SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 4

Chemical Formulas

WRITING IONIC EQUATIONS

At times, chemists choose to show only the substances that react in the chemical action. These equations are called ionic equations because they stress the reaction and production of ions. If we look at the preceding equation, we see the complete cast of “actors”:

Reactants

          2HCl(aq) releases   →   2H+ (aq) + 2Cl(aq) in solution

          Zn(s) stay as           →   Zn(s) particles                          

Products

          ZnCl2(aq) releases   →   Zn2+(aq) + 2Cl(aq) in solution

          H2(g) stay as           →   H2(g) molecules

Writing the complete reaction using these results, we have:

2H+(aq) + 2Cl(aq) + Zn(s) → Zn2+(aq) + 2Cl(aq) + H2(g)

Notice that nothing happened to the chloride ion. It appears the same on both sides of the equation. It is referred to as a spectator ion. In writing the net ionic equation, spectator ions are omitted, so the net ionic equation is:

2H+(aq) + Zn(s) → Zn2−(aq) + H2(g)

TIP 

In net ionic equations, do not show “spectator” ions that do not change.