Question

A solution of zinc nitrate is mixed with a solution of potassium hydroxide. A precipitate forms. Complete and balance the equation for this reaction, including the phases of each reactant and product.

Short answer

The balanced chemical equation for the reaction between zinc nitrate and potassium hydroxide, including the phases, is: \(Zn(NO_3)_2 (aq) + KOH (aq) \rightarrow Zn(OH)_2 (s) + KNO_3 (aq)\). In this reaction, a precipitate of zinc hydroxide (Zn(OH)₂) is formed.

Step-by-step solution

Identify the reactants and their phases

The reactants in this reaction are zinc nitrate and potassium hydroxide. Zinc nitrate is a compound formed between Zinc (Zn) and nitrate ions (NO₃⁻), and its chemical formula is Zn(NO₃)₂. Potassium hydroxide is a compound formed between potassium (K⁺) and hydroxide ions (OH⁻), and its chemical formula is KOH. Both reactants are in the dissolved state (aqueous), indicated by the (aq) symbol. Reactants: Zinc nitrate: Zn(NO₃)₂ (aq) Potassium hydroxide: KOH (aq)

Identify the products of the reaction

The reaction between zinc nitrate and potassium hydroxide is a double displacement reaction, which occurs when the ions of two compounds exchange places in an aqueous solution to form two new compounds. Positive ions (cations): Zn²⁺ from zinc nitrate, K⁺ from potassium hydroxide Negative ions (anions) : NO₃⁻ from zinc nitrate, OH⁻ from potassium hydroxide New compounds formed: Zinc hydroxide: Zn(OH)₂ Potassium nitrate: KNO₃

Determine the phases of the products

Zinc hydroxide (Zn(OH)₂) is insoluble in water and will form a precipitate (solid) in the solution, indicated by the (s) symbol. Potassium nitrate (KNO₃) is highly soluble in water and will remain in the aqueous phase (aq). Products: Zinc hydroxide: Zn(OH)₂ (s) Potassium nitrate: KNO₃ (aq)

Write the chemical equation and balance it

Now, we will combine the reactants and products, including their phases, in a chemical equation: Zn(NO₃)₂ (aq) + KOH (aq) → Zn(OH)₂ (s) + KNO₃ (aq) This equation is already balanced, as the number of each type of atom is equal on both sides.

Confirm the formation of a precipitate

As mentioned in Step 3, zinc hydroxide (Zn(OH)₂) is insoluble in water and will form a solid precipitate in the solution. This confirms that the precipitate forms as described in the problem statement. Final balanced chemical equation: Zn(NO₃)₂ (aq) + KOH (aq) → Zn(OH)₂ (s) + KNO₃ (aq)

Key concepts

Double Displacement Reaction

A double displacement reaction, also known as a metathesis reaction, is a type of chemical reaction where the cation and anion of two different compounds swap places, resulting in the formation of two new compounds. This primarily occurs in aqueous solutions where compounds are dissolved in water and their ions are free to move and react.

In the case of the zinc nitrate and potassium hydroxide reaction, we see this exchange in action. The zinc (Zn²⁺) from zinc nitrate swaps with the potassium (K⁺) from potassium hydroxide. After the exchange, we end up with zinc hydroxide and potassium nitrate. It's important to remember that in these reactions, the total number of each type of ion remains constant; they're just rearranged into a new configuration.

Understanding the concept of double displacement is crucial for predicting the products of a reaction and writing balanced chemical equations, as it guides us towards the correct pairing of ions to form the new compounds.

Precipitate Formation

Precipitate formation occurs when a product of a chemical reaction is insoluble in the reaction mixture and settles out as a solid. This phenomenon typically happens in aqueous solutions when two soluble salts react and one of the resulting products has low solubility in water.

In the textbook exercise, when zinc nitrate and potassium hydroxide are mixed, a precipitate of zinc hydroxide forms. In chemical terms, the hydroxide (OH⁻) ions from potassium hydroxide combine with zinc ions (Zn²⁺) from zinc nitrate to produce zinc hydroxide, which is not soluble in water, causing it to form as a solid (precipitate).

This process can be used to separate or remove ions from a solution, identify the presence of certain ions, and is also fundamental in various industrial processes. It is a dramatic indication of a chemical change, usually evidenced by a cloudiness or solid particles appearing in the previously clear solution.

Balanced Chemical Equation

A balanced chemical equation is a representation of a chemical reaction using symbols and formulas to show the relationship between the reactants and products. It follows the law of conservation of mass, meaning the number of atoms for each element must be the same on both sides of the equation.

In balancing the chemical equation for the reaction between zinc nitrate and potassium hydroxide, we ensure that the number of atoms of each element is equal on the reactant and product sides of the equation. For the exercise in question, the balanced chemical equation is:
\[\text{Zn(NO}_3\text{)}_2\text{(aq)} + 2\text{KOH(aq)} \rightarrow \text{Zn(OH)}_2\text{(s)} + 2\text{KNO}_3\text{(aq)}\]
Each reactant and product is also labeled with its phase, such as (aq) for aqueous or soluble substances and (s) for solids, notably the precipitate. Writing a correctly balanced chemical equation is fundamental in chemistry because it provides crucial information such as the proportions of reactants needed and the quantity of products formed.