9. Acids & Bases

Question 1

In general, what determines the strength of an acid?

Answer 1

An acid is strong if it completely dissociates in water. This can be thought of as how stable the conjugate base is (since if it is very stable, the reaction will favor this low energy side).

Question 2

What is Ka and Kb?

Answer 2

These are the equilibrium products for acids and bases.

Ka = [products] / [reactants]

note that a larger Ka value indicates more products are favored at equilibrium which indicates that the acid is strong.

Question 3

What are the 6 strongest acids (where Ka > 1)? Is HF a strong acid?

Answer 3

HCl, HBr, HI
HClO4 - perchloric acid
H2SO4 - sulfuric acid
HNO3 - nitric acid

HF is not a strong acid because F is such a small atom that the H-F bond is strong (bond strength increases when bond length decreases). Additionally, smaller anions are worse at stabilizing a negative charge.

Question 4

What determines a strong base?

Answer 4

We look back to the equilibrium product. Stronger bass favor their products more. So a strong base will produce lots of its conjugate acid in water.

Thus a larger Kb value, a stronger base

Question 5

What are the known strong bases? Weak bases?

Answer 5

Any group 1 hydroxides or oxides - NaOH, Li2O, KOH
Some group 2 hydroxides - Ba(OH)2, Ca(OH)2
metal amides - NaNH2

Weaker bases are ammonia (NH3) and amines as-well as the conjugates of strong acids.

Question 6

What is the auto-ionization of water?

Answer 6

Water is amphoteric which means it can react with itself as both an acid and base.

Kw = [H+][OH-] = 1.0 x 10^-14

like any equilibrium constant, this varies with temperature.

Question 7

What is the formula for pH?

Answer 7

pH = -log[H+] and thus, [H+] = 10^pH

pOH = -log[OH-] and thus, [OH-] = 10^-pOH

pOH + pH = 14

Question 8

How do you identify stronger acids and bases using pKa and pKb?

Answer 8

the smaller the pKa or pKb, the stronger the acid or base.

pKa = -log(Ka)
pKb = -log(Kb)

Question 9

t or f, Kw = Ka x Kb = [H+][OH-] = 1.0 x 10^-14

Answer 9

true

Question 10

if 99% of H+ ions are removed from a solution that was originally at pH of 3, what is the new pH?

Answer 10

the new solution has 1/100th the amount of H+ as the original. 100 is two factors of 10, which means the pH scale (log scale) has changed by 2.

New pH = 5 (since H+ is lost, more basic now)

Question 11

How can you calculate the pH of a strong acid (e.g. HClO4) if you are given the concentration of the acid?

Answer 11

strong acids completely dissociate and thus whateever the concentration of the acid will equal the concentration of H+ ions in solution.

If you have 0.5 moles of HCl, then you have 0.5 moles of H+ in solution. Find molarity and get pH = -log[H+]

Question 12

How can you calculate the pH of a weak acid if you are given its concentration (e.g. 0.2 moles / liter)?

Answer 12

Weak acids do not dissociate completely. When the acid is placed in solution (1L), it is at 0.2 moles / liter. It will then reach equilibrium by dissociating x amount.

acid concentration = 0.2 - x (omit this cause x is small)
H+ concentration = x
Y concentration = x

Ka = [produdcts] / [reactants]

ka = x^2 / 0.2

we know Ka so we can solve for x and then use this concentration to find pH.

Note: typically the starting concentrations for the products are 0. However, if something has been dissolved in solution, we must add their concentration to the equilibrium concentration of the weak acid.

Question 13

What is a neutralization reaction?

Answer 13

When an acid reacts with a base, they often neutralize each other producing some salt and water.

HCl + NaOH –> NaCl + H2O

When acids and bases react in equimolar concentrations, we get complete neutralization.

Question 14

Hydrolysis of salts: a salt is an ionic compound composed of a cation and anion. When these dissolve in water, the solution may become acidic or basic. Explain how you determine this (hint: periodic table, conjugates).

Answer 14

Group 1 cations do not react with water
Large group 2 cations do not react with water

conjugate bases on strong acids (i.e. weak bases) do not react with water

NaCl dissolves in water, Na+ (group 1 metal) does not react with water. Cl- which is the conjugate of HCl also does not react. Thus, pH = 7.

Question 15

How does Be(2+) and Mg(2+) react in water?

Answer 15

These are the only group 2 elements that react in water (and are acidic). Larger group 2 elements do not react.

Question 16

How does NH4Cl react in water?

Answer 16

NH4Cl dissolves into NH4+ and Cl-. As we know, Cl- does not react. NH4+ will react with water and will be acidic as it gives up protons.

Question 17

What is a buffer solution?

Answer 17

A weak acid and its conjugate base (or vice versa) in roughly equal concentrations.

Question 18

If we have a buffer solution containing a weak acid and its conjugate base, what is the equation for pH (Henderson-Hasselbalch equation?

Answer 18

pH = pKa + log([base] / [acid])

if we had a base with its conjugate acid then,

pOH = pKb + log([acid] / [base])

the log part is just a product of the equilibrium equation. Base dissociation has acid in its products so its on top in the equation.

Question 19

What is an indicator?

Answer 19

An indicator is a weak acid that undergoes some color change when it is converted into its conjugate base.

Question 20

What is an acid-base titration? Why is is performed?

Answer 20

Acid-base titrations are performed to find the identity of a solution. We slowly add a strong base (titrant) to a weak acid or a strong acid to a weak base until we have fully converted all of the acid to its base form (or all of the base to its acid form).

Question 21

What is a titration cure? Explain the equivalence point and half-equivalence point. How do you determine the pKa or pKb of the unknown acid or base?

Answer 21

A titration curve measures pH (y-axis) vs volume of titrant added (x-axis).
Base titrant example
The start is called the buffer period because as you add base, the acid in solution shifts its equilibrium to produce more H+ (i.e. buffering the solution). Thus pH rises slowly. Eventually at some point, pH rises rapidly. At this point, all of the acid in solution has been neutralized by the added base. The volume of titrant to achieve this is the equivalence point. The half equivalence point is then the volume of titrant (base) where the acid in solution exists in equal parts with its conjugate base
[acid] = [base]

pH = pKa + log([base] / [acid])
log([base] / [acid]) = 1

pH = pKa

The pH at the half-equivalence point is the pKa of the unknown acid.

Question 22

What is the equivalence point for
a strong base titrating a weak acid
a strong acid titrating a weak base
a strong acid titrating a strong base (vice versa)

Answer 22

a strong base titrating a weak acid: Equiv > 7
a strong acid titrating a weak base: Equiv < 7
a strong acid titrating a strong base (vice versa): Equiv = 7

Question 23

Explain [A] x Va = [B] x Vb.

Answer 23

This equation tells us how to find the concentration of an acid or base needed to neutralize another acid or base.
[A] x Va = [B] x Vb

using the titrants added volume and known concentration as-well as the unknown solutions volume, we can find its concentration.

Question 24

explain the titration curve of a polyprotic acid.

Answer 24

there will be two equivalence points for a diprotic acid.