Problem 38 - Chemistry The Molecular Science

Question

Write an overall balanced equation for the precipitation reaction that occurs when aqueous lead(II) nitrate is mixed with an aqueous solution of potassium chloride. Name each reactant and product. Indicate the state of each substance \((\mathrm{s}, \ell, \mathrm{g},\) or aq \()\).

Step-by-Step Solution

Answer

Pb(NO₃)₂(aq) + 2KCl(aq) → PbCl₂(s) + 2KNO₃(aq), with reactants: lead(II) nitrate, potassium chloride; products: lead(II) chloride, potassium nitrate.

Step 1: Identify Reactants

The reactants in the precipitation reaction are aqueous lead(II) nitrate, which is denoted as Pb(NO₃)₂(aq), and aqueous potassium chloride, denoted as KCl(aq).

Step 2: Determine Products

When lead(II) nitrate reacts with potassium chloride, lead(II) chloride (PbCl₂) and potassium nitrate (KNO₃) are formed. Lead(II) chloride will form as a solid precipitate (s), while potassium nitrate remains in aqueous form (aq).

Step 3: Write Unbalanced Equation

Write the unbalanced chemical equation for the reaction:\[ \text{Pb(NO}_3)_2(aq) + \text{KCl}(aq) \rightarrow \text{PbCl}_2(s) + \text{KNO}_3(aq) \]

Step 4: Balance the Equation

The reaction needs to be balanced in terms of the number of atoms on each side. The balanced equation is:\[ \text{Pb(NO}_3)_2(aq) + 2\text{KCl}(aq) \rightarrow \text{PbCl}_2(s) + 2\text{KNO}_3(aq) \]

Step 5: Name Reactants and Products

The reactants are lead(II) nitrate and potassium chloride. The products are lead(II) chloride, which is a solid, and potassium nitrate, which remains aqueous.

Step 6: Indicate the States

In the balanced equation, indicate the physical states: lead(II) nitrate and potassium chloride are aqueous (aq), lead(II) chloride is a solid (s), and potassium nitrate is aqueous (aq).

Key Concepts

Balance Chemical Equations

Balancing chemical equations ensures that there are equal numbers of each type of atom on both sides of the equation. This is important because it reflects the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. To balance an equation:

• First, write the unbalanced equation based on the reactants and products.
• Look at the number of each type of atom present in the reactants and products.
• Add coefficients in front of compounds to balance these atoms.

For example, in the reaction between lead(II) nitrate and potassium chloride, the unbalanced equation is: \[ \text{Pb(NO}_3)_2(aq) + \text{KCl}(aq) \rightarrow \text{PbCl}_2(s) + \text{KNO}_3(aq) \]To balance it, notice there's 2 nitrate \(NO_3^-\) groups and 2 potassium \(K^+\) ions needed in the products for every molecule of reactant used, resulting in the balanced equation:\[ \text{Pb(NO}_3)_2(aq) + 2\text{KCl}(aq) \rightarrow \text{PbCl}_2(s) + 2\text{KNO}_3(aq) \]Balancing ensures the same number of each atom appears on both sides of the equation.

Chemical Reaction Identification

Chemical reactions can be identified based on the changes in substances and their properties. A key feature of reactions is that the substances you start with, the reactants, turn into new substances, the products. To identify the precipitation reaction in this exercise:

• Recognize that mixing leads to the formation of a different compound as a solid, indicating a precipitation reaction.
• In the given example, mixing lead(II) nitrate and potassium chloride forms lead(II) chloride as a solid precipitate.
• The reaction is complete when the solid separates from the solution.

Precipitation reactions occur when cations and anions in aqueous solutions combine to form an insoluble ionic solid, which is distinct from the remaining ions still dissolved in the solution. This forms the basis for recognizing such reactions.

Physical States in Chemistry

Understanding physical states is crucial in describing chemical reactions accurately. In chemistry, compounds can exist in different states: solid (s), liquid (l), gas (g), and aqueous (aq) which helps predict the behavior in reactions. For the precipitation reaction:

• Compounds that are dissolved in water are labeled as aqueous (aq), meaning they are dispersed as ions.
• Substances forming solids are denoted as (s), indicating they do not dissolve under reaction conditions.

For example, in the reaction:\[ \text{Pb(NO}_3)_2(aq) + 2\text{KCl}(aq) \rightarrow \text{PbCl}_2(s) + 2\text{KNO}_3(aq) \]both lead(II) nitrate and potassium chloride start as aqueous solutions. Lead(II) chloride, however, forms a solid, while potassium nitrate remains in aqueous form. Understanding these states allows chemists to deduce which compounds are soluble and possible precipitates.