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As seen from the equation CH
4 + 2 O
2 → CO
2 + 2 H
2O, a coefficient of 2 must be placed before the oxygen gas on the reactants side and before the water on the products side in order for, as per the law of conservation of mass, the quantity of each element does not change during the reaction
P4O10 + 6 H2O → 4 H3PO4
This chemical equation is being balanced by first multiplying H3PO4 by four to match the number of P atoms, and then multiplying H2O by six to match the numbers of H and O atoms.
The law of conservation of mass dictates that the quantity of each element does not change in a chemical reaction. Thus, each side of the chemical equation must represent the same quantity of any particular element. Likewise, the charge is conserved in a chemical reaction. Therefore, the same charge must be present on both sides of the balanced equation.
One balances a chemical equation by changing the scalar number for each chemical formula. Simple chemical equations can be balanced by inspection, that is, by trial and error. Another technique involves solving a system of linear equations.
Balanced equations are written with smallest whole-number coefficients. If there is no coefficient before a chemical formula, the coefficient 1 is understood.
The method of inspection can be outlined as putting a coefficient of 1 in front of the most complex chemical formula and putting the other coefficients before everything else such that both sides of the arrows have the same number of each atom. If any fractional coefficient exists, multiply every coefficient with the smallest number required to make them whole, typically the denominator of the fractional coefficient for a reaction with a single fractional coefficient.
As an example, seen in the above image, the burning of methane would be balanced by putting a coefficient of 1 before the CH4:
1 CH4 + O2 → CO2 + H2O
Since there is one carbon on each side of the arrow, the first atom (carbon) is
435
balanced.
Looking at the next atom (hydrogen), the right-hand side has two atoms, while the left-hand side has four. To balance the hydrogens, 2 goes in front of the H2O, which yields:
1 CH4 + O2 → CO2 + 2 H2O
Inspection of the last atom to be balanced (oxygen) shows that the right-hand side has four atoms, while the left-hand side has two. It can be balanced by putting a
2 before O2, giving the balanced equation:
CH4 + 2 O2 → CO2 + 2 H2O
This equation does not have any coefficients in front of CH4 and CO2, since a coefficient of 1 is dropped.
Ionic equations
An ionic equation is a chemical equation in which electrolytes are written as dissociated ions. Ionic equations are used for single and double displacement reactions that occur inaqueous solutions. For example, in the following precipitation reaction:
the full ionic equation is:
In this reaction, the Ca2+ and the NO3− ions remain in solution and are not part of the reaction. That is, these ions are identical on both the reactant and product side of the chemical equation. Because such ions do not participate in the reaction, they are called spectator ions. A net ionic equation is the full ionic equation from which the spectator ions have been removed. The net ionic equation of the proceeding reactions is:
or, in reduced balanced form,
In a neutralization or acid/base reaction, the net ionic equation will usually be:
H+(aq) + OH−(aq) → H2O(l)
There are a few acid/base reactions that produce a precipitate in addition to the water molecule shown above. An example is the reaction of barium hydroxide with phosphoric acid, which produces not only water but also the insoluble salt barium phosphate. In this reaction, there are no spectator ions, so the net ionic equation is the same as the full ionic equation.
Double displacement reactions that feature a carbonate reacting with an acid have the net ionic equation:
If every ion is a "spectator ion" then there was no reaction, and the net ionic equation is null. [11]
An acid-base reaction is a chemical reaction that occurs between an acid and a base. Several concepts exist which provide alternative definitions for the reaction mechanisms involved and their application in solving related problems. Despite several similarities in definitions, their importance becomes apparent as different methods of analysis when applied to acid-base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. Historically, the first of these scientific concepts of acids and bases was provided by the French chemistAntoine Lavoisier, circa 1776.[12]
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