Question

Explain why the following reactions take place. (a) \(\mathrm{CrCl}_{3}+3 \mathrm{NaOH} \longrightarrow \mathrm{Cr}(\mathrm{OH})_{3}+3 \mathrm{NaCl}\) (b) \(\mathrm{ZnO}+2 \mathrm{HBr} \longrightarrow \mathrm{ZnBr}_{2}+\mathrm{H}_{2} \mathrm{O}\)

Short answer

Reaction (a) occurs because a salt of a weak base and strong acid reacts with a strong base to form a weak base and a salt. Reaction (b) occurs because a basic oxide reacts with a strong acid to form a salt and water.

Step-by-step solution

Understand Reaction (a)

The reaction between Chromium(III) chloride \(\mathrm{CrCl}_{3}\) and sodium hydroxide (NaOH) is a double displacement reaction in basic solution. Chromium(III) chloride, being a salt of a weak base (Cr(OH)3) and a strong acid (HCl), reacts with a strong base (NaOH) to form Chromium(III) hydroxide \(\mathrm{Cr}(\text{OH})_{3}\) and sodium chloride (NaCl).

Understand Reaction (b)

Zinc oxide (ZnO) reacts with hydrobromic acid \(\mathrm{HBr}\) in an acid-base reaction. ZnO is a basic oxide and reacts with \(\mathrm{HBr}\), a strong acid, to form zinc bromide \(\mathrm{ZnBr}_{2}\), a salt, along with water \(\mathrm{H}_{2}\mathrm{O}\).

Key concepts

Double Displacement Reaction

Double displacement reactions, also known as metathesis reactions, are a type of chemical process where parts of two ionic compounds are exchanged, forming two new compounds. The general form of a double displacement reaction is \(AB + CD \rightarrow AD + CB\).In the context of the provided exercise, reaction (a) \(\mathrm{CrCl}_{3}+3 \mathrm{NaOH} \longrightarrow \mathrm{Cr}(\mathrm{OH})_{3}+3 \mathrm{NaCl}\)is a classic example of a double displacement reaction involving the exchange of anions between two reactants. Here, the chromium trades its chloride ions with the sodium's hydroxide ions resulting in the formation of chromium(III) hydroxide and sodium chloride.

Acid-Base Reaction

Acid-base reactions, often called neutralization reactions, occur when an acid and a base react to form salt and water. These reactions are central to the study of chemistry and involve the transfer of protons from the acid to the base.When looking at reaction (b) \(\mathrm{ZnO}+2 \mathrm{HBr} \longrightarrow \mathrm{ZnBr}_{2}+\mathrm{H}_{2} \mathrm{O}\), we see that zinc oxide, which functions as a base, reacts with hydrobromic acid to yield zinc bromide and water. This exemplifies the neutralization process where the acid \(\mathrm{HBr}\) donates a proton to the base \(\mathrm{ZnO}\), leading to the production of water and zinc bromide, a salt.

Chemical Reaction Equations

Chemical reaction equations provide a concise way for scientists to convey information about the chemical changes taking place during a reaction. These equations outline the reactants, the substances that undergo change, and the products, the substances that are produced.In our case, the reactions are represented using chemical formulas, where the left side of the arrow indicates the reactants, and the right side shows the products. For instance, reaction (a) correctly balances the number of atoms on both sides, adhering to the law of conservation of mass. It's important to note that the coefficients in the reaction \(\mathrm{CrCl}_{3}+3 \mathrm{NaOH} \longrightarrow \mathrm{Cr}(\mathrm{OH})_{3}+3 \mathrm{NaCl}\) ensure that the same number of each type of atom is present before and after the reaction, which is a fundamental rule in writing chemical equations.

Stoichiometry

Stoichiometry is the field of chemistry that relates to the quantitative relationships between reactants and products in a chemical reaction. It involves calculations based on the balanced equations and allows chemists to predict the amount of substances needed or produced in a given reaction.The stoichiometry of the given reactions can be understood by looking at the coefficients in front of each compound in the balanced equations. For example, in reaction (a), the '3' in front of \(\mathrm{NaOH}\) and \(\mathrm{NaCl}\) indicates that three moles of sodium hydroxide react with one mole of chromium(III) chloride to produce three moles of sodium chloride and one mole of chromium(III) hydroxide. These stoichiometric coefficients are essential for calculations involving reactant and product quantities.