Problem 52 - Chemistry and Chemical Reactivity

Question

Balance the following equations, and then classify each as a precipitation, acid-base, or gas-forming reaction. (a) \(\mathrm{K}_{2} \mathrm{CO}_{3}(\mathrm{aq})+\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq}) \rightarrow \mathrm{CuCO}_{3}(\mathrm{s})+\mathrm{KNO}_{3}(\mathrm{aq})\) (b) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{PbCl}_{2}(\mathrm{s})+\mathrm{HNO}_{3}(\mathrm{aq})\) (c) \(\mathrm{MgCO}_{3}(\mathrm{s})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{CO}_{2}(\mathrm{g})\)

Step-by-Step Solution

Answer

(a) Precipitation; (b) Precipitation; (c) Gas-forming.

Step 1: Balance Equation (a)

The unbalanced equation is \( \mathrm{K}_{2}\mathrm{CO}_{3}(\mathrm{aq})+\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq}) \rightarrow \mathrm{CuCO}_{3}(\mathrm{s})+\mathrm{KNO}_{3}(\mathrm{aq}) \). To balance it, ensure that the number of each type of atom is equal on both sides:- Copper (Cu): 1 Cu on both sides- Carbon (C): 1 C on both sides- Oxygen (O): 3 O in CuCO₃, 3 O in one NO₃−, 3 O in KNO₃, and 3 O in KNO₃; balanced overall- Potassium (K): 2 K from K₂CO₃ on the left, 2 K needed on the right, thus 2 KNO₃- Nitrate (NO₃): 2 nitrate ions with Cu(NO₃)₂, thus 2 KNO₃ on the rightBalanced Equation: \( \mathrm{K}_{2}\mathrm{CO}_{3}(\mathrm{aq})+\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq}) \rightarrow \mathrm{CuCO}_{3}(\mathrm{s})+2\mathrm{KNO}_{3}(\mathrm{aq}) \).

Step 2: Classify Reaction (a)

In the balanced equation for (a), \( \mathrm{CuCO}_{3} \) is a solid precipitate forming from aqueous reactants. Therefore, this is a precipitation reaction.

Step 3: Balance Equation (b)

The unbalanced equation is \( \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{PbCl}_{2}(\mathrm{s})+\mathrm{HNO}_{3}(\mathrm{aq}) \). Ensure quantities are equal on both sides:- Lead (Pb): 1 Pb on both sides- Chlorine (Cl): 2 Cl in PbCl₂ on the right, so 2 HCl on the left- Hydrogen (H): 2 H in 2 HCl- Nitrate (NO₃): 2 NO₃ in \( \mathrm{Pb}(\mathrm{NO}_{3})_{2} \) on the left, 2 needed on the right, thus 2 HNO₃Balanced Equation: \( \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{PbCl}_{2}(\mathrm{s})+2\mathrm{HNO}_{3}(\mathrm{aq}) \).

Step 4: Classify Reaction (b)

In the balanced equation for (b), \( \mathrm{PbCl}_{2} \) forms as a solid precipitate from aqueous reactants, indicative of a precipitation reaction.

Step 5: Balance Equation (c)

The unbalanced equation is \( \mathrm{MgCO}_{3}(\mathrm{s})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+\mathrm{H}_{2}\mathrm{O}(\ell)+\mathrm{CO}_{2}(\mathrm{g}) \). Balance the equation:- Magnesium (Mg): 1 Mg on both sides- Carbon (C): 1 C on both sides- Chlorine (Cl): 2 Cl in MgCl₂ on the right, so 2 HCl needed on the left- Hydrogen (H): 2 H in 2 HCl- Oxygen (O): 3 O in CO₂ and H₂O, matching 3 O in MgCO₃Balanced Equation: \( \mathrm{MgCO}_{3}(\mathrm{s})+2\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+\mathrm{H}_{2}\mathrm{O}(\ell)+\mathrm{CO}_{2}(\mathrm{g}) \).

Step 6: Classify Reaction (c)

In the balanced equation for (c), \( \mathrm{CO}_{2} \) gas is formed, indicating this is a gas-forming reaction.

Key Concepts

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two solutions combine to form an insoluble solid, known as a precipitate. Imagine mixing two clear liquids and suddenly seeing a solid form – that's precipitation at work! This happens when the ions in the solution come together to form a compound that does not dissolve in water.

In the examples given, like the reaction between potassium carbonate (\(\mathrm{K}_2\mathrm{CO}_3\)) and copper(II) nitrate (\(\mathrm{Cu(NO_3)_2}\)), a solid copper carbonate (\(\mathrm{CuCO}_3\)) forms as a precipitate. This reaction can be represented by the balanced equation:

• \( \mathrm{K}_{2}\mathrm{CO}_{3}(\mathrm{aq}) + \mathrm{Cu} \left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq}) \rightarrow \mathrm{CuCO}_{3}(\mathrm{s}) + 2 \mathrm{KNO}_{3}(\mathrm{aq}) \)

Recognizing a precipitation reaction involves identifying the formation of a solid product when two aqueous solutions are mixed. This makes precipitation reactions a valuable tool in creating specific compounds or removing unwanted ions from solutions.

Gas-Forming Reactions

Gas-forming reactions are fascinating because they result in the formation of a gas, often visible as bubbles. These reactions are particularly engaging as they demonstrate a clear transformation from solid or liquid reactants to a gaseous product.

In the exercise, magnesium carbonate (\(\mathrm{MgCO_3}\)) reacts with hydrochloric acid to yield magnesium chloride, water, and carbon dioxide gas. The balanced equation is:

• \( \mathrm{MgCO}_{3}(\mathrm{s}) + 2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell) + \mathrm{CO}_{2}(\mathrm{g}) \)

The release of carbon dioxide is a hallmark of gas-forming reactions. These reactions are not only interesting to watch but also have practical applications in processes like baking and in industrial reactions where gas generation is necessary.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, involving the transfer of a proton (H⁺) from the acid to the base. Understanding these reactions is key to mastering many chemical processes. Although the original exercise didn't include a direct acid-base reaction example, let's explore how they generally work.

When an acid, such as hydrochloric acid (\(\mathrm{HCl}\)), reacts with a base, such as sodium hydroxide (\(\mathrm{NaOH}\)), they form water and salt. This neutralization can be represented by:

• \( \mathrm{HCl}(\mathrm{aq}) + \mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}\mathrm{O}(\ell) + \mathrm{NaCl}(\mathrm{aq}) \)

The concept of neutralization is central in many scientific and industrial processes, and these reactions are employed in everything from digestion in our bodies to maintaining swimming pool pH levels. Recognizing and balancing acid-base reactions is crucial for chemistry students and professionals alike.