Ch.4 - Reactions in Aqueous Solution

Problem 6

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Chapter 4, Problem 6

Which of the following ions will always be a spectator ion in a precipitation reaction? (a) Cl- (b) NO3- (c) NH4+ (d) S2- (e) SO42-

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Identify the concept of a spectator ion: Spectator ions are ions that do not participate in the actual chemical reaction and remain unchanged in the solution.

Understand the context of a precipitation reaction: In a precipitation reaction, two soluble salts react to form an insoluble salt (precipitate) and other ions remain in solution.

Recall the solubility rules: Nitrate ions (NO3-) are always soluble in water, meaning they do not form precipitates with any common cations.

Analyze each option: Consider the solubility of compounds formed by each ion with common cations. Nitrate (NO3-) is known for forming soluble compounds with all cations.

Conclude which ion is always a spectator: Based on the solubility rules, NO3- is always a spectator ion in precipitation reactions because it does not form insoluble compounds.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Spectator Ions

Spectator ions are ions that do not participate in the actual chemical reaction during a precipitation reaction. They remain in the solution and do not change their oxidation state or form any precipitate. Identifying spectator ions is crucial for understanding the net ionic equation, which only includes the ions that undergo a change.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to form an insoluble compound, known as a precipitate. This process is driven by the formation of a solid that separates from the solution, and it is essential to recognize the solubility rules to predict which combinations of ions will lead to precipitation.

Solubility Rules

Solubility rules are guidelines that help predict whether a compound will dissolve in water or form a precipitate. For example, most nitrates (NO3-) and ammonium (NH4+) salts are soluble, while sulfides (S2-) and carbonates (CO3^2-) are often insoluble. Understanding these rules is key to determining which ions will remain in solution and which will precipitate.

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Solubility Rules

Related Practice

Use the molecular representations shown here to classify each compound as a nonelectrolyte, a weak electrolyte, or a strong electrolyte (see Figure 4.6 for the element color scheme). (a)

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Textbook Question

The concept of chemical equilibrium is very important. Which one of the following statements is the most correct way to think about equilibrium? (a) If a system is at equilibrium, nothing is happening. (b) If a system is at equilibrium, the rate of the forward reaction is equal to the rate of the back reaction. (c) If a system is at equilibrium, the product concentration is changing over time.

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Textbook Question

You are presented with a white solid and told that due to careless labeling it is not clear if the substance is barium chloride, lead chloride, or zinc chloride. When you transfer the solid to a beaker and add water, the solid dissolves to give a clear solution. Next an Na2SO41aq2 solution is added and a white precipitate forms. What is the identity of the unknown white solid?

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Open Question

The labels have fallen off three bottles containing powdered samples of metals; one contains zinc, one contains lead, and the other contains platinum. You have three solutions at your disposal: 1 M sodium nitrate, 1 M nitric acid, and 1 M nickel nitrate. How could you use these solutions to determine the identities of each metal powder?

Open Question

In each of the following pairs, indicate which has the higher concentration of Cl^- ion: (a) 0.10 M AlCl₃ solution or a 0.25 M LiCl solution (b) 150 mL of a 0.05 M MnCl₃ solution or 200 mL of 0.10 M KCl solution (c) a 2.8 M HCl solution or a solution made by dissolving 23.5 g of KCl in water to make 100 mL of solution.

Open Question

You want to analyze a silver nitrate solution. What mass of NaCl is needed to precipitate Ag+ ions from 45.0 mL of 0.2500 M AgNO3 solution?