A chemical equation communicates the mass relationships between
reactants and products in a reaction; you may recall that mass is
never created or destroyed: a chemical equation must follow this
rule
The coefficients in front of each formula tell you how many moles
of each reactant are required for the reaction to go to completion
Therefore the mass relationship is indirect and implied; to find
the mass relationships you must find the molar masses of each
substance
For an equation to be considered ‘balanced’ it has to
have the same amount of each element on the left and on the right
The substances on the left side undergo a reaction: they are the
reactants
The substances on the right side of the equation are the products
of the reaction
One special type of chemical equation is called a total ionic equation in which all the ions are shown separately since they are really separate when they are in solution
This type of chemical reaction can be reduced to a net ionic equation by removing all the components that do not actually react but rather stay in solution
Here is an example of a net ionic equation series:
2KI + Pb(NO3)2 → PbI2 + 2KNO3 regular equation
2K+ + 2I- + Pb2+ 2NO3- → PbI2 + 2K+ + 2NO3- total ionic equation
2I- + Pb2+ → PbI2 net ionic equation
Balancing Chemical Equations
There are a few simple steps that will enable you to balance any
chemical equation
Count the atoms of each element in the equation in its
unbalanced form. Make a table for the reactant side (left) and
one for the product side (right) showing the quantity of each
element or polyatomic ion.
Balance the atom or polyatomic
ion that appears only once on each side by inserting a coefficient
in front of the whole formula. Note: you cannot change
the subscripts in the formulas since this would change the substance
in the reaction! Sometimes an element or polyatomic ion will appear
more than once on one side. Save these instances for later.
Balance the other elements and polyatomic ions one by one,
leaving O and H for last.
Update your table for each change you make to the equation.
When you are done, each element or polyatomic ion will have the
same number next to it in both the reactant table and the
product table.
Some other things to keep in mind:
Treat polyatomic ions as units if they appear on both sides of
an equation, as they typically will. Do not count the atoms
inside the polyatomic ion since you might get them confused
with the same atoms that are not part of the ion.
Sometimes a change you make in the middle of the balancing
process will cause one of the other elements or polyatomic ions
to go out of balance. Don’t let this bother you: you will
just go back and update the coefficients as you go along.
Think in terms of the mathematical concept of least common
multiples (LCMs). You can figure out how many atoms or ions are
on each side of the equation; just find the LCM of the two
numbers to balance that unit.
The concept of the LCM is important because the ratios between
substances in the reaction should be expressed in lowest terms
Finally, make sure you have the correct formulas for all of
your reactants: incorrect formulas lead to incorrect equations!
Chemical Reactions
Some changes, such as melting, evaporating, or freezing are purely
physical changes
Physical changes involve changes of state and are accompanied by
either a loss or a gain in energy
Chemical reactions also involve changes in energy and very often
changes in state
A chemical reaction can be recognized for what it is if the
chemical properties of the products are different from those of the
reactants
For instance, you know a chemical reaction has occurred when you
strike a match; once you have lit it and blown it out, you cannot
relight it: the chemical properties of the stuff in the
match’s head have changed
This change in chemical properties comes about because of the
rearrangement of atoms into new compounds
Information in Chemical Equations
Chemical equations tell you the molar ratios among all the substances involved
In the equation 2H2 + O2 → 2H2O you could say that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water
You could also say that two moles of hydrogen react with one mole of oxygen to form two moles of water
Say you wanted to know how many moles of hydrogen you would need (there is plenty of oxygen in the air so we don’t need to worry about that) to get 10 moles of water
Look at the coefficients in the chemical equation: there is a 1:1 relationship between hydrogen and water
To get 10 moles of water you would need 10 moles of hydrogen; how many moles of oxygen did we use?
One thing to note is that reactions seldom go to completion; that is, it is seldom true that you have all product and no more reactants
The amount of each product shown in the chemical equation represents the theoretical yield of the reaction
The amount of each product actually collected represents the actual yield
Chemical Reaction Types
Combustion: the combination of an element with oxygen to form an oxide; it is usually accompanied by the release of a large amount of heat; hydrocarbon combustion reactions usually result in the formation of CO2 and H2O; if ashes remain after such a reaction it is because the reaction did not go to completion and some of the carbon failed to combust
Synthesis: the combination of simpler materials to form more complex materials; combustion is actually a type of synthesis; another example is photosynthesis: 6CO2 + H2O → C6H12O6 + 6O2
Decomposition: the breaking down of one substance into several others; an example is the electrolysis of water which reverses the synthesis reaction already referred to
Displacement: a single displacement results in one element taking the place of another; for example take the reaction of zinc with HCl to form zinc chloride and hydrogen gas; it is possible to predict displacement reactions with an activity series; there is a table in every chemistry book that you can refer to; elements at the top of the list are more reactive than elements at the bottom and if two elements are ‘competing’ in a reaction, the one that is higher up will react first; also, elements at the top of the list are capable of displacing elements below them but the reverse is not true
