Question

Write balanced molecular, ionic, and net ionic equations for the following pairs of reactants. If all ions cancel, indicate that no reaction (N.R.) takes place. (a) ammonium sulfide and sodium hydroxide (b) chromium(III) sulfate and potassium carbonate (c) silver nitrate and chromium(III) acetate (d) strontium hydroxide and magnesium chloride

Short answer

For (a) no reaction occurs (N.R.), for (b) the net ionic equation is \( Cr^{3+}(aq) + 3 CO_3^{2-}(aq) \rightarrow Cr_2(CO_3)_3(s) \), for (c) the net ionic equation is \( 3 Ag^+(aq) + Cr(C_2H_3O_2)_3(aq) \rightarrow Ag_3Cr(C_2H_3O_2)_3(s) \), and for (d) the net ionic equation is \( Sr^{2+}(aq) + 2 OH^{-}(aq) + Mg^{2+}(aq) + 2 Cl^{-}(aq) \rightarrow SrCl_2(s) + Mg(OH)_2(s) \).

Step-by-step solution

- Write the unbalanced molecular equations

Combine the reactants to produce the expected products. For (a), ammonium hydroxide and sodium sulfide are formed. For (b), potassium sulfate and chromium(III) carbonate are produced. For (c), silver acetate and chromium(III) nitrate are the products. For (d), strontium chloride and magnesium hydroxide are expected.

- Balance the molecular equations

Balance the chemical equations from Step 1 according to the law of conservation of mass. Ensure each element has the same number of atoms on both sides of the equation.

- Write the total ionic equations

Break all soluble ionic compounds into their respective ions to form the total ionic equations. Insoluble compounds, liquids, and gases are written in their molecular form.

- Identify spectator ions

Spectator ions are the ions that do not change during the reaction and appear on both the reactant side and the product side of the ionic equation.

- Write the net ionic equations

Cancel out the spectator ions from the total ionic equations to find the net ionic equations for each reaction. If all ions cancel each other out, indicate that no reaction has occurred (N.R.).

Key concepts

Molecular Equations

Molecular equations are representations of chemical reactions where the compounds are expressed as molecules, not ions. In these equations, you'll find the reactants listed on the left side of the arrow and the products on the right, just like in a standard equation. For example, when ammonium sulfide reacts with sodium hydroxide, the unbalanced molecular equation would simply show the reactants and expected products:

\text(\(\text{(NH4)_2S} + \text{NaOH} \rightarrow \text{NH4OH} + \text{Na_2S}\)).

The key in balancing molecular equations is to ensure that the number of each type of atom is the same on both sides of the equation. This respects the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.

Ionic Equations

Ionic equations take the molecular equation a step further by representing soluble ionic compounds as separate ions. The strong electrolytes are divided into the ions they form in solution, providing a more detailed depiction of what's actually occurring in the reaction. For the same reaction between ammonium sulfide and sodium hydroxide, an ionic equation would look like this:

\text(\((NH_4^+)_2S^{2-} + 2Na^+ + 2OH^- \rightarrow 2NH_4^+ + OH^- + 2Na^+ + S^{2-}\)).

This format can help to visualize the movement and exchange of ions during the chemical process more clearly, giving insights into the driving forces behind reactions.

Net Ionic Equations

Net ionic equations simplify the picture even further by only showing the ions that directly participate in the reaction. They exclude the spectator ions—those that don’t undergo a chemical change. Writing the net ionic equation involves canceling out these spectator ions from both sides of the ionic equation.

For instance, using the previous example, the spectator ions (\text(\(Na^+\))) would cancel leaving us with the net ionic equation:

\text(\((NH_4^+)_2S^{2-} + 2OH^- \rightarrow 2NH_4^+ + OH^- + S^{2-}\)).

This representation hones in on the crux of the chemical change, helping students understand the core interaction without the distraction of inactive components.

Chemical Reaction Balancing

Balancing chemical reactions is a fundamental skill in chemistry that ensures the principle of matter conservation is respected. It involves manipulating the coefficients of the substances in the reaction to equalize the number of atoms for each element on both sides. Instructions for balancing typically start with balancing atoms of elements that appear in only one reactant and one product, then moving on to more complex molecules.

For example, in balancing the reaction between chromium(III) sulfate and potassium carbonate, one would begin by balancing chromium, then sulfur, and finally, oxygen and potassium. The balanced equation not only confirms the reaction obeys the laws of chemistry but also provides a stoichiometric ratio which is essential in calculating reactants or products in quantitative analysis.

Spectator Ions

Spectator ions are ions in an ionic equation that don't participate in the reaction—they appear unchanged on both the reactant and product sides of the equation. Identifying those ions is crucial when transitioning from the total ionic to the net ionic equation, as they provide no useful information about the reaction's chemistry. For example, in a reaction between silver nitrate and chromium(III) acetate:

\text(\(3Ag^+ + 3NO_3^- + Cr^{3+} + 3C_2H_3O_2^- \rightarrow 3AgC_2H_3O_2 + Cr^{3+} + 3NO_3^-\)),

the \text(\(NO_3^-\)) and \text(\(Cr^{3+}\)) ions are spectator ions. Recognizing spectator ions helps clarify the core changes in a reaction and is an essential step in writing net ionic equations.