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) \(\mathrm{NaCl}+\mathrm{MgSO}_{4} \longrightarrow\)

Short answer

(a) NR; (b) Balanced: Li2CO3 + 2HNO3 → 2LiNO3 + H2O + CO2; Net Ionic: CO3^2- + 2H+ → H2O + CO2; (c) Balanced: LiCl + AgNO3 → LiNO3 + AgCl↓; Net Ionic: Ag+ + Cl- → AgCl↓; (d) NR.

Step-by-step solution

Reaction (a) LiOH + HCN

First, identify the products that might form when lithium hydroxide (LiOH) reacts with hydrogen cyanide (HCN). This could lead to a double displacement reaction, forming water (H2O) and lithium cyanide (LiCN). However, both are weak electrolytes and no significant reaction occurs. Therefore, we write: \[ \text{LiOH + HCN} \rightarrow NR \] NR stands for 'No Reaction'.

Reaction (b) Li2CO3 + HNO3

Consider the reaction between lithium carbonate (Li2CO3) and nitric acid (HNO3). This is a typical acid-carbonate reaction leading to the formation of a salt, water, and carbon dioxide. The balanced molecular equation is: \[ \text{Li}_{2}\text{CO}_{3} + 2\text{HNO}_{3} \rightarrow 2\text{LiNO}_{3} + \text{H}_{2}\text{O} + \text{CO}_{2}\uparrow \] For the net ionic equation, LiNO3 is soluble and dissociates completely while H2O remains as liquid and CO2 is a gas: \[ \text{CO}_{3}^{2-} + 2\text{H}^{+} \rightarrow \text{H}_{2}\text{O} + \text{CO}_{2}\uparrow \]

Reaction (c) LiCl + AgNO3

This is a classic precipitation reaction, where silver chloride (AgCl), an insoluble solid, forms. The balanced molecular equation is: \[ \text{LiCl} + \text{AgNO}_{3} \rightarrow \text{LiNO}_{3} + \text{AgCl} \downarrow \] For the net ionic equation, LiNO3 is soluble whereas AgCl precipitates: \[ \text{Ag}^{+} + \text{Cl}^{-} \rightarrow \text{AgCl} \downarrow \]

Reaction (d) NaCl + MgSO4

Here, sodium chloride (NaCl) and magnesium sulfate (MgSO4) are both strong electrolytes and soluble in water. Neither significant reaction nor precipitation occurs as all ions remain in solution: \[ \text{NaCl} + \text{MgSO}_{4} \rightarrow NR \] No net ionic equation is necessary.

Key concepts

Double Displacement Reactions

Double displacement reactions, also known as metathesis reactions, are a type of chemical reaction wherein components of two compounds swap places. The typical form of these reactions is \[ AB + CD \rightarrow AD + CB \]Here A and B are parts of one compound, while C and D are parts of another. In these reactions:

• Anions and cations exchange partners, forming new compounds.
• They often occur in aqueous solutions because ions rely on water to move and collide.
• The driving force behind these reactions is often the formation of a solid, gas, or water.

In our example, reactions between lithium compounds and acids can involve double displacement, but only some of them proceed to form products. Understanding these reactions provides insight into how compounds interact and the conditions necessary for a reaction to occur.

Net Ionic Equations

Net ionic equations simplify the representation of chemical reactions by only including the species that undergo a change. This way we focus on the essence of the reaction, stripping away spectator ions that remain unchanged. Writing a net ionic equation involves:

• Identifying all the ions present in the solution before the reaction occurs.
• Recognizing the ions or molecules that undergo chemical change.
• Omitting spectator ions, which are ions not involved in the actual chemical change.

For example, when reacting LiCl with AgNO₃, the net ionic equation only features the formation of insoluble AgCl from aqueous ions, as this is the essential change happening. Using net ionic equations helps to fundamentally grasp what is chemically significant in a reaction.

Precipitation Reactions

Precipitation reactions are a subtype of double displacement reactions that result in the formation of an insoluble solid from two aqueous solutions. This solid is called a precipitate and it can be visually observed as a cloudiness or solid particles within the solution. Key aspects of precipitation reactions include:

• Identifying the potential for a reaction by considering the solubility of the products.
• Recognizing that the formation of a precipitate drives the reaction towards completion.
• Constructing a balanced equation, including the insoluble product as a solid (usually indicated by \( \downarrow \)).

An example from the exercise is the reaction of LiCl and AgNO₃ where AgCl is the precipitate forming. Understanding precipitation reactions is crucial when predicting products and determining reaction feasibility.

Solubility Rules

Solubility rules are guidelines that help predict the solubility of ionic compounds in water. These rules enable chemists to determine whether a compound will dissolve or form a precipitate in a given reaction. Important points to consider:

• Compounds containing alkali metal ions (e.g., Li⁺, Na⁺) and ammonium (NH₄⁺) are generally soluble.
• Nitrates (NO₃⁻), acetates (C₂H₃O₂⁻), and most perchlorates (ClO₄⁻) are typically soluble.
• Carbonates (CO₃²⁻) and phosphates (PO₄³⁻) are often insoluble, except when paired with alkali metals or NH₄⁺.
• Chlorides (Cl⁻), bromides (Br⁻), and iodides (I⁻) are soluble, except with Ag⁺, Pb²⁺, and Hg₂²⁺.

Using these rules, students can predict the outcomes of chemical reactions, like the formation of an insoluble AgCl, and understand why certain reactions do not proceed. This understanding aids in learning which compounds are likely to react under specific conditions.