reaction should be possible, that is, its flow is acceptable from the point of view of the well-known physical and chemical laws.For example:
Zn + 2HCl = ZnCl2 + H2 - such a reaction between zinc and hydrochloric acid is possible;

2Au + 6HCl = 2AuCl3 + 3H2 - but the reaction between gold and the hydrochloric acid is not possible, though, zinc and gold - metals.The reason for the arrangement of zinc and gold "electrochemical voltage range."Zinc is located far to the left of hydrogen and gold, respectively, far to the right.
the left side of the equation is written formulas of substances, reactive ("starting materials"), and on the right side of the equation - the formula of substances produced by the reaction ("Product").
While talking about equations,
we must firmly remember: mathematical rule "from the relocation of sum does not change" does not apply here.In other words, in a molecular chemical reaction equation in any case can not swap the left and right part.From this completely changes its meaning.For example:
AgNO3 + KBr = KNO3 + AgBr (precipitate) - reaction in this form can be written, it proceeds to the end, as formed poorly soluble substance - silver bromide;

AgBr + KNO3 = AgNO3 + KBr - but in such a way (although you just swapped the left and right sides), it will not do.
coefficients must be chosen so that the number of atoms of each element in the left and right sides was the same.For example:
Na2SO4 + 2BaCl = 2NaCl + BaSO4.
is easy to ensure that both the left and the right side of the equation contains two atoms of sodium four oxygen atoms, two atoms of barium, the two chlorine atoms and one sulfur atom.
To make it easier to make the molecular reaction equations, you must immediately remember a simple rule: the reaction is going to end, if at least one of the reaction products leaving the reaction zone, ie escapes as a gas or precipitate,If a sparingly soluble substance.For example:
H2SO4 + Na2CO3 = Na2SO4 + H2CO3.
forms carbonic acid (H2CO3) is so unstable that almost instantaneously decomposed into carbon dioxide and water.Therefore, the reaction can be immediately written in final form:
H2SO4 + Na2CO3 = Na2SO4 + H2O + CO2.