Problem 45 - 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 hydrobromic acid with calcium hydroxide solution b. the reaction of solid aluminum hydroxide with nitric acid c. the reaction of aqueous hydrogen cyanide with calcium hydroxide solution d. the neutralization of lithium hydroxide solution by aqueous hydrogen cyanide

Step-by-Step Solution

Answer

Write balanced molecular and net ionic equations for each reaction.

Step 1: Understanding molecular equation

To begin, we will write down the balanced molecular equations for each reaction, including the proper phase labels (s for solid, l for liquid, g for gas, and aq for aqueous). This step involves understanding the reactants and products.

Step 2: Write molecular equation for part a

For the neutralization of hydrobromic acid (HBr, aq) with calcium hydroxide (Ca(OH)_2, aq), the molecular equation is: \[2 \text{HBr} (aq) + \text{Ca(OH)}_2 (aq) \rightarrow 2 \text{H}_2\text{O}(l) + \text{CaBr}_2 (aq)\]

Step 3: Write net ionic equation for part a

Cancel the spectator ions to derive the net ionic equation for part a: \[2 \text{H}^+ (aq) + 2 \text{OH}^- (aq) \rightarrow 2 \text{H}_2\text{O} (l)\] This simplifies to: \[\text{H}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{H}_2\text{O} (l)\]

Step 4: Write molecular equation for part b

For the reaction of solid aluminum hydroxide (Al(OH)_3, s) with nitric acid (HNO_3, aq), the molecular equation is: \[\text{Al(OH)}_3 (s) + 3 \text{HNO}_3 (aq) \rightarrow \text{Al(NO}_3)_3 (aq) + 3 \text{H}_2\text{O} (l)\]

Step 5: Write net ionic equation for part b

Identify and cancel the spectator ions to write the net ionic equation: \[\text{Al(OH)}_3 (s) + 3 \text{H}^+ (aq) \rightarrow \text{Al}^3^+ (aq) + 3 \text{H}_2\text{O} (l)\]

Step 6: Write molecular equation for part c

For the reaction of aqueous hydrogen cyanide (HCN, aq) with calcium hydroxide (Ca(OH)_2, aq), the molecular equation is: \[2 \text{HCN} (aq) + \text{Ca(OH)}_2 (aq) \rightarrow 2 \text{H}_2\text{O} (l) + \text{Ca(CN)}_2 (aq)\]

Step 7: Write net ionic equation for part c

Cancel the spectator ions to form the net ionic equation: \[2 \text{HCN} (aq) + 2 \text{OH}^- (aq) \rightarrow 2 \text{CN}^- (aq) + 2 \text{H}_2\text{O} (l)\] This simplifies to: \[\text{HCN} (aq) + \text{OH}^- (aq) \rightarrow \text{CN}^- (aq) + \text{H}_2\text{O} (l)\]

Step 8: Write molecular equation for part d

For the neutralization of lithium hydroxide (LiOH, aq) by hydrogen cyanide (HCN, aq), the molecular equation is: \[\text{LiOH} (aq) + \text{HCN} (aq) \rightarrow \text{LiCN} (aq) + \text{H}_2\text{O} (l)\]

Step 9: Write net ionic equation for part d

After identifying and eliminating the spectator ions, the net ionic equation is: \[\text{OH}^- (aq) + \text{HCN} (aq) \rightarrow \text{CN}^- (aq) + \text{H}_2\text{O} (l)\]

Key Concepts

Molecular Equation

A molecular equation is a way of representing chemical reactions where all reactants and products are noted in their molecular form. This includes showing their phase labels, such as solid (\(s\)), liquid (\(l\)), gas (\(g\)), and aqueous (\(aq\)).
A molecular equation is particularly useful for gaining an overview of the reaction and identifying the substances involved. It helps in visualizing the changes that take place during the reaction.
For example, the reaction between hydrobromic acid (\( ext{HBr}\)) and calcium hydroxide (\( ext{Ca(OH)}_2\)) can be written as:

• \[2 \, \text{HBr} \, (aq) + \text{Ca(OH)}_2 \, (aq) \rightarrow 2 \text{H}_2 \text{O} \, (l) + \text{CaBr}_2 \, (aq)\]

By writing the molecular equation, you can see what is initially present and what is formed as a result of the reaction.

Neutralization Reactions

Neutralization reactions occur when an acid and a base react to form water and a salt. This type of reaction generally results in the neutralization of the acid and base properties.
A classic example of a neutralization reaction is when hydrochloric acid reacts with sodium hydroxide, forming water and sodium chloride:

• \[\text{HCl} \, (aq) + \text{NaOH} \, (aq) \rightarrow \text{NaCl} \, (aq) + \text{H}_2 \text{O} \, (l)\]

During neutralization, the hydrogen ions \(\text{H}^+\) from the acid react with the hydroxide ions \(\text{OH}^-\) from the base, producing water (\(\text{H}_2\text{O}\)), which is a characteristic outcome of these reactions.
This type of reaction is essential in many chemical processes, such as titration in laboratories, and is fundamental in understanding pH balance.

Aqueous Solutions

Aqueous solutions are formed when a substance, typically a solid, is dissolved in water. In chemical reactions, they are denoted with the phase label (\(aq\)).
Many reactions occur because the reactants are in aqueous solutions, allowing for easy interaction and mixing of ions. Aqueous solutions are pivotal in neutralization reactions, as they typically involve acids and bases dissolved in water.
Additionally, when writing chemical equations, it's crucial to portray which substances are in aqueous form since it impacts solubility and reaction dynamics. An example of this is the reaction of solid aluminum hydroxide with nitric acid:

• \[\text{Al(OH)}_3 \, (s) + 3 \text{HNO}_3 \, (aq) \rightarrow \text{Al(NO}_3\text{)}_3 \, (aq) + 3 \text{H}_2 \text{O} \, (l)\]

Here, the nitric acid is in aqueous form, making it an effective reactant to dissolve and interact with the aluminum hydroxide.

Spectator Ions

Spectator ions are ions in a reaction that do not partake in the chemical change. They remain unchanged before and after the reaction.
When writing net ionic equations, these ions are often removed from the equation to highlight the active participants in the reaction. This makes it easier to understand which ions are forming new substances.
For reactions occurring in aqueous solutions, such as the neutralization of lithium hydroxide by hydrogen cyanide:

• \[\text{LiOH} \, (aq) + \text{HCN} \, (aq) \rightarrow \text{LiCN} \, (aq) + \text{H}_2\text{O} \, (l)\]

In this case, the lithium ion \(\text{Li}^+\) serves as a spectator ion, which doesn’t contribute directly to the formation of water.
The net ionic equation focuses instead on these key reactants and products:

• \[\text{OH}^- \, (aq) + \text{HCN} \, (aq) \rightarrow \text{CN}^- \, (aq) + \text{H}_2\text{O} \, (l)\]

Understanding spectator ions is crucial for simplifying and understanding chemical reactions.