Question

Write net ionic equations for the following reactions: (a) \(\mathrm{Mg}(s)+\mathrm{CuCl}_{2}(a q) \longrightarrow \mathrm{MgCl}_{2}(a q)+\mathrm{Cu}(s)\) (b) \(2 \mathrm{KCl}(a q)+\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q) \longrightarrow\) \(\mathrm{PbCl}_{2}(s)+2 \mathrm{KNO}_{3}(a q)\) (c) \(2 \mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}(a q)+3 \mathrm{Na}_{2} \mathrm{~S}(a q) \longrightarrow\) \(\mathrm{Cr}_{2} \mathrm{~S}_{3}(s)+6 \mathrm{NaNO}_{3}(a q)\)

Short answer

(a) \(\mathrm{Mg}(s) + \mathrm{Cu}^{2+}(aq) \rightarrow \mathrm{Mg}^{2+}(aq) + \mathrm{Cu}(s)\); (b) \(\mathrm{Pb}^{2+}(aq) + 2\mathrm{Cl}^-(aq) \rightarrow \mathrm{PbCl}_2(s)\); (c) \(2\mathrm{Cr}^{3+}(aq) + 3\mathrm{S}^{2-}(aq) \rightarrow \mathrm{Cr}_2\mathrm{S}_3(s)\)

Step-by-step solution

Write the balanced molecular equation for (a)

The balanced chemical reaction provided is:\[ \mathrm{Mg}(s) + \mathrm{CuCl}_2(aq) \rightarrow \mathrm{MgCl}_2(aq) + \mathrm{Cu}(s) \]

Determine the aqueous species for (a)

Identify and split all the aqueous ionic compounds into their ions:- \(\mathrm{CuCl}_2(aq)\) becomes \(\mathrm{Cu}^{2+}(aq) + 2\mathrm{Cl}^-(aq)\)- \(\mathrm{MgCl}_2(aq)\) becomes \(\mathrm{Mg}^{2+}(aq) + 2\mathrm{Cl}^-(aq)\)

Identify the net ionic equation for (a)

Remove the spectator ions \(\mathrm{Cl}^-\) and write the net ionic equation:\[ \mathrm{Mg}(s) + \mathrm{Cu}^{2+}(aq) \rightarrow \mathrm{Mg}^{2+}(aq) + \mathrm{Cu}(s) \]

Write the balanced molecular equation for (b)

The balanced chemical reaction provided is:\[ 2\mathrm{KCl}(aq) + \mathrm{Pb}(\mathrm{NO}_3)_2(aq) \rightarrow \mathrm{PbCl}_2(s) + 2\mathrm{KNO}_3(aq) \]

Determine the aqueous species for (b)

Identify and split all the aqueous ionic compounds into their ions:- \(\mathrm{KCl}(aq)\) becomes \(\mathrm{K}^+(aq) + \mathrm{Cl}^-(aq)\)- \(\mathrm{Pb}(\mathrm{NO}_3)_2(aq)\) becomes \(\mathrm{Pb}^{2+}(aq) + 2\mathrm{NO}_3^-(aq)\)- \(\mathrm{KNO}_3(aq)\) becomes \(\mathrm{K}^+(aq) + \mathrm{NO}_3^-(aq)\)

Identify the net ionic equation for (b)

Remove the spectator ions \(\mathrm{K}^+\) and \(\mathrm{NO}_3^-\) and write the net ionic equation:\[ \mathrm{Pb}^{2+}(aq) + 2\mathrm{Cl}^-(aq) \rightarrow \mathrm{PbCl}_2(s) \]

Write the balanced molecular equation for (c)

The balanced chemical reaction provided is:\[ 2\mathrm{Cr}(\mathrm{NO}_3)_3(aq) + 3\mathrm{Na}_2\mathrm{S}(aq) \rightarrow \mathrm{Cr}_2\mathrm{S}_3(s) + 6\mathrm{NaNO}_3(aq) \]

Determine the aqueous species for (c)

Identify and split all the aqueous ionic compounds into their ions:- \(\mathrm{Cr}(\mathrm{NO}_3)_3(aq)\) becomes \(\mathrm{Cr}^{3+}(aq) + 3\mathrm{NO}_3^-(aq)\)- \(\mathrm{Na}_2\mathrm{S}(aq)\) becomes \(2\mathrm{Na}^+(aq) + \mathrm{S}^{2-}(aq)\)- \(\mathrm{NaNO}_3(aq)\) becomes \(\mathrm{Na}^+(aq) + \mathrm{NO}_3^-(aq)\)

Identify the net ionic equation for (c)

Remove the spectator ions \(\mathrm{Na}^+\) and \(\mathrm{NO}_3^-\) and write the net ionic equation:\[ 2\mathrm{Cr}^{3+}(aq) + 3\mathrm{S}^{2-}(aq) \rightarrow \mathrm{Cr}_2\mathrm{S}_3(s) \]

Key concepts

Chemical Reactions

Chemical reactions are fascinating processes where substances, known as reactants, change into new substances, called products. In a chemical reaction, bonds between atoms in the reactants are broken and new bonds are formed in the products. This process can be represented using balanced chemical equations, where the number of each type of atom is the same on both sides of the equation.
To begin understanding a reaction, it is important to consider different states of matter involved, such as solid, liquid, gas, or aqueous (dissolved in water). In the context of net ionic equations, we focus especially on reactions in aqueous solutions. It is these reactions where we can clearly see ions interacting with each other.
For example, in reaction (a) of the exercise provided, - Solid magnesium (\(\text{Mg(s)}\)) reacts with copper(II) chloride (\(\text{CuCl}_2\text{{(aq)}}\)), resulting in magnesium chloride (\(\text{MgCl}_2\text{{(aq)}}\)) and solid copper (\(\text{Cu(s)}\)). This equation describes how substances transform from one form to another during a reaction.

Spectator Ions

Spectator ions are ions that exist in the solution both before and after a chemical reaction and do not participate directly in the reaction itself. These ions help balance charges in the solution but do not alter in form or participate in the formation of products. Understanding spectator ions is crucial for simplifying equations to their net ionic forms, which show only the species that actually participate in the reaction.
In net ionic equations, spectator ions are removed to highlight the core reaction. For instance, in reaction (a), after identifying \(\text{MgCl}_2(aq)\)'s ions as \(\text{Mg}^{2+}(aq)\) and \(\text{Cl}^-(aq)\), and \(\text{CuCl}_2(aq)\) as \(\text{Cu}^{2+}(aq)\) and \(\text{Cl}^-(aq)\), we spot chloride ions as spectators.
Thus, the crucial interaction between magnesium and copper ions is expressed, excluding the unchanged chloride ions.

Aqueous Ionic Compounds

Aqueous ionic compounds are substances that dissolve in water to form ions. These solutions allow ions to move freely, enabling them to participate in reactions. These are essential in net ionic equations where the focus is on the ions involved in the chemical changes.
When ionic compounds dissolve in water, they split into their constituent cations (positively charged ions) and anions (negatively charged ions). This dissociation is critical in writing both molecular and net ionic equations.

• For example, \(\text{KCl(aq)}\) dissociates into \(\text{K}^+(aq)\) and \(\text{Cl}^-(aq)\).
• Similarly, \(\text{Pb(NO}_3)_2(aq)\) splits into \(\text{Pb}^{2+}(aq)\) and two \(\text{NO}_3^-(aq)\) ions.

Understanding these compounds and their dissociation in water is essential for predicting and balancing chemical reactions. This knowledge simplifies complex reactions by expressing them as interactions between essential ions, making equations easier to work with and understand.