Question

Aqueous chromium(III) sulfate reacts with aqueous sodium hydroxide to form a precipitate of chromium(III) hydroxide and a solution of sodium sulfate. Write a net ionic equation for this reaction.

Short answer

The net ionic equation for the reaction is \[2Cr^{3+} (aq) + 6OH^- (aq) \rightarrow 2Cr(OH)_3 (s)\]

Step-by-step solution

Write the balanced chemical equation

First step is to write the balanced chemical equation for the reaction. Chromium(III) sulfate has the formula \(Cr_2(SO_4)_3\), and sodium hydroxide has the formula \(NaOH\). On reaction, chromium(III) hydroxide (\(Cr(OH)_3\)) and sodium sulfate (\(Na_2SO_4\)) are formed. So, the balanced chemical equation is: \[Cr_2(SO_4)_3 (aq) + 6NaOH (aq) \rightarrow 2Cr(OH)_3 (s) + 3Na_2SO_4 (aq)\]

Write the total ionic equation

Next, write the total ionic equation. This involves breaking up all the (aq) species into their respective ions: \[2Cr^{3+} (aq) + 3SO_4^{2-} (aq) + 6Na^+ (aq) + 6OH^- (aq) \rightarrow 2Cr(OH)_3 (s) + 6Na^+ (aq) + 3SO_4^{2-} (aq)\]

Write the net ionic equation

Finally, write the net ionic equation. Cancel out the common ions (spectator ions) on both sides of the equation, leaving only the species that actually participate in the reaction: \[2Cr^{3+} (aq) + 6OH^- (aq) \rightarrow 2Cr(OH)_3 (s)\] This is the net ionic equation for the reaction.

Key concepts

Chemical Reactions in Chemistry

Chemical reactions are the core of chemistry, involving the transformation of substances through the breaking and forming of chemical bonds. In a chemical reaction, reactants, which are the starting substances, are converted into products, which are the substances formed as a result of the reaction. During this process, the atoms get rearranged to create new molecules, and this change is depicted through a chemical equation.

Understanding the different types of chemical reactions is crucial for students as it helps them predict the outcome of reactions. For instance, in the exercise provided, an aqueous solution of chromium(III) sulfate reacts with sodium hydroxide to create new substances, illustrating a type of reaction called a double displacement reaction. These reactions are fundamental in predicting the behavior of substances in different environments, be it in a laboratory or within biological systems.

Balancing Chemical Equations

Balancing chemical equations is a vital skill in chemistry, ensuring that the law of conservation of mass is upheld in chemical reactions. This law states that mass is neither lost nor gained in a reaction. To balance an equation, one must ensure that the number of atoms for each element is equal on both the reactant and product sides of the equation.

For instance, in the provided solution, the balanced equation \[Cr_2(SO_4)_3 (aq) + 6NaOH (aq) \rightarrow 2Cr(OH)_3 (s) + 3Na_2SO_4 (aq)\] reflects equal numbers of each type of atom on both sides. This process requires a clear understanding of stoichiometry and the ability to manipulate coefficients—the numbers in front of formulas—to balance the equation correctly. Mastering this allows students to further delve into more complex exercises such as calculating reaction yields and understanding reaction mechanisms.

Precipitation Reactions

Precipitation reactions are a subtype of chemical reactions where two soluble salts react in solution to form one or more insoluble products, known as a precipitate. In the context of the textbook exercise, the chromium(III) hydroxide formed is the precipitate, easily identified because its physical state is denoted with an (s) for solid, distinguishing it from the aqueous reactants and products.

Writing net ionic equations is an extension of understanding precipitation reactions, as they focus on the entities that change during the reaction. The equation derived in Step 3 from the exercise, \[2Cr^{3+} (aq) + 6OH^- (aq) \rightarrow 2Cr(OH)_3 (s)\], effectively illustrates the formation of the precipitate with only the ions directly involved. These equations omit spectator ions, promoting a streamlined perspective. Learning about precipitation reactions equips students with the knowledge to predict when and how compounds will form insoluble combinations—a valuable skill for applications ranging from industrial chemical processes to environmental science.