Problem 77 - Chemistry and Chemical Reactivity

Question

In each of the following cases, decide whether a precipitate will form when mixing the indicated reagents, and write a balanced equation for the reaction. (a) \(\mathrm{NaBr}(\mathrm{aq})+\mathrm{AgNO}_{3}(\mathrm{aq})\) (b) \(\mathrm{KCl}(\mathrm{aq})+\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Precipitates form in both reactions: (a) AgBr and (b) PbCl2.

Step 1: Predict Possible Products for Reaction (a)

When mixing \(\mathrm{NaBr}(\mathrm{aq})\) and \(\mathrm{AgNO}_3(\mathrm{aq})\), the ions involved are \(\mathrm{Na}^+, \mathrm{Br}^-\), \(\mathrm{Ag}^+\), and \(\mathrm{NO}_3^-\). The possible new products are \(\mathrm{NaNO}_3\) and \(\mathrm{AgBr}\). We hypothesize that \(\mathrm{AgBr}\) might precipitate based on solubility rules.

Step 2: Apply Solubility Rules to Reaction (a)

According to solubility rules, most bromides are soluble, but silver bromide (\(\mathrm{AgBr}\)) is an exception and is insoluble in water. Therefore, \(\mathrm{AgBr}\) will form as a precipitate.

Step 3: Write Balanced Equation for Reaction (a)

The balanced chemical equation is: \[ \mathrm{NaBr}(\mathrm{aq}) + \mathrm{AgNO}_3(\mathrm{aq}) \rightarrow \mathrm{AgBr}(\mathrm{s}) + \mathrm{NaNO}_3(\mathrm{aq}) \] This shows the precipitation of \(\mathrm{AgBr}\).

Step 4: Predict Possible Products for Reaction (b)

For the reaction \(\mathrm{KCl}(\mathrm{aq}) + \mathrm{Pb}(\mathrm{NO}_3)_2(\mathrm{aq})\), the ions present are \(\mathrm{K}^+, \mathrm{Cl}^-\), \(\mathrm{Pb}^{2+}\), and \(\mathrm{NO}_3^-\). The products could be \(\mathrm{KNO}_3\) and \(\mathrm{PbCl}_2\). We suspect that \(\mathrm{PbCl}_2\) might precipitate based on solubility rules.

Step 5: Apply Solubility Rules to Reaction (b)

Solubility rules indicate that most chlorides are soluble, except for those of silver, lead (II), and mercury. \(\mathrm{PbCl}_2\) is one of these exceptions and is therefore insoluble in water, hence it will precipitate.

Step 6: Write Balanced Equation for Reaction (b)

The balanced chemical equation for the reaction is:\[ \mathrm{2KCl}(\mathrm{aq}) + \mathrm{Pb}(\mathrm{NO}_3)_2(\mathrm{aq}) \rightarrow \mathrm{PbCl}_2(\mathrm{s}) + 2\mathrm{KNO}_3(\mathrm{aq}) \]This equation confirms the formation of \(\mathrm{PbCl}_2\) as a precipitate.

Key Concepts

Solubility Rules

In chemistry, understanding whether a substance will dissolve in water or form a solid is crucial. This is where solubility rules come into play. Solubility rules are guidelines that help predict the solubility of ionic compounds in water.
They are essential for determining if a precipitation reaction will occur. Here are some key points:

• Most nitrate salts (e.g., \( \text{NO}_3^- \)) are soluble in water.
• Most salts of sodium (\( \text{Na}^+ \)), potassium (\( \text{K}^+ \)), and ammonium (\( \text{NH}_4^+ \)) are soluble.
• Common chlorides, bromides, and iodides are soluble except when paired with silver (\( \text{Ag}^+ \)), lead (\( \text{Pb}^{2+} \)), and mercury (\( \text{Hg}_2^{2+} \)).
• Sulfates (\( \text{SO}_4^{2-} \)) are generally soluble, with exceptions like barium sulfate (\( \text{BaSO}_4 \)) and lead sulfate (\( \text{PbSO}_4 \)).

By applying these rules, we can predict whether a reaction will produce a precipitate, such as \( \text{AgBr} \) or \( \text{PbCl}_2 \), which are insoluble in water.

Balanced Chemical Equations

A balanced chemical equation is like a recipe in chemistry—it tells us how much of each reactant is needed to form products. The main concept behind balancing equations is the conservation of mass. This means the number of atoms for each element should be the same on both sides of the equation.
Here’s a simple guideline to keep in mind:

• Identify the reactants and products in the reaction.
• Write down the chemical formulas of reactants on the left and products on the right.
• Count the number of atoms for each element in the reactants and products.
• Add coefficients (numbers in front of molecules) to balance the number of atoms for each element.

For example, for the reaction between \( \text{KCl} \) and \( \text{Pb(NO}_3)_2 \), the balanced equation \( 2\text{KCl} + \text{Pb(NO}_3)_2 \rightarrow \text{PbCl}_2 + 2\text{KNO}_3 \) ensures the mass is conserved, with equal numbers of potassium, chlorine, lead, and nitrate ions on both sides.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two soluble salts in an aqueous solution react to form an insoluble solid, called a precipitate. This solid forms because it is not soluble in water according to solubility rules.
Understanding precipitation reactions can be broken down into these steps:

• Identify the ions present in the solution before the reaction.
• Determine possible combinations of cations and anions, predicting new compounds.
• Apply solubility rules to these compounds to see which, if any, will form a precipitate.

Using an example: mixing \( \text{NaBr} \) and \( \text{AgNO}_3 \) forms \( \text{AgBr} \), which is insoluble and precipitates out of the solution. Recognizing these reactions often involves spotting the formation of a solid that is opaque or different in color from the rest of the solution.