Question

When aqueous solutions of sodium carbonate and calcium bromide mix, a white solid forms. (a) Identify the solid. (b) Write a balanced molecular equation for the reaction. (c) Write a balanced ionic equation for the reaction. (d) Identify any spectator ions in the reaction. (c) Write a balanced net ionic equation for the reaction.

Short answer

The precipitate formed is calcium carbonate. The balanced molecular, ionic, and net ionic equations are \[Na2CO3(aq) + CaBr2(aq) \rightarrow 2NaBr(aq) + CaCO3(s)\], \[2Na^+(aq) + CO3^{2-}(aq) + Ca^{2+}(aq) + 2Br^-(aq) \rightarrow 2Na^+(aq) + 2Br^-(aq) + CaCO3(s)\], and \[CO3^{2-}(aq) + Ca^{2+}(aq) \rightarrow CaCO3(s)\] respectively. The spectator ions are Na^+ and Br^-.

Step-by-step solution

Identify the Solid

The reactants are sodium carbonate (Na2CO3) and calcium bromide (CaBr2). When combined, the cation of one compound combines with the anion of another compound. So, we can predict the products to be sodium bromide (NaBr) and calcium carbonate (CaCO3). Among these, calcium carbonate is the solid that forms as a precipitate.

Write a balanced molecular equation

The balanced molecular equation is given by: \[Na2CO3(aq) + CaBr2(aq) \rightarrow 2NaBr(aq) + CaCO3(s)\]. This represents the stoichiometry of the reaction.

Write balanced ionic equation

Ionic equations are written by splitting covalently bonded atoms or ions. The ionic equation thus becomes: \[2Na^+(aq) + CO3^{2-}(aq) + Ca^{2+}(aq) + 2Br^-(aq) \rightarrow 2Na^+(aq) + 2Br^-(aq) + CaCO3(s)\].

Identify the spectator ions

Spectator ions are those ions that do not participate in the reaction. They are present on both sides of the ionic equation. Here, the spectator ions are sodium ions (Na^+) and bromide ions (Br^-).

Write balanced net ionic equation

A net ionic equation includes only the particles that participate in the reaction. So, the spectator ions found in Step 4 are removed from the ionic equation: \[CO3^{2-}(aq) + Ca^{2+}(aq) \rightarrow CaCO3(s)\].

Key concepts

Chemical Equations

Chemical equations are vital representations in chemistry that show the reactants and products of chemical reactions. A balanced chemical equation ensures that the same number of atoms for each element is present on both sides, respecting the Law of Conservation of Mass. For example, in the reaction between sodium carbonate and calcium bromide, the balanced molecular equation is:
\[ Na_2CO_3(aq) + CaBr_2(aq) \rightarrow 2NaBr(aq) + CaCO_3(s) \]
Here, 'aq' denotes an aqueous solution, meaning the substance is dissolved in water, and 's' signifies a solid, which in this case is the precipitate. Balancing this reaction makes it clear how much of each substance is involved in the reaction and allows for further analysis, like identifying the precipitate and writing ionic and net ionic equations.

Ionic Equations

Ionic equations enhance our understanding of reactions by showcasing the actual species in solution that are involved in the reaction process. These species include ions – atoms, or groups of atoms, that have a net charge due to the loss or gain of electrons. In an aqueous environment, many compounds dissociate into their respective ions. The balanced ionic equation for the reaction between sodium carbonate and calcium bromide demonstrates this:
\[ 2Na^+(aq) + CO_3^{2-}(aq) + Ca^{2+}(aq) + 2Br^-(aq) \rightarrow 2Na^+(aq) + 2Br^-(aq) + CaCO_3(s) \]
Writing the equation in this form clarifies which ions are reactants and which are products. Such detail is essential for identifying the ions that directly participate in the formation of the solid precipitate.

Spectator Ions

Spectator ions are like bystanders in a chemical reaction: present but not actively participating in the creation of the product. They appear unchanged on both sides of an ionic equation. In our example, when sodium carbonate reacts with calcium bromide, the sodium ions (Na+) and bromide ions (Br−) are spectator ions. They remain in the aqueous environment before and after the reaction:
\[ Spectator ions: Na^+(aq), Br^-(aq) \]
Identifying these ions is a step toward simplifying the reaction down to the essentials, which is exactly the role of the net ionic equation.

Net Ionic Equations

The net ionic equation is the distilled essence of the reaction, showing only the species that participate in the formation of the product. Spectator ions are omitted, as they do not change during the course of the reaction. The net ionic equation for the formation of calcium carbonate from sodium carbonate and calcium bromide is succinctly expressed as:
\[ CO_3^{2-}(aq) + Ca^{2+}(aq) \rightarrow CaCO_3(s) \]
This equation reveals that carbonate ions (CO_3^2−) from sodium carbonate and calcium ions (Ca^2+) from calcium bromide combine to form the precipitate, calcium carbonate (CaCO_3), without the distraction of other ions. Understanding net ionic equations can greatly aid students in analyzing the core components of chemical reactions.