Problem 62 - Chemistry and Chemical Reactivity

Question

Classify each of the following reactions as a precipitation, acid-base, or gas-forming reaction. Show states for the products \((s, \ell, g, a q),\) and then balance the completed equation. Write the net ionic equation. (a) \(\mathrm{Fe}(\mathrm{OH})_{3}(\mathrm{s})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}+\mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{FeCO}_{3}(\mathrm{s})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(a) Acid-base reaction; (b) Gas-forming reaction.

Step 1: Identify Reaction Types

In reaction (a), Fe(OH)₃ (a solid) reacts with HNO₃ (aq), which is indicative of an acid-base reaction between a metal hydroxide and an acid, producing a salt and water. In reaction (b), FeCO₃ reacts with HNO₃, forming Fe(NO₃)₂, CO₂, and water; this is a gas-forming reaction because carbon dioxide gas is produced.

Step 2: Write Product States

For reaction (a): Fe(OH)₃(s) + HNO₃(aq) → Fe(NO₃)₃(aq) + H₂O(l). Reaction (b) is: FeCO₃(s) + HNO₃(aq) → Fe(NO₃)₂(aq) + CO₂(g) + H₂O(l).

Step 3: Balance the Equations

For reaction (a), the balanced equation is: Fe(OH)₃(s) + 3HNO₃(aq) → Fe(NO₃)₃(aq) + 3H₂O(l). For reaction (b), the balanced equation is: FeCO₃(s) + 2HNO₃(aq) → Fe(NO₃)₂(aq) + CO₂(g) + H₂O(l).

Step 4: Write Net Ionic Equations

For reaction (a), the net ionic equation is Fe(OH)₃(s) + 3H⁺(aq) → Fe³⁺(aq) + 3H₂O(l). For reaction (b), the net ionic equation is FeCO₃(s) + 2H⁺(aq) → Fe²⁺(aq) + CO₂(g) + H₂O(l).

Key Concepts

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two soluble salts in aqueous solutions are combined, resulting in the formation of an insoluble salt, known as the precipitate. This occurs because the resulting product is not soluble in water, causing it to separate out of the solution as a solid.
To identify a precipitation reaction, look at the products of the reaction. A solid precipitate indicates that this type of reaction has occurred. For example, if an equation shows a product with a (s) for solid, it's indicative of a precipitation reaction.
The classic rule for predicting precipitation reactions is to understand the solubility rules of common ions. For instance:

• Sulfates (SO₄²⁻) are generally soluble except when combined with lead (Pb²⁺), barium (Ba²⁺), or calcium (Ca²⁺).
• Nitrates (NO₃⁻) and most alkali metals are typically soluble.
• Chlorides are soluble, except for those of silver (Ag⁺), lead, and mercury (Hg₂²⁺).

Acid-Base Reactions

An acid-base reaction is a chemical reaction that occurs between an acid and a base. It typically results in the formation of water and a salt. In reaction (a) from the exercise, iron(III) hydroxide, a base, reacts with nitric acid.
The typical form of an acid-base reaction is:\[ \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} \]When identifying these reactions, notice the presence of an acid, which often contains hydrogen ions (H⁺), and a base, which typically contains hydroxide ions (OH⁻). They combine to form water (H₂O), a neutral and stable compound.
In the net ionic equation, you can often see that it simplifies to:\[ \text{H⁺(aq)} + \text{OH⁻(aq)} \rightarrow \text{H₂O(l)} \]This simplicity is because many anions and cations are spectators and do not participate directly in the reaction.
Remember, strong acids and strong bases dissociate completely in water, which can influence calculations and predictions of the reaction products.

Gas-Forming Reactions

Gas-forming reactions are a fascinating type of chemical reaction where the formation of gas, like carbon dioxide ( CO₂ ) or hydrogen ( H₂ ), is the driving factor. These reactions often occur when an acid reacts with a carbonate or bicarbonate.
In the given exercise, the reaction of FeCO₃ with HNO₃ is an example. When you see carbonates ( CO₃^{2-} ) reacting with acids, anticipate the formation of CO₂ gas, along with water and salt.
Common gas-forming reactions can include reactions where the products are gasses like oxygen ( O₂ ), sulfur dioxide ( SO₂ ), or ammonia ( NH₃ ).
A good way to anticipate a gas-forming reaction is to look for reactants known to release gas on reaction, such as:

• Carbonates and bicarbonates, which usually form CO₂.
• Ammonium compounds, when heated, often produce NH₃.
• Sulfites reacting with acids typically yield SO₂.

These reactions often have visible signs such as bubbling or effervescence due to the gas escaping the reaction mixture.