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Topic 5: Formulae, Equations and Amounts of Substance
5.4 Stoichiometry
5.4.2 Limiting Reagents
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What is stoichiometry the quantitative relationship between?
Reactants and products
Stoichiometry helps predict the masses or
volumes
Stoichiometry involves determining the exact amounts of substances involved in a reaction using
balanced chemical equations
.
In the reaction
2
H
2
+
2H_{2} +
2
H
2
+
O
2
→
2
H
2
O
O_{2} \rightarrow 2H_{2}O
O
2
→
2
H
2
O
, what is the stoichiometric ratio between hydrogen and water?
2:2
Chemical equations use chemical
formulas
A balanced chemical equation ensures the number of
atoms
of each element is equal on both sides.
Balance the equation for methane reacting with oxygen:
C
H
4
+
CH_{4} +
C
H
4
+
O
2
→
C
O
2
+
O_{2} \rightarrow CO_{2} +
O
2
→
C
O
2
+
H
2
O
H_{2}O
H
2
O
C
H
4
+
CH_{4} +
C
H
4
+
2
O
2
→
C
O
2
+
2O_{2} \rightarrow CO_{2} +
2
O
2
→
C
O
2
+
2
H
2
O
2H_{2}O
2
H
2
O
Limiting reagents are reactants that are completely
consumed
The limiting reagent determines the
maximum yield
of a reaction.
What law is followed when balancing chemical equations?
Law of Conservation of Mass
In the balanced equation
C
H
4
+
CH_{4} +
C
H
4
+
2
O
2
→
C
O
2
+
2O_{2} \rightarrow CO_{2} +
2
O
2
→
C
O
2
+
2
H
2
O
2H_{2}O
2
H
2
O
, the stoichiometric ratio between oxygen and water is 2:2
Limiting reagents are reactants that are completely consumed during a chemical reaction, limiting the amount of
product
In the reaction2H_{2} + O_{2} \rightarrow 2H_{2}O</latex>, if 4 moles of
H
2
H_{2}
H
2
react with 1 mole of
O
2
O_{2}
O
2
, which is the limiting reagent?
O
2
O_{2}
O
2
The limiting reagent is the reactant that is left over after a reaction is completed.
False
Match the term with its definition:
Limiting Reagent ↔️ Reactant completely consumed
Excess Reagent ↔️ Reactant left over
To identify a limiting reagent, first write the
balanced
chemical equation.
Steps to identify a limiting reagent in a reaction:
1️⃣ Write the balanced chemical equation
2️⃣ Convert reactant masses to moles
3️⃣ Calculate the mole ratio
4️⃣ Determine which reactant is completely consumed first
What is the first step to calculate the amount of product formed using the limiting reagent?
Identify the limiting reagent
The balanced chemical equation is used to find the mole ratio between the
limiting reagent
and the product.
Calculate the moles of product formed by multiplying the moles of the limiting reagent by the mole
ratio
What is stoichiometry?
Quantitative relationship in reactions
How are the moles of product calculated using the limiting reagent in stoichiometry?
Multiply by mole ratio
If
O
2
O_{2}
O
2
is the limiting reagent with 1 mole in the reaction
2
H
2
+
2H_{2} +
2
H
2
+
O
2
→
2
H
2
O
O_{2} \rightarrow 2H_{2}O
O
2
→
2
H
2
O
, 2 moles of
H
2
O
H_{2}O
H
2
O
are formed.
The limiting reagent ensures that calculations are based on the reactant that is completely
consumed
What is stoichiometry defined as in chemistry?
Quantitative relationships
How are chemical reactions represented in chemistry?
Chemical equations
A balanced chemical equation ensures the number of
atoms
of each element is equal on both sides.
What is the balanced equation for CH_{4} + O_{2} \rightarrow CO_{2} + H_{2}O</latex>?
C
H
4
+
CH_{4} +
C
H
4
+
2
O
2
→
C
O
2
+
2O_{2} \rightarrow CO_{2} +
2
O
2
→
C
O
2
+
2
H
2
O
2H_{2}O
2
H
2
O
What is the role of limiting reagents in a chemical reaction?
Control maximum yield
In the reaction
2
H
2
+
2H_{2} +
2
H
2
+
O
2
→
2
H
2
O
O_{2} \rightarrow 2H_{2}O
O
2
→
2
H
2
O
, if 4 moles of
H
2
H_{2}
H
2
react with 1 mole of
O
2
O_{2}
O
2
,
O
2
O_{2}
O
2
is the limiting reagent
The concept of limiting reagents aligns with
stoichiometry
in chemical reactions.
Steps to identify limiting reagents in a chemical reaction
1️⃣ Write the balanced chemical equation
2️⃣ Convert reactant masses to moles
3️⃣ Calculate the mole ratio
4️⃣ Determine which reactant is completely consumed first
What must be determined after calculating the mole ratio to identify the limiting reagent?
Reactant completely consumed
For the reaction 2H_{2} + O_{2} \rightarrow 2H_{2}O</latex>, if 4 moles of
H
2
H_{2}
H
2
react with 1 mole of
O
2
O_{2}
O
2
,
O
2
O_{2}
O
2
is the limiting reagent.
To calculate the amount of product formed, the first step is to identify the limiting
reagent
What are excess reagents in a chemical reaction?
Reactants in excess
In
2
H
2
+
2H_{2} +
2
H
2
+
O
2
→
2
H
2
O
O_{2} \rightarrow 2H_{2}O
O
2
→
2
H
2
O
, if 4 moles of H_{2}</latex> react with 1 mole of
O
2
O_{2}
O
2
,
H
2
H_{2}
H
2
is the excess reagent.
To solve stoichiometry problems involving limiting reagents, the first step is to write the balanced chemical
equation
In the reaction
N
2
(
g
)
+
N_{2}(g) +
N
2
(
g
)
+
3
H
2
(
g
)
→
2
N
H
3
(
g
)
3H_{2}(g) \rightarrow 2NH_{3}(g)
3
H
2
(
g
)
→
2
N
H
3
(
g
)
, if 28 grams of
N
2
N_{2}
N
2
react with 12 grams of
H
2
H_{2}
H
2
, which is the limiting reagent?
N
2
N_{2}
N
2
In the reaction
N
2
(
g
)
+
N_{2}(g) +
N
2
(
g
)
+
3
H
2
(
g
)
→
2
N
H
3
(
g
)
3H_{2}(g) \rightarrow 2NH_{3}(g)
3
H
2
(
g
)
→
2
N
H
3
(
g
)
, 2 moles of
N
H
3
NH_{3}
N
H
3
are formed if 1 mole of
N
2
N_{2}
N
2
is the limiting reagent.
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