9/ mol 42reacted < 9.9 mol 42left over

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?*g9,?*mol

?*g,.92

?mol0

?*mol9

?g02.99

?mol?g2/9. 2

2

2

2

2

2

22 =

c. To figure out ho many grams of >'*9are left over" e first need to figure out ho

many grams ill react ith the ?21

9'

9'

9'

2

9'

2

22 *>g39.9

*>mol

*>g/,.2

?mol0

*>mol2

?g02.99

?mol?g2/9. =

4o" e take the difference beteen the amount e started ith and the amount that

ill react1

99. g >'*9initially ; 39.9 g >'*9reacted < 09. g >'*9left over

d. The percent yield of >?2is the actual yield divided by the theoretical yield1

30.A99>?g22

>?g//yieldpercent

2

2==

In this e!periment" e ill e!plore the concept of limiting reactant as it relates to a

reaction beteen a strong acid and a strong base. Bou ill be using acid-base indicators and

titrations to accomplish this. In acid-base titrations" an indicator dye is often used to determinehen the titration is complete. &or most indicators" the endpoint occurs hen the solution

changes color $e.g." clear solution pink solution or vice versa%.

Cemember that titrations compare the moles of reactants on the basis of the reactions

stoichiometry" hich is determined from the balanced chemical reaction. &or e!ample" if to

substances" 5 and =" react in a 21 stoichiometric ratio2"then one mole of = reacts ith 2 molesof 5. If e kno the moles of one substance $i.e" 5%" then this ratio permits us determine the

concentration of the other $=% at and only at the point here both reactants are limiting $neither

is in e!cess%D this point is referred to as the e./i0alence 1!intof the titration. The titration mustbe stopped as close to the equivalence point as possible. This e!perimentally determined point is

referred to as the en#1!int. If the volume of the titrant measured at the endpoint is belo or

above the volume required to achieve an e!act equivalence point" the results ill be inaccurate.

Provided that a suitable indicator is used" there is very little difference beteen the end

point and the equivalence point. These organic dye compounds e!hibit a change in color over a

narro p* range. The table belo lists the color changes hich are observed for several

Is this the right number of significant figures+ $int: Cemember the rules for addition and subtraction.%2Cemember that for chemical reactions" the stoichiometric ratio is simply the ratio of the stoichiometric coefficients

of the to substances in the balanced chemical reaction. In the above e!ample" 25 7 = products" the

stoichiometric ratio of 5 to = is 21.

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indicators over a particular p* range. $5n acidic solution has a p* of less than 3" a basic

solution has a p* of greater than 3" and a neutral solution has a p* of 3.%

In#icat!r 1& Range C!l!r CangeEthyl Ced '.9-/., >olorless to red

>resol Ced 3.9-,., Bello to red

=romothymol blue :.9-3.: Bello to bluePhenol red :.'-,.9 Bello to Ced

Phenolphthalein ,.9-.: >olorless to pink

Thymolphthalein ./-9.: >olorless to blue

In this lab you ill use phenolphthalein to determine hether the e!cess reactant is the

acid or base. If e!cess acid is present" the resulting solution ill be acidic $#hat color ill the

solution be+%. If the base is in e!cess" the solution ill be basic after the reaction $again" hat

color+%. The number of moles of e!cess acid and base ill be determined by titration using thesame indicator. #hen indicators are used in a titration" the point at hich the indicator changes

color is the titration end point.

The reaction that e ill be using today is the reaction beteen *>l and 4a?*1

*>l 7 4a?* 4a>l 7 *2?

There are some items of information that are necessary for understanding the calculations

in this lab1

The stoichiometric ratio of reactants hich can be obtained from the balanced equation.

The moles of the unreacted e!cess reactant are determined using an acid-base titration.

PROCEDURE:

Safety Information

*>l and *4?0are strong acids and 4a?* and F?* are strong bases. If you spill any of

these solutions on yourself" flush the area thoroughly ith ater" and inform your

instructor. If your skin comes in contact ith 4a?* or F?* solutions" you ill feel

your skin become (slimy) as oils on your skin are converted to soap. Cinse your handsith plenty of ater if you notice this feeling.

>lean up any spills of 4a?* pellets immediately. >ontact stockroom personnel to help clean up any large spills of acid or base solution.

