Problem 51 - Introductory Chemistry Atoms First

Question

You need calcium sulfate, but there is none in the lab. However, there are lots of other ionic compounds on the shelves. (a) Propose a synthesis for calcium sulfate. (b) Write a complete ionic equation for the synthesis. (c) Write a net ionic equation for the synthesis. (d) Write the equation you wrote for (b) as an English sentence.

Step-by-Step Solution

Answer

In the proposed synthesis, calcium chloride (CaCl₂) reacts with sodium sulfate (Na₂SO₄) to form calcium sulfate (CaSO₄) and sodium chloride (NaCl). The complete ionic equation is: Ca²⁺ (aq) + 2 Cl⁻ (aq) + 2 Na⁺ (aq) + SO₄²⁻ (aq) → CaSO₄ (s) + 2 Na⁺ (aq) + 2 Cl⁻ (aq). The net ionic equation is: Ca²⁺ (aq) + SO₄²⁻ (aq) → CaSO₄ (s). In an English sentence, calcium ions from a calcium chloride solution and sulfate ions from a sodium sulfate solution react to precipitate solid calcium sulfate.

Step 1: Propose a synthesis for calcium sulfate

To synthesize calcium sulfate, we can combine calcium-containing compounds with sulfate-containing compounds. Let's use calcium chloride (CaCl₂) and sodium sulfate (Na₂SO₄), which are common ionic compounds available in most laboratories. CaCl₂ (aq) + Na₂SO₄ (aq) → CaSO₄ (s) + 2 NaCl (aq) This reaction will produce calcium sulfate (CaSO₄), a desired product, and sodium chloride (NaCl) as a byproduct.

Step 2: Write a complete ionic equation for the synthesis

Now, we will write the complete ionic equation for the reaction. The soluble salts dissociate into ions in the aqueous solution. Ca²⁺ (aq) + 2 Cl⁻ (aq) + 2 Na⁺ (aq) + SO₄²⁻ (aq) → CaSO₄ (s) + 2 Na⁺ (aq) + 2 Cl⁻ (aq)

Step 3: Write a net ionic equation for the synthesis

To write the net ionic equation, we will cancel out the spectator ions (ions that remain unchanged during the reaction) from the complete ionic equation. In this case, spectator ions are Na⁺ and Cl⁻. Ca²⁺ (aq) + SO₄²⁻ (aq) → CaSO₄ (s) This is the net ionic equation for the synthesis of calcium sulfate.

Step 4: Write the equation as an English sentence

In this reaction, calcium ions (Ca²⁺) from a calcium chloride solution and sulfate ions (SO₄²⁻) from a sodium sulfate solution react to precipitate solid calcium sulfate (CaSO₄).

Key Concepts

Ionic Compounds

Ionic compounds are essential to chemistry because they form the basis of many reactions. They consist of positive and negative ions held together by electrostatic forces, known as ionic bonds. These ions are typically metal cations and non-metal anions. For example:

• Calcium chloride (CaCl₂) involves calcium ions, Ca²⁺, and chloride ions, Cl⁻.
• Sodium sulfate (Na₂SO₄) consists of sodium ions, Na⁺, and sulfate ions, SO₄²⁻.

Ionic compounds are notable for their high melting points and their ability to conduct electricity when dissolved in water. This conductivity is due to the ions being free to move and carry charge. When dissolved in water, they break apart into their constituent ions, a process known as dissociation. Understanding ionic compounds is crucial for predicting how they will react with one another.

Net Ionic Equation

The net ionic equation is a streamlined version of a chemical equation that shows only the ions and molecules directly involved in the chemical reaction. In this way, it removes any spectator ions, which are ions that remain unchanged throughout the reaction. By focusing only on the reacting participants, net ionic equations present a clearer depiction of the chemical reaction.

• For example, in the reaction to produce calcium sulfate, the net ionic equation is: \(\text{Ca}^{2+} (\text{aq}) + \text{SO}_4^{2-} (\text{aq}) \rightarrow \text{CaSO}_4 (\text{s})\).

This equation indicates that calcium ions and sulfate ions are the only ones participating in forming the solid precipitate, calcium sulfate. Understanding net ionic equations helps students identify which elements and compounds are actively participating in, and are crucial to, a chemical reaction.

Complete Ionic Equation

A complete ionic equation provides a detailed view of all the soluble ionic compounds in a reaction as dissociated ions. This allows chemists to see clearly how the constituent ions interact before forming new compounds.

• In our calcium sulfate synthesis reaction, the complete ionic equation is: \( \text{Ca}^{2+} (\text{aq}) + 2\text{Cl}^{-} (\text{aq}) + 2\text{Na}^{+} (\text{aq}) + \text{SO}_4^{2-} (\text{aq}) \rightarrow \text{CaSO}_4 (\text{s}) + 2\text{Na}^{+} (\text{aq}) + 2\text{Cl}^{-} (\text{aq}) \).

This equation accounts for all ions present in the solutions and reflects the actual state of the substances involved before any factors like precipitation occur. It's a useful step in writing chemical equations, as it fully captures all components initially present in the reaction medium.

Precipitation Reaction

Precipitation reactions are a type of chemical reaction where two solutions containing soluble salts are combined, resulting in the formation of an insoluble solid known as the precipitate. This usually occurs due to the exchange of cations and anions between the reacting ionic compounds.

• In our example, the mixing of calcium chloride and sodium sulfate results in the formation of calcium sulfate. Calcium sulfate is the precipitate that forms as a solid out of the solution because it’s not soluble in water.

Precipitation reactions are essential in removing ions from solutions and are utilized in various scientific and industrial processes. Recognizing and predicting these reactions is crucial for fields such as environmental science, pharmacology, and chemical manufacturing. They offer a way to extract and isolate certain compounds from a mixture by taking advantage of their solubility properties.