9/26/2011
6.22 – Limiting Reactant (Reagent)
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Limiting Reactant - The reactant in a chemical
reaction that limits the amount of product that
can be formed.
The reaction will stop when all of the limiting
reactant is consumed.
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Excess Reactant - The reactant in a chemical
reaction that remains when a reaction stops
when the limiting reactant is completely
consumed.
consumed
The excess reactant remains because there is
nothing with which it can react.
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In “live” reactions, not 100% of all reactants
present convert into product(s)
One reactant, usually, runs out first. This is
the limiting reactant; the other is the excess
reactant.
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The number of
sundaes possible
is limited by the
amount of syrup,
syrup
the limiting
reactant.
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Deciding which reactants are the limiting reagents and the
reactants in excess
1.
Write the balanced chemical equation for the chemical
reaction
2.
Calculate the available moles of each reactant in the
chemical reaction
3.
Use the mole ratio from the equation to calculate how
many grams of the other reagent you’d need to react both
to completion OR calculate the moles required.
4.
Either you have an excess of the first reagent, or an
excess of the second.
5.
The one that is not in excess is the limiting reagent.
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Suppose 2.00g of NaCl reacts with 5.00g of
AgNO3 to form NaNO3 and AgCl.
Determine the limiting reactant.
NaCl + AgNO3 Æ NaNO3 + AgCl
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•
Suppose 20.0 grams of magnesium hydroxide,
Mg(OH)2, reacts with 40.0 grams of phosphoric
acid, H3PO4 to form magnesium phosphate,
Mg3(PO4)2, and water, H2O.
3Mg(OH)2 + 2H3PO4 Æ Mg3(PO4)2 + 6H2O
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Determine the limiting reactant.
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2.55 grams of silver nitrate and 1.97 grams of
potassium iodide are each dissolved in water and
combined.
The solutions react to form a precipitate of silver
iodide and an aqueous solution of potassium
nitrate.
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What is the limiting reactant?
How much excess reactant is left after reaction?
AgNO3(aq) + KI(aq) Æ AgI(ppt) + KNO3(aq)
169.01 g/mol
166.00 g/mol
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Once you have determined the limiting
reactant, you can determine the excess
reactant.
reactant
This allows you to calculate the amount
of excess reactant left over, after the
reaction has gone to completion.
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We can then calculate the anticipated
amounts of product(s) formed.
When
so, use ONLY the
h doing
d
h limiting
l
reactant amount.
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We can also introduce a previously
discussed/used component to calculations –
STP.
Remember: 1mol = 22
4L @ STP!
22.4L
This applies for gasses and you will be told
whether or not to deal with STP.
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a)
b)
c)
If 52.64L of H2 gas reacts with 119.39L N2
gas to make ammonia gas (NH3):
Determine the limiting reactant @ STP.
H
h excess reactant
t t remains
i after
ft
How
much
reaction (in L)?
How many grams of NH3 can you produce?
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Ammonia reacts with oxygen to produce nitrogen monoxide
plus liquid water.
NH3(g) + O2(g) Æ NO(g) + H2O(l)
1.
2.
3.
4.
Determine the limiting reactant if 100g of ammonia and 100g of
oxygen are present at the start.
start
What is the excess reactant and how many grams of the excess
reactant will remain when the reaction reaches completion.
How many grams of nitrogen monoxide will be produced if the
reaction goes to completion?
How many grams of water will be produced if the reaction goes
to completion.
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Iron (III) oxide reacts with carbon monoxide to
produce iron and carbon dioxide.
1.
2.
3.
What is the limiting reagent if 50g of iron (III)
oxide and 67g of carbon monoxide are
present at the beginning of the reaction?
What is the excess reagent, and how many
grams of it will remain after the reaction
proceeds to completion?
How many grams of each product should be
expected if the reaction goes to completion?
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Solid sodium reacts violently with water to produce sodium
hydroxide and hydrogen gas:
2Na(s) + 2H2O(l) Æ 2NaOH(aq) + H2(g)
1.
2.
3.
What is the limiting reagent if 25g of sodium and 40.2g
of water are present at the beginning of the reaction?
What is the excess reagent, and how many grams of it
will remain after the reaction proceeds to completion?
How many grams of sodium hydroxide and how many
liters (at STP) of hydrogen gas should be expected if the
reaction goes to completion?
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Aluminum reacts with chlorine gas to produce aluminum
chloride:
2Al(s) + 2Cl2(g) Æ 2AlCl3(s)
1.
2.
3.
What is the limiting reagent if 29.3g of aluminum and
34.6L (at STP) of chlorine gas are present at the
b i i
beginning
off th
the reaction?
ti ?
What is the excess reagent, and how many grams (or
liters at STP) of it will remain after the reaction proceeds
to completion?
How many grams of aluminum chloride should be
expected if the reaction goes to completion?
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