Problem 20 - Chemistry and Chemical Reactivity

Question

Complete and balance the following acid-base reactions. Name the reactants and products. (a) \(\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+\mathrm{KOH}(\mathrm{aq}) \longrightarrow\) (b) \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}(\mathrm{aq})+\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{s}) \longrightarrow\) $$\left(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\right.$$ is oxalic acid, an acid capable of donating two $$\left.\mathrm{H}^{+} \text {ions. }\right)$$

Step-by-Step Solution

Answer

(a) Phosphoric acid + potassium hydroxide → potassium phosphate + water. (b) Oxalic acid + calcium hydroxide → calcium oxalate + water.

Step 1: Identify Reactants and Predict Products for Reaction (a)

Reaction (a) involves phosphoric acid, \( \mathrm{H}_3\mathrm{PO}_4 \), and potassium hydroxide, \( \mathrm{KOH} \). Phosphoric acid is triprotic, meaning it can donate up to three hydrogen ions, while potassium hydroxide is a strong base. The reaction between an acid and a base will produce a salt and water. The products of this reaction will be water and potassium phosphate, \( \mathrm{K}_3\mathrm{PO}_4 \), according to the formula: \[ \mathrm{H}_3\mathrm{PO}_4(\mathrm{aq}) + 3\mathrm{KOH}(\mathrm{aq}) \rightarrow \mathrm{K}_3\mathrm{PO}_4(\mathrm{aq}) + 3\mathrm{H}_2\mathrm{O}(\ell) \]

Step 2: Balance Reaction (a)

Let's balance the chemical equation from Step 1. The balanced equation should have equal numbers of each type of atom on both sides. Starting with the phosphoric acid and potassium hydroxide, we write:\[ \mathrm{H}_3\mathrm{PO}_4(\mathrm{aq}) + 3\mathrm{KOH}(\mathrm{aq}) \rightarrow \mathrm{K}_3\mathrm{PO}_4(\mathrm{aq}) + 3\mathrm{H}_2\mathrm{O}(\ell) \] The equation is already balanced: 1 phosphorus, 3 potassium, 4 oxygen, and 6 hydrogen atoms are present on each side.

Step 3: Identify Reactants and Predict Products for Reaction (b)

Reaction (b) involves oxalic acid, \( \mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4 \), and calcium hydroxide, \( \mathrm{Ca(OH)}_2 \). Oxalic acid can donate two hydrogen ions per molecule. The resulting products of this acid-base reaction would be calcium oxalate, \( \mathrm{CaC}_2 \mathrm{O}_4 \), and water, which results from the neutralization reaction.\[ \mathrm{H}_2\mathrm{C}_2\mathrm{O}_4(\mathrm{aq}) + \mathrm{Ca(OH)}_2(\mathrm{s}) \rightarrow \mathrm{CaC}_2\mathrm{O}_4(\mathrm{s}) + 2 \mathrm{H}_2\mathrm{O}(\ell) \]

Step 4: Balance Reaction (b)

Now, balance the equation from Step 3 by ensuring an equal number of each type of atom on both sides of the equation.Starting with the oxalic acid and calcium hydroxide, our balanced chemical equation becomes:\[ \mathrm{H}_2\mathrm{C}_2\mathrm{O}_4(\mathrm{aq}) + \mathrm{Ca(OH)}_2(\mathrm{s}) \rightarrow \mathrm{CaC}_2\mathrm{O}_4(\mathrm{s}) + 2 \mathrm{H}_2\mathrm{O}(\ell) \] This equation is now balanced: 1 calcium, 2 carbons, 5 oxygens, and 4 hydrogens are on each side.

Step 5: Name the Reactants and Products

For reaction (a):- Reactants: Phosphoric acid (\( \mathrm{H}_3\mathrm{PO}_4 \)), potassium hydroxide (\( \mathrm{KOH} \)).- Products: Potassium phosphate (\( \mathrm{K}_3\mathrm{PO}_4 \)), water (\( \mathrm{H}_2\mathrm{O} \)).For reaction (b):- Reactants: Oxalic acid (\( \mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4 \)), calcium hydroxide (\( \mathrm{Ca(OH)}_2 \)).- Products: Calcium oxalate (\( \mathrm{CaC}_2 \mathrm{O}_4 \)), water (\( \mathrm{H}_2\mathrm{O} \)).

Key Concepts

Chemical Equations

Chemical equations are symbolic representations of chemical reactions, illustrating the transition of reactants to products. Each equation is expressed in terms of the molecular formulas of the substances involved. For example, when phosphoric acid reacts with potassium hydroxide, the reaction can be written as:

• Reactants: \( \mathrm{H}_3\mathrm{PO}_4 + 3\mathrm{KOH} \)
• Products: \( \mathrm{K}_3\mathrm{PO}_4 + 3 \mathrm{H}_2\mathrm{O} \)

Balancing a chemical equation is crucial. It ensures that the same number of each type of atom is present on both sides of the equation. This reflects the law of conservation of mass, indicating that atoms are neither created nor destroyed in chemical reactions. To balance an equation, adjust the coefficients before each formula to match the number of atoms in the reactants and products.

Phosphoric Acid

Phosphoric acid, represented by the chemical formula \( \mathrm{H}_3\mathrm{PO}_4 \), is a triprotic acid. Triprotic means it can donate three hydrogen ions \( \mathrm{H}^+ \). This property makes phosphoric acid versatile in various reactions.In acid-base reactions, phosphoric acid reacts with bases like potassium hydroxide. It forms a salt called potassium phosphate \( \mathrm{K}_3\mathrm{PO}_4 \) and water. This specific reaction showcases the triprotic nature as three hydrogen ions are replaced by potassium ions. Phosphoric acid is well recognized for its role in fertilizers and in the food and beverage industry as a food additive. Its ability to donate multiple protons makes it essential in chemistry, especially in buffer solutions.

Oxalic Acid

Oxalic acid, denoted by \( \mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4 \), is a dicarboxylic acid. Being dicarboxylic means it has two carboxyl groups \( \mathrm{COOH} \) and can donate two protons. Oxalic acid is found in many plants, including spinach and rhubarb.In chemical reactions, oxalic acid can effectively neutralize strong bases like calcium hydroxide \( \mathrm{Ca(OH)}_2 \). The reaction between oxalic acid and calcium hydroxide forms calcium oxalate \( \mathrm{CaC}_2 \mathrm{O}_4 \), a water-insoluble salt, and water. Calcium oxalate is considered significant in medical contexts, such as kidney stone formation. In an educational scenario, examining oxalic acid reactions helps illustrate the fundamental concepts of acid-base interactions and provides real-world applications, such as in cleaning agents or rust removers due to the acidic properties.

Neutralization Reaction

Neutralization reactions are a class of chemical reactions where an acid and a base react to form water and a salt. This process typically occurs in aqueous solutions. A classic example includes the reaction between oxalic acid and calcium hydroxide.

• Reactants: Acid \( \mathrm{H}_2 \mathrm{C}_2 \mathrm{O}_4 \) and Base \( \mathrm{Ca(OH)}_2 \)
• Products: Calcium oxalate \( \mathrm{CaC}_2 \mathrm{O}_4 \) and Water \( 2\mathrm{H}_2\mathrm{O} \)

The essence of a neutralization reaction lies in the ability of acids and bases to effectively "cancel" one another out, producing water—and often a salt. This concept is essential in various areas of science and industry. It is fundamental in titration, which chemists use to determine the concentration of an unknown solution. Understanding neutralization allows students to grasp how different substances interact, which is essential for further studies in chemistry and related fields.