Problem 63 - 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 reactants and products \((s, \ell, g, a q).\) (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. Equations are balanced.

Step 1: Balance Equation (a)

The unbalanced equation is \(\mathrm{CuCl}_{2} (aq) + \mathrm{H}_{2} \mathrm{S} (aq) \rightarrow \mathrm{CuS} (s) + \mathrm{HCl} (aq)\).- 1 \(\mathrm{Cu}\) is on both sides.- 2 \(\mathrm{Cl}\) on the reactant side; thus, write 2 in front of \(\mathrm{HCl}\) on the product side.- Write the balanced equation: \[\mathrm{CuCl}_{2} (aq) + \mathrm{H}_{2} \mathrm{S} (aq) \rightarrow \mathrm{CuS} (s) + 2 \mathrm{HCl} (aq)\]This is a precipitation reaction because \(\mathrm{CuS}\) forms as a solid.

Step 2: Balance Equation (b)

The unbalanced equation is \(\mathrm{H}_{3} \mathrm{PO}_{4} (aq) + \mathrm{KOH} (aq) \rightarrow \mathrm{H}_{2} \mathrm{O} (\ell) + \mathrm{K}_{3} \mathrm{PO}_{4} (aq)\).- 3 \(\mathrm{K}\) are needed on the left side. Write 3 in front of \(\mathrm{KOH}\).- Adjust hydrogens accordingly, 6 \(\mathrm{H}\) on the left; thus, write 3 in front of \(\mathrm{H}_{2} \mathrm{O}\).- Write the balanced equation: \[\mathrm{H}_{3} \mathrm{PO}_{4} (aq) + 3 \mathrm{KOH} (aq) \rightarrow 3 \mathrm{H}_{2} \mathrm{O} (\ell) + \mathrm{K}_{3} \mathrm{PO}_{4} (aq)\]This is an acid-base reaction because \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (acid) and \(\mathrm{KOH}\) (base) react.

Step 3: Balance Equation (c)

The unbalanced equation is \(\mathrm{Ca} (s) + \mathrm{HBr} (aq) \rightarrow \mathrm{H}_{2} (g) + \mathrm{CaBr}_{2} (aq)\).- 2 \(\mathrm{Br}\) needed on reactant side. Write 2 in front of \(\mathrm{HBr}\).- Adjust hydrogens accordingly. \(\mathrm{H}_{2}\) is present as a product.- Write the balanced equation: \[\mathrm{Ca} (s) + 2 \mathrm{HBr} (aq) \rightarrow \mathrm{H}_{2} (g) + \mathrm{CaBr}_{2} (aq)\]This is a gas-forming reaction because \(\mathrm{H}_{2}\) gas is produced.

Step 4: Balance Equation (d)

The unbalanced equation is \(\mathrm{MgCl}_{2} (aq) + \mathrm{NaOH} (aq) \rightarrow \mathrm{Mg} (\mathrm{OH})_{2} (s) + \mathrm{NaCl} (aq)\).- We need 2 \(\mathrm{NaOH}\) because 2 \(\mathrm{OH}\) are needed on the product side.- Adjust and write the balanced equation: \[\mathrm{MgCl}_{2} (aq) + 2 \mathrm{NaOH} (aq) \rightarrow \mathrm{Mg} (\mathrm{OH})_{2} (s) + 2 \mathrm{NaCl} (aq)\]This is a precipitation reaction as \(\mathrm{Mg} (\mathrm{OH})_{2}\) is a solid formed.

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions react to form an insoluble solid, known as a precipitate. The role of solubility is critical here: when ions in the solution pair up to form a compound that doesn't dissolve in water, a solid forms.
A typical example from the above exercise is when \[ \text{MgCl}_2 (aq) + 2 \text{NaOH} (aq) \rightarrow \text{Mg(OH)}_2 (s) + 2 \text{NaCl} (aq) \] This reaction results in the formation of magnesium hydroxide, \( \text{Mg(OH)}_2 \), as a solid precipitate.
Key points to look for in a precipitation reaction:

• Formation of an insoluble solid (precipitate).
• Reactants are typically aqueous solutions.
• Product includes a solid not soluble in water.

Precipitation reactions are essential in various applications, such as water treatment and the production of certain paints and pigments.

Acid-Base Reactions

Acid-base reactions involve the transfer of one or more protons \( \text{(H}^+ \text{)} \) from an acid to a base. The classic outcome is the formation of water and a salt. This type of reaction often involves a neutralization, where the acidic and basic properties cancel each other out.
Consider the following balanced equation from the exercise:\[\text{H}_3\text{PO}_4 (aq) + 3 \text{KOH} (aq) \rightarrow 3 \text{H}_2\text{O} (\ell) + \text{K}_3\text{PO}_4 (aq)\]In this reaction, \( \text{H}_3\text{PO}_4 \) is an acid and \( \text{KOH} \) is a base. The protons from phosphoric acid react with hydroxide ions, resulting in water.
The essential features of acid-base reactions include:

• A proton transfer from an acid to a base.
• The formation of water, when a base containing hydroxide \( \text{(OH}^-\text{)} \) reacts with an acid.
• The formation of a salt, in this case, potassium phosphate, \( \text{K}_3\text{PO}_4 \).

These reactions are fundamental to processes in both biological systems and industrial applications.

Gas-Forming Reactions

Gas-forming reactions are characterized by the generation of a gas as one of the reaction products. These reactions are often surprising, sometimes even appearing explosive due to the rapid release and expansion of gas.
From our exercise, the reaction:\[\text{Ca} (s) + 2 \text{HBr} (aq) \rightarrow \text{H}_2 (g) + \text{CaBr}_2 (aq)\]shows calcium metal reacting with hydrobromic acid to produce hydrogen gas, \( \text{H}_2 (g) \).
Highlights of gas-forming reactions include:

• The appearance of bubbles indicating gas production.
• Involve a metal reacting with an acid or some specific other compounds.
• Formation of gas can be used in practical applications like culinary arts with baking soda in cakes.

Such reactions are central to a variety of contexts, from laboratory demonstrations to the functioning of airbags in automobiles.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are processes where there is a transfer of electrons between species. This shift in electrons leads to changes in the oxidation states of the elements involved.
For the reactions mentioned in the exercise, redox reactions seem less applicable directly, but it's important to understand their fundamental nature.
Key features of redox reactions include:

• Exchange of electrons, where oxidation involves loss and reduction involves gain of electrons.
• An increase in oxidation state represents oxidation, whereas a decrease represents reduction.
• They are involved in processes like combustion, corrosion, and in batteries where they generate electrical current.

Understanding these reactions is crucial for insight into energy production and the workings of biological systems.