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Chapter 4: Problem 203

Match the following: List II (Heat of List I \(\quad\) neutralization) A. \(\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{NaOH}\) (p) \(13.7 \mathrm{Kcal}\) B. \(\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{NH}_{4} \mathrm{OH}\) \((\mathrm{q})>13.7 \mathrm{kcal}\) C. \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{NH}_{4} \mathrm{OH}\) \((\mathrm{r})<13.7 \mathrm{kcal}\) D. \(\mathrm{NaOH}+\mathrm{HF}\) (s) cannot be said

Short Answer

Expert verified
A _{(p)}, B _{(q)}, C _{(r)}, D _{(s)}.

Step by step solution

01

Understand the Heat of Neutralization

The heat of neutralization is the heat change that occurs when an acid and a base react to form water and a salt. The heat of neutralization for strong acids and strong bases is typically around 13.7 kcal because the reaction mainly involves the formation of water from H鈦� and OH鈦� ions. However, for weak acids or weak bases, the heat of neutralization can be less, as energy is also used to ionize the weak acid or base.
02

Match A - Sulfuric Acid and Sodium Hydroxide

Sulfuric acid (_H_2_SO_4_) and sodium hydroxide (_NaOH_) are both strong, so the reaction between them will have a heat of neutralization close to 13.7 kcal. Therefore, _{A ightarrow (p)}.
03

Match B - Sulfuric Acid and Ammonium Hydroxide

Sulfuric acid (_H_2_SO_4_) is strong, but ammonium hydroxide (_NH_4_OH_) is weak, which means the ionization of _NH_4_OH_ uses additional heat energy. This results in a heat of neutralization greater than 13.7 kcal. Therefore, _{B ightarrow (q)}.
04

Match C - Acetic Acid and Ammonium Hydroxide

Acetic acid (_CH_3_COOH_) and ammonium hydroxide (_NH_4_OH_) are both weak. The heat of neutralization will be much less than 13.7 kcal as energy is required to dissociate both weak species. Hence, _{C ightarrow (r)}.
05

Match D - Sodium Hydroxide and Hydrofluoric Acid

Hydrofluoric acid (_HF_) is a weak acid. Predicting its heat of neutralization is complicated due to its anomalous behavior, so the heat of neutralization can't be easily stated. Hence, _{D ightarrow (s)}.

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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 a type of acid that completely dissociate in water. This means they ionize fully, releasing all available hydrogen ions (H鈦�) into the solution. Strong acids include common examples like sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)), hydrochloric acid (\( \mathrm{HCl} \)), and nitric acid (\( \mathrm{HNO}_3 \)).
When a strong acid reacts with a strong base, the heat of neutralization is fairly consistent, commonly around 13.7 kcal/mol.
  • This constancy occurs because the reaction predominantly involves the combination of H鈦� ions from the acid with OH鈦� ions from the base, forming water (\( \mathrm{H}_2\mathrm{O} \)).
  • Due to complete ionization, no additional energy is needed to break apart the acid into its ions, unlike with weak acids.
Understanding this helps in examining reactions involving strong acids where consistent energy changes are expected.
Weak Acids
Weak acids are characterized by their partial ionization in water. Unlike strong acids, they do not dissociate completely, releasing only a fraction of the hydrogen ions into the solution. Examples of weak acids include acetic acid (\( \mathrm{CH}_3\mathrm{COOH} \)) and hydrofluoric acid (\( \mathrm{HF} \)).
Because weak acids do not fully ionize:
  • More energy is required to ionize the remaining molecules during a neutralization reaction.
  • This results in a lower heat of neutralization compared to strong acids.
The unique behavior of weak acids affects their reactions with both strong and weak bases, often resulting in a heat of neutralization that is less than the standard 13.7 kcal/mol.
Strong Bases
Strong bases are substances that fully ionize and release hydroxide ions (OH鈦�) in a solution. Examples include sodium hydroxide (\( \mathrm{NaOH} \)) and potassium hydroxide (\( \mathrm{KOH} \)). When a strong base neutralizes a strong acid, the reaction is straightforward and primarily involves the combination of H鈦� and OH鈦� ions to form water.
  • This process is highly efficient and requires no additional energy to break apart base molecules.
  • The heat of neutralization for such reactions often remains around 13.7 kcal/mol, reflecting the direct interaction of fully available ions.
In reactions involving weak acids, a strong base might influence the overall energy change due to differences in ionization levels.
Weak Bases
Weak bases only partially ionize in water, producing fewer hydroxide ions (OH鈦�) compared to strong bases. A common example is ammonium hydroxide (\( \mathrm{NH}_4\mathrm{OH} \)).
In a neutralization reaction:
  • The heat of neutralization involving a weak base tends to be higher than expected if paired with a strong acid. This is because additional energy is required to dissociate weak base molecules.
  • When both the acid and base are weak, even more energy is needed, often resulting in a much lower heat of neutralization.
Recognizing the role of weak bases in neutralization helps explain why heat changes deviate from standard values seen with strong acids and bases.

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Most popular questions from this chapter

Which of \(\mathrm{CH}_{4}\) (g), \(\mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{~g})\) and \(\mathrm{CH}_{3} \mathrm{OH}\) (I) provides the most energy per gram upon combustion and which provides the least? $$ \begin{array}{r} \mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(l) \\ \Delta \mathrm{H}^{\circ}=-890 \mathrm{~kJ} \end{array} $$ \(2 \mathrm{C}_{2} \mathrm{H}_{2}(\mathrm{~g})+5 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 4 \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(l)\) $$ \Delta \mathrm{H}^{\circ}=-2599 \mathrm{~kJ} $$ \(2 \mathrm{CH}_{3} \mathrm{OH}(\mathrm{l})+3 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{~g})+4 \mathrm{H}_{2} \mathrm{O}(l)\) $$ \Delta \mathrm{H}^{\circ}=-1453 \mathrm{~kJ} $$ a. \(\mathrm{C}_{2} \mathrm{H}_{2}\) provides the most energy per gram and \(\mathrm{CH}_{3} \mathrm{OH}\) the least. b. \(\mathrm{C}_{2} \mathrm{H}_{2}\) provides the most energy per gram and \(\mathrm{CH}_{4}\) the least. c. \(\mathrm{CH}_{4}\) provides the most energy per gram and \(\mathrm{C}_{2} \mathrm{H}_{2}\) the least. d. \(\mathrm{CH}_{4}\) provides the most energy per gram and \(\mathrm{CH}_{3} \mathrm{OH}\) the least.

The enthalpy change for a reaction depends upon: a. The differences in initial or final temperatures of involved substances b. The nature of intermediate reaction steps c. Use of different reactants for the same product d. The physical states of reactants and products

Correct statement among the following is/are a. mass plus energy of the universe remains always constant while entropy of the universe remains increasing continuously b. an exothermic reaction with \(\Delta \mathrm{S}\) being positive, will be spontaneous only at high temperature c. in a reversible process the system always in equilibrium with surroundings d. in any cyclic process \(\Delta \mathrm{X}=0\) where Xs a state fuction.

In thermodynamics, a process is called reversible when a. surroundings and system change into each other b. there is no boundary between system and surroundings c. the surroundings are always in equilibrium with the system d. the system changes into the surroundings spontaneously [IIT 2001]

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