Acid-Base Equilibria: Chapter 14

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Questions and Answers

Explain why, according to the Arrhenius theory, $SO_3$ is neither an acid nor a base, despite reacting with water to produce $H^+$ ions.

Arrhenius theory defines acids as substances that produce $H^+$ ions in water and bases as substances that produce $OH^-$ ions in water. $SO_3$ does not contain H atoms, so it is not an acid.

How does the Bronsted-Lowry definition broaden the definition of acids and bases compared to the Arrhenius definition?

The Bronsted-Lowry definition defines acids as proton ($H^+$) donors and bases as proton acceptors, regardless of whether they produce $H^+$ or $OH^-$ ions in water directly. This allows for a wider range of substances to be classified as acids or bases.

In the reaction $NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$, identify the conjugate acid-base pairs.

$NH_3$ (base) and $NH_4^+$ (conjugate acid) form one pair, and $H_2O$ (acid) and $OH^-$ (conjugate base) form the other pair.

Explain why H+ is said to never exist alone in water.

H+ is a bare proton and is highly reactive. It immediately bonds with a water molecule to form a hydronium ion ($H_3O^+$). Signup and view all the answers

If the concentration of $H_3O^+$ in a solution is $1.0 \times 10^{-3}$ M, is the solution acidic, basic, or neutral? Briefly explain.

The solution is acidic. In an acidic solution, the concentration of $H_3O^+$ is greater than the concentration of $OH^-$, and is greater than $1.0 \times 10^{-7}$ M. Signup and view all the answers

If the hydroxide ion concentration $[OH^-]$ in a solution is $0.010$ M, calculate the hydronium ion concentration $[H_3O^+]$ and indicate whether the solution is acidic, basic, or neutral.

Using the ion product constant of water, $K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}$, we find $[H_3O^+] = 1.0 \times 10^{-14} / 0.010 = 1.0 \times 10^{-12}$ M. Since $[OH^-] > [H_3O^+]$, the solution is basic. Signup and view all the answers

Explain the relationship between pH and the concentration of hydronium ions ($H_3O^+$). How does an increase in hydronium ion concentration affect the pH?

pH is defined as the negative logarithm of the hydronium ion concentration: $pH = -log[H_3O^+]$. Therefore, an increase in $[H_3O^+]$ results in a lower pH value. Signup and view all the answers

What is the significance of the 'p' in pH, pOH, and pKa, and how does it relate to the value it precedes?

The 'p' indicates a negative logarithm (-log) of the value it precedes. For instance, pH is the negative logarithm of $[H_3O^+]$, pOH is the negative logarithm of $[OH^-]$, and pKa is the negative logarithm of Ka. Signup and view all the answers

Explain how to determine the number of significant figures in a pH value that is derived from a hydronium ion concentration.

For logarithms, only the numbers to the right of the decimal point are significant. So, if $[H_3O^+]$ has two significant figures, the pH will have two digits after the decimal point. Signup and view all the answers

Why is it usually sufficient to only consider the first ionization step when calculating the pH of a polyprotic acid solution?

Each subsequent ionization step is significantly weaker than the prior one ($K_{a1} > K_{a2} > K_{a3}$). The contribution of $H^+$ from later ionization steps is typically negligible compared to the first ionization. Signup and view all the answers

How does the strength of a binary acid change as you move from left to right across the periodic table? What trend explains this?

Acidity increases from left to right. This is due to increasing electronegativity, which makes the H atom more easily ionized. Signup and view all the answers

Explain how to determine the pH of a neutral salt solution.

Combinations of pH-neutral salts include: $Li^+$, $Na^+$, $K^+$, $Ca^{2+}$, $Sr^{2+}$, $Ba^{2+}$, $Cl^-$, $Br^-$, $I^-$, $NO_3^-$, $ClO_4^-$. Strong acids and bases (minus $H_2SO_4$): $LiOH$, $NaOH$, $KOH$, $Ca(OH)_2$, $Sr(OH)_2$, $Ba(OH)_2$, $HCl$, $HBr$, $HI$, $HNO_3$, $HClO_4$. Signup and view all the answers

What two factors influence the acidity of oxyacids?

The electronegativity of the central atom (Y) and the number of oxygen atoms bonded to the central atom (Y). Signup and view all the answers

Briefly explain the concept of percent ionization for a weak acid. How is it calculated?

Percent ionization is the percentage of a weak acid that ionizes in solution. It is calculated as the ratio of the equilibrium concentration of $H_3O^+$ to the initial concentration of the acid, multiplied by 100%. Signup and view all the answers

What are the main components of a buffer solution, and how do they work together to resist changes in pH?

A buffer solution consists of a weak acid and its conjugate base. The weak acid neutralizes added base, and the conjugate base neutralizes added acid, minimizing changes in pH. Signup and view all the answers

Explain the difference between buffer range and buffer capacity. Which factor determines the pH of the buffer, and which determines how much acid or base the buffer can neutralize?

Buffer range is the pH range over which the buffer is effective. Buffer capacity is the amount of acid or base the buffer can neutralize before its pH changes significantly. The $pK_a$ of the weak acid determines the pH, and the concentrations of the weak acid/conjugate base determine the capacity. Signup and view all the answers

What is the Henderson-Hasselbalch equation, and under what conditions is it most useful for calculating the pH of a buffer solution?

The Henderson-Hasselbalch equation is: $pH = pK_a + log \frac{[conjugate base]}{[weak acid]}$. It is most useful when the 'x is small' approximation is valid, i.e., when the changes in concentration due to ionization are small compared to the initial concentrations. Signup and view all the answers

Explain what is meant by the equivalence point in a titration. How does the pH at the equivalence point differ for titrations involving strong acids/bases versus weak acids/bases?

