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
Formation of a solid (precipitate) within a clear solution
Evolution of a gas
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 dioxide
Write the reaction using chemical symbols
NO + O NO
Balance the chemical equation
2NO + 1O2 2 NO2
*** Balancing equations***
Remember these rules when balancing an equation
An equation can be balanced only by adjusting the coefficients of formulas.
Never introduce extraneous atoms to balance.
NO + O 2NO + O 2
Never change a formula for the purpose of balancing an equation.
NO + O 2NO 3
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 multiplication.
- 1 - Problem: Balance C H O6+ 14 4O + H2O 2 2
Balance C: C H 6 +14 4 6 CO2+ H O 2 2
Balance H: C H 6 +14 46 CO 2 7H O 2 2
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 O . 2
Balance O: C H 6 +141542) O 6 CO 2 7H O 2 2
Now remove the fraction by multiplying the whole equation by 2.
2C 6 O14 14 O 12 2O + 14 H O 2 2
Remember to check both sides to see that both sides contain equal amounts of each element
Problem: Write a balanced equation for the complete combustion of C H , a gaso8in18
Solution: 2C H + 25O 16CO + 18H O
8 18 2 2 2
States of Matter
The states of matter in a chemical equation are represented by subscript symbols next to the
(g) gas (l) liquid (s) solid
Example:2C H 8 18(l) 25O 2(g) 16CO 2(g)+ 18H O2 (l)
- 2 - Another common symbol which is used for reactants or products disso