Laws of chemical combination,
Chemical equations and
stoichiometry
Outline
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Four laws of chemical combination
Chemical equations
Stoichiometry
The mole concept
Examples
Tutor marked assignments (TMAs)
Laws of chemical combination
• Law of conservation of mass
– ‘Matter can be neither created nor destroyed’
• Law of definite proportions
– ‘A pure compound always contain the same elements combined
in the same proportions by mass’
The mass of a pure compound is constant regardless of it modes of
preparation.
• Law of multiple proportions
– ‘If two elements A and B can form more than one compound (such as AB,
A2B, AB2, etc), then the several masses of a which separately combine with
a fixed mass of B are in a simple ratio.
Examples of compounds that show multiple proportions are CO and CO2;
CuO and Cu2O; NO, N2O, NO2, N2O3 and N2O5.
Laws of chemical combination contd
• Law of reciprocal proportions/equivalent proportions
– ‘the respective masses of elements B and C that combines
with a fixed mass of element A are the masses of B and C
that will combine with each other or a simple multiple of
these masses’
Examples of compounds that show law of reciprocal
proportions are NH3 and H2O (A is Hydrogen, B is Nitrogen
and C is Oxygen). Furthermore, B and C can form any of
NO, NO2, N2O, etc.
N2 + O2 → 2NO
It can be shown that the reacting ratio of B and C is an
integer or ratio of integers
Chemical equations
• A chemical equation is a representation of
chemical reaction by means of elemental symbols
and formulae of compound or ions.
• Types of equations:
1. normal/molecular 2. ionic 3. nuclear
• Conventionally a chemical equation is written as
Or,
Reactants → Products
A + B → C+ D
• Example: 2H2O + O2 → 2H2O
• Our task is to balance equations by providing appropriate
stoichiometric coefficients such 2, 1 and 2 in the above example.
Balancing Chemical Equations
• Methods are:
– By Inspection
– By Algebraic approach
– By Oxidation number/ion-electron transfer (for
redox reactions)
Goto TMAs for illustrations.
• Note: Nuclear equations require special
methods for balancing.
Stoichiometry
• Stoichiometry deals with mass (and volume)
relationships between elements and
compounds especially in chemical reactions.
• It is based on three major concepts/principles:
– The law of conservation of mass,
– The atomic mass/molecular mass concept,
– The mole concept.
The mole concept
• The mole concept permits expression of quantity of matter based on the
number of atoms/particles rather than its mass.
• Definitions of the mole
– The mole is the relative atomic/molecular of a substance expression
expressed in gramme
– The mole is the amount of substance that contains particles as there
are atoms in exactly 12.000g of carbon-12.
– One mole of a substance contains a constant number of its particles
called the Avogadro number, NA which is 6.023 x 1023 particles
The particle could be atoms, molecules, electrons, ion, formula
particles, etc.
– For gaseous systems, one mole of a gas occupies 22.4dm3 at stp
– Thus, the mole is taken as the SI unit of amount of substance.
The mole concept contd
Illustrations on the mole concept
• 1 mole of H2O Ξ 18g Ξ 6.023 x 1023 molecules of water
• 1 mole of NaCl Ξ 58.5g Ξ 6.023 x 1023 formula particles
• 1 mole of Cu2+ Ξ 63.5g Ξ 6.023 x 1023 ions of Cu2+
Thus following formulas apply:
n = m/M
… (i)
N = nNA
… (ii)
N/NA = m/M
… (iii)
where n is number of moles, m = mass of substance, M = molar
mass, N = number of particles and NA = Avogadro number
For gaseous systems:
n = (CV)/1000
… (iv)
where C is concentration in mole/dm3 and V is volume of gas in cm3.
Examples on mole concept
• Calculate the number of moles in
(a)15.8g of KMnO4
(b) 3.14 x 1022 atoms of magnesium.
Solution
(a) n = 15.8g/158g/mol = 0.100 mol
(b) n = 3.14 x 1022 atoms/6.023 x 1023atoms/mole
= 0.0521 mol
Tutor Marked Assignments/Exercises
1. Three oxides of nitrogen are N2O, NO and NO2.
find the ratio of oxygen that combines with 14g
of nitrogen. Which law does these compounds
illustrate?
2. State the law of reciprocal proportions and give
two example of compounds that illustrate it.
3. An oxide of Cu has 1.30g of Cu and a mass of
1.62. Another oxide has a mass of 2.02g and
contains 0.40g of oxygen. Which law is obeyed
by the compounds?
Tutor Marked Assignments/Exercises
contd
4.Balance the following chemical equations
a.
H2S + SO2 → H2O + S
b. CuO + NH3 → Cu +H2O + N2
c.
NH3 + Cl2 → NH4Cl + N2
d. I- + Cl2 → I2 + Cle. MnO4- + Fe2+ → Mn2+ + Fe3+ (in Acidic medium)
Tutor Marked Assignments/Exercises
contd
5. Analysis of a compound shows that it contains
2.6% of Al. If 2.0g of the compound was used,
find the number of Al atoms in it.
6. A sample of 0.500g of impure CaCO3 was
dissolved in 50.0cm3 of 0.100moldm-3 HCl and
the remaining acid was titrated against 5.0cm3
of 0.12moldm-3 NaOH solution. Find the
percentage of CaCO3 in the sample.
7. What is the number of moles of sulphur atoms
present in 500g of CuSO4.5H2O?