The addition of 4a?* to ater is an e!othermic processD add the 4a?* sloly" in small

portions to the solution.

A2 Organi3ing Gr!/1 4!r5

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Bou ill ork in groups of eight. Each student ill prepare one solution by adding different

masses of 4a?* pellets to 09.99 mL *>l solution and then analy6ing that solution. 5s a group"

you must make sure that the amounts 4a?* used span the range of about 9.2 to 2.2 g 4a?*"ith the points spaced out somehat evenly across that range.

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B2 Pre1aring te S!l/ti!ns2

. 5n 4a?* pellet eighs appro!imately 9.2 grams. In a clean dry beaker" eigh out the massof 4a?* assigned to you ithin G 9.2 grams. This can be conveniently done by using the

Tarefunction on the balance.

a. Place a small" empty beaker on the balance" and press the (Tare) buttonD the readingshould go to 9.999" but may vary G 9.992 g.

b. Hsing a spatula" add 4a?* pellets until you get to the desired mass. $o not handle

the pellets ith your fingers" and clean up any spilled 4a?* pellets.%c. Cecord the e!act mass of the 4a?*.

d. Make sure that you replace the lid on the 4a?* bottle" since 4a?* is hygroscopic.

2. >lean a 2/9 mL Erlenmeyer flask" and rinse it three times ith small portions of I ater.ry the outside of the flaskD it is not necessary to dry the inside $the presence of a small

amount of I ater has no effect on the amount of acid or base present" and therefore no

effect on the titration or results.%. &rom the buret labeled .9 M *>l" deliver 09.99 mL of

*>l solution into your beaker. =ecause it is important to use the same volume of *>l ineach run" use the folloing procedure1

a. &ill the buret to Just above the 9.99 mark using the ash bottle labeled .9 M *>l.b. rain the buret into a aste container until the meniscus is as close to the 9.99 mark

as possible.

c. Place your Erlenmeyer flask under the buretD open the stopcock and drain the buret tothe 09.99 mL mark. 8ince some liquid may cling to the sides of the buret" ait about

one minute" and if the meniscus is above the 09.99 mL mark" open the stopcock and

again drain it to the 09.99 mark.

d. Cecord the e!act concentration of the *>l.

0. )lowly add the 4a?* pellets to your flask" sirling the flask gently as you do so. $The

dissolution of the 4a?* and its reaction ith the acid produce a lot of heat" hich may causesplattering if the pellets are added all at once.% 8irl the flask to completely dissolve the

4a?*. Make sure that you label your flasks in order to avoid confusion.

C2 Set/1 6!r te Titrati!ns2

. #orking as a group" obtain appro!imately /9 mL of .9 M *4?0and /9 mL of .9 MF?* in to separate beakers. *a&e sure that you record the actual concentration of the

N+!and + from the la"els on the stoc& "ottles.

2. &ill to burets ith the *4?0solution" and to ith the F?* solution. Make sure to first

rinse the burets ith to 0-/ mL portions of the solution you are filling them ith. Label the

burets as F?* or *4?0so that they can be identified later.

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D2 Determining te Limiting Reagent an# Per6!rming te Titrati!n2

Each member of your group must record the data for their titration during this portion of the lab.Each student should titrate their solution according to the folloing procedure.

. 5dd 0 drops of phenolphthalein to the solution being titrated. =ased on the color of theindicator in your solution" determine hether your solution is acidic or basic" hether *>l or

4a?* is the e!cess reagent" and hether you ill use *4?0or F?* to titrate your solution.

If you are uncertain of this choice" check ith your instructor before titrating. Cecord yourconclusions in section of the Lab Ceport. If you fail to add the phenolphthalein" it ill be

difficult to perform this e!perimentK

2. =ring your flask to the buret required for your titration. =efore starting each titration" theburet used should be filled to near the 9.9 mark. Cemove any bubbles from the stopcock by

draining a small amount of solution into a aste container. Cecord the position of the

meniscus as the initial volume.

0. Titrate your solution to the appropriate endpoint $from clear to pink if titrating ith F?*"

and from pink to clear if titrating ith *4?0%. In each case" a permanent transition shouldoccur ith the addition of no more than one drop of solution.

'. Cecord the final volume on the buret.

