Problem 55 - Chemistry and Chemical Reactivity

Question

Balance each of the following equations, and classify them as precipitation, acid-base, gas-forming, or oxidation-reduction reactions. Show states for the products \((\mathrm{s}, \ell, \mathrm{g}, \mathrm{aq}),\) and then balance the completed equation. (a) \(\mathrm{CuCl}_{2}+\mathrm{H}_{2} \mathrm{S} \rightarrow \mathrm{CuS}+\mathrm{HCl}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}+\mathrm{KOH} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{K}_{3} \mathrm{PO}_{4}\) (c) \(\mathrm{Ca}+\mathrm{HBr} \rightarrow \mathrm{H}_{2}+\mathrm{CaBr}_{2}\) (d) \(\mathrm{MgCl}_{2}+\mathrm{NaOH} \rightarrow \mathrm{Mg}(\mathrm{OH})_{2}+\mathrm{NaCl}\)

Step-by-Step Solution

Answer

(a) Precipitation; (b) Acid-base; (c) Gas-forming; (d) Precipitation.

Step 1: Balance Equation (a)

For the reaction \( \mathrm{CuCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2} \mathrm{S}(\mathrm{g}) \rightarrow \mathrm{CuS}(\mathrm{s}) + \mathrm{HCl}(\mathrm{aq}) \):1. Balance copper: There is 1 Cu on each side.2. Balance sulfur: There is 1 S on each side.3. Balance chlorine: There are 2 Cl on the reactant side, so need 2 HCl on the product side.4. Balance hydrogen: There are 2 H on the product side which matches with 2 H in HCl.The balanced equation is: \( \mathrm{CuCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2} \mathrm{S}(\mathrm{g}) \rightarrow \mathrm{CuS}(\mathrm{s}) + 2\mathrm{HCl}(\mathrm{aq}) \).This is a precipitation reaction due to the formation of solid CuS.

Step 2: Balance Equation (b)

For the reaction \( \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq}) + \mathrm{KOH}(\mathrm{aq}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\ell) + \mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \):1. Balance potassium: 3 KOH gives 3 K.2. Balance phosphate: There is 1 PO₄ on each side.3. Balance oxygen and hydrogen by forming water: Combine OH from KOH and H from H₃PO₄. 4. Balance hydrogens: Form 3 H₂O from 3 OH and remaining H.The balanced equation is: \( \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq}) + 3 \mathrm{KOH}(\mathrm{aq}) \rightarrow 3 \mathrm{H}_{2} \mathrm{O}(\ell) + \mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \).This is an acid-base reaction as it involves the neutralization of an acid by a base.

Step 3: Balance Equation (c)

For the reaction \( \mathrm{Ca}(\mathrm{s}) + \mathrm{HBr}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g}) + \mathrm{CaBr}_{2}(\mathrm{aq}) \):1. Balance calcium: There is 1 Ca on both sides.2. Balance bromine: 2 HBr are needed to form CaBr₂.3. Balance hydrogen: 2 H are needed to form \( \mathrm{H}_{2} \).The balanced equation is: \( \mathrm{Ca}(\mathrm{s}) + 2 \mathrm{HBr}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g}) + \mathrm{CaBr}_{2}(\mathrm{aq}) \).This is a gas-forming reaction due to the generation of hydrogen gas.

Step 4: Balance Equation (d)

For the reaction \( \mathrm{MgCl}_{2}(\mathrm{aq}) + \mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s}) + \mathrm{NaCl}(\mathrm{aq}) \):1. Balance magnesium: There is 1 Mg on both sides.2. Balance chlorine: There are 2 Cl on the reactant side, so need 2 NaCl on the product side.3. Balance sodium: 2 NaOH are needed to form 2 NaCl.4. Balance hydroxide: 2 OH from NaOH match the Mg(OH)₂.The balanced equation is: \( \mathrm{MgCl}_{2}(\mathrm{aq}) + 2 \mathrm{NaOH}(\mathrm{aq}) \rightarrow \mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s}) + 2 \mathrm{NaCl}(\mathrm{aq}) \).This is a precipitation reaction due to the formation of solid Mg(OH)₂.

Key Concepts

Types of Chemical Reactions

Chemical reactions can be grouped into different types based on the nature of the reactants and the products they form. Understanding the types of reactions helps predict the products and balance the chemical equation.

• Precipitation Reaction: This involves the formation of a solid (or precipitate) when two aqueous solutions are mixed.
• Acid-Base Reaction: When an acid and a base react, they produce water and a salt. This is commonly known as a neutralization reaction.
• Gas-Forming Reaction: Certain reactions result in the formation of a gas as a product.
• Oxidation-Reduction Reaction: Sometimes known as redox reactions, these involve the transfer of electrons between two substances.

Classifying reactions helps in numerous applications, such as identifying reaction conditions and determining energy changes.

Precipitation Reaction

A precipitation reaction occurs when two soluble salts react in aqueous solution to form one or more insoluble products. These insoluble products are called precipitates. Such reactions are typically represented by a balanced chemical equation specifying the physical states of reactants and products.
For example, consider the reaction between copper(II) chloride and hydrogen sulfide:

• The reaction can be written as: \( \mathrm{CuCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2} \mathrm{S}(\mathrm{g}) \rightarrow \mathrm{CuS}(\mathrm{s}) + 2\mathrm{HCl}(\mathrm{aq}) \)
• Copper(II) sulfide forms as a solid precipitate, while hydrogen chloride remains in the aqueous phase.

Recognizing precipitation reactions is vital, especially in laboratory settings, as these reactions show color changes, cloudiness, or solid formation.

Acid-Base Reaction

Acid-base reactions are vital in both chemistry and everyday life. They involve the reaction between an acid and a base, producing water and a salt. This type of reaction is a classic example of neutralization, where hydrogen ions \( (\mathrm{H}^+) \) from the acid combine with hydroxide ions \( (\mathrm{OH}^-) \) from the base to produce water.

• Considering the exercise example: \( \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq}) + 3 \mathrm{KOH}(\mathrm{aq}) \rightarrow 3 \mathrm{H}_{2} \mathrm{O}(\ell) + \mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \)
• The reaction between phosphoric acid and potassium hydroxide results in water as a product and potassium phosphate as the salt.

These reactions are fundamental in various chemical processes including digestion, metabolic pathways, and many industrial applications.

Gas-Forming Reaction

Gas-forming reactions occur when reactants produce a gas as one of the products. These reactions are important in many applications like pharmaceuticals, food industry processes, and scientific demonstrations due to the noticeable evolution of gas bubbles.

• From our exercise: \( \mathrm{Ca}(\mathrm{s}) + 2 \mathrm{HBr}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g}) + \mathrm{CaBr}_{2}(\mathrm{aq}) \)
• Calcium reacts with hydrobromic acid to produce hydrogen gas and calcium bromide.

Gas-forming reactions are easy to spot because bubbles form as the gas escapes into the surroundings. Understanding these reactions aids in creating controlled environments for reactions where gases are desired products.