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Chemical equation

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A chemical equation is the same thing as a chemical reaction.[1] The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers. The first chemical equation was diagrammed by Jean Beguin in 1615.

Reading chemical equations

Each formula is preceded by an optional scalar number (if no scalar number is written, it is implied that the number is 1). The scalar numbers indicate the relative quantity of molecules in the reaction. For instance, the string 2H2O + 3CH4 represents a mixture containing 2 molecules of H2O for every 3 molecules of CH4.

  • The two sides of the equation are separated by an arrow. If the reaction is non-reversible, a right-arrow (→) is used, indicating that the left side represents the mixture of chemicals before the reaction, and the right side indicates the mixture after the reaction. For a reversible reaction, a two-way arrow is used. For example the equation 4Na + O2 → 2Na2O represents a non-reversible reaction. In this reaction, sodium (Na) and oxygen (O2) are converted to a single molecule, Na2O (containing 2 sodium atoms and 1 oxygen atom). We can also see that for every 4 sodium atoms at the beginning of the reaction, a single O2 molecule will participate, and 2 Na2O molecules will result.
  • A chemical equation does not imply that all reactants are consumed in a chemical process. For instance a limiting reactant determines how far a reaction can go.
  • In an ionic equation balancing of charge also takes place. In a molecular equation all reactants are written as molecules.

Balancing chemical equations

The law of conservation of mass dictates 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. Similarly, the charge is conserved in a chemical reaction. Therefore, the same charge must be present on both sides of the unbalanced equation.

One balances a chemical equation by changing the scalar number for each molecular formula. Simple chemical equations can be balanced by inspection, that is, by trial and error. Another technique involves solving a system of linear equations.

Example #1: Na + O2 → Na2O

In order for this equation to be balanced, there must be an equal amount of Na on the left hand side as on the right hand side. As it stands now, there is 1 Na on the left but 2 Na's on the right. This problem is solved by putting a 2 in front of the Na on the left hand side:

2Na + O2 → Na2O

In this there are 2 Na atoms on the left and 2 Na atoms on the right. In the next step the oxygen atoms are balanced as well. On the left hand side there are 2 O atoms and the right hand side only has one. This is still an unbalanced equation. To fix this a 2 is added in front of the Na2O on the right hand side. Now the equation reads:

2Na + O2 → 2Na2O

Notice that the 2 on the right hand side is "distributed" to both the Na2 and the O. Currently the left hand side of the equation has 2 Na atoms and 2 O atoms. The right hand side has 4 Na's total and 2 O's. Again, this is a problem, there must be an equal amount of each chemical on both sides. To fix this 2 more Na's are added on the left side. The equation will now look like this:

4Na + O2 → 2Na2O

This equation is a balanced equation because there is an equal number of atoms of each element on the left and right hand sides of the equation.

Example #2:

P4 + O2 → 2P2O5

This equation is not balanced because there is an unequal amount of O's on both sides of the equation. The left hand side has 4 P's and the right hand side has 4 P's. So the P atoms are balanced. The left hand side has 2 O's and the right hand side has 10 O's.


To fix this unbalanced equation a 5 in front of the O2 on the left hand side is added to make 10 O's on both sides resulting in

P4 + 5O2 → 2P2O5

The equation is now balanced because there is an equal amount of substances on the left and the right hand side of the equation.

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 which occur in aqueous solutions. For example in the following precipitation reaction:

CaCl2(aq) + 2AgNO3(aq) → Ca(NO3)2(aq) + 2AgCl(s)

the full ionic equation would be:

Ca2+ + 2Cl- + 2Ag+ + 2NO3- → Ca2+ + 2NO3- + 2AgCl(s)

and the net ionic equation would be:

2Cl-(aq) + 2Ag+(aq) → 2AgCl(s)

or, in reduced balanced form,

Ag+ + Cl- → AgCl(s)

In this aqueous reaction the Ca2+ and the NO3- ions remain in solution and are not part of the reaction. They are termed spectator ions and do not participate directly in the reaction, as they exist with the same oxidation state on both the reactant and product side of the chemical equation. They are only needed for charge balance of the original reagents.

In a neutralization or acid/base reaction, the net ionic equation will usually be:

H+ + OH- → H2O

There are a few acid/base reactions that produce a precipitate in addition to the water molecule shown above. An example would be the reaction of barium hydroxide with phosphoric acid because the insoluble salt barium phosphate is produced in addition to water.

Double displacement reactions that feature a carbonate reacting with an acid have the net ionic equation:

2 H+ + CO32- → H2O + CO2

NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq).

If every ion is a "spectator ion", then there was no reaction, and the net ionic equation is null.

See also

References

  1. ^ IUPAC Compendium of Chemical Terminology


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