Question

Complete and balance the molecular, ionic, and net ionic equations for the following reactions. (a) \(\mathrm{HNO}_{3}+\mathrm{Cr}(\mathrm{OH})_{3} \longrightarrow\) (b) \(\mathrm{HClO}_{4}+\mathrm{NaOH} \longrightarrow\) (c) \(\mathrm{Cu}(\mathrm{OH})_{2}+\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2} \longrightarrow\) (d) \(\mathrm{ZnO}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow\)

Short answer

(a) Net Ionic: \(3H^{+} + 3OH^{-} \longrightarrow 3H_{2}O\). (b) Net Ionic: \(H^{+} + OH^{-} \longrightarrow H_{2}O\). (c) Net Ionic: Same as molecular due to weak acid. (d) Net Ionic: \(ZnO + 2H^{+} \longrightarrow Zn^{2+} + H_{2}O\).

Step-by-step solution

Balance Molecular Equations

For each reaction, identify the reactants and products, then balance the number of atoms for each element on both sides of the equation.

Write Ionic Equations

For each balanced molecular equation, write the ionic equation by dissociating all strong electrolytes (strong acids, strong bases, and soluble salts) into their constituent ions.

Write Net Ionic Equations

Cancel out the spectator ions that appear on both sides of the ionic equation to get the net ionic equation, which shows the species that actually participate in the chemical reaction.

Solution for (a)

Molecular: \(3HNO_{3} + Cr(OH)_{3} \longrightarrow Cr(NO_{3})_{3} + 3H_{2}O\). Ionic: \(3H^{+} + 3NO_{3}^{-} + Cr^{3+} + 3OH^{-} \longrightarrow Cr^{3+} + 3NO_{3}^{-} + 3H_{2}O\). Net Ionic: \(3H^{+} + 3OH^{-} \longrightarrow 3H_{2}O\).

Solution for (b)

Molecular: \(HClO_{4} + NaOH \longrightarrow NaClO_{4} + H_{2}O\). Ionic: \(H^{+} + ClO_{4}^{-} + Na^{+} + OH^{-} \longrightarrow Na^{+} + ClO_{4}^{-} + H_{2}O\). Net Ionic: \(H^{+} + OH^{-} \longrightarrow H_{2}O\).

Solution for (c)

Molecular: \(Cu(OH)_{2} + 2HC_{2}H_{3}O_{2} \longrightarrow Cu(C_{2}H_{3}O_{2})_{2} + 2H_{2}O\). Ionic: Not required as \(HC_{2}H_{3}O_{2}\) is a weak acid and does not dissociate completely. Net Ionic: Same as the molecular equation.

Solution for (d)

Molecular: \(ZnO + H_{2}SO_{4} \longrightarrow ZnSO_{4} + H_{2}O\). Ionic: \(ZnO + 2H^{+} + SO_{4}^{2-} \longrightarrow Zn^{2+} + SO_{4}^{2-} + H_{2}O\). Net Ionic: \(ZnO + 2H^{+} \longrightarrow Zn^{2+} + H_{2}O\).

Key concepts

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry, ensuring that the same number of each type of atom appears on both sides of the reaction. It reflects the Law of Conservation of Mass, which states that matter cannot be created or destroyed. A balanced equation allows chemists to understand the ratio of reactants that will react and the products that will form.

When balancing equations, it's important to follow certain steps: First, write down all the reactants and products. Then balance the elements one at a time, using coefficients (numbers in front of the chemical formulas) to equalize the number of atoms on each side. Start with the most complex molecule, and save hydrogen and oxygen for last, as they are often present in multiple compounds. After balancing, make sure to double-check that all atoms balance and that the charge is the same on both sides if the reaction involves ions.

Molecular Equations

Molecular equations are representations of reactions where the formulas of the reactants and products are written as if they were molecules, regardless of their actual form in solution. It is the standard form of representing chemical reactions and shows the chemicals in the form they are initially added to the reaction - for example, solid, liquid, or aqueous. Molecular equations provide a quick overview of the entire reaction without specifying the ionic character of compounds.

For instance, when you mix solutions of lead (II) nitrate and potassium iodide, the molecular equation would simply show the reactants and the solid lead (II) iodide and aqueous potassium nitrate that form, not indicating which are in ionic form in the solution.

Ionic Equations

Ionic equations provide a closer look compared to molecular equations by showing the dissociated ions when the reaction is taking place in an aqueous solution. Strong electrolytes, such as most salts, strong acids, and strong bases, dissociate completely into ions in water. The ionic equation reflects this by breaking down these compounds into their constituent ions.

Writing ionic equations involves listing all of the ions present as either reactants or products. This includes both the ions that participate in the reaction (reacting ions) and those that don't (spectator ions). The ionic equation thus gives more information about the forms of substances involved in the reaction and is useful for understanding the actual chemical changes occurring.

Acid-Base Reactions

Acid-base reactions, also known as neutralization reactions, occur when an acid and a base react to form water and a salt. The general form of the molecular equation for this process is acid + base → salt + water. Acids typically donate a proton (H⁺ ion), while bases accept a proton, resulting in the formation of water. The remaining ions from the acid and base form the salt that is a product of the reaction.

For instance, the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) forms water and sodium chloride (NaCl), which in a molecular equation, is simply written as: HCl + NaOH → NaCl + H₂O. Understanding the nature of acid-base reactions is essential for applications ranging from industrial chemistry to biological processes.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions containing soluble salts are combined, resulting in the formation of an insoluble product known as a precipitate. These reactions are vital in many areas of chemistry, including analytical chemistry and environmental engineering, because they can be used to isolate or remove specific ions from a solution.

The process can be predicted using solubility rules, which help to determine whether a solid will form when two ionic compounds are mixed in water. For example, when a solution of silver nitrate is mixed with a solution of sodium chloride, a visible precipitate of silver chloride forms. The equation illustrating this is: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), where 's' denotes the solid precipitate formed.

Spectator Ions

Spectator ions are ions present in a chemical reaction that do not participate in the actual reaction process; they remain unchanged in both the reactants and the products. In ionic equations, they appear on both sides of the chemical equation and are not involved in the formation of the precipitate, gas, or water. They are called 'spectator' ions because they are essentially 'watching' the other ions participate in the reaction.

When writing net ionic equations, these spectator ions are omitted, as they are not essential to the reaction's outcome. Net ionic equations thus focus only on the ions and molecules that undergo chemical change. Identifying spectator ions helps simplify complex reactions, making it clearer to see the substances that are actually interacting during the chemical process.