Question

Write a balanced molecular equation and a net ionic equation for the reaction that occurs when (a) solid \(\mathrm{CaCO}_{3}\) reacts with an aqueous solution of nitric acid; (b) solid iron(II) sulfide reacts with an aqueous solution of hydrobromic acid.

Short answer

(a) Balanced molecular equation: \( CaCO_{3}(s) + 2 HNO_{3}(aq) \rightarrow Ca(NO_{3})_{2}(aq) + H_{2}CO_{3}(aq) \) Net ionic equation: \( CaCO_{3}(s) + 2 H^{+}(aq) \rightarrow Ca^{2+}(aq) + CO_{2}(g) + H_{2}O(l) \) (b) Balanced molecular equation: \( FeS(s) + 2 HBr(aq) \rightarrow FeBr_{2}(aq) + H_{2}S(g) \) Net ionic equation: \( FeS(s) + 2 H^{+}(aq) \rightarrow Fe^{2+}(aq) + H_{2}S(g) \)

Step-by-step solution

Write the unbalanced molecular equation

First, write the chemical formulas for calcium carbonate (CaCO3) and nitric acid (HNO3). Then, predict the products of the reaction, which are calcium nitrate (Ca(NO3)2) and carbonic acid (H2CO3). Unbalanced equation: CaCO3(s) + HNO3(aq) → Ca(NO3)2(aq) + H2CO3

Balance the molecular equation

Balance the equation by adjusting the coefficients of the reactants and products. Balanced equation: CaCO3(s) + 2 HNO3(aq) → Ca(NO3)2(aq) + H2CO3(aq)

Write the net ionic equation

Separate all strong electrolytes into their respective ions and remove spectator ions to obtain the net ionic equation. Note that carbonic acid, H2CO3, will decompose into water (H2O) and carbon dioxide (CO2). Net ionic equation: CaCO3(s) + 2 H+(aq) → Ca2+(aq) + CO2(g) + H2O(l) (b) Solid iron(II) sulfide reacts with an aqueous solution of hydrobromic acid

Write the unbalanced molecular equation

First, write the chemical formulas for iron(II) sulfide (FeS) and hydrobromic acid (HBr). Then, predict the products of the reaction, which are iron(II) bromide (FeBr2) and hydrogen sulfide (H2S). Unbalanced equation: FeS(s) + HBr(aq) → FeBr2(aq) + H2S

Balance the molecular equation

Balance the equation by adjusting the coefficients of the reactants and products. Balanced equation: FeS(s) + 2 HBr(aq) → FeBr2(aq) + H2S(g)

Write the net ionic equation

Separate all strong electrolytes into their respective ions and remove spectator ions to obtain the net ionic equation. Net ionic equation: FeS(s) + 2 H+(aq) + 2 Br-(aq) → Fe2+(aq) + 2 Br-(aq) + H2S(g) After removing the spectator ions 2 Br-(aq), we get the final net ionic equation: Net ionic equation: FeS(s) + 2 H+(aq) → Fe2+(aq) + H2S(g)

Key concepts

Balancing Equations

In chemistry, balancing equations is a crucial skill. It ensures that the chemical equation follows the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction.
This means that the number of atoms for each element must be the same on both sides of the equation.

• Start by writing down the unbalanced equation with the correct chemical formulas for the reactants and products.
• Identify elements that are unbalanced by comparing the number of atoms of each element on both sides of the equation.
• Use coefficients to balance the atoms, starting with the most complex molecule.
• Ensure that each side of the equation has equal numbers of each type of atom.
• Finally, check your work to confirm that the equation is balanced.

Balancing chemical equations is essential for understanding the stoichiometry of a reaction, which refers to the proportional relationship between reactants and products in a chemical reaction.

Net Ionic Equations

Net ionic equations are used to highlight the chemical species that are directly involved in a reaction. This is particularly useful for understanding reactions in solutions where many ions do not participate in the reaction.
To write a net ionic equation, follow these steps:

• Write the balanced molecular equation including all reactants and products.
• Identify the strong electrolytes, which are substances that completely dissociate into ions in solution.
• Separate these strong electrolytes into their constituent ions.
• Identify and cancel out the spectator ions, which are ions that appear on both sides of the equation and do not change.
• The remaining ions represent the net ionic equation.

Net ionic equations can simplify complex chemical equations by focusing on the actual chemical changes occurring.

Molecular Equations

Molecular equations represent the complete chemical formulas of all reactants and products in a chemical reaction. They provide a macroscopic view of the reaction as opposed to the ionic view.

• Write the formulas of each reactant and product in the reaction, maintaining their chemical integrity.
• Ensure the correct representation of physical states: (s) for solids, (l) for liquids, (g) for gases, and (aq) for aqueous solutions.
• Molecular equations don't reflect the ionic nature of compounds in solution; they focus on the overall chemical change.

Despite being less precise about ionic dissociation, molecular equations are the starting point for deriving net ionic equations, as they outline the reactants and products involved.

Acid-Base Reactions

Acid-base reactions are a type of chemical reaction that involves the transfer of protons ( H+ ) between reactants.

• Acids can donate protons, while bases can accept protons during the reaction.
• Acid-base reactions are essential for many biological and environmental processes.
• These reactions often result in the formation of water and a salt.
• A key concept in acid-base chemistry is the understanding of strong and weak acids and bases.
• Strong acids and bases fully dissociate into ions in solution, while weak acids and bases do not.

Balancing acid-base reactions involves ensuring that you account for all protons transferred. By simplifying to net ionic equations, you can see the exchange of protons more clearly, offering insights into the nature of the reaction.