Problem 99 - General Chemistry

Question

Decide whether a reaction occurs for each of the following. If it does not, write \(N R\) after the arrow. If it does, write the balanced molecular equation; then write the net ionic equation. a. \(\mathrm{LiOH}+\mathrm{HCN} \longrightarrow\) b. \(\mathrm{Li}_{2} \mathrm{CO}_{3}+\mathrm{HNO}_{3} \longrightarrow\) C. \(\mathrm{LiCl}+\mathrm{AgNO}_{3} \longrightarrow\) d. \(\mathrm{LiCl}+\mathrm{MgSO}_{4} \longrightarrow\)

Step-by-Step Solution

Answer

a. Yes, \(\mathrm{LiOH + HCN} \rightarrow \mathrm{LiCN + H_2O}\), net ionic: \(\mathrm{OH^- + HCN} \rightarrow \mathrm{CN^- + H_2O}\). b. Yes, \(\mathrm{Li_2CO_3 + HNO_3} \rightarrow \mathrm{LiNO_3 + CO_2 + H_2O}\), net ionic: \(\mathrm{CO_3^{2-} + 2H^+} \rightarrow \mathrm{CO_2 + H_2O}\). c. Yes, \(\mathrm{LiCl + AgNO_3} \rightarrow \mathrm{LiNO_3 + AgCl}\), net ionic: \(\mathrm{Cl^- + Ag^+} \rightarrow \mathrm{AgCl}\). d. No, reaction.

Step 1: Determine Reaction Occurrence for Reaction a

For the reaction between \(\mathrm{LiOH}\) and \(\mathrm{HCN}\), compare with known reaction types. \(\mathrm{LiOH}\) is a strong base, while \(\mathrm{HCN}\) is a weak acid. A weak acid and strong base reaction generally occurs to form water and the conjugate base. Since \(\mathrm{HCN}\) is weak, this forms a weak electrolyte, ensuring a reaction will occur. Write the balanced molecular equation:\[ \mathrm{LiOH (aq) + HCN (aq)} \rightarrow \mathrm{LiCN (aq) + H_2O (l)} \]For the net ionic equation, since \(\mathrm{LiOH}\) and \(\mathrm{LiCN}\) contain the spectator ion \(\mathrm{Li}^+\), eliminate it:\[ \mathrm{OH}^- (aq) + \mathrm{HCN} (aq) \rightarrow \mathrm{CN}^- (aq) + \mathrm{H_2O (l)} \]

Step 2: Determine Reaction Occurrence for Reaction b

For the reaction between \(\mathrm{Li}_2\mathrm{CO}_3\) and \(\mathrm{HNO}_3\), an acid reacting with a carbonate generally results in gas formation (carbon dioxide), indicating a reaction occurs. Write the balanced molecular equation:\[ \mathrm{Li}_2\mathrm{CO}_3 (s) + 2\mathrm{HNO}_3(aq) \rightarrow 2\mathrm{LiNO}_3 (aq) + \mathrm{CO}_2 (g) + \mathrm{H}_2\mathrm{O} (l) \]For the net ionic equation, remove the spectator ions \(\mathrm{Li}^+\) and \(\mathrm{NO}_3^-\) to emphasize the formation of gases and liquids:\[ \mathrm{CO}_3^{2-} (aq) + 2\mathrm{H}^+ (aq) \rightarrow \mathrm{CO}_2 (g) + \mathrm{H}_2\mathrm{O} (l) \]

Step 3: Determine Reaction Occurrence for Reaction c

For \(\mathrm{LiCl}\) and \(\mathrm{AgNO}_3\), consider if a precipitate forms. Silver chloride (\(\mathrm{AgCl}\)) is known to be insoluble in water, ensuring a reaction occurs. Write the balanced molecular equation:\[ \mathrm{LiCl (aq) + AgNO}_3 \mathrm{(aq)} \rightarrow \mathrm{LiNO}_3 (aq) + \mathrm{AgCl (s)} \]For the net ionic equation, eliminate \(\mathrm{Li}^+\) and \(\mathrm{NO}_3^-\), leaving:\[ \mathrm{Cl}^- (aq) + \mathrm{Ag}^+ (aq) \rightarrow \mathrm{AgCl (s)} \]

