LIMITING REACTANT

1
LIMITING REACTANT The reactant that gives the
least number of product moles limits the
reaction.
To understand this concept, lets suppose you
were an elf working for Santa Claus and your job
was to make candy canes. You take one red stick
and one white stick then twist them around to
make one candy cane. The ratio of red to white is
11. Time is ticking and you find that you have
24 red stick and 17 white sticks left. What is
the maximum number of candy canes you can make?
The answer is 17 candy canes! The white sticks
limit the amount of product you could make. In
chemistry we do not use sticks but MOLES to
determine which starting material will limit the
maximum amount of product that can be produced in
a chemical reaction.
2
LIMITING REACTANT
a) Sodium metal reacts with oxygen to produce
sodium oxide. If 5.00 g of sodium reacted with
5.00 grams of oxygen, how many grams of product
is formed? 4 Na (s) O2(g) ? 2 Na2O (s)
Start with what is given, calculate the amount of
product that can be theoretically made but do it
twice (once for each reactant) 5.00g Na (1 mole
Na) ( 2 mole Na2O)( 62 g Na2O) 6.74 g of
Na2O 23 g Na 4 mole Na 1
mol Na2O 5.00g O2 (1 mole O2) ( 2 mole Na2O)( 62
g Na2O) 19.38 g of Na2O 32 g O2
1 mole O2 1 mol Na2O Notice you can not
have two different masses produced for the same
product in one reaction vessel! So in this case,
Na (sodium) limits how much sodium oxide is
produced. The correct answer is 6.74 g of sodium
oxide.
Wrong answer
3
LIMITING REACTANT
b) How much oxygen was used in this reaction and
how much of each reactant was leftover (in
excess)? 4 Na (s) O2(g) ? 2 Na2O (s)
There are two methods used to answer this
question. The Law of Conservation of mass and
Stoichiometry. The amount of O2 used to make 6.74
g of Na2O is calculated by 5.00g Na (1 mole Na)
( 1 mole O2)( 32 g O2) 1.74 g of O2 was used
23 g Na 4 mole Na 1 mol O2 Or
use the Law of Conservation of mass Mass of
product (6.74 g) mass of limiting reactant
(5.00 g) mass of other reactant, in this case
oxygen (1.74 g). The amount of oxygen (O2)
leftover can be calculated by subtracting the
starting mass of oxygen from the used mass.
5.00g 1.74 g 3.26 g The amount of sodium (Na)
leftover at the end of the reaction is 0.00 g
(zero), since it was the limiting reactant and
was completely consumed in the reaction.
4
LIMITING REACTANT
How many grams of solid are formed when 10.0 g of
lead reacts with 10.0 g of phosphoric acid? 3
Pb2 H3PO4 ? Pb3(PO4)2 (s) 3 H2 (g)
Start with what is given but do it twice 10.0g
Pb (1 mole Pb) ( 1 mole Pb3(PO4)2)(811 g
Pb3(PO4)2) 13.1 g Pb3(PO4)2 207 g
3 mole Pb 1 mole
Pb3(PO4)2 10.0g H3PO4 (1 mole H3PO4)( 1 mol
Pb3(PO4)2)(811 g Pb3(PO4)2)41.4 g Pb3(PO4)2
98 g 2 mole H3PO4 1
mole Pb3(PO4)2 You can not have two different
answers for one question so in this case lead
limits how much lead(II) phosphate that can be
produced. The correct answer is 13.1 g.
Wrong answer
5
PRACTICE PROBLEM 19
1. How much AgCl product will be produced if
100.00 g of BaCl2 reacted with excess AgNO3? 2.
How many moles of carbon dioxide could be
produced from 220.0 g of C2H2 and 545.0 g of
O2? 3. How many grams of CO2 can be produced
by the reaction of 35.5 grams of C2H2 and 45.9
grams of O2? 4. In the reaction between CH4
and O2, if 25.0 g of CO2 are produced, what is
the minimum amount of each reactant needed? 5.
Cu 2 AgNO3 ? Cu(NO3)2 2 Ag. When 10.0
g of copper was reacted with 60.0 g of silver
nitrate solution, 30.0 g of silver was obtained.
What is the percent yield of silver obtained?
137.57 g
13.63 mol
50.5 g
9.09 g of CH4 36.4 g of O2
88.3
6
GROUP STUDY PROBLEM 19
______1. Which reactant will produce the least
amount of AgCl product if reacted with AgNO3?
a)100.00 g BaCl2 b) 400.0 g NaCl c) 200.0
g CsCl ______2. How many moles of CO2 can be
produced by the reaction of 5.0 grams of C2H4 and
12.0 grams of O2? ______3. How many grams of
carbon dioxide could be produced from 2.0 g of
C2H4 and 5.0 g of O2? ______4. In the
reaction between CH4 and O2, if 18.0 g of CO2 are
produced, how many grams of water are
produced? ______5. Cu 2 AgNO3 ? Cu(NO3)2
2 Ag. When 50.0 g of copper was reacted
with 300.0 g of silver nitrate solution, 149 g of
silver was obtained. What is the percent yield
of silver obtained?