The equivalence point is when the moles of acid are stoichiometrically equal to the moles of base. For strong acid/strong base titrations, the pH = 7 at the equivalence point. For strong acid/weak base, pH < 7, and for weak acid/strong base, pH > 7. Signup and view all the answers

In a titration, what is the role of an indicator and what property makes it useful for determining the end point of the titration?

An indicator is a chemical that changes color when the pH changes. Its color change signals the end point of the titration, ideally coinciding with, or close to, the equivalence point. Signup and view all the answers

Describe the two-step procedure involved in calculating the pH curve for the titration of a weak acid with a strong base and why the approach is necessary.

The two-step procedure first involves a stoichiometry problem to determine the amount of weak acid and conjugate base formed after the addition of strong base. Then, an equilibrium problem, often using the Henderson-Hasselbalch equation, is solved to determine the pH. Signup and view all the answers

How does the strength of a weak acid influence its percent ionization, and what happens to the percent ionization as a weak acid solution is diluted?

Stronger weak acids have a higher percent ionization compared to weaker weak acids. As a weak acid solution is diluted, the percent ionization increases. Signup and view all the answers

When titrating a solution of $HCl$ with $NaOH$, what is the net ionic equation?

The net ionic equation is $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$. Signup and view all the answers

When carrying out a titration, should you use moles or millimoles in calculations?

You can use either moles or millimoles (mmol). Signup and view all the answers

Besides a strong acid and strong base, list some other components of pH-neutral salts:

Combinations of pH-neutral salts include: $Li^+$, $Na^+$, $K^+$, $Ca^{2+}$, $Sr^{2+}$, $Ba^{2+}$, $Cl^-$, $Br^-$, $I^-$, $NO_3^-$, $ClO_4^-$. Signup and view all the answers

In a titration of a weak acid with a strong base, is the solution acidic, basic, or neutral at the equivalence point?

In the titration of a weak acid with a strong base, the solution will be basic at the equivalence point. Signup and view all the answers

When calculating pH curves, what two types of problems will need to be solved in sequence in a weak acid-strong base titration?

A stoichiometry problem, followed by an equilibrium problem. Signup and view all the answers

In a titration of H2SO4, why is it important to use H2SO4 as [H3O+]0?

Because H2SO4 is an anomaly in that is it a strong acid in its first reaction, and then a good acid in its second reaction. Signup and view all the answers

If you are tasked to compute the pH of a substance, and there are two acid dissociation constants, $K_{a_1}$ and $K_{a_2}$, which one do you use?

You would use $K_{a_1}$. Signup and view all the answers

If you are given a substance known to contain bases, and it does not contain $OH^-$ ions, how can it still be considered a base?

Following Arrhenius theory, $NH_3$, $Na_2CO_3$ and $Na_2O$ do not contain $OH^-$ ions; However, they react in water to produce $OH^-$ ions. Signup and view all the answers

Why are bases said to feel slippery when touched?

Bases feel slippery because they react with the oils on your skin to form soap. Signup and view all the answers

What are the characteristics of acids?

Some characteristics of acids: sour taste, corrode / dissolve metals, neutralize bases, and turn litmus red. Signup and view all the answers

Is $H_20$ an acid or a base following the Bronsted-Lowry definition?

$H_20$ can be either an acid or a base, depending on the reaction. It is amphoteric. Signup and view all the answers

How does the addition of an acid influence pH levels?

To compute the pH, you must calculate $[H_3O^+]$ by subtracting moles of added $OH^-$ from the initial number of moles of $H_3O^+$. Signup and view all the answers

How does adding $0.01$ mol $HCl$ influence the acid, and how does that compare to blood?

Adding $0.01$ mol $HCl$ to $1L$ blood: pH 7.4 $\rightarrow$ 7.2. Adding $0.01$ mol $HCl$ to $1L$ water: pH 7.0 $\rightarrow$ 2.0 Signup and view all the answers

Explain the steps needed to produce a good buffer?

Effective buffers have: weak acids with $pK_a$ close to the desired pH, equal concentrations of weak acid and conjugate base, and higher concentrations of both acid and base. Signup and view all the answers

What happens to binary acids as you progress down the periodic table?

Acidity increases going down the PT due to decreasing bond strength. Signup and view all the answers

During an acid ($HA$) equilibrium reaction ($HA + H_2O \rightleftharpoons A^- + H_3O^+$), what does it mean if $Q < K$?

If $Q < K$, the system shifts to products ($H_3O^+$ and $A^-$). Signup and view all the answers

How can the strength of an acid be measured regarding the equilibrium constant?

Acid srength is measured by the size of the equilibrium constant in a reaction with $H_2O$. Signup and view all the answers

What happens in acid ($HA$) equilibrium reaction ($HA + H_2O \rightleftharpoons A^- + H_3O^+$) as the system is diluted?

Percent ionization increases with dilution. Signup and view all the answers

In polyprotic acids, what does it mean if $K_{a1} > K_{a2} > K_{a3}$?

Removing one H+ makes removal of next harder. Signup and view all the answers

In $H_2SO_4$, what can be applied from the second ionization?

For $H_2SO_4$ use $[H_2SO_4]$ as $[H_3O^+]_0$ in the 2nd ionization. Signup and view all the answers

What typically happens if you find yourself doing three ICE tables during a calculation?

If you find yourself incredibly bored, calculate $[PO_4^{3-}]$ in a 1.0 M solution of $H_3PO_4$. With 3 ICE tables, it's the Mount Everest of equilibrium problems. Signup and view all the answers

Flashcards

What is an acid?

A substance that tastes sour, corrodes metals, neutralizes bases, and turns litmus red.

What is a base?

A substance that tastes bitter, feels slippery, neutralizes acids, and turns litmus blue.