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) \(\mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})\) (b) \(\mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq})+\mathrm{FeCl}_{3}(\mathrm{aq})\)

Short answer

(a) No precipitate forms. (b) A precipitate of \( \mathrm{FePO}_{4} \) forms.

Step-by-step solution

Identify possible products

When mixing two ionic solutions, exchange the anions and cations to predict possible products. (a) For \( \mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq}) + \mathrm{Mg}(\mathrm{NO}_{3})_{2}(\mathrm{aq}) \), the possible products after the exchange are \( \mathrm{MgSO}_{4} \) and \( \mathrm{NaNO}_{3} \). (b) For \( \mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq}) + \mathrm{FeCl}_{3}(\mathrm{aq}) \), the possible products are \( \mathrm{FePO}_{4} \) and \( \mathrm{KCl} \).

Check solubility of products

Consult a solubility chart to determine the solubility of each possible product. (a) \( \mathrm{MgSO}_{4} \) is soluble in water, and \( \mathrm{NaNO}_{3} \) is also soluble, so no precipitate forms.(b) \( \mathrm{FePO}_{4} \) is insoluble in water, meaning a precipitate will form, whereas \( \mathrm{KCl} \) is soluble.

Write the balanced chemical equation

Based on the solubility in Step 2, write the balanced equation for the reaction where a precipitate forms.(a) No precipitation reaction occurs, so no balanced equation is needed.(b) The balanced equation for the formation of precipitate \( \mathrm{FePO}_{4} \) is: \[ \mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq}) + \mathrm{FeCl}_{3}(\mathrm{aq}) \rightarrow \mathrm{FePO}_{4}(\mathrm{s}) + 3\mathrm{KCl}(\mathrm{aq}) \]

Key concepts

Solubility Rules

Solubility rules are essential guidelines that help us predict whether a substance will dissolve in water, forming an aqueous solution, or remain as a solid precipitate. These rules categorize ionic compounds based on their solubility characteristics and are crucial in determining the outcome of precipitation reactions.
Understanding these rules, such as chlorides being generally soluble except with silver, lead, and mercury, allows students to decide whether a precipitate will form when two aqueous solutions mix. For example, in the case of mixing \( \mathrm{K}_{3}\mathrm{PO}_{4}(\mathrm{aq}) \) and \( \mathrm{FeCl}_{3}(\mathrm{aq}) \), referring to the solubility rules helps us identify that a precipitate, \( \mathrm{FePO}_{4} \), forms because it's generally insoluble.

• Sulfates are mostly soluble, except for those of barium, calcium, and lead.
• Nitrates and acetates are always soluble.
• Phosphates and carbonates are generally insoluble unless paired with alkaline metals or ammonium.

Balanced Chemical Equations

Balanced chemical equations represent the accurate stoichiometry of a chemical reaction. This means that the number of atoms for each element involved in the reaction must be the same on both sides of the equation.
Balancing chemical equations does not only help fulfill the Law of Conservation of Mass, but also ensures that we accurately represent the process occurring during a reaction.
For example, once we know precipitation occurs when \( \mathrm{K}_{3}\mathrm{PO}_{4}(\mathrm{aq}) \) reacts with \( \mathrm{FeCl}_{3}(\mathrm{aq}) \), writing a balanced equation becomes crucial:\[ \mathrm{K}_{3}\mathrm{PO}_{4}(\mathrm{aq}) + \mathrm{FeCl}_{3}(\mathrm{aq}) \rightarrow \mathrm{FePO}_{4}(\mathrm{s}) + 3\mathrm{KCl}(\mathrm{aq}) \].
This equation indicates that three moles of potassium chloride form, maintaining atomic balance across the entire reaction. While balancing, remember to:

• Count atoms of each element on the reactant and product sides.
• Use coefficients to equalize the number of atoms for each element.
• Re-assess the equation to ensure that it remains balanced even after adjusting coefficients.

Ionic Compounds

Ionic compounds are the result of cations and anions coming together to form a compound with an overall neutral charge. They're typically composed of metals and nonmetals.
The properties of ionic compounds are crucial when predicting the outcome of reactions, especially precipitation reactions. These compounds often dissolve in water to form electrolytic solutions that conduct electricity. The process in which these compounds dissociate into their ions is key to understanding solubility and precipitation reactions.
For instance, when \( \mathrm{Na}_{2}\mathrm{SO}_{4}(\mathrm{aq}) \) mixes with \( \mathrm{Mg}(\mathrm{NO}_{3})_{2}(\mathrm{aq}) \), the resulting ionic exchange suggests potential products, \( \mathrm{MgSO}_{4} \) and \( \mathrm{NaNO}_{3} \). Both remain dissolved due to their solubility, thus no precipitate forms.
Remember these traits about ionic compounds:

• High melting and boiling points due to strong ionic bonds.
• Solubility varies, and is determinant in reaction outcomes.
• Conduct electricity when dissolved in water, forming ionic solutions.

Chemical Reactions

Chemical reactions involve the transformation of substances through breaking and forming of chemical bonds, leading to the production of new materials. Understanding the fundamentals of chemical reactions helps predict and describe real-world changes, such as rusting or energy release in combustion.
In the reactions shared in the exercise, we're looking at precipitation reactions. A key aspect of chemical reactions that produce precipitates is the formation of insoluble compounds. This is why knowing the solubility rules is essential to predict if products will be soluble or precipitate as solids.
For example, in the reaction between \( \mathrm{K}_{3}\mathrm{PO}_{4}(\mathrm{aq}) \) and \( \mathrm{FeCl}_{3}(\mathrm{aq}) \), the insoluble \( \mathrm{FePO}_{4} \) forms as a solid precipitate, illustrating how chemical reactions yield new compounds that can exhibit entirely different properties from the reactants.
Highlighting aspects of chemical reactions:

• Involves reactants turning into products through bond rearrangements.
• Requires energy changes, either absorbing or releasing energy.
• May change physical properties like color, state, and temperature.