Chapter 12 notes
Part 2
Limiting Reagents
Rarely in nature are reactants in a
chemical reaction present in the exact
ratios specified by the balanced
equation. One or more reactants are
in excess and the reaction proceeds
until all of one reactant is used up.


Limiting reactant limits the extent of
the reaction and determines the
amount of the product
Excess reactants a portion of all
the other reactants remains after the
reaction stops, these are the excess
reactants
1 banana + 5 s.berries + 2 scoops
i.c. + 8oz milk  1 shake
if you had:


8 bananas , 40 s.berries, 10 scoops i.c., and a
gallon of milk
how many shakes could you make? what limits
you?
bananas berries
8/1 = 8 40/5 = 8
ice cream
10/ 2 = 5
milk
128oz/8 = 16
New shipment arrives
320 oz bananas, 80oz berries, 80 scoops i.c., 3
gal milk
(1 banana weighs 8oz, 1 berry weighs 0.5oz, 1
gal=128oz)


what will “limit” production of shakes now?
what will be “in excess” ?
Determining amt of product
expected should be based on
the reagent (reactant) not
present in excess.
Use the limiting reactant to
solve the problem.
10.0g NaOH are reacted with 10.0g HCl to form ?
Salt
NaOH + HCl →
10.0g
?g
NaCl
+
H2O 10.0g
1st: change g of each reactant to moles and ÷
by coeffic.
2nd: Which is smaller? That is the limiting
reactant.
3rd: Use the limiting reactant to solve the
problem.
4th: To find out how much is leftover of
excess reactant use the limiting
reactant to solve for the one in
excess (as if it were unknown). This is
theoretically how much was “used.”
5th: Subtract the “used” g from the “given” g
to find unused
Lime (CaO) is obtained by heating limestone
CaCO3

CaO + CO2
Lime is important in the production of cement,
glass, steel…heated w/ coke (type of carbon)
forms calcium carbide
2CaO(s) + 5C(s)  2 CaC2(s) + CO2(g)
CaC2(s) + 2H2O(ℓ) → C2H2(g) + Ca(OH)2(aq)
acetylene – an important fuel for welding and a
chemical base for many industrial chemicals
like vinyl chloride
Suppose your company took delivery of 17.35 kg
of CaO and 12.30 kg of coke. You supervised
the mixing and heating of these.
How much CaC2 would your boss expect you to
produce?
He has promised to sell 23.00 kg of your product
(CaC2) to the acetylene producer by next week.
Do you have enough reagents on hand to meet
the demand? If not, how much more do you
need to order?
Examples
2NO + 5H2 →
150g
20g
limiting _______
mol of H2O ____
2NH3 + 2H2O
4mol
6mol
CaO + 3C → CaC2 + CO
?g
limiting ________
CaC2 _______g
Examples:
C2H4 + 3O2 → 2CO2 + 2H2O
280g
320g
?g
limiting________
CO2 ______g
Zn + 2HCl → ZnCl2 + H2
130g
100g
?g
limiting _________
H2 _______g