acid base

Question 1

Define an Arrhenius acid and base. Explain the limitation of the Arrhenius model for acids and bases.

Answer 1

An Arrhenius acid is one that increases the proton concentration in water. An Arrhenius base is one that increases the hydroxide concentration in water.

Arrehenius model is limited because it restricts our view of acids and bases to only reactions involving water. This limits the number of reactions that can be perceived to be of acid/base character.

Question 2

Define a Bronsted/Lowry acid and base.

Answer 2

A Bronsted/Lowry (BL) acid is one that donates a proton in a chemical reaction. A BL base is one that accepts a proton in a chemical reaction.

This is a functional definition, that is, it is based on the “behaviour” of a chemical in a reaction. It is therefore possible for a chemical to act as a BL acid in one reaction and a BL base in another.

Also note that this definition implies that for a chemical to act as a BL acid, there must be another chemical that can serve as a BL base.

Question 3

Define conjugate acid/base pair. Why is it important to recognize such a pair?

Answer 3

A conjugate acid base pair are a pair of molecules/ions which differ by the presence of a single proton. For example, H₃O⁺ and H₂O. In this example, H₃O⁺ is the conjugate acid and H₂O is the conjugate base.

It is important to recognize conjugate acid base pairs because knowing the properties of the conjugate acid, will in turn tell us the property of the conjugate base. For example, HBr is a strong acid and therefore its conjugate base (Br^-) is a negligible base.

Question 4

Define amphoteric.

Answer 4

We say a molecule/ion is amphoteric when it can serve as both a BL acid and base. In a particular chemical reaction, a single molecule or ion does not simultaneously donate and accept a proton, but rather an amphoteric molecule/ion has the potential to do both. Hydrogen carbonate (HCO3-) is a good example of an amphoteric ion. In the presence of a strong base, it will donate a proton.

HCO₃^- + OH^- → CO₃^2- + H₂O

But in the presence of H₂O, it is more likely to accept a proton.

HCO₃^- + H₂O ⇔ H₂CO₃ + OH^-

Question 5

What is the autoionization of water? Explain why this reaction is important to understanding acid base chemistry.

Answer 5

The autoionization of water is a reaction that occurs in pure water and in ALL water solutions.

H₂O + H₂O ⇔ H₃O⁺ + OH^- ; K_w = 1x10^-14 (at 25°C)

It is this reaction that dictates the relative concentrations of H₃O⁺ and OH^- in every aqueous situation according to the equation: [H₃O⁺][OH^-] = K_w

If we have pure water at 25°C, then [H₃O⁺] = [OH^-] = 1 x10^-7 M. This relationship forms the basis of the pH scale, where pH = -log [H₃O⁺] = 7.00 for neutral water.

For every non-neutral water solution, then [H₃O⁺] > [OH^-] (acidic) or [H₃O⁺] < [OH^-] (basic), but the equation:

[H₃O⁺][OH^-] = K_w still applies.

Question 6

Define basic solution.

Answer 6

This is a solution where [H₃O⁺] -]. This is different than labelling the solution as an base. The solution may contain BL acids and bases - and collectively all we know is that [H₃O⁺] -].

Question 7

Define acidic solution.

Answer 7

This is a solution where [H₃O⁺] > [OH^-]. This is different than labelling the solution as an acid. The solution may contain BL acids and bases - and collectively all we know is that [H₃O⁺] > [OH^-].

Question 8

Define neutral solution.

Answer 8

This is a solution where [H₃O⁺] = [OH^-]. This is different than labelling the solution as pure water. The solution may contain BL acids and bases - and collectively all we know is that [H₃O⁺] = [OH^-].

Question 9

Define strong acid. List the “super six”.

Answer 9

A strong acid (HA) is one that is essentially 100% efficient in donating its proton to water.

HA + H₂O → H₃O⁺ + A^-

In Chem 122, we assume this reaction is so efficient we do not report an equilibrium constant. The “super six” (the strong acids you need to know) are:

1. HCl
2. HBr
3. HI
4. HNO₃
5. H₂SO₄
6. HClO₄

Question 10

Define weak acid. Provide some common examples.

Answer 10

A weak acid (HA) is one that is only partially effective in donating its proton to water.

