Chapter 15

Consider the ammonium ion. (a) Write the chemical formula for this ion, complete with its charge. (b) Write the formula for ammonium chloride. (c) Ammonium chloride is an electrolyte. Explain why this is so.

Write a balanced equation to show what happens when \(\mathrm{CaBr}_{2}\) dissolves in water. Use the \((a q)\) symbol when necessary.

What do we mean by saying that some molecular compounds dissolve in water and also dissociate? Give an example of such a compound.

Two beakers each contain 1 L of water. Then \(0.1\) mole of HCl gas is dissolved in one beaker, and \(0.1\) mole of HF gas is dissolved in the other beaker. The HCl solution causes a light bulb to glow intensely, and the HF solution causes the same bulb to glow dimly. Explain these observations.

To which side does the dissociation equilibrium lie for a strong electrolyte? For a weak electrolyte?

Consider the three molecular compounds \(\mathrm{HCl}\), \(\mathrm{CH}_{3} \mathrm{COOH}\), and \(\mathrm{H}_{2} \mathrm{SO}_{4} \cdot\) (a) When they dissolve in water they all produce a common ion. What is it? (b) Why is it proper to call all three compounds electrolytes? (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) is the only weak electrolyte among the three compounds above. Write an equilibrium to show this using a set of unequal-length double arrows.

A particular molecular electrolyte has a dissociation equilibrium constant of \(8.2 \times 10^{-6}\). Is this electrolyte strong or weak? Explain.

A particular molecular electrolyte has a dissociation equilibrium constant of approximately \(10^{8} .\) Is it appropriate to ignore equilibrium for this electrolyte? Explain.

Write the formulas and names of two strong molecular electrolytes and two weak molecular electrolytes. For all four, write a balanced dissociation equation.

Name the acids: (a) \(\mathrm{HCl}\) (b) \(\mathrm{HNO}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) HF (e) \(\mathrm{CH}_{3} \mathrm{COOH}\) (f) \(\mathrm{NH}_{1} \mathrm{Cl}\).

True or false? The acidity of a given volume of 1.0 \(\mathrm{M}\) hydrofluoric acid is the same as that of the same volume of \(1.0 \mathrm{M}\) hydrochloric acid. Explain your answer.

What does diprotic mean when applied to an acid? Give an example, and show both dissociation equilibrium equations.

Go back in the chapter and examine the values of the equilibrium constants for the three dissociations of the triprotic phosphoric acid. Give a reason for why each successive dissociation yields a weaker acid. (Hint: Acids must give up a proton, and opposite charges attract.)

Other than water, which species would you expect to find in the highest concentration in an aqueous solution of \(\mathrm{CH}_{3} \mathrm{COOH}\) ? Explain.

\(\mathrm{HNO}_{3}\) is a strong acid, whereas \(\mathrm{HNO}_{2}\) is a weak acid. Likewise, \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid, whereas \(\mathrm{H}_{2} \mathrm{SO}_{3}\) is a weak acid. Likewise, \(\mathrm{HClO}_{3}\) is a strong acid, whereas \(\mathrm{HClO}_{2}\) and \(\mathrm{HClO}\) are weak acids. (a) After water, what would be the predominant species present in a solution of \(\mathrm{HNO}_{3} ?\) (b) After water, what would be the predominate species present in a solution of \(\mathrm{HNO}_{2} ?\) (c) For acids of the general formula \(\mathrm{HXO}_{n}\) where \(\mathrm{X}\) is a nonmetal atom (these are called oxy-acids), how does the acid strength depend on the value of \(n\) ?

Draw a dot diagram for acetic acid, and indicate which proton is the one that dissociates.

What is the value of the equilibrium constant for the dissociation of acetic acid's proton? What does this value tell you?

What is the value of the equilibrium constant for the dissociation of the first proton of sulfuric acid? What does this value tell you?

True or false? Because carbonic acid, \(\mathrm{H}_{2} \mathrm{CO}_{3}\), is diprotic, it is a stronger acid than \(\mathrm{HCl}\), which is only monoprotic. Explain your answer.

