Balancing Chemical Equations Balance the equation Na(s) + H2O(/)—? NaOH(aq) + H2 (g) Balance these equations by providing the missing coefficients:

Answer Step 1 Begin by counting each kind of atom on the two sides of the arrow. There are one Na, one O, and two H on the left side, and one Na, one O, and three H on the right. The Na and O atoms are balanced, but the number of H atoms is not. To increase the number of H atoms on the left, let’s try placing the coefficient 2 in front of H2O: Na(s)+2H2O(l) NaOH(aq)+H2(g). Although beginning this way does not balance H, it does increase the number of reactant H atoms, which we need to do. (Also, adding the coefficient 2 on H2O unbalanced O, but we will take care of that after we balance H.) Now that we have 2 H2O on the left, we balance H by putting the coefficient 2 in front of NaOH: Na (aq)+2H2O2NaOH+H2 Balancing H in this way brings O into balance, but now Na is unbalanced, with one Na on the left and two on the right. To rebalance Na, we put the coefficient 2 in front of the reactant: 2Na+2H2O 2NaOH + H2 We now have two Na atoms, four H atoms, and two O atoms on each side. The equation is balanced. Comment Notice that we moved back and forth, placing a coefficient in front of H2O, then NaOH, and finally Na. In balancing equations, we often find ourselves following this pattern of moving back and forth from one side of the arrow to the other, placing coefficients first in front of a formula on one side and then in front of a formula on the other side until the equation is balanced. You can always tell if you have balanced your equation correctly by checking that the number of atoms of each element is the same on the two sides of the arrow, and that you’ve chosen the smallest set of coefficients that balances the equation.