CHEM 1001 Lecture Notes - Triethylene Glycol, Chemical Equation, Nitric Oxide
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Chemistry Chapter Four
Chemical Reactions and Chemical Equations
A chemical reaction is a process in which one set of substances (reactants) is
converted to a new set of substances (products). The main evidences that prove
that a chemical reaction has occurred are:
a) Colour change
b) Formation of a solid (precipitate) within a clear solution
c) Evolution of a gas
d) Evolution or absorption of heat
To write the shorthand representation of a chemical equation certain steps must be
followed. Consider the chemical reaction: nitrogen monoxide + oxygen nitrogen
1. Write the reaction using chemical symbols
NO + O2 NO2
2. Balance the chemical equation
2NO + 1O2 2 NO2
*** Balancing equations***
Remember these rules when balancing an equation
1. An equation can be balanced only by adjusting the coefficients of formulas.
2. Never introduce extraneous atoms to balance.
NO + O2 NO2 + O
3. Never change a formula for the purpose of balancing an equation.
NO + O2 NO3
When balancing equations, using these strategies will result in efficient balancing of
Balance elements that occur in only one compound of each side first.
Balance free elements last.
Balance unchanged polyatomics (or other groups of atoms) as groups.
Fractional coefficients are acceptable and can be cleared at the end by
Problem: Balance C6H14O4 + O2 CO2 + H2O
Balance C: C6H14O4 + O2 6 CO2 + H2O
Balance H: C6H14O4 + O2 6 CO2 + 7H2O
At this point, the right side contains 19 O atoms whereas the left only contains 4. To
get 15 more O atoms we need to add 15/2 more O atoms by multiplying it with O2.
Balance O: C6H14O4 + (15/2) O2 6 CO2 + 7H2O
Now remove the fraction by multiplying the whole equation by 2.
2C6H14O4 + 15 O2 12 CO2 + 14 H2O
Remember to check both sides to see that both sides contain equal amounts of each
Problem: Write a balanced equation for the complete combustion of C8H18, a gasoline
Solution: 2C8H18 + 25O2 16CO2 + 18H2O
States of Matter
The states of matter in a chemical equation are represented by subscript symbols
next to the compound.
(g) gas (l) liquid (s) solid
Example: 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O(l)
Another common symbol which is used for reactants or products dissolved in water is
(aq): aqueous solution.
A simple chemical equation cannot convey the information of how to carry out the
reaction in an experiment. However there are symbols that tell us what must be done
for the reaction to occur.
Example: which means to heat up as in the decomposition reaction of silver oxide:Δ
2 Ag2O (s) Δ 4 Ag (s) + O2 (g)