Chemistry General I Laboratories
CHE 1120
CHE
Limiting Reactants
Limiting
02/13 02/14
Experiment 5
Experiment
Objective:
Determine the theoretical yield
Determine the percent yield
Make CuI2
Develop an understanding of Limiting Reactant
Six Types of Reactions
Six
Combustion: C10H8 + 12 O2 10 CO2 + 4 H2O
Synthesis: A + B AB ;
8 Fe + S8 8 FeS
Decomposition: AB A + B ; 2 H2O 2 H2 + O2
Single Displacement: A + BC AC + B ;
Mg + 2 H2O Mg(OH)2 + H2
Double displacement: AB + CD AD + CB
Pb(NO3)2 + 2 KI PbI2 + 2 KNO3
Acid/Base: HA + BOH H2O + BA;
HBr + NaOH NaBr + H2O
Limiting Reagent
Limiting
-reagent that is all used up in the reaction
5 Easy Steps
1. Balance the equation
2. Convert all reactants to moles
3. Use balanced equation to convert reactants to product (if more
than one product do the same one for all the reactants)
4. Which reactant produced the smallest amount of product?
This is your limiting reactant!
5. Take the moles of the product formed by the limiting reactant
and figure out how much of the other reactant is left over.
Limiting Reagent Cont
Limiting
Recipe:
2 T.S + 2 Chz + 1Crust 1 pizza
Have: 3 T.S, 2 Chz, and 3 crust
How many pizzas can each ingredient make?
3 T.S
1 pizza
= 1.5 pizzas
2 T.S
2 Chz
1 pizza
= 1 pizzas
Which ingredient makes the fewest
number of pizzas?
How many cans of T.S. used?
1 pizza
2 Chz
2 T.S.
= 2 T.S. used
1 pizza
3 Crust
1 pizza
1 Crust
= 3 pizzas
How much T.S. was in excess?
Excess = initial - used
Excess T.S. = 3 -2 = 1 T.S. excess
Example
Example
Start with: 75.0 mL of a 3.0 M solution of Na2CrO4(aq) and 125 mL
of a 2.5 M solution of AgNO3(aq). What is the limiting reagent and
how much excess reagent is left?
Na2CrO4 + AgNO3
Ag2CrO4 + NaNO3
I. Balance Equation:
Na2CrO4 + 2AgNO3
II. Convert reactants to moles
Na2CrO4: 3.0 mols
Ag2CrO4 + 2NaNO3
0.075 L
0.225 mols
L
2.5 mols
AgNO3 :
L
0.125 L
0.312 mols
Example Cont.
Example
III. Convert reactants to products - Ag2CrO4
0.225 mols Na2CrO4
Na2CrO4:
0.225 mols Ag2CrO4
1 mol Na2CrO4
0.312 mols AgNO3
AgNO3 :
1 mol Ag2CrO4
1 mol Ag2CrO4
0.156 molsAg2CrO4
2 mols AgNO3
IV. Which reactant is the limiting reagent?
AgNO3 is the limiting reagent
Example Cont.
Example
V. Determine the amount of excess reagent
- 0.156 mols of Ag2CrO4 will be formed
- How many moles of Na2CrO4 will be used?
0.156 mols Ag2CrO4
1 mol Na2CrO4
0.156 mols Na2CrO4 (used)
1 mol Ag2CrO4
- Excess Na2CrO4 = 0.225 mols 0.156 mols = 0.069 mols Na2CrO4 (excess)
Type of Reaction in the Experiment
Type
-Precipitation Reaction (i.e. double displacement): a reaction that results in
the formation of a solid substance (precipitate)
AgNO3(aq) + NaCl(aq)
AgNO3(aq) + NaOH(aq)
AgCl(s) + NaNO3(aq)
AgOH(s) + NaNO3(aq)
2NaI(aq) + HgCl2(aq)
HgI2(s) + 2NaCl(aq)
*2CuSO4(aq) + 4KI(aq)
2CuI(s) + I2(aq) + 2K2SO4 (aq)
Lab Info
Lab
- Use the following equation:
2Cu(NO3)2(aq) + 4KI(aq)
2CuI(s) + I2(aq) + 4KNO3(aq)
- Safety note: reaction will produce iodine
- leaves a yellow stain on skin/clothes
- if allergic please notify TA, IMMEDITELY
- Determine which reactant is the limiting reagent
Lab Procedure
TA will assign certain volumes
of reagents to each group
1
1a 1b 2
2a 2b
test 2, labeled 2a ,2b are duplicates
of 1, 1a, 1b respectively
-wipe off any fingerprints
-weigh to the nearest 0.0001g
-do not touch them with fingers
1
KI
Cu(NO3)2
-Add reagents
-mix
Let sit for 5mins
1b
1a
1
1
Remove liquid
Using a pipet
Place to the side
Lab Procedure Cont
1
Wash with ~3mL of MeOH
and shake
1
Cloudy
5 mins
Weigh test
tube, to
nearest
0.0001g
1
dry in oven
~20 mins
place in beaker
Clear
1
Lab Procedure Cont
Lab
** Testing the Supernatants
1a
1b
add 1mL Cu(NO3)2
add 1mL KI
Record observations
Record observations
Use a (+) for more precipitate formed
Use a (-) for no more precipitate formed
Prelab Tables
Prelab
CuI
Vol.