HA + H₂O ⇔ H₃O⁺ + A^-

Rather than 100% efficient, this is usually only 5% or less efficient. We usually describe the strength of a weak acid in terms of the the equilibrium constant for the above reaction, called the K_a. The K_a can be calculated using the following expression:

K_a = [H₃O⁺] [A^-]/ [HA]

Typical examples of weak acids are HF, CH₃COOH (carboxylic acids), and NH₄⁺.

Question 11

Define strong base. List the “super nine”.

Answer 11

A strong base (MOH) is an ionic compound that is essentially 100% efficient in ionizing in water.

MOH → M⁺ + OH^-

Note that this definition is not consistent with the BL model. The main reason for this discrepancy is that the common strong bases are ionic compounds. In Chem 122, we assume this ionization is so efficient we do not report an equilibrium constant. The “super nine” (the strong bases you need to know) are:

1. LiOH
2. NaOH
3. KOH
4. RbOH
5. CsOH
6. Mg(OH)₂
7. Ca(OH)₂
8. Sr(OH)₂
9. Ba(OH)₂

Question 12

Define weak base. Provide some common examples.

Answer 12

A weak acid (B) is one that is only partially effective in accepting a proton from water.

B + H₂O ⇔ OH^- + BH⁺

Rather than 100% efficient, this is usually only 5% or less efficient. We usually describe the strength of a weak base in terms of the the equilibrium constant for the above reaction, called the K_b. The K_b can be calculated using the following expression:

K_b = [OH^-] [BH⁺]/ [B]

Typical examples of weak bases are F^- (anions - which are not negligible bases), CH₃NH₂ (amines), and NH₃ (ammonia).

Question 13

Define negligible base. List the six common examples. Why is it important to know the negligible bases?

Answer 13

A negligible base is an anion which essentially does not react with water (as a base or even as an acid). We know the neglible bases because they are the conjugate bases of the “super six” strong acids. Therefore the negligible bases are:

1. Cl^-
2. Br^-
3. I^-
4. NO₃^-
5. HSO₄^-
6. ClO₄^-

It is important to know these because when these anions are present in water, we know they will not react with water, nor will they affect the pH of water.

Question 14

Define negligible acid. List the nine common examples. Why is it important to know the negligible acids?

Answer 14

A negligible acid is a cation which essentially does not react with water (as a acid or even as an base). We know the neglible acid because they are the conjugate acids of the “super nine” strong bases. Therefore the negligible acids are:

1. Li⁺
2. Na⁺
3. K⁺
4. Rb⁺
5. Cs⁺
6. Mg²⁺
7. Ca²⁺
8. Sr²⁺
9. Ba²⁺

It is important to know these because when these cations are present in water, we know they will not react with water, nor will they affect the pH of water.

Question 15

List the five flow + one in determining what pH calculation to apply.

Answer 15

The five flow + one are:

1. super six or super nine? - strong acid or base
2. weak molecular acid or base with K_a or K_b?
3. ionic compound (NAWB)
4. ionic compound (WANB)
5. ionic compound (NANB)
6. + one is weak conj acid base pair

Question 16

Will the following compound result in an acidic, basic or neutral solution? NaCN

Answer 16

Basic - this is an example of NAWB

Question 17

Will the following compound result in an acidic, basic or neutral solution? NaF

Answer 17

Basic - this is an example of NAWB

Question 18

Will the following compound result in an acidic, basic or neutral solution? NaCl

Answer 18

Neutral - this is an example of NANB

Question 19

Will the following compound result in an acidic, basic or neutral solution? FeCl₂

Answer 19

Acidic - this is an example of WANB

Question 20

Will the following compound result in an acidic, basic or neutral solution? Ca(NO₂)₂

Answer 20

Basic - this is an example of NAWB

Question 21

Will the following compound result in an acidic, basic or neutral solution? NiBr

Answer 21

Acidic - this is an example of WANB

Question 22

Will the following compound result in an acidic, basic or neutral solution? SrI

Answer 22

Neutral - this is an example of NANB

Question 23

Will the following compound result in an acidic, basic or neutral solution? Co(NO₃)₂

Answer 23

Acidic - this is an example of WANB

Question 24

Describe the two types of titration reactions for which you will need to do pH calculations.

