The
calculation of limiting reagents and excess reagents is a real-life application
of the stoichiometric relationship between reactants and products.
Limiting Reagent: this is the reagent which gets consumed entirely in the chemical reaction. It determines how long a chemical reaction
will last or the amount of products to be produced. In other words, the limiting
reagent or reactant stops the chemical reaction.
Excess Reagent: this is the reactant that could continue to react if the other reactant is still available for the reaction.
Let us look at this example, and assume that all
the requirements of producing a tricycle are three wheels and a wheel steering
and a tricycle company has 600 wheels and 250 wheel-steerings for the production
of tricycles.
Which of the
materials will finish first and how many of the other materials will be left
over?
3 wheels + 1
steering = 1tricycle
The first
thing to do is to find the number of tricycles that can be produced by each of
the materials.
3wheels = 1tricycle
600 wheels =
1/3 X 600 = 200 tricycles.
1 steering =
1 tricycle
250 steering
= 250 tricycles.
The material
that will finish first is the wheel because it produced the least amount of
tricycles, so, the wheel is the limiting material.
Since 200 tricycles
will be produced, the material in excess is steering.
The amount
of excess wheel steering is 250 -200 = 50 steering.
Now, let us
apply the same principle to chemical reactions.
Question One:
In the Haber process
of ammonia, 10cm3 of nitrogen combines with 20cm3 of
hydrogen to produce ammonia gas. Calculate the:
1) Limiting reagent or reactant
2) Volume of excess reagent or reactant
The equation
for the reaction is
N2(g)
+ 3H2(g) → 2NH3(g)
Calculating the Limiting Reactant
To find the
limiting reagent, we need to calculate the volume of ammonia produced by each
of the reactants.
From the equation,
1cm3 of
nitrogen = 2cm3 of ammonia
10 cm3 of
nitrogen = 2/1 * 10 = 20 cm3 of ammonia
Similarly,
3 cm3 of
hydrogen = 2cm3 of ammonia
20cm3
of hydrogen = 2/3 * 20 = 13.3 cm3 of ammonia
Since hydrogen
produced less volume of ammonia, it is the limiting reactant or reagent.
Calculating the volume of excess
reagent
To find the
volume of excess reagent, we need to calculate the volume of nitrogen required
to combine completely with the limiting reagent.
From the
equation,
3cm3
of hydrogen = 1cm3 of nitrogen
20cm3
of hydrogen = 1/3 *20 = 6.7cm3 of nitrogen
Volume of
excess nitrogen = 10 – 6.7 = 3.3cm3
To be continued in the next class.
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