Problem 20 - General Chemistry: Principles and Modern Applications

Question

Complete each of the following as a net ionic equation. If no reaction occurs, so state. (a) \(\mathrm{Ca}^{2+}+2 \mathrm{I}^{-}+2 \mathrm{Na}^{+}+\mathrm{CO}_{3}^{2-} \longrightarrow\) (b) \(\mathrm{Ba}^{2+}+\mathrm{S}^{2-}+2 \mathrm{Na}^{+}+\mathrm{SO}_{4}^{2-} \longrightarrow\) (c) \(2 \mathrm{K}^{+}+\mathrm{S}^{2-}+\mathrm{Ca}^{2+}+2 \mathrm{Cl}^{-} \longrightarrow\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Ca}^{2+}+ \mathrm{CO}_{3}^{2-}\rightarrow \mathrm{CaCO}_3\) (precipitate), (b) \(\mathrm{Ba}^{2+} + \mathrm{SO}_{4}^{2-} \longrightarrow \mathrm{BaSO_4}\) (precipitate), (c) No reaction.

Step 1: Identify the compounds

The compounds are formed by pairing a cation (+) with an anion (-). For (a) we have \(\mathrm{CaI}_2\) and \(\mathrm{Na}_2 \mathrm{CO}_3\), for (b) we have \(\mathrm{Na}_2 \mathrm{S}\) and \(\mathrm{BaSO}_4\) and for (c) we have \(\mathrm{K}_2 \mathrm{S}\) and \(\mathrm{CaCl}_2\).

Step 2: Full ionic equations

(a) \(\mathrm{Ca}^{2+}+2 \mathrm{I}^{-}+2 \mathrm{Na}^{+}+\mathrm{CO}_{3}^{2-} \longrightarrow \mathrm{Ca^{2+}}+2\mathrm{I^{-}}+2\mathrm{Na^{+}} +\mathrm{CO_3^{2-}}\),\n(b) \(\mathrm{Ba}^{2+}+\mathrm{S}^{2-}+2 \mathrm{Na}^{+}+\mathrm{SO}_{4}^{2-} \longrightarrow \mathrm{Ba^{2+}}+\mathrm{SO_4^{2-}}+2\mathrm{Na^{+}}+\mathrm{S^{2-}}\),\n(c) \(2 \mathrm{K}^{+}+\mathrm{S}^{2-}+\mathrm{Ca}^{2+}+2 \mathrm{Cl}^{-}\longrightarrow 2\mathrm{K^{+}}+\mathrm{S^{2-}}+\mathrm{Ca^{2+}}+2\mathrm{Cl^{-}}\).

Step 3: Identify and exclude the spectator ions

The ions that do not change from reactants to products are the spectator ions. Exclude them to get the net ionic equations.\n(a) \(\mathrm{Ca}^{2+} + \mathrm{CO}_{3}^{2-} \longrightarrow \mathrm{CaCO_3}\) (precipitate),\n(b) \(\mathrm{Ba}^{2+} + \mathrm{SO}_{4}^{2-} \longrightarrow \mathrm{BaSO_4}\) (precipitate),\n(c) No reaction.

Key Concepts

Spectator Ions

In chemical reactions, especially those occurring in aqueous solutions, not all ions participate in forming a new compound. Some ions remain unchanged before and after the reaction. These are called spectator ions.

• For instance, if you have a reaction involving sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)) ions in water, they might not contribute to forming a precipitate or a new substance.
• Instead, they stay in the solution just as they were in the beginning.

To find the net ionic equation, identify spectator ions and exclude them from the equation. This helps to highlight the actual chemical change. For example, in reaction (a) from the exercise, \(\text{Ca}^{2+}\) combines with \(\text{CO}_3^{2-}\) to form \(\text{CaCO}_3\) while other ions like \(\text{Na}^+\) and \(\text{I}^-\) are spectator ions that remain in the solution.

Chemical Reactions

Chemical reactions are processes where substances, the reactants, are transformed into new substances, the products. These transformations involve the making and breaking of chemical bonds.
In simple reactions involving ionic compounds, ions can recombine to form new products, often changing the state of matter.

• A common type of chemical reaction occurs when ionic compounds in solution form new compounds through precipitation, gas evolution, or neutralization.
• Consider reaction (b) in the exercise where \(\text{Ba}^{2+}\) ions react with \(\text{SO}_4^{2-}\) ions to form \(\text{BaSO}_4\), a solid precipitate.

Every chemical reaction seeks to be balanced, meaning the same number of each type of atom appears on both sides of the equation. Understanding chemical reactions involves recognizing these processes and predicting the substances formed during reactions.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two soluble salts in aqueous solution react to form one or more insoluble products, known as precipitates.
This type of reaction occurs when cations and anions in the solution combine and form a compound that does not dissolve in water.

• An example from the exercise is reaction (a), where \(\text{Ca}^{2+}\) and \(\text{CO}_3^{2-}\) form \(\text{CaCO}_3\), a chalky solid that falls out of solution.
• These reactions are important in various practical applications, including water treatment and in the formation of scale and sediments.

To predict whether a precipitation reaction will occur, solubility rules are often used. These rules help determine whether a compound will dissolve or form a precipitate when mixed in aqueous solutions.