Question

Write net ionic equations for the reaction, if any, that occurs when aqueous solutions of the following are mixed. a. chromium(III) chloride and sodium hydroxide b. silver nitrate and ammonium carbonate c. copper(II) sulfate and mercury(I) nitrate d. strontium nitrate and potassium iodide

Short answer

The net ionic equations for the given reactions are as follows: a. \(Cr^{3+} (aq) + 3OH^- (aq) \rightarrow Cr(OH)_3 (s)\) b. \(2Ag^+ (aq) + CO_3^{2-} (aq) \rightarrow Ag_2CO_3 (s)\) c. \(Hg_2^{2+} (aq) + SO_4^{2-} (aq) \rightarrow Hg_2SO_4 (s)\) d. \(Sr^{2+} (aq) + 2I^- (aq) \rightarrow SrI_2 (s)\)

Step-by-step solution

a. chromium(III) chloride and sodium hydroxide

1. Write the balanced molecular equation for the reaction: CrCl3 (aq) + 3NaOH (aq) -> Cr(OH)3 (s) + 3NaCl (aq) 2. Rewrite the equation using the ionic form of each aqueous species: Cr^3+ (aq) + 3Cl^- (aq) + 3Na^+ (aq) + 3OH^- (aq) -> Cr(OH)3 (s) + 3Na^+ (aq) + 3Cl^- (aq) 3. Remove spectator ions and write the net ionic equation: Cr^3+ (aq) + 3OH^- (aq) -> Cr(OH)3 (s)

b. silver nitrate and ammonium carbonate

1. Write the balanced molecular equation for the reaction: 2AgNO3 (aq) + (NH4)2CO3 (aq) -> 2Ag2CO3 (s) + 2NH4NO3 (aq) 2. Rewrite the equation using the ionic form of each aqueous species: 2Ag^+ (aq) + 2NO3^- (aq) + 2NH4^+ (aq) + CO3^2- (aq) -> Ag2CO3 (s) + 2NH4^+ (aq) + 2NO3^- (aq) 3. Remove spectator ions and write the net ionic equation: 2Ag^+ (aq) + CO3^2- (aq) -> Ag2CO3 (s)

c. copper(II) sulfate and mercury(I) nitrate

1. Write the balanced molecular equation for the reaction: CuSO4 (aq) + Hg2(NO3)2 (aq) -> Cu(NO3)2 (aq) + Hg2SO4 (s) 2. Rewrite the equation using the ionic form of each aqueous species: Cu^2+ (aq) + SO4^2- (aq) + Hg2^2+ (aq) + 2NO3^- (aq) -> Cu^2+ (aq) + 2NO3^- (aq) + Hg2SO4 (s) 3. Remove spectator ions and write the net ionic equation: Cu^2+ (aq) + SO4^2- (aq) + Hg2^2+ (aq) -> Cu^2+ (aq) + 2NO3^- (aq) + Hg2SO4 (s)

d. strontium nitrate and potassium iodide

1. Write the balanced molecular equation for the reaction: Sr(NO3)2 (aq) + 2KI (aq) -> SrI2 (s) + 2KNO3 (aq) 2. Rewrite the equation using the ionic form of each aqueous species: Sr^2+ (aq) + 2NO3^- (aq) + 2K^+ (aq) + 2I^- (aq) -> SrI2 (s) + 2K^+ (aq) + 2NO3^- (aq) 3. Remove spectator ions and write the net ionic equation: Sr^2+ (aq) + 2I^- (aq) -> SrI2 (s)

Key concepts

Aqueous Solution Reactions

When substances dissolve in water, they create aqueous solutions, meaning the solutes are dispersed throughout the solvent at the molecular or ionic level. In a chemical reaction, such as the ones mentioned in your exercises, the dissolved substances can react to form new compounds. These reactions often involve the exchange of ions between the compounds when mixed in water. Understanding how these reactions occur is essential in chemistry, particularly when predicting the outcomes of mixing various compounds and determining the resultant products.

For example, when chromium(III) chloride mixes with sodium hydroxide in an aqueous solution, they react to form chromium(III) hydroxide and sodium chloride. Here, different ions in the solution interact to form a new substance, which could be a precipitate if it's insoluble in water, as with chromium(III) hydroxide.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid called a precipitate. This solid emerges from the solution and can often be observed as a cloudiness or deposition at the bottom of the reaction vessel. Through the examples given in the exercise, such as the formation of silver carbonate from the reaction of silver nitrate and ammonium carbonate, we see the practical application of this concept. Identifying the solubility rules helps us predict when a precipitate will form. For instance, most carbonates are insoluble except those of alkali metals and ammonium, making silver carbonate an insoluble precipitate in this scenario.

It is essential to recognize that not all reactions in aqueous solutions form precipitates. Only the combinations of ions that result in the formation of an insoluble product lead to a precipitation reaction.

Spectator Ions

Spectator ions are ions in an aqueous solution that do not participate in the actual chemical reaction — they do not change their oxidation state or form any part of the precipitate. These ions can be thought of as 'bystanders', watching the reaction occur without getting involved. When writing net ionic equations, it is standard practice to remove spectator ions as they do not affect the reaction's outcome. For instance, in the reaction between strontium nitrate and potassium iodide, potassium and nitrate ions are present before and after the reaction occurs and do not participate in forming the strontium iodide precipitate. Recognition of spectator ions streamlines the chemical equations to focus only on the reacting species, making it easier to understand the core changes occurring in a reaction.

Chemical Equation Balancing

The art of chemical equation balancing is critical to accurately represent what occurs during a chemical reaction. To follow the law of conservation of mass, the number of atoms for each element must be the same on both the reactants and products sides of the equation. The balancing process ensures that the equation adheres to this principle. When writing a balanced chemical equation, start by listing all the reactants and products, then adjust coefficients — the numbers in front of chemical formulas — to obtain the same number of atoms for each element on both sides of the equation.

Balancing chemical equations can sometimes be tricky, but there are systematic approaches to guide you through the process. Balancing is not just a formal requirement; it has practical implications, allowing chemists to predict the amounts of substances consumed and produced in a given reaction, which is foundational for quantitative chemical analysis.