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 + O 2 NO 2
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 + O 2 NO + 2
3. Never change a formula for the purpose of balancing an equation.
NO + O 2 NO 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. Problem: Balance C H O 6 14 4O + H2O 2 2
Balance C: C H 6 14O 4 6 CO2+ H O 2 2
Balance H: C H O + O 6 CO + 7H O
6 14 4 2 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 14(14/2) 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
Problem: Write a balanced equation for the complete combustion of C H , a gasolin8 18
Solution: 2C H8 18+ 25O 216CO + 18H 2 2
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: 2C H + 25O 16CO + 18H O
8 18(l) 2(g) 2(g) 2 (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: Δ which means to heat up as in the decomposition reaction of silver oxide:
2 Ag O2 (s)Δ 4 Ag (s)+ O 2 (g) General equations and Stoichiometry
Stoichiometry includes all the quantitative relationships involving atomic and formula
masses, chemical formulas, and chemical equations.
Consider: 2H 2 (g) O 2(g) 2H O 2 (l)
This means that : 2x molecules H 2 (g) x molecules O 2(g) 2x molecules H O 2 (l)
And since x = Avogadro’s number which represents 1 mole, we can also write:
2 mol H 2 (g) 1 mol O 2(g) 2 mol H O 2 (l)
The coefficients in the equation allow us to make statements such as:
• Two moles of H 2 are produced for every two moles of H consumed. 2
• Two moles of H O a2e produced for every one mole of O consumed. 2
• Two moles of H are2consumed for every one mole of O consumed. 2
These statements can be turned into conversion factors called stoichiometric
factors. A stoichiometric factor relates the amounts, on a mole basis, of any two
substances involved in a chemical reaction. Thus a stoichiometric factor is also a
Relating the Numbers of Moles of Reactant and Products
Problem: How many moles of CO are pro2uced in the combustion of 2.72 mol of
triethylene glycol, C H O , in an excess of O ?
6 14 4 2
An excess of oxygen gas means that triethylene glycol is the limiting reactant.
Balance the equation: 2 C 6 14+ 45O 122O +14H O 2 2
? mol CO =2[2.72 mol C H O 6 (14m4l CO / 2 mol C2H O )] = 166314ol4CO 2
Chemical Reactions in Solutions
Most chemical reactions take place in solution partly because mixing the reactants
helps to achieve the close contact between atoms, ions, or molecules necessary for
the reaction to occur.
Solvent determines whether the solution exists as a solid, liquid, or a gas. However,
for learning purposes, the aqueous (aq) solution will be used as a solvent.
Solute is the material being dissolved by the solvent.
Aqueous reactions can be grouped into three general categories: Precipitation
reactions, acid-base neutralization reactions and oxidation-reduction (red ox)