Step 4: Determine Reaction Occurrence for Reaction d

For \(\mathrm{LiCl}\) and \(\mathrm{MgSO}_4\), check for any formation of an insoluble product. Both \(\mathrm{Li}_2\mathrm{SO}_4\) and \(\mathrm{MgCl}_2\) are soluble in water. As no precipitate is formed, there is no reaction. Write 'NR' (No Reaction) after the arrow:\[ \mathrm{LiCl (aq) + MgSO}_4 \mathrm{(aq)} \rightarrow \mathrm{NR} \]

Key Concepts

Net Ionic Equation

A net ionic equation is the essence of a chemical reaction that removes spectator ions, showing only the species that are actively involved in the chemical change. This form of equation helps students understand which elements or compounds actually contribute to the reaction. To write a net ionic equation, follow these steps:

• Write the balanced molecular equation.
• Separate all aqueous compounds into ions, but keep solids, liquids, and gases intact.
• Identify and eliminate spectator ions - ions that appear on both the reactant and product sides unchanged.
• Re-write the equation with only the species that undergo a chemical change.

For example, in the reaction between lithium hydroxide and hydrocyanic acid, officially it forms lithium cyanide and water. However, when focusing only on the ions that change, the net ionic equation becomes: \[ \mathrm{OH}^- (aq) + \mathrm{HCN} (aq) \rightarrow \mathrm{CN}^- (aq) + \mathrm{H_2O} (l) \] ensuring only the hydroxide and cyanide ions' transformation is noted.

Precipitation Reaction

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid, known as a precipitate. This type of reaction is a key type of chemical change, commonly seen in laboratory and real-world applications.When performing these reactions, follow these considerations:

• Mix the solutions of two ionic compounds in water.
• Identify potential precipitates formed using solubility rules.
• For the reaction of lithium chloride and silver nitrate, for example, the precipitate formed is silver chloride (\(\mathrm{AgCl}\)), known for its insolubility in water.

Write the balanced molecular equation:\[ \mathrm{LiCl} (aq) + \mathrm{AgNO}_3 (aq) \rightarrow \mathrm{LiNO}_3 (aq) + \mathrm{AgCl} (s) \]The net ionic equation focuses on the formation of the \(\mathrm{AgCl}\) precipitate:\[ \mathrm{Cl}^- (aq) + \mathrm{Ag}^+ (aq) \rightarrow \mathrm{AgCl} (s) \]Understanding which ions make up the precipitate helps illustrate the core occurrence of the reaction.

Acid-Base Reaction

Acid-base reactions are fundamental chemical reactions involving the transfer of protons (\(\mathrm{H}^+\)) between an acid and a base. These reactions typically result in the formation of water along with an ionic compound.Key components to remember:

• An acid releases \(\mathrm{H}^+\) ions into the solution.
• A base provides \(\mathrm{OH}^-\) ions.
• Reactions between strong bases and weak acids help illustrate incomplete dissociation, such as the case with lithium hydroxide reacting with hydrocyanic acid.

The balanced molecular equation shows water and the salt for this reaction:\[ \mathrm{LiOH} (aq) + \mathrm{HCN} (aq) \rightarrow \mathrm{LiCN} (aq) + \mathrm{H_2O} (l) \]The net ionic equation highlights significant ion interactions, often simplifying to:\[ \mathrm{OH}^- (aq) + \mathrm{HCN} (aq) \rightarrow \mathrm{CN}^- (aq) + \mathrm{H_2O} (l) \]This equation emphasizes the formation of water as the hydroxide ions neutralize the acidic hydrogen ions.

Solubility Rules

Solubility rules help determine whether a compound will dissolve in water, which is crucial for predicting the outcomes of chemical reactions. Here are some basic rules:

• Most nitrate (\(\mathrm{NO}_3^-\)) salts are soluble.
• Salts containing alkali metal ions (such as lithium, \(\mathrm{Li}^+\)) are typically soluble.
• Chlorides, such as \(\mathrm{Cl}^-\), are generally soluble except when paired with silver, lead, or mercury ions.
• Carbonates (\(\mathrm{CO}_3^{2-}\)) are often insoluble except when with alkali metals.

Using these rules, one can predict reactions such as the formation of a precipitate in the reaction between \(\mathrm{LiCl}\) and \(\mathrm{AgNO}_3\). Knowing these rules aids in understanding whether a reaction will occur and what the precipitate might be, highlighting their importance in the study of chemical reactions.