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Chapter 15: Problem 31

For each of the given pairs, determine which solution is more acidic. All are water solutions. Explain your answer. (a) \(1 M \mathrm{HCl}\) or \(1 M \mathrm{H}_{2} \mathrm{SO}_{4}\) ? (b) \(1 \mathrm{M} \mathrm{HCl}\) or \(1 M \mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) ?

Short Answer

Expert verified
(a) 1 M H₂SO₄ is more acidic. (b) 1 M HCl is more acidic.

Step by step solution

01

Determine the Nature of Each Acid

Identify whether the acids are strong or weak. - HCl (hydrochloric acid) is a strong acid. - H₂SO₄ (sulfuric acid) is a strong acid. - HC₂H₃O₂ (acetic acid) is a weak acid.
02

Compare the First Pair

For the pair of HCl and H₂SO₄, considering both are strong acids: - HCl dissociates completely to give 1 mol of H⁺ ions per mole of HCl.- H₂SO₄ partially dissociates to give 2 mol of H⁺ ions per mole of H₂SO₄ (one in the first dissociation and one in the second dissociation). Since H₂SO₄ provides more H⁺ ions, it is more acidic.
03

Compare the Second Pair

For the pair of HCl and HC₂H₃O₂, considering one is a strong acid and the other is a weak acid: - HCl dissociates completely in water, yielding a high concentration of H⁺ ions. - HC₂H₃O₂ does not dissociate completely, yielding fewer H⁺ ions compared to HCl at the same concentration. Therefore, HCl is more acidic.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Strong Acids
Strong acids are acids that completely dissociate in water. This means they break up entirely into their ions when dissolved. For instance, hydrochloric acid (HCl) is a strong acid. When HCl is added to water, it dissociates completely into H⁺ and Cl⁻ ions.

This complete dissociation results in a high concentration of hydrogen ions (H⁺) in the solution. More hydrogen ions make the solution more acidic.

Examples of strong acids include:
  • Hydrochloric acid (HCl)
  • Sulfuric acid (H₂SO₄) - although its second hydrogen dissociation is not complete
  • Nitric acid (HNO₃)
Understanding the nature of strong acids helps us compare their acidic strength effectively.
Weak Acids
Weak acids do not dissociate completely in water. This means that only a small fraction of the acid molecules break into their ions. Most of the acid remains intact in the solution. An example of a weak acid is acetic acid (HC₂H₃O₂), commonly known as vinegar.

When acetic acid is dissolved in water, it partially dissociates into H⁺ and C₂H₃O₂⁻ ions. Because the dissociation is incomplete, the concentration of hydrogen ions (H⁺) is lower compared to a solution of a strong acid at the same concentration.

Examples of weak acids include:
  • Acetic acid (HC₂H₃O₂)
  • Formic acid (HCOOH)
  • Citric acid (C₆H₈O₇)
Knowing whether an acid is strong or weak helps us predict the acidity of its solution.
Dissociation in Water
Dissociation is the process by which an acid separates into ions when dissolved in water. For strong acids, dissociation is complete, resulting in many free H⁺ ions in the solution. For weak acids, dissociation is only partial, producing fewer H⁺ ions.

To understand dissociation better, consider these points:
  • Complete Dissociation: Strong acids like HCl dissociate fully, so in a 1 M solution of HCl, nearly all HCl molecules become H⁺ and Cl⁻ ions.
  • Partial Dissociation: Weak acids, such as acetic acid, dissociate only slightly. In a 1 M solution of acetic acid, most of the molecules remain as HC₂H₃O₂, with only a few breaking up into H⁺ and C₂H₃O₂⁻ ions.
This difference in dissociation explains why strong acids create more acidic solutions than weak acids at the same concentration. More H⁺ ions from complete dissociation mean a lower pH and higher acidity.

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Most popular questions from this chapter

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