Stoichiometry/Chemical Arithmetic
Definition
The branch of
chemistry which deals with the quantitative
relationship between reactants and products of a chemical reaction
as given by a balanced chemical equation is called Stoichiometry (Greek word; stoicheion meaning "element"
and metron meaning "measurement”).
OR
“Stoichiometry is the
study of quantitative relationships between
the amounts of reactants and products in chemical reactions as given by
balanced chemical equations”. It is quantitative
chemistry which is concerned with
calculations involved in the inter-conversion of matter based on balance chemical
equation.
Explanation
Stoichiometry tell us
that how much amount of product will be formed from specific amount of
reactants or for the specific amount of product formation how much amount of
reactant will be needed.
Etymology
(Meaning of the Word)
The word stoichiometry
derives from two Greek words;
stoicheion
meaning "element" and
metron
meaning "measurement”.
Stoichiometry deals
with calculations about the masses and volumes of reactants and products
involved in a chemical reaction. It is a very mathematical part of chemistry.
Stoichiometric Amounts
The amount of
reactants and products in balanced chemical equations are called Stoichiometric
Amounts. According to Law of
Conservation of Mass, the weight of reactants is equal to weight of products.
For Example;
In following equation,
4 g H2, 32 g O2 and 36 g H2O are called
Stoichiometric Amount It is seen that
weight of reactants is equal to weight of products which is in accordance with
Law of Conservation of Mass.
Stoichiometric Coefficient
The coefficient of each
reactants and products in balanced chemical equations is called Stoichiometric coefficient.
Stoichiometric
coefficients will be always equal to number of moles.
Limitation
of chemical equations
They do not tell about
the
1. Reaction conditions like temperature
and pressure
2. Rate of reaction
3. Physical state of reactants and
products
4. Mechanism of reaction
5. Feasibility of reaction
Factors governing balanced chemical
equation
1. Number of atoms of reactants and
products.
2. Masses of reactants and products.
Factors
Not affecting balanced chemical equation
1. Number of moles of reactants and
products.
2. Number of molecules of reactants and
products.
3. Volume of reactants and products.
Conditions
for Stoichiometric Calculations
For stoichiometric
calculation, a balanced chemical equation is of great importance.
Stoichiometric calculations are based on the following conditions:
1. All the reactants must be completely
converted into the products.
2. The side-reactions must not occur.
3. The law of conservation of mass and the
law of definite proportions must be obeyed.
Steps for doing stoichiometric
problems
Method I
There are four steps in solving a stoichiometry problem:
Step
I: A balanced molecular equation is
written first.
Step
II: Given and required quantities
are underlined.
Step
III: The atomic/molecular masses
(moles) or molar volumes of underlined substances are written.
Step
IV: Calculation is done by Unitary or
Mole Method.
Method II
There are four steps in solving a stoichiometry problem:
Stoichiometric Relationships
The stoichiometric
calculations from the balanced chemical equations involve three types of
relationships:
1. Mass-Mass Relationship.
2. Mass-Volume Relationship.
3. Volume-Volume Relationship.
1. Mass – Mass Relationship
Such relationships are
helpful in determining unknown mass of a reactant or product from the given
mass of reactant or product with the help of balanced chemical equation.
If we are given the
mole of one substance, we can calculate the mass of the other substance and
vice versa.
If we are given the
mass of one substance, we can calculate the mass of the other substance and
vice-versa.
Mass-mass
relationships are of following types:
(i) Mass-mole relationship
(ii) Mole-mass relationship
(iii) g-g relationship
(iv) Mole-mole relationship
2. Mass – Volume Relationship
The Mass – Volume
relationship calculation are used when either one of the reactant or product is
in gaseous state. The mass – volume relationships are useful is determining the
unknown volume or mass of reactants or products from a known volume or mass of
some substances in a chemical reaction.
Mass-volume
relationships are of following types:
(i) Volume-mole relationship
(ii) Mole-volume relationship
(iii) g-volume relationship (Mass-Volume
relationship)
(iv) Volume-g relationship (Volume-mass
relationship)
The following points
are used in Mass – Volume Relationship:
(a) 1 mole (1 g/mole) of every gas always
occupies 22.4 dm3 (22.4 litre) or 22400 cm3 or 0.0224 m3
at S.T.P. It means that when mass is taken in grams,
the volume is taken in dm3 (lit) or cm3.
(b) 1 ounce – mole (ounce
molecular mass) of any gas at S.T.P. always occupies 22.4 cubic feet. It means
when mass is taken in ounce; volume is measured in cubic feet.
3. Volume – Volume Relationship
Volume-Volume
Relationship is used for determination of volumes of gases. The volume-volume
relationships are useful in determining unknown volume of reactants or products
from a known volume of reactants or products.
It is based on Gay-Lussac’s Law of Combining Volumes which states that:
“Gases
combine or form in chemical reactions in the ratio of simple whole numbers by volumes
provided at same temperature and pressure” OR “Under similar conditions of
temperature and pressure, the gases react in simple ratio of their volumes. The
ratio of the volumes of gases is same as the ratios of their molecules in a
balanced equation”. e.g.:
According to
Avogadro’s Law, 1 mole of any gas at standard condition of temperature and
pressure (S.T.P. 0°C and 1 atm.) always occupies 22.4 dm3. This is called Molar Volume.
1
mole of any gas at S.T.P. = 22.4
dm3 = 6.02 x 1023 molecules
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