Problem 16 - Introductory Chemistry Atoms First

Question

Write the intact-compound equation and the net ionic equation for the neutralization of aqueous hydrofluoric acid by aqueous calcium hydroxide. What is the name of the salt that forms?

Step-by-Step Solution

Answer

The balanced intact-compound equation for the neutralization of aqueous hydrofluoric acid (HF) by aqueous calcium hydroxide (Ca(OH)\(_2\)) is: HF (aq) + Ca(OH)\(_2\)(aq) → CaF\(_2\)(s) + 2H2O (l) The net ionic equation is: H\(^+\)(aq) + F\(^-\)(aq) + Ca\(^{2+}\)(aq) + 2OH\(^-\)(aq) → CaF\(_2\)(s) + 2H2O (l) The salt formed is calcium fluoride (CaF\(_2\)).

Step 1: 1. Write the balanced intact-compound equation

First, we need to write the balanced chemical equation for the neutralization reaction between hydrofluoric acid (HF) and calcium hydroxide (Ca(OH)\(_2\)): HF (aq) + Ca(OH)\(_2\)(aq) → CaF\(_2\)(s) + 2H2O (l)

Step 2: 2. Separate the intact-compound equation into individual ions

Now, let's dissociate the strong electrolytes (aqueous strong acids and bases) into their individual ions: HF (aq) → H\(^+\)(aq) + F\(^-\)(aq) Ca(OH)\(_2\)(aq) → Ca\(^{2+}\)(aq) + 2OH\(^-\)(aq)

Step 3: 3. Write the net ionic equation by eliminating spectator ions

We can now write the net ionic equation. Spectator ions, which do not participate in the reaction, are eliminated. In this case, there are no spectator ions, so we rewrite the previous ions and combine them to form the net ionic equation: H\(^+\)(aq) + F\(^-\)(aq) + Ca\(^{2+}\)(aq) + 2OH\(^-\)(aq) → CaF\(_2\)(s) + 2H2O (l)

Step 4: 4. Identify the salt formed and provide its name

In the balanced intact-compound equation, the salt formed is CaF\(_2\), which is called calcium fluoride.

Key Concepts

Balanced Chemical Equation

A balanced chemical equation represents a chemical reaction where the number of atoms for each element is equal on both the reactant and product sides. This is crucial as it reflects the law of conservation of mass. In the context of a neutralization reaction, such as the reaction between hydrofluoric acid (HF) and calcium hydroxide (Ca(OH)_2), balancing ensures that the same number of hydrogen, fluorine, calcium, and oxygen atoms appear on both sides of the equation.
For our example, the balanced equation is:\[\text{HF (aq) + Ca(OH)}_2\text{ (aq) → CaF}_2\text{ (s) + 2H}_2\text{O (l)}\]

• HF: a weak acid that provides F\(^-\) ions.
• Ca(OH)\(_2\): a strong base that supplies Ca\(^{2+}\) and OH\(^-\) ions.
• Products: Calcium fluoride (CaF\(_2\)) is an insoluble salt, and water is formed.

Balancing is achieved when each type of atom is counted and matched, showcasing conservation of mass in any chemical reaction.

Net Ionic Equation

The net ionic equation focuses only on the species that actually participate in the reaction, excluding any that remain unchanged as spectator ions. This simplifies reactions and emphasizes the actual chemistry taking place.
For the neutralization reaction of HF and Ca(OH)\(_2\), the full ionic breakdown is:\[\text{H}^+\text{(aq)} + \text{F}^-\text{(aq)} + \text{Ca}^{2+}\text{(aq)} + 2\text{OH}^-\text{(aq)} → \text{CaF}_2\text{(s)} + 2\text{H}_2\text{O(l)}\]This shows the formation of the solid CaF\(_2\) as the primary reaction product, indicating the removal of spectator ions. However, if we can identify any such ions, we would remove them to form simpler net ionic equations. Here, fluoride ions and hydroxide ions directly participate in forming calcium fluoride and water, so no spectators are present.

Spectator Ions

Spectator ions are ions in an electrolyte solution that do not directly participate in the formation of a precipitate, gas, or other non-dissolved substance in a chemical reaction. They remain unchanged throughout the reaction process and can be removed from the net ionic equation.
In our specific neutralization example of HF with Ca(OH)\(_2\), typically in many reactions, some ions do not change and act merely as bystanders. However, in this case:

• All ions introduced from the reactants are consumed to form the products.
• No ions remain unreacted.

Thus, there are no spectator ions to speak of in this scenario as every ion undergoes a transformation, contributing to product formation.

Calcium Fluoride

Calcium fluoride (CaF\(_2\)) is an insoluble ionic compound and significant product in the described neutralization reaction. It forms from the combination of calcium cations (Ca\(^{2+}\)) and fluoride anions (F\(^-\)). This substance appears as a white crystalline solid in typical laboratory settings. Its formation can be simplified by understanding the interaction between the two ions as a classic double replacement reaction.
Key Characteristics:

• Insoluble in water at room temperature, differentiating it from many other salt compounds.
• Occurs naturally as the mineral fluorite, widely used in the commercial production of certain glasses and enamels.

Understanding calcium fluoride’s pivotal role as a product in neutralization not only deepens knowledge of specific chemical reactions but highlights its importance in various practical applications.