Question

Complete and balance the following molecular equations, and then write the net ionic equation for each: (a) \(\mathrm{HBr}(a q)+\mathrm{Ca}(\mathrm{OH})_{2}(a q) \longrightarrow\) (b) \(\mathrm{Cu}(\mathrm{OH})_{2}(s)+\mathrm{HClO}_{4}(a q) \longrightarrow\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}(\mathrm{~s})+\mathrm{HNO}_{3}(a q) \longrightarrow\)

Short answer

The net ionic equations for the given reactions are: (a) \(2 \mathrm{H^+}(a q) + 2\mathrm{OH^-}(a q) \longrightarrow 2\mathrm{H}_{2}\mathrm{O}(l)\) (b) \(\mathrm{Cu(OH)_2}(s) + 2\mathrm{H^+}(a q) \longrightarrow \mathrm{Cu^{2+}}(a q) + 2\mathrm{H}_{2}\mathrm{O}(l)\) (c) \(\mathrm{Al(OH)_3}(s) + 3\mathrm{H^+}(a q) \longrightarrow \mathrm{Al^{3+}}(a q) + 3\mathrm{H}_{2}\mathrm{O}(l)\)

Step-by-step solution

Balancing the molecular equations

First, we need to balance each given molecular equation: (a) For the first equation, the balanced molecular equation is: \(2 \mathrm{HBr}(a q)+\mathrm{Ca}(\mathrm{OH})_{2}(a q) \longrightarrow \mathrm{CaBr}_{2}(a q) + 2\mathrm{H}_{2}\mathrm{O}(l)\) (b) For the second equation, the balanced molecular equation is: \(\mathrm{Cu}(\mathrm{OH})_{2}(s)+2\mathrm{HClO}_{4}(a q) \longrightarrow \mathrm{Cu(ClO_4)_2}(a q) + 2\mathrm{H}_{2}\mathrm{O}(l)\) (c) For the third equation, the balanced molecular equation is: \(\mathrm{Al}(\mathrm{OH})_{3}\mathrm{(s)}+3\mathrm{HNO}_{3}(a q) \longrightarrow \mathrm{Al(NO_3)_3}(a q) + 3\mathrm{H}_{2}\mathrm{O}(l)\)

Writing the complete ionic equations

Now let's write the complete ionic equations: (a) \(2 \mathrm{H^+}(a q) + 2 \mathrm{Br^-}(a q) + \mathrm{Ca^{2+}}(a q) + 2\mathrm{OH^-}(a q) \longrightarrow \mathrm{Ca^{2+}}(a q) + 2\mathrm{Br^-}(a q) + 2\mathrm{H}_{2}\mathrm{O}(l)\) (b) \(\mathrm{Cu(OH)_2}(s) + 2\mathrm{H^+}(a q) + 2 \mathrm{ClO_4^-}(a q) \longrightarrow \mathrm{Cu^{2+}}(a q) + 2 \mathrm{ClO_4^-}(a q) + 2\mathrm{H}_{2}\mathrm{O}(l)\) (c) \(\mathrm{Al(OH)_3}(s) + 3\mathrm{H^+}(a q) + 3 \mathrm{NO_3^-}(a q) \longrightarrow \mathrm{Al^{3+}}(a q) + 3\mathrm{NO_3^-}(a q) + 3 \mathrm{H}_{2}\mathrm{O}(l)\)

Finding the net ionic equations

Now we can write the net ionic equation for each reaction: (a) \(2 \mathrm{H^+}(a q) + 2\mathrm{OH^-}(a q) \longrightarrow 2\mathrm{H}_{2}\mathrm{O}(l)\) (b) \(\mathrm{Cu(OH)_2}(s) + 2\mathrm{H^+}(a q) \longrightarrow \mathrm{Cu^{2+}}(a q) + 2\mathrm{H}_{2}\mathrm{O}(l)\) (c) \(\mathrm{Al(OH)_3}(s) + 3\mathrm{H^+}(a q) \longrightarrow \mathrm{Al^{3+}}(a q) + 3 \mathrm{H}_{2}\mathrm{O}(l)\) So, these are the net ionic equations for the given reactions.

Key concepts

Balancing Chemical Equations

Balancing chemical equations is crucial in chemistry to comply with the Law of Conservation of Mass, which states that matter cannot be created or destroyed in an isolated system. An unbalanced chemical equation does not reflect the same quantity of each element on both sides of the equation. To balance an equation, we adjust the coefficients—the numbers in front of the compounds or elements—without altering the actual formulas of the substances involved.

For instance, consider the reaction between hydrobromic acid (HBr) and calcium hydroxide (Ca(OH)₂). The initial unbalanced equation is simply HBr(aq) + Ca(OH)₂(aq) —>. To achieve balance, we need two hydrogen atoms and two bromine atoms on both sides. Therefore, we write 2HBr(aq) + Ca(OH)₂(aq) -> CaBr₂(aq) + 2H₂O(l). The coefficients 2 in front of HBr and H₂O ensure each element has the same number of atoms on both sides of the equation.

Complete Ionic Equations

Complete ionic equations show all of the ions present in a chemical reaction when dissolved in water. Unlike a molecular equation that shows compounds in their entire form, complete ionic equations break down aqueous compounds into their respective ions. Spectator ions, which are present but don't participate in the reaction, are usually omitted in the net ionic equation.

To illustrate, when hydrobromic acid (HBr) reacts with calcium hydroxide (Ca(OH)₂), breaking up the aqueous compounds into ions, we get: 2H⁺(aq) + 2Br^-(aq) + Ca²⁺(aq) + 2OH^-(aq) -> Ca²⁺(aq) + 2Br^-(aq) + 2H₂O(l). Here, Ca²⁺(aq) and Br^-(aq) are spectator ions and not included in the net ionic equation.

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationship between reactants and products in a chemical reaction. It is based on the balanced chemical equation and allows chemists to determine how much of each reactant is needed to produce a certain amount of product. The stoichiometric coefficients (the numbers in front of each species in the balanced equation) tell us the ratio in which reactants combine and products form.

Taking the balanced equation, 2HBr(aq) + Ca(OH)₂(aq) -> CaBr₂(aq) + 2H₂O(l), as an example, the coefficients indicate that 2 moles of hydrobromic acid react with 1 mole of calcium hydroxide to produce 1 mole of calcium bromide and 2 moles of water. This proportional relationship is foundational in predicting the amounts of substances consumed and produced in a given reaction.