Answer 24

The two kinds of titration reactions are:

weak acid with strong base (strong base in burette)

weak base with strong acid (strong acid in burette)

Question 25

Which kind of titration reacton is described by the equation below. Will the solution at the equivalence point be acidic or basic?

HNO₂ + KOH → KNO₂ + H₂O

Answer 25

HNO₂ + KOH → KNO₂ + H₂O

This is a titration of a weak acid (HNO₂) with a strong base (KOH). The solution at the equivalence point will be basic due to the formation of the weak base (NO₂^-). Remember that the ionic compound seperates into its ions, K⁺ and NO₂^- which are NAWB.

Question 26

Which kind of titration reacton is described by the equation below. Will the solution at the equivalence point be acidic or basic?

HCl + NaCN → NaCl + HCN

Answer 26

HCl + NaCN → NaCl + HCN

This is a titration of a weak base (CN^-) with a strong acid (HCl). The solution at the equivalence point will be acidic due to the formation of the weak acid (HCN).

Question 27

Which kind of titration reacton is described by the equation below. Will the solution at the equivalence point be acidic or basic?

HNO₃ + CH₃NH₂ → NO₃^- + CH₃NH₃⁺

Answer 27

HNO₃ + CH₃NH₂ → NO₃^- + CH₃NH₃⁺

This is a titration of a weak base (CH₃NH₂) with a strong acid (HNO₃). The solution at the equivalence point will be acidic due to the formation of the weak acid CH₃NH₃⁺.

Question 28

Which kind of titration reacton is described by the equation below. Will the solution at the equivalence point be acidic or basic?

HCl + NaCN → NaCl + HCN

Answer 28

HF + NaOH → NaF+ H₂O

This is a titration of a weak acid (HF) with a strong base (NaOH). The solution at the equivalence point will be basic due to the formation of the weak base (F^-). Remember that the ionic compound seperates into its ions, Na⁺ and F^- which are NAWB.

Question 29

What information is needed and what equation would you use to find the pH ½-way through the titration:

HNO₂ + KOH → KNO₂ + H₂O

Answer 29

HNO₂ + KOH → KNO₂ + H₂O

For this titration the K_a (or pK_a) for HNO₂ is needed. The pH ½-way through the titration can be calculated with the Henderson Hasselbach equation:

pH = pK_a + log [NO₂^-]/[HNO₂]

Because ½-way through the titration [NO₂^-] = [HNO2], then pH = pK_a

Question 30

What information is needed and what equation would you use to find the pH ½-way through the titration:

HCl + NaCN → NaCl + HCN

Answer 30

HCl + NaCN → NaCl + HCN

For this titration, the K_a (or pK_a) for HCN is needed. The pH ½-way through the titration can be calculated with the Henderson Hasselbach equation:

pH = pK_a + log [CN^-]/[HCN]

Because ½-way through the titration [CN^-] = [HCN], then pH = pK_a

Question 31

Define a buffer system.

Answer 31

A buffer system is a solution that resists a change in pH upon the addition of strong acid or strong base. A buffer system consists of a conjugate acid/base pair which are present in at least a 1/10 concentration ratio.

Question 32

What equation can be used to calculate the pH of a buffer system?

Answer 32

The Henderson-Hasselbach equation is typically used to calculate the pH of a buffer system, because, as required of use of this equation, substantial amounts of BOTH conjugate acid (HA) and conjugate base (A^-) are present. Therefore the pH = pK_a + log [A^-]/ [HA].

Question 33

A strong acid (HCl) is added to a buffer system consisting of H₂PO₄^- and HPO₄^2-. Which anion is involved in “neutralizing” the strong acid? Write the appropriate chemical equation.

Answer 33

When a strong acid is added to a buffer sustem, the strong acid will react with the weak conjuagate base, which in this case is HPO₄^2-. The corresponding chemical reaction is: H₃O⁺ + HPO₄^2- → H₂PO₄^- + H₂O

The effect of this reaction will be to consume any of the added strong acid, decrease the concentration of the conjugate base (HPO₄^2-), increase the concentration of the conjugate acid (H₂PO₄^-) with essentially only a minor change in pH.

(Note that any HCl added will be directly converted to H₃O⁺ and this is what is used in the above reaction.)