Just because substances come in contact with each other does not mean a chemical reaction must occur. Predicting whether a reaction will take place is not always easy. In general, if one element is more active than another, the more active element will replace the less active element in a compound - thus a chemical reaction will take place. The table below lists metals in order of reactivity, or tendency to react. A metal on the table will usually displace any below it on the table.
Coefficients are the large numbers in front of chemical formulas in a chemical equation.
Coefficients indicate the number of molecules. Think of this as the ratio of the molecules involved in the chemical reaction. The oxidation number of an element indicates the number of electrons gained or lost when forming compounds. A positive oxidation number means the atom looses electrons and a negative oxidation number means the atom gains electrons.