Problem 11 - Chemistry The Molecular Nature of Matter

Question

How can you tell that the following is a net ionic equation? $$ \mathrm{Al}^{3+}(a q)+3 \mathrm{OH}^{-}(a q) \longrightarrow \mathrm{Al}(\mathrm{OH})_{3}(s) $$

Step-by-Step Solution

Answer

The equation is a net ionic equation because it shows only the ions that participate in forming the precipitate (the aluminum hydroxide solid), and does not include any spectator ions.

Step 1: Identify the state symbols

Examine the equation and identify the state symbols of each compound or ion, which indicate their phase (solid, liquid, gas, or aqueous). In this equation, the aluminum ion \text{Al}^{3+} and the hydroxide ion \text{OH}^{-} have the symbol (aq) which means they are in an aqueous solution. The aluminum hydroxide, \(\text{Al}(\text{OH})_{3}\), has the symbol (s) indicating it is a solid.

Step 2: Check the reactants and products

The equation should represent the actual species involved in the reaction while omitting the spectator ions. In this equation, \text{Al}^{3+}(aq) and \(\text{OH}^{-}(aq)\) are the reactants that undergo a chemical change to form the product, \(\text{Al}(\text{OH})_{3}(s)\). There are no spectator ions present.

Step 3: Determine if the equation shows the essence of the chemical reaction

A net ionic equation illustrates only the chemical species that participate in the change during the reaction. Since the given equation shows the formation of a precipitate from ions in solution and does not include any spectator ions, it represents the net chemical change.

Key Concepts

Chemical Reactions

Chemical reactions are the processes by which substances, known as reactants, are transformed into different substances called products. During a reaction, the bonds between atoms in the reactants are broken and new bonds are formed to create the products.

To understand these transformations, chemists use chemical equations that symbolically represent the reactants and products, as well as their physical states—indicated by symbols like (s) for solids, (l) for liquids, (g) for gases, and (aq) for substances dissolved in water (aqueous solutions).

Reactants and Products

For example, in the net ionic equation \(\mathrm{Al}^{3+}(aq) + 3\mathrm{OH}^{-}(aq) \longrightarrow \mathrm{Al}(\mathrm{OH})_{3}(s)\), \(\mathrm{Al}^{3+}(aq)\) and \(\mathrm{OH}^{-}(aq)\) are the reactants, and \(\mathrm{Al}(\mathrm{OH})_{3}(s)\) is the product—an insoluble solid called a precipitate.

Indicating Phase Changes

Representing phase changes is crucial in a chemical reaction. The (aq) symbol tells us that the reactants are in aqueous solution, readily interacting with each other, while the (s) indicates that the product is a solid precipitate that falls out of the solution as a new substance.

Precipitate Formation

Precipitate formation is a critical aspect in understanding chemical reactions, especially in aqueous solutions. A precipitate is an insoluble solid that emerges from the reaction of two soluble substances.

During the reaction, ions from the dissolved substances rearrange and form new compounds. If the product of this rearrangement is insoluble in water, it will appear as a solid, which is observed as the precipitate. The net ionic equation highlights this transformation by focusing solely on the ions that partake in the formation of the precipitate.

Identifying a Precipitate

The equation from the exercise \(\mathrm{Al}^{3+}(aq) + 3\mathrm{OH}^{-}(aq) \longrightarrow \mathrm{Al}(\mathrm{OH})_{3}(s)\) demonstrates this: \(\mathrm{Al}(\mathrm{OH})_{3}\) cannot remain dissolved and instead solidifies and separates from the solution.

The identification of the solid phase \((s)\) following the reaction's arrow indicates that the reaction leads to precipitate formation, a common outcome in many chemical processes involving ionic compounds in solution.

Spectator Ions

Spectator ions are ions present in a solution that do not participate in the chemical reaction. These ions merely 'observe' the reaction—they are unchanged and do not affect the outcome of the reaction.

In writing net ionic equations, it is important to remove spectator ions to simplify the equation and focus on the actual chemical change. By eliminating these ions, net ionic equations present a clearer and more concise description of the reaction.

Net Ionic Equations Without Spectator Ions

As shown in the provided exercise, the equation does not have any spectator ions since all the ions listed are directly involved in forming the precipitate. No ions are left unchanged on both sides of the equation, which signifies the essence of a net ionic equation.

Correct identification of spectator ions and their omission from net ionic equations allow chemists and students alike to better understand the essential changes occurring in a chemical reaction.