Problem 69 - Introductory Chemistry Atoms First

Question

Aqueous solutions of calcium chloride and potassium carbonate are combined. (a) Write the formulas for both reactants. (b) Does a precipitation reaction occur? Explain. (c) If it does, write a net ionic equation for the reaction.

Step-by-Step Solution

Answer

(a) CaCl2 and K2CO3 (b) Yes, a precipitation reaction occurs because calcium carbonate (CaCO3) is insoluble. (c) Net ionic equation: \(Ca^{2+} (aq) + CO_3^{2-} (aq) \rightarrow CaCO_3 (s)\)

Step 1: Write the formulas for the reactants

First, we need to write the chemical formulas for calcium chloride and potassium carbonate. Calcium is a group 2 element and has a charge of +2, and chloride is a group 17 element with a charge of -1. Potassium is a group 1 element with a charge of +1, and carbonate is a polyatomic ion with a charge of -2. Therefore, the chemical formulas for calcium chloride and potassium carbonate are CaCl2 and K2CO3, respectively.

Step 2: Determine if a precipitation reaction occurs

To determine if a precipitation reaction occurs, we need to consider the solubility rules for ionic compounds. In general, we can say that all alkali metal (group 1) salts and ammonium salts are soluble, and all nitrates, acetates, and chlorates are also soluble. Most chloride, sulfate, and nitrate salts are soluble except when combined with certain metal ions. In this reaction, when the two solutions are mixed, calcium ions (Ca2+) from the calcium chloride solution and carbonate ions (CO3 2-) from the potassium carbonate solution may combine to form calcium carbonate (CaCO3) and potassium ions (K+) and chloride ions (Cl-) may combine to form potassium chloride (KCl). According to the solubility rules, most chloride salts are soluble, so KCl is soluble. However, most carbonate salts are insoluble, with some exceptions. Calcium carbonate is one of those insoluble salts. Since CaCO3 is insoluble, a precipitation reaction occurs.

Step 3: Write the balanced molecular equation

Before writing the net ionic equation, we need to write the balanced molecular equation for the reaction: CaCl2 (aq) + K2CO3 (aq) → CaCO3 (s) + 2 KCl (aq)

Step 4: Write the net ionic equation

Now that we have the balanced molecular equation, we can write the net ionic equation. First, split all soluble compounds into their individual ions: Ca2+ (aq) + 2 Cl- (aq) + 2 K+ (aq) + CO3 2- (aq) → CaCO3 (s) + 2 K+ (aq) + 2 Cl- (aq) Now, remove the spectator ions (ions that appear on both sides of the equation) to get the net ionic equation: Ca2+ (aq) + CO3 2- (aq) → CaCO3 (s) So the net ionic equation for the reaction is: Ca2+ (aq) + CO3 2- (aq) → CaCO3 (s)

Key Concepts

Understanding Solubility Rules

Solubility rules help us predict whether a substance will dissolve in water. These rules are crucial for determining the likelihood of a precipitation reaction in chemical mixtures.

Some key solubility guidelines include:

• All nitrates (NO₃⁻) are soluble.
• Most chloride salts (Cl⁻) are soluble, except when paired with silver (Ag⁺), lead (Pb²⁺), or mercury (Hg₂²⁺).
• All alkali metal (group 1) salts and ammonium (NH₄⁺) compounds are soluble.
• Carbonate (CO₃²⁻) salts are typically insoluble, except for those of alkali metals and ammonium.

When calcium chloride (CaCl₂) and potassium carbonate (K₂CO₃) are mixed, calcium carbonate (CaCO₃)—an insoluble salt—forms, indicating a precipitation reaction. Understanding these rules is key to predicting such reactions.

Crafting a Net Ionic Equation

The net ionic equation reveals the actual chemical change in a reaction by showing only the particles that participate in the reaction.

Here's how to create it:

• Start with a balanced molecular equation.
• Split all soluble ionic compounds into their individual ions.
• Remove spectator ions—ions that remain unchanged in the reaction.

For the reaction between calcium chloride and potassium carbonate, the net ionic equation is:
\[\text{Ca}^{2+} (aq) + \text{CO}_{3}^{2-} (aq) \rightarrow \text{CaCO}_{3} (s)\]
This equation highlights the formation of calcium carbonate, showing the essence of the precipitation reaction.

Exploring Calcium Chloride

Calcium chloride, with the chemical formula CaCl₂, is a soluble ionic compound.

Key characteristics include:

• Formed from calcium (Ca²⁺), a group 2 alkaline earth metal, and chloride (Cl⁻), a group 17 halogen.
• Highly soluble in water, making it useful in de-icing and as a drying agent due to its hygroscopic nature (it absorbs moisture).

When dissolved in water, it dissociates into calcium ions and chloride ions. In the context of reactions, calcium ions can combine with other anions like carbonate to form insoluble compounds like calcium carbonate, resulting in precipitation.

Understanding Potassium Carbonate

Potassium carbonate (K₂CO₃) is a soluble salt made from potassium (K⁺) and carbonate (CO₃²⁻) ions.

Key properties include:

• Highly soluble in water because potassium (an alkali metal) forms soluble salts.
• Widely used in industries such as glassmaking and soap production due to its alkaline nature.

In a chemical reaction, the carbonate ions can sometimes form insoluble compounds, such as calcium carbonate, when mixed with specific cations like calcium ions. This behavior makes potassium carbonate a common compound in precipitation reactions.