each element the number of atoms in the reactants are equal to the number of atoms in the products. 2 H 2(g) + O 2(g) à 2 H 2 O(g). In addition, the coefficients must be reduced to the smallest set of whole numbers . For example, in the equation, 4 H 2 + 2 O 2 à 4 H 2 O, all the coefficients are multiples of 2. Thus, they must be divided by 2. Another example is H 2 + 1/2 O 2 à H 2 O. Both sides need to be multiplied by 2, to obtain the smallest whole numbers. 7.2 Types of reactions Description of Types of Reactions Examples In combination reaction , two or more reactants combine to form one product. 2Mg( s ) + O 2 ( g ) à 2MgO( s ) In decomposition , a reactant breaks down into two or more products. 2KClO 3 ( s ) à 2KCl( s ) + 3O 2 ( g )
2 In single replacement , one element takes the place of a different element in a reacting compound. Zn( s ) + 2HCl( aq ) à ZnCl 2 ( aq ) + H 2 ( g ) In double replacement , anions and cations of two reactants switch places. Mg(OH) 2 ( s ) +2HCl( aq ) à MgCl 2 ( aq ) + 2H 2 O( l ) 7.4 The mole Mole is used by chemists as a counting unit. 1 mole = 6.02 x 10 23 objects , such quantity is called Avogadro number, which is the same number as the atoms in 12 grams of 12 C.