Question

Identify the precipitate (if any) that forms when the following solutions are mixed, and write a balanced equation for each reaction. (a) \(\mathrm{NH}_{4} \mathrm{I}\) and \(\mathrm{CuCl}_{2},\) (b) LiOH and \(\mathrm{MnCl}_{2}\), (c) \(\mathrm{K}_{3} \mathrm{PO}_{4}\) and \(\mathrm{CoSO}_{4}\)

Short answer

Precipitates: (a) CuI, (b) Mn(OH)2, (c) Co3(PO4)2.

Step-by-step solution

Analyzing Reaction (a)

To determine if a precipitate forms when mixing \(\text{NH}_4\text{I}\) and \(\text{CuCl}_2\), examine the possible products: \(\text{NH}_4\text{Cl}\) and \(\text{CuI}\). \(\text{CuI}\) is insoluble in water, indicating that \(\text{CuI}\) forms as a precipitate. The balanced equation is: \[\text{2 NH}_4\text{I} (aq) + \text{CuCl}_2(aq) \rightarrow \text{2 NH}_4\text{Cl} (aq) + \text{CuI}(s)\].

Analyzing Reaction (b)

Mixing \(\text{LiOH}\) and \(\text{MnCl}_2\) forms \(\text{LiCl}\) and \(\text{Mn(OH)}_2\). \(\text{Mn(OH)}_2\) is insoluble in water, indicating the formation of a precipitate. The balanced equation is: \[\text{2 LiOH} (aq) + \text{MnCl}_2 (aq) \rightarrow \text{2 LiCl} (aq) + \text{Mn(OH)}_2(s)\].

Analyzing Reaction (c)

For the mixture of \(\text{K}_3\text{PO}_4\) and \(\text{CoSO}_4\), calculate the potential reaction products: \(\text{K}_2\text{SO}_4\) and \(\text{Co}_3(\text{PO}_4)_2\). \(\text{Co}_3(\text{PO}_4)_2\) is insoluble in water and forms a precipitate. The balanced equation is: \[2 \text{K}_3\text{PO}_4 (aq) + 3 \text{CoSO}_4 (aq) \rightarrow 3 \text{K}_2\text{SO}_4 (aq) + \text{Co}_3(\text{PO}_4)_2(s)\].

Key concepts

Precipitate Formation

In chemical reactions, a precipitate is a solid that emerges from a liquid solution. This occurs when certain ions in a solution react to form an insoluble compound. The formation of a precipitate often indicates that a chemical reaction has taken place. When two aqueous solutions are mixed, the cations from one solution may combine with the anions from the other to create such an insoluble solid. For example, when mixing ammonium iodide (\(\text{NH}_4\text{I}\)) with copper(II) chloride (\(\text{CuCl}_2\)), copper(I) iodide (\(\text{CuI}\)) forms as a precipitate because it doesn't dissolve in water. This characteristic is crucial for identifying precipitation reactions.

Solubility Rules

Solubility rules are guidelines that help predict whether an ionic compound dissolves in water. These rules are based on the general properties of ions and their interactions in a solution. Understanding solubility rules can tell you which products will precipitate out of a reaction. Some general rules include:

• Most nitrates (NO₃^-) and acetates (CH₃COO^-) are soluble.
• Alkali metal ions like Li⁺, Na⁺, and K⁺ tend to form soluble compounds.
• Most halides (Cl^-, Br^-, I^-) are soluble, except when paired with Ag⁺, Pb²⁺, and Hg₂²⁺.
• Sulfates (SO₄^2-) are generally soluble except for those paired with Ba²⁺, Sr²⁺, and Pb²⁺.
• Hydroxides (OH^-) are generally insoluble, except for those of alkali metals and some alkaline earth metals like Ba(OH)₂.

In our examples, using these rules helps verify which compound will precipitate. For instance, manganese(II) hydroxide (\(\text{Mn(OH)}_2\)) found in reaction b is insoluble, and cobalt(II) phosphate (\(\text{Co}_3(\text{PO}_4)_2\)) from reaction c does not dissolve in water.

Balancing Chemical Equations

Balancing chemical equations is an essential skill in chemistry that ensures the conservation of mass. It involves ensuring that the number of atoms for each element is the same on both sides of the equation. This is crucial because chemical reactions involve the rearrangement of atoms, not their creation or destruction.
To balance equations, follow these simple steps:

• Write down the unbalanced equation and list each element present.
• Count the number of atoms for each element in both the reactants and products.
• Adjust coefficients before compound formulas to balance atoms one element at a time.
• Repeat the process until all elements are balanced, checking your work carefully.

In the exercise examples, each equation showcases balanced reactions, like: \[\text{2 NH}_4\text{I} (aq) + \text{CuCl}_2(aq) \rightarrow \text{2 NH}_4\text{Cl} (aq) + \text{CuI}(s)\]This ensures that two ammonium salts and one copper halide on both sides maintain balance, adhering strictly to the law of conservation of mass.