Cu(NO3)2
0.50M
Vol. KI
0.50M
A
1.0mL
6.0mL
B
2.0mL
5.0mL
C
3.0mL
4.0mL
D
4.0mL
3.0mL
E
5.0mL
2.0mL
F
6.0mL
Moles of
Cu(NO3)2
1.0mL
Remember: # mols = M * V
Moles
of KI
Moles of
Moles of
ppt based ppt based
on moles on moles of
of
KI
Cu(NO3)2
Class Data
Class
Vol.
Cu(NO3)2
0.50M
Vol. KI
0.50M
A
1.0mL
1.0mL
6.0mL
6.0mL
B
2.0mL
2.0mL
5.0mL
5.0mL
C
3.0mL
3.0mL
4.0mL
4.0mL
D
4.0mL
4.0mL
3.0mL
3.0mL
E
5.0mL
5.0mL
2.0mL
2.0mL
F
6.0mL
6.0mL
Mass of ppt
in test tubes
Cu(NO3)2
test
KI test
1.0mL
1.0mL
Make sure to complete(in lab manual) and attach to
post lab report!
Post Lab Table
Get from
prelab
Balanced Equation:
Group A
moles of Cu(NO3)2
Theoretical moles
of CuI based on
Cu(NO3)2
moles of KI
Theoretical moles
of CuI based on KI
Avg. mass of CuI*
Avg. moles of CuI*
Limiting Reactant
(Cu(NO3)2 or KI)
% error of moles
of CuI
Group B
Group C
Group D
Group E
Group F
Table for Results for Limiting Reagent Lab
Data Anaylsis
A student was assigned the combination in Group A. Group A consists of 1.0mL of
0.5M Cu(NO3)2 and 6.0mL of 0.5M KI. The student obtained 0.0910 and 0.0940 g for
trial 1 and 2, respectively. A (+) result occurred when performing the Cu(NO3)2 test.
Balanced Equation:
MM(KI) = 166.1
2Cu(NO3)2 (aq) + 4KI(aq) 2CuI(s) + I2(aq) +
4KNO3 (aq)
Group A
MM(CuI) = 190.5
MM(Cu(NO3)2) = 187.6
moles of
Cu(NO3)2
0.0005
Theoretical
moles of
CuI based
on Cu(NO3)2
moles of KI
0.0005
Theoretical
moles of
CuI based
on KI
Avg. mass
of CuI*
0.0015
Avg. moles
of CuI*
0.000485
Limiting
Reactant
(Cu(NO3)2 or
KI)
% error of
moles of
CuI
Cu(NO3)2
0.003
0.0925
3.09%
Group B
Group C
Group D
Group E
Group F
Additional Problems
-Read over sections 3.8-3.10 (pgs 99-108) in Chang book.
-Problems 3.84, 3.86, 3.90, 3.92 (pg 114) in Chang book
-Aluminum reacts with sulfur gas to form aluminum sulfide.
Initially 1.18 mol of Al and 2.25 mols of S are combined.
a. write a balance equation for reaction
2Al(s) + 3S(g) Al2O3
b. What is the limiting reagent? Al
c What is the theoretical yield of aluminum sulfide in mols?
0.590 mols
d. How many moles of excess reactant remain unreacted?
0.48 mols
Lab 4: Exp 5 Checklist
Completed online quiz # 5
By 10pm Sunday (02/19/12)
Prelab for Lab 4 Exp. 5 completed
Hand written in lab notebook. Complete
Prelab questions for Exp. 5
Postlab for Lab 3 Exp. 3 completed, Typed.
Exp. 4 parts B,C,D is due as well. Complete
prelab questions 1-6 on for exp. 4 as well.
Attach to post lab for exp. 3.
Bring Lab Coat, goggles, lab notebook to lab