Question

It takes one molecule of \(\mathrm{NaOH}\) to neutralize one molecule of \(\mathrm{HCl}\), but it takes two molecules of \(\mathrm{NaOH}\) to neutralize one molecule of \(\mathrm{H}_{2} \mathrm{SO}_{4}\). Explain why this is true.

Short answer

The difference in the number of NaOH molecules needed to neutralize HCl and H₂SO₄ is due to the fact that HCl has one hydrogen ion (H⁺) per molecule, while H₂SO₄ has two hydrogen ions (H⁺) per molecule. Therefore, it takes two molecules of NaOH, each providing one hydroxide ion (OH⁻), to neutralize the two hydrogen ions (H⁺) in one molecule of H₂SO₄. Conversely, only one molecule of NaOH is needed to neutralize the single hydrogen ion (H⁺) in one molecule of HCl.

Step-by-step solution

Identify the chemical reaction with HCl

When sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl), the reaction can be represented by the following balanced chemical equation: \[ \mathrm{NaOH} + \mathrm{HCl} \rightarrow \mathrm{NaCl} + \mathrm{H}_{2} \mathrm{O} \] In this reaction, one hydroxide ion (OH⁻) from the NaOH reacts with one hydrogen ion (H⁺) from the HCl, forming one molecule of water (H₂O). The sodium ions (Na⁺) and the chloride ions (Cl⁻) combine to form sodium chloride (NaCl) as a product.

Identify the chemical reaction with H₂SO₄

When sodium hydroxide (NaOH) reacts with sulfuric acid (H₂SO₄), the reaction can be represented by the following balanced chemical equation: \[ 2 \mathrm{NaOH} + \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4} + 2 \mathrm{H}_{2} \mathrm{O} \] In this reaction, two hydroxide ions (OH⁻) from two molecules of NaOH react with two hydrogen ions (H⁺) from one molecule of H₂SO₄, forming two molecules of water (H₂O). The sodium ions (Na⁺) from the two NaOH molecules and the sulfate ion (SO₄²⁻) from the H₂SO₄ combine to form sodium sulfate (Na₂SO₄) as a product.

Compare the number of H⁺ ions in each reaction

In the reaction with HCl, there is only one hydrogen ion (H⁺) in the hydrochloric acid molecule that needs to be neutralized. This means that one molecule of NaOH, with its one OH⁻ ion, is enough to neutralize the acid. On the other hand, in the reaction with H₂SO₄, there are two hydrogen ions (H⁺) in the sulfuric acid molecule that need to be neutralized. In this case, it takes two molecules of NaOH, each providing one OH⁻ ion, to neutralize the acid.

Conclusion

The difference in the number of NaOH molecules needed to neutralize HCl and H₂SO₄ is because HCl contains only one hydrogen ion (H⁺) per molecule, while H₂SO₄ contains two hydrogen ions (H⁺) per molecule. As a result, it takes two molecules of NaOH to provide enough hydroxide ions (OH⁻) to neutralize the two hydrogen ions (H⁺) in one molecule of H₂SO₄, while only one molecule of NaOH is needed to neutralize the single hydrogen ion (H⁺) in one molecule of HCl.

Key concepts

Chemical Reactions

Chemical reactions involve the breaking and forming of bonds between atoms to produce new substances. A crucial aspect of understanding these reactions is identifying the reactants, which are the starting materials, and the products, which are the substances formed. In an acid-base neutralization reaction like the ones involving \(\text{NaOH, HCl,}\) and \(\text{H}_{2}\text{SO}_{4}\), the acid (\(\text{HCl or H}_{2}\text{SO}_{4}\)) reacts with the base (\(\text{NaOH}\)) to eliminate acidic properties, resulting in the formation of water and a salt. To put it simply:

• An acid and a base react to form salt and water.
• Hydrogen ions from the acid combine with hydroxide ions from the base to make water \((\text{H}_2\text{O})\).
• Sodium ions from the base combine with the other ions from the acid to form a salt, like \(\text{NaCl}\) or \(\text{Na}_2\text{SO}_4\).

The steps and sequence of how these ions combine help to determine the specific outcomes and products of the reaction, and this forms the foundation of the chemical reaction process.

Stoichiometry

Stoichiometry, a core concept in chemistry, deals with the quantitative relationships between the amounts of reactants and products in a chemical reaction. It helps determine how much of each substance is needed or produced in a reaction. In our acid-base neutralization context:

• When \(1\) mole of \(\text{HCl}\) reacts with \(1\) mole of \(\text{NaOH}\), \(1\) mole of water and \(1\) mole of \(\text{NaCl}\) are produced.
• For \(\text{H}_{2}\text{SO}_{4}\), since it contains \(2\) hydrogen ions, \(2\) moles of \(\text{NaOH}\) are required.
• The reaction produces \(2\) moles of water and \(1\) mole of \(\text{Na}_{2}\text{SO}_{4}\).

Through stoichiometry, we quantify the precise amounts of reactants needed, helping us predict the outcomes accurately. This concept provides the mathematical framework to balance chemical equations, convert units, and ensure all reactants are fully utilized.

Balanced Chemical Equations

Balanced chemical equations are fundamental in expressing the actual stoichiometry of a chemical reaction. They ensure that the number of atoms of each element is the same on both sides of the reaction equation. This follows the Law of Conservation of Mass, where matter is neither created nor destroyed in a chemical reaction.For instance:

• The balanced equation \(\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}\) shows an equal number of each type of atom on both sides.
• In the equation \(2\text{NaOH} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{Na}_{2}\text{SO}_{4} + 2\text{H}_{2}\text{O}\), doubling the \(\text{NaOH}\) ensures all hydrogen ions are neutralized, balancing the equation concerning all involved elements.

Balancing helps to convey that the reaction has been accurately represented, reflecting the exact proportions as dictated by stoichiometry. Without balance, our understanding of the chemical process and prediction of reactant and product quantities would be flawed.