\text { What is the Arrhenius definition of a base? }

What is meant by acid-base neutralization? Write a net ionic equation to go along with your answer.

What kinds of compounds are typically strong bases?

When aqueous calcium hydroxide reacts with aqueous hydrochloric acid in a neutralization reaction to yield a neutral solution, the resulting solution is found to conduct electricity very well. (a) Explain why the neutral solution conducts electricity. (b) Write a complete ionic equation that goes along with your explanation in (a).

Consider the neutralization reaction that occurs upon the reaction of aqueous potassium hydroxide with aqueous hydrochloric acid. (a) Write a complete ionic equation for this neutralization reaction. (b) Write a net ionic equation for this neutralization reaction.

How many moles of \(\mathrm{Ba}(\mathrm{OH})_{2}\) does it take to neutralize \(0.50\) mole of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) ? Why does the resulting solution after the neutralization no longer conduct electricity? Why does the solution then conduct electricity after an excess of \(\mathrm{Ba}(\mathrm{OH})_{2}\) is added?

How many moles of \(\mathrm{Ba}(\mathrm{OH})_{2}\) does it take to neutralize \(2.5\) moles of \(\mathrm{HNO}_{3}\) ?

Write a balanced equation for what happens when gaseous \(\mathrm{NH}_{3}\) is dissolved in water.

Use the Bronsted-Lowry definition to explain why \(\mathrm{NH}_{3}\) is a base in water.

When ammonia gas is dissolved in water, is the water behaving as an acid, as a base, or neither? Explain.

Solid ammonium chloride, \(\mathrm{NH}_{4} \mathrm{Cl}\), reacts with solid sodium hydroxide to produce ammonia gas, water, and sodium chloride, \(\mathrm{NaCl}\). (a) Write a balanced equation for this reaction. (b) According to the Bronsted-Lowry definition, which species is the acid and which species is the base? Explain.

The equilibrium constant for the reaction \(\mathrm{NH}_{4}^{+}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftarrows \mathrm{NH}_{3}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\) is \(5.6 \times 10^{-10}\) (a) Is a solution of ammonium ion very acidic or only slightly acidic? (b) Is water acting as an acid or a base according to the Bronsted-I owry definition? Explain.

The carbonate ion, \(\mathrm{CO}_{3}^{2}\), is a weak base. (a) Write the equilibrium equation that shows how this ion makes water basic. (b) Which species is the acid and which species is the base according to the Brønsted-Lowry definition? (c) The equilibrium constant for the reaction between \(\mathrm{CO}_{3}^{2-}\) and water is \(2.1 \times 10^{-4}\) What does this tell you about the strength of Che 2 a base?

Hydride ion, \(\mathrm{H}\), is an exceptionally strong base, reacting with water to produce lots of hydroxide ion and \(\mathrm{H}_{2}\) gas. The \(K_{\mathrm{eq}}\) for this reaction is huge. (a) Write the balanced equation for the reaction between hydride and water. (b) Explain why \(\mathrm{H}_{2}\) gas forms. (Hint: Use the Bronsted-Lowry definition of base.)

The oxide ion, \(\mathrm{O}^{2-}\), present in sodium oxide \(\left(\mathrm{Na}_{2} \mathrm{O}\right)\) reacts violently with water to produce a highly basic solution. The hydride ion, \(\mathrm{H}^{-}\), in sodium hydride (NaH) does the same. (a) Write a balanced total ionic equation for the reaction of sodium oxide with water. (b) In terms of the Bronsted-Lowry definition, how are oxide and hydride similar? (c) What is it about the hydride and oxide ions that allow them to do what they do in water?

Ammonia \(\left(\mathrm{NH}_{3}\right)\) can act as a base in water. (a) Suppose methane \(\left(\mathrm{CH}_{4}\right)\) could also act as a base in water. Write a chemical equation to show this. (b) In fact, methane cannot act as a base. Why not? (Hint: One of the products you drew violates an important rule for carbon.) (c) By comparing \(\mathrm{NH}_{3}\) to \(\mathrm{CH}_{4}\), what can you say must exist on a species for it to serve as a Bronsted-Lowry base?

Ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), and hydride ion, \(\mathrm{H}^{-}\), react to produce \(\mathrm{H}_{2}\) gas and the \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\) anion. Arrhenius could not tell you what is going on, but Bronsted and Lowry would have no trouble. How would they explain this reaction?

How would the Bronsted-Lowry theory explain that ammonia is a weak base?

Acetic acid is a weak acid. Upon losing a proton, acetic acid yields the acetate anion. (a) Draw a dot diagram for the acetate anion. (b) Hydrogen atoms that are bound to carbon atoms are not acidic. Knowing this, is it possible for the acetate anion to serve as an acid? Explain. (c) In fact, acetate ion is a weak base. Write a chemical equilibrium with unequal-length arrows showing how acetate in water produces a weakly basic solution. (d) Is water acting as an acid or a base? Explain.

Why aren't ionic compounds like \(\mathrm{NaOH}\) and \(\mathrm{Mg}(\mathrm{OH})_{2}\) weak bases?

Are a \(1 \mathrm{M}\) solution of ammonia, \(\mathrm{NH}_{3}\), and a \(1 \mathrm{M}\) solution of lithium hydroxide, I iOH, equally basic, or is one solution more basic than the other? Explain.

In water, the hydrogen phosphate ion, \(\mathrm{HPO}_{4}^{2-}\), can act both as a weak base and as a weak acid. (a) Write an equilibrium equation that shows hydrogen phosphate acting as a weak acid in water. (b) Write an equilibrium equation that shows bisulfate acting as a weak base in water. (c) What information would you need to help you determine whether a solution of hydrogen phosphate is going to be slightly acidic or slightly basic? How would you use that information to find out?

Amines are organic compounds that contain an \(\mathrm{NH}_{2}\) group, and water-soluble amines are weak bases in water. For example, the compound methylamine, \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\), is a weak base. (a) Draw a dot diagram for methylamine. (b) Using dot diagrams, show the equilibrium reaction between methylamine and water. (c) To which side does the equilibrium in part (b) lie? What did we tell you that allowed you to figure out the answer? (d) The similar compound ethane, \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\), does not act as a weak base. Why can methylamine act as a weak base but \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\) can't? (Hint: Draw a dot diagram for \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}_{3}\).) (e) Is it appropriate to call methylamine an electrolyte? If so, is it weak or strong? Explain.

To be a weak base in water, a molecular compound must also be a weak electrolyte. What must be one of the ions it produces in water?

(a) List all the weak bases phosphoric acid, \(\mathrm{H}_{3} \mathrm{PO}_{4}\), can produce via successive losses of its protons. (b) Of the bases you listed, which has no ability to serve as a weak acid? (c) Phosphorous acid, \(\mathrm{H}_{3} \mathrm{PO}_{3}\), is a diprotic acid, even though there are three \(\mathrm{H}\) atoms in the formula. Draw a Lewis dot diagram for phosphorous acid. (Hint: Only H bound directly to \(\mathrm{O}\) is acidic).

Consider the autoionization of water. (a) What do we mean by autoionization of water (b) Write a balanced equation to go along with your explanation. (c) An alternate name for autoionization of water is autodissociation. Exactly how is water dissociating?

When water undergoes autoionization, is it serving as an acid, a base, or neither? Explain.

Does the equilibrium for water autoionization lie to the left or to the right? What constant verifies your answer?

Write the mathematical expression that allows you to solve for the \(\mathrm{OH}^{-}\) concentration in water when you know only the \(\mathrm{H}_{3} \mathrm{O}^{+}\) concentration.

Is it possible to obtain water at \(25{ }^{\circ} \mathrm{C}\) that contains absolutely no ions of any sort? Explain.

What is the molar concentration of hydronium ion and hydroxide ion in pure water at \(25{ }^{\circ} \mathrm{C}\) ?