Problem 46 - General Chemistry

Question

For each of the following, write the molecular equation, including phase labels. Then write the net ionic equation. Note that the salts formed in these reactions are soluble. a. the neutralization of lithium hydroxide solution by aqueous perchloric acid b. the reaction of barium hydroxide solution and aqueous nitrous acid c. the reaction of sodium hydroxide solution and aqueous nitrous acid d. the neutralization of aqueous hydrogen cyanide by aqueous strontium hydroxide

Step-by-Step Solution

Answer

The net ionic equation for all reactions shows the formation of water: OH$^-$(aq) + H$^+$(aq) → H$_2$O(l).

Step 1: Determine Reactants

For each reaction, identify the reactants and write their formulas with phase labels (aq for aqueous solutions). - a. Lithium hydroxide solution (LiOH) and perchloric acid (HClO$_4$). - b. Barium hydroxide solution (Ba(OH)$_2$) and nitrous acid (HNO$_2$). - c. Sodium hydroxide solution (NaOH) and nitrous acid (HNO$_2$). - d. Hydrogen cyanide (HCN) and strontium hydroxide (Sr(OH)$_2$).

Step 2: Write Molecular Equations

Translate the reactants into products, considering they form water (H$_2$O) and a salt. Each equation should indicate the phase of each compound: - a. LiOH(aq) + HClO$_4$(aq) → LiClO$_4$(aq) + H$_2$O(l) - b. Ba(OH)$_2$(aq) + 2HNO$_2$(aq) → Ba(NO$_2$)$_2$(aq) + 2H$_2$O(l) - c. NaOH(aq) + HNO$_2$(aq) → NaNO$_2$(aq) + H$_2$O(l) - d. 2HCN(aq) + Sr(OH)$_2$(aq) → Sr(CN)$_2$(aq) + 2H$_2$O(l)

Step 3: Write Complete Ionic Equations

Separate each aqueous compound into its ions for the complete ionic equation: - a. Li$^+$(aq) + OH$^-$(aq) + H$^+$(aq) + ClO$_4^-$ → Li$^+$(aq) + ClO$_4^-$ + H$_2$O(l) - b. Ba$^{2+}$(aq) + 2OH$^-$(aq) + 2H$^+$(aq) + 2NO$_2^-$(aq) → Ba$^{2+}$(aq) + 2NO$_2^-$(aq) + 2H$_2$O(l) - c. Na$^+$(aq) + OH$^-$(aq) + H$^+$(aq) + NO$_2^-$(aq) → Na$^+$(aq) + NO$_2^-$(aq) + H$_2$O(l) - d. 2H$^+$(aq) + 2CN$^-$(aq) + Sr$^{2+}$(aq) + 2OH$^-$(aq) → Sr$^{2+}$(aq) + 2CN$^-$(aq) + 2H$_2$O(l)

Step 4: Cancel Spectator Ions for Net Ionic Equations

Identify and cancel the ions that appear on both sides of the ionic equation. These are spectator ions: - a. Net ionic equation: OH$^-$(aq) + H$^+$(aq) → H$_2$O(l) - b. Net ionic equation: 2OH$^-$(aq) + 2H$^+$(aq) → 2H$_2$O(l) - c. Net ionic equation: OH$^-$(aq) + H$^+$(aq) → H$_2$O(l) - d. Net ionic equation: 2OH$^-$(aq) + 2H$^+$(aq) → 2H$_2$O(l)

Key Concepts

Neutralization Reactions

Neutralization reactions are a fundamental concept in chemistry where an acid and a base react to form water and a salt. The basic idea is quite simple: the hydrogen ions (H\(^+\)) from the acid combine with the hydroxide ions (OH\(^-\)) from the base to create water (H₂O). This reaction typically results in a solution with a pH closer to neutral. For instance, when lithium hydroxide (LiOH) reacts with perchloric acid (HClO₄), they form water and lithium perchlorate (LiClO₄).

• Acid + Base → Salt + Water
• Common acids: HCl, HNO₂, H₂SO₄
• Common bases: NaOH, KOH, Ba(OH)₂

Neutralization is a key process in various industrial applications and environmental processes, like wastewater treatment. Understanding this can help in calculating the correct amounts of acid/base needed to neutralize specific solutions.

Aqueous Solutions

An aqueous solution is simply a solution where the solvent is water. Represented with the phase label \((aq)\), it indicates that a substance is dissolved in water, affecting its chemical behavior. In our exercises, several substances such as lithium hydroxide, perchloric acid, and nitrous acid are in aqueous form. This state allows electrolytes to dissociate into ions, which is crucial for conducting electricity and facilitating reactions.

• Water as a universal solvent
• Enables ion dissociation
• Critical for biological and chemical reactions

Understanding aqueous solutions is essential because many reactions, including neutralization, occur in this medium. Thus, it forms the backbone of many chemical processes, both naturally and in the lab.

Spectator Ions

In chemical reactions, particularly in aqueous solutions, some ions do not participate in the reaction itself and are known as spectator ions. They are involved as part of the reactive species but remain unchanged during the reaction. For example, in the neutralization reaction of lithium hydroxide and perchloric acid, the lithium ions (Li\(^+\)) and perchlorate ions (ClO₄\(^-\)) remain unchanged as spectators.
These ions can be identified when writing complete ionic equations:

• Present in reactants and products
• Do not affect the net ionic equation
• Examples: Na\(^+\), K\(^+\), NO₃\(^-\)

Eliminating spectator ions from equations simplifies them, revealing the net ionic equation which indicates the actual chemical change occurring. This simplification aids in understanding the core reaction without unnecessary details.

Phase Labels

Phase labels are important in chemistry as they indicate the physical state of a substance in a reaction. These labels are crucial for understanding the conditions under which a reaction occurs and predicting the behavior of reactants and products. Common phase labels include:

• \((s)\) for solid
• \((l)\) for liquid
• \((g)\) for gas
• \((aq)\) for aqueous solution

In our example equations, all reactants are in aqueous form \((aq)\), which facilitates the dissociation into ions, except for water produced \((l)\). Including phase labels ensures that students and chemists alike have a complete picture of what physical states they are working with, which is vital for safety and experimental accuracy.