Electrolytes, Ionic Reactions, and Precipitation

Strong Electrolytes

Strong electrolytes are substances that dissociate nearly 100% into ions in solution. Because they produce a high concentration of ions, they conduct electric current very efficiently.

For example, when 1 mole of sodium chloride ($NaCl$) dissolves, it produces 2 moles of ions:

$$\begin{aligned} 1\text{m } NaCl(s) &\rightarrow 1\text{m } Na^+(aq) + 1\text{m } Cl^-(aq) \end{aligned}$$

Similarly, sulfuric acid ($H_2SO_4$) dissociates to produce more ions per mole of solute:

$$\begin{aligned} 1\text{m } H_2SO_4(s) &\rightarrow 2\text{m } Na^+(aq) + 1\text{m } SO_4^{2-}(aq) \end{aligned}$$

Common strong electrolytes include salts like $NaCl$ and strong acids like $HNO_3$ and $HCl$.

Weak Electrolytes

Weak electrolytes only partially dissociate into ions. In these solutions, most of the substance remains in its original molecular form, with only a small fraction converted into ions. Consequently, they are only slightly conductive. A classic example is vinegar (an aqueous solution of acetic acid, $CH_3CO_2H$), which exists in a state of equilibrium:

$$\begin{aligned} CH_3CO_2H &\rightleftharpoons CH_3CO_2^-(aq) + H^+(aq) \end{aligned}$$

Exam Tip: Solubility $\neq$ Electrolyte Strength Do not confuse how much a substance dissolves (solubility) with how well it conducts electricity (electrolyte strength).

• Sugar dissolves completely in water but is a non-electrolyte because it doesn't form ions.
• $BaSO_4$ is very poorly soluble, but the tiny amount that does dissolve dissociates completely into $Ba^{2+}$ and $SO_4^{2-}$, making it a strong electrolyte.

Precipitation and Solubility Guidelines

A precipitation reaction occurs when two aqueous solutions are mixed and an insoluble product forms, falling out of the solution as a solid called a precipitate. To predict whether a precipitate will form, chemists use solubility guidelines.

Key Solubility Rules

To predict the outcome of a reaction, examine the products formed. If either product is insoluble, a precipitate will form.

Generally Soluble Compounds:

• Salts containing Group I elements ($Li^+, Na^+, K^+, Cs^+, Rb^+$) and Ammonium ($NH_4^+$).
• Salts containing the Nitrate ion ($NO_3^-$).
• Salts containing halides ($Cl^-, Br^-, I^-$), except for those containing $Ag^+, Pb^{2+},$ or $(Hg_2)^{2+}$.
• Most sulfate salts ($SO_4^{2-}$), except for $CaSO_4, BaSO_4, PbSO_4, Ag_2SO_4,$ and $SrSO_4$.
• Hydroxides of Group I elements.

Frequently Insoluble Compounds:

• Most silver salts (except $AgNO_3$ and $Ag(C_2H_3O_2)$).
• Most transition metal sulfides (e.g., $CdS, FeS, ZnS, Ag_2S$).
• Carbonates (e.g., $CaCO_3, BaCO_3, FeCO_3, PbCO_3$).
• Phosphates (e.g., $Ca_3(PO_4)_2, Ag_3PO_4$).
• Chromates (e.g., $PbCrO_4, BaCrO_4$).

Worked Example: Predicting a Precipitate

Question: Predict whether a precipitate will form in the following reaction:

$$\begin{aligned} 3AgNO_3(aq) + Na_2S(aq) &\rightarrow Ag_2S(s) + 2NaNO_3(aq) \end{aligned}$$

Step 1: Analyze the products. The products are $Ag_2S$ and $NaNO_3$.

Step 2: Apply solubility rules.

• For $NaNO_3$: Nitrates are generally soluble, so $NaNO_3$ will remain in solution.
• For $Ag_2S$: Sulfides of transition metals (including silver) are highly insoluble.

Conclusion: Because $Ag_2S$ is insoluble, a precipitate will form.

Important Rule Precedence: When rules seem to contradict, specific rules take precedence. For example, while most bromides ($Br^-$) are soluble, silver bromide ($AgBr$) is insoluble because the rule stating "most silver salts are insoluble" takes precedence.

Net Ionic Equations

To focus only on the species actually participating in the chemical change, we use net ionic equations. This involves removing "spectator ions"—ions that appear on both sides of the equation unchanged and do not participate in the formation of the precipitate.

For the reaction between $AgNO_3$ and $NaCl$:

1. Molecular Equation: $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$
2. Complete Ionic Equation: $Ag^+(aq) + NO_3^-(aq) + Na^+(aq) + Cl^-(aq) \rightarrow AgCl(s) + Na^+(aq) + NO_3^-(aq)$
3. Net Ionic Equation: $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$

In this case, $Na^+$ and $NO_3^-$ are the spectator ions.