Problem 13 - Chemistry Matter and Change

Question

Evaluate Aqueous phosphoric acid and aqueous calcium hydroxide react to form solid calcium phosphate and water. Write a balanced chemical equation for this reaction.

Step-by-Step Solution

Answer

The balanced chemical equation is 2 H₃PO₄ (aq) + 3 Ca(OH)₂ (aq) → Ca₃(PO₄)₂ (s) + 6 H₂O (l).

Step 1: Write the Unbalanced Equation

First, we determine the chemical formulas of the reactants and products: Phosphoric acid is H₃PO₄, calcium hydroxide is Ca(OH)₂, calcium phosphate is Ca₃(PO₄)₂, and water is H₂O. The unbalanced equation is: H₃PO₄ (aq) + Ca(OH)₂ (aq) → Ca₃(PO₄)₂ (s) + H₂O (l).

Step 2: Balance Phosphorus Atoms

In the reactants, we have 1 phosphorus atom in H₃PO₄, and in the products, we have 2 phosphorus atoms in Ca₃(PO₄)₂. To balance phosphorus, we need 2 molecules of H₃PO₄: 2 H₃PO₄ (aq) + Ca(OH)₂ (aq) → Ca₃(PO₄)₂ (s) + H₂O (l).

Step 3: Balance Calcium Atoms

In the reactants, there is 1 calcium atom in Ca(OH)₂, while in the products, there are 3 calcium atoms in Ca₃(PO₄)₂. To balance calcium, we need 3 molecules of Ca(OH)₂: 2 H₃PO₄ (aq) + 3 Ca(OH)₂ (aq) → Ca₃(PO₄)₂ (s) + H₂O (l).

Step 4: Balance Hydrogen Atoms

In the reactants, we now have 6 hydrogen atoms from 2 H₃PO₄ and 6 hydrogen atoms from 3 Ca(OH)₂, a total of 12 hydrogen atoms. In the products, each molecule of H₂O contains 2 hydrogen atoms, therefore, we need 6 H₂O to balance: 2 H₃PO₄ (aq) + 3 Ca(OH)₂ (aq) → Ca₃(PO₄)₂ (s) + 6 H₂O (l).

Step 5: Verify Oxygen Atoms

In the reactants, we have 8 oxygen atoms from 2 H₃PO₄ and 6 oxygen atoms from 3 Ca(OH)₂, a total of 14 oxygen atoms. In the products, we have 8 oxygen atoms in Ca₃(PO₄)₂ and 6 oxygen atoms in 6 H₂O, for a total of 14 oxygen atoms. The equation is balanced. The balanced equation is: 2 H₃PO₄ (aq) + 3 Ca(OH)₂ (aq) → Ca₃(PO₄)₂ (s) + 6 H₂O (l).

Key Concepts

Understanding Chemical Reactions

Chemical reactions are processes where reactant molecules transform into products through the breaking and formation of bonds. In the reaction between phosphoric acid and calcium hydroxide, the reactants are substances that interact to form new substances known as products.
This specific reaction occurs in an aqueous environment and results in the formation of solid calcium phosphate and water. During a chemical reaction:

• Atoms are rearranged but not created or destroyed.
• Bond energies can change, leading to potential energy changes.
• Reactions often absorb or release energy.

To describe what happens, we write chemical equations that show the reactants and products, separated by an arrow. Writing a balanced equation is crucial because it ensures the same number of each type of atom on both sides of the equation, adhering to the law of conservation of mass.

Exploring Stoichiometry

Stoichiometry is the study of the quantitative relationships between reactants and products in a chemical reaction. It allows you to predict the amount of product formed and the amounts of reactants needed. In our reaction, stoichiometry helps balance the equation:

• First, we find the correct formulas for all reactants and products.


• Identify and compare the number of atoms for each element in the reactants and products.


• Adjust the coefficients for each compound until the number of atoms for each element is balanced on both sides of the equation.

In the case of phosphoric acid and calcium hydroxide, the balanced equation serves as a stoichiometric map, showing the proportionate amounts needed and produced. Correct coefficients ensure that the ratios of the molecules are consistent with the rearrangement of matter in the chemical reaction.

An Insight Into Phosphoric Acid

Phosphoric acid, with the chemical formula H₃PO₄, is a common chemical used in a variety of applications like fertilizers, food flavoring, and industrial processes. It is a triprotic acid, meaning it can donate three hydrogen ions (H⁺) per molecule in a reaction.
This ability to donate multiple protons makes it a versatile reactant in many chemical reactions. In our specific balanced chemical equation, phosphoric acid reacts with a base, calcium hydroxide, to form calcium phosphate and water. This type of reaction is known as a neutralization reaction where an acid and a base react to create water and a salt.

• H₃PO₄ acts by neutralizing the hydroxide ions from Ca(OH)₂.
• In doing so, it helps produce the insoluble compound, calcium phosphate.

Understanding the properties and reactions of phosphoric acid is crucial in both academic studies and real-world applications.

The Role of Calcium Phosphate

Calcium phosphate, with the chemical formula Ca₃(PO₄)₂, is a common compound used in dietary supplements and fertilizers. It is also an important mineral in biological systems, particularly in bone formation. Within the context of our chemical reaction,

• Calcium phosphate is formed as a precipitate.
• It is the product of the reaction between calcium ions from Ca(OH)₂ and phosphate ions from H₃PO₄.


• The solid formation demonstrated in the equation highlights the reaction's completion as a precipitation reaction.

When combined with other compounds, calcium phosphate plays a crucial role in articulating reactions that involve the transfer of structural frameworks. Such understanding assists in fields as varied as pharmacology, agriculture, and materials science.