E2 Anal"sis !6 In#i0i#/al Titrati!n Res/lts

>omplete this section of your lab report" shoing any required calculations. Bou can consult

ith your partner or other group members" but you should do your on calculations and give

your on observations and ansers.

72 Anal"sis !6 Gr!/1 Data

>omplete the group analysis section of your lab report.

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LABREPORT

Limiting Reactants

4ame ate Ceport rade

Partner 8ection

A2 Organi3ing Gr!/1 4!r5

4ame N mass 4a?* 4ame N mass 4a?*

88888888888888888888 8888888888 88888888888888888888 8888888888

88888888888888888888 8888888888 88888888888888888888 8888888888

88888888888888888888 8888888888 88888888888888888888 8888888888

88888888888888888888 8888888888 88888888888888888888 8888888888

DATAANAL9SISPER7ORMEDB9EAC&STUDENTONT&EIRSOLUTION

B2 Pre1aring te S!l/ti!ns C2 Set/1 6!r te Titrati!ns2

5ctual Mass of 4a?* used Moles of 4a?* used

Oolume of *>l >oncentration of *>l

>oncentration of *4?0 >oncentration of F?*

D2 Determining te Limiting Reagent an# Per6!rming te Titrati!n2

>olor of indicator added to solution

Is the solution acidic or basic+

#hich reagent is in e!cess+

This reagent ill be titrated ith *4?0 or F?* $circle one%

-$f you are uncertain of this choice, chec& with your instructor "efore titrating.

I6 "!/r e;cess reagent is NaO&< g! t! secti!n E' )12 '(*2

I6 "!/r e;cess reagent is &Cl< g! t! secti!n E$ )12 ''*2

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E'2 Anal"sis !6 In#i0i#/al Titrati!n Res/lts )NaO& in e;cess*

$n this procedure, you will "e titrating unreacted Na+.

>oncentration of *4?0Titrant

Initial Oolume of *4?0

&inal Oolume of *4?0

Oolume of *4?0

Hse the titration data recorded above to complete the folloing section.

. >alculate the total number of moles of 4a?* that ere added to your reaction mi!ture.

2. Hse the volume and concentration of the *4?0used during the titration to determine ho

many moles of the 4a?* ere left overafter the reaction beteen the *>l and the 4a?*.

0. *o many moles of the 4a?* ere consumedduring the reaction beteen *>l and 4a?*+

'. &rom the moles of 4a?* that ere consumed $calculated in step 0%" determine the number of

moles and grams of 4a>l that ere produced during the reaction beteen *>l and 4a?*.

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E$2 Anal"sis !6 In#i0i#/al Titrati!n Res/lts )&Cl in e;cess*

$n this procedure, you will "e titrating unreacted /l.

>oncentration of F?* Titrant

Initial Oolume of F?*

&inal Oolume of F?*

Oolume of F?*

Hse the titration data recorded above to complete the folloing section.

. >alculate the total number of moles of *>l that ere added to your reaction mi!ture.

2. Hse the volume and concentration of the F?* used during the titration to determine ho

many moles of the *>l ere left overafter the reaction beteen the *>l and the 4a?*.

0. *o many moles of the *>l ere consumedduring the reaction beteen *>l and 4a?*+

'. &rom the moles of *>l that ere consumed $calculated in step 0%" determine the number of

moles and grams of 4a>l that ere produced during the reaction beteen *>l and 4a?*.

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72 Anal"sis !6 Gr!/1 Data

. roup ata Table1

4ameTarget g

4a?*

5ctual g

4a?*

Moles

4a?*

Oolume

*>lsolution

Moles

*>l

Indicator

color

Limiting

Ceactant

Titrant

Oolume

Moles

4a>lformed

Moles

e!cess4a?*

Moles

e!cess*>l

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2. ?n the graph paper provided" plot the moles of 4a?* used on thex-a!is versus the folloing

quantities on they-a!is1

moles of 4a>l formedmoles of e!cess 4a?*

moles of e!cess *>l

0. E!plain the observed trends in your graph in light of the concept of the limiting reagent.

'. =riefly sketch ho your graph ould have looked if this e!periment ould have been carriedout using a fi!ed amount of 4a?* and varying the amount of *>l.

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4ame

,3