Problem 37 - Chemistry and Chemical Reactivity

Question

Balance the following equations, and then write the net ionic equation. Show states for all reactants and products (s, \(\ell, \mathrm{g},\) aq). (a) the reaction of silver nitrate and potassium iodide to give silver iodide and potassium nitrate (b) the reaction of barium hydroxide and nitric acid to give barium nitrate and water (c) the reaction of sodium phosphate and nickel(II) nitrate to give nickel(II) phosphate and sodium nitrate

Step-by-Step Solution

Answer

(a) Ag⁺ + I⁻ → AgI; (b) OH⁻ + H⁺ → H₂O; (c) 3Ni²⁺ + 2PO₄³⁻ → Ni₃(PO₄)₂.

Step 1: Write the Balanced Molecular Equation (a)

For the reaction between silver nitrate (AgNO₃) and potassium iodide (KI) to form silver iodide (AgI) and potassium nitrate (KNO₃), the equation is:\[ \text{AgNO}_3 (\text{aq}) + \text{KI} (\text{aq}) \rightarrow \text{AgI} (\text{s}) + \text{KNO}_3 (\text{aq}) \] In this equation, all elements are already balanced.

Step 2: Write the Net Ionic Equation (a)

In the reaction of silver ions and iodide ions, the net ionic equation includes only the ions and precipitate that change:\[ \text{Ag}^+ (\text{aq}) + \text{I}^- (\text{aq}) \rightarrow \text{AgI} (\text{s}) \] Here, the spectator ions \(\text{K}^+\) and \(\text{NO}_3^-\) are omitted as they do not participate in the formation of the precipitate.

Step 3: Write the Balanced Molecular Equation (b)

For the reaction between barium hydroxide (Ba(OH)₂) and nitric acid (HNO₃) to produce barium nitrate (Ba(NO₃)₂) and water, the balanced equation is:\[ \text{Ba(OH)}_2 (\text{aq}) + 2\text{HNO}_3 (\text{aq}) \rightarrow \text{Ba(NO}_3)_2 (\text{aq}) + 2\text{H}_2\text{O} (\ell) \] Both elements and charges are balanced here.

Step 4: Write the Net Ionic Equation (b)

The net ionic equation involves the reaction between hydroxide ions and hydrogen ions to form water:\[ \text{OH}^- (\text{aq}) + \text{H}^+ (\text{aq}) \rightarrow \text{H}_2\text{O} (\ell) \] Barium and nitrate ions are spectator ions and are not included in the net ionic equation.

Step 5: Write the Balanced Molecular Equation (c)

For the reaction between sodium phosphate (Na₃PO₄) and nickel(II) nitrate (Ni(NO₃)₂), the equation is:\[ 2\text{Na}_3\text{PO}_4 (\text{aq}) + 3\text{Ni(NO}_3)_2 (\text{aq}) \rightarrow \text{Ni}_3(\text{PO}_4)_2 (\text{s}) + 6\text{NaNO}_3 (\text{aq}) \] Upon inspection, all elements and charge are balanced in this equation.

Step 6: Write the Net Ionic Equation (c)

The net ionic equation focuses on the formation of solid nickel(II) phosphate and removes spectator ions:\[ 3\text{Ni}^{2+} (\text{aq}) + 2\text{PO}_4^{3-} (\text{aq}) \rightarrow \text{Ni}_3(\text{PO}_4)_2 (\text{s}) \] The sodium and nitrate ions are spectator ions and do not participate in the actual reaction.

Key Concepts

Net Ionic Equations

Net ionic equations provide a deeper look into chemical reactions by focusing only on the components involved in forming the product, typically a solid, water, or gas. This type of equation strips away the spectator ions, which do not change state or participate directly in the reaction.

To write a net ionic equation, start with the balanced molecular equation. Then, identify and remove the spectator ions. What remains is the net ionic equation, featuring only the species that undergo transformation.

• Example (a): During the reaction between silver nitrate and potassium iodide, the net ionic equation pinpoints the creation of silver iodide (AgI) as a precipitate without including potassium and nitrate ions.
• Example (b): For the reaction between barium hydroxide and nitric acid, the pertinent transformation is the formation of water from hydroxide and hydrogen ions.

This approach clarifies which ions or molecules are responsible for driving the chemical change.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid, known as a precipitate. These reactions are common in double displacement reactions, where ions exchange partners, resulting in at least one insoluble product.

By understanding solubility rules, we can predict whether a precipitate will form. This typically occurs if the product, such as silver iodide in the reaction between silver nitrate and potassium iodide, is insoluble in water.

Precipitation reactions are valuable in countless applications, from purifying compounds to detecting specific ions in analytical chemistry.

• Example (c): When sodium phosphate reacts with nickel(II) nitrate, nickel(II) phosphate precipitates, illustrating the principle of precipitation reactions.

Recognizing and predicting precipitation can be key in laboratory practices.

Spectator Ions

Spectator ions are ions present in a chemical reaction that do not take part in forming the product. They remain unchanged in both reactants and products' ionic equations.

Spectator ions are identified and omitted when writing net ionic equations, allowing a clearer view of the substantive chemical changes. They "watch" the reaction unfold without engaging actively in the abstraction or formation of products.

• For example, in the reaction between silver nitrate and potassium iodide, potassium ( K^+ ) and nitrate ( NO_3^- ) ions remain spectator ions.
• Similarly, in the reaction involving barium hydroxide and nitric acid, barium and nitrate are spectator ions.

By focusing on the active participants, we gain a streamlined version of the equation highlighting the core chemical processes.

Molecular Equations

Molecular equations are the initial representation of chemical reactions, showing all reactants and products as intact compounds. These equations display compounds in their molecular form, providing a straightforward view of the reaction as a whole.

Writing molecular equations is the first step in chemical equation balancing. It ensures that each type of atom appears equally on both sides of the equation, following the law of conservation of mass.

• For instance, in example (a), the reaction of silver nitrate with potassium iodide is expressed as follows: AgNO₃(aq) + KI(aq) → AgI(s) + KNO₃(aq).
• Similarly, in example (b), barium hydroxide reacts with nitric acid, which can be expressed to highlight the aqueous and liquid states as: Ba(OH)₂(aq) + 2HNO₃(aq) → Ba(NO₃)₂(aq) + 2H₂O(ℓ).

Once a balanced molecular equation is established, we can derive further insights through complete and net ionic equations.