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Chem 110 > Block 5: Acids and Bases > Flashcards

Block 5: Acids and Bases Flashcards

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1
Q

What is the equation to calculate pH from [H3O+]?

A

pH = -log[H3O+]

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2
Q

What is the equation to calculate [H3O+] from pH?

A

[H3O+] = 10^-pH

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3
Q

What is the definition of an acid?

A

A substance that can donate a proton to another substance (proton donor)

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4
Q

What is the definition of a base?

A

A substance that can accept a proton from another substance (proton acceptor)

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5
Q

What is a conjugate acid?

A

A species formed after a base has accepted a proton

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6
Q

What is a conjugate base?

A

A species formed after an acid has donated a proton

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7
Q

What does ‘amphiprotic’ mean?

A

A species which can sometimes be an acid and sometimes a base

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8
Q

What affects the position of an equilibrium in a given system?

A

The ability of an acid/base to accept/donate a proton- related to their strength.

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9
Q

Where will eq lie if HA is a better proton donor to B than HB+ is to A-?

A

It will lie to the right- products favored

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10
Q

Where will eq lie if HB+ is a better proton donor to A- than HA is to B?

A

It will lie to the left- reactants favored

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11
Q

What molecule is used to reference the strength of acids?

A

Water

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12
Q

If an acid is strong, will its conjugate base be weak or strong?

A

Weak (NB this works vice versa)

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13
Q

If a base is strong, will its conjugate acid be weak or strong?

A

Weak (NB this works vice versa)

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14
Q

What is the equilibrium constant?

A

Ka = [A-][H3O+]/[HA]

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15
Q

What does not appear in the eq constant?

A

Water

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16
Q

Why doesn’t water appear in the eq constant?

A

The equation is only a ratio of the real concentrations, and as water is pure water it has a concentration of 1, causing both its value and units to cancel out. Therefore as it is a (l), it doesn’t feature. NB this also applies to (s).

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17
Q

What is denoted by a larger Ka value?

A

A stronger acid

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18
Q

What is the formula for finding pKa?

A

pKa = -log(Ka)

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19
Q

What does a smaller pKa denote?

A

A stronger acid

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20
Q

Why does pKa = 0 for H3O+ with H2O?

A

Because the same species are on both sides of the equation

H3O+ + H2O H3O+ + H2O

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21
Q

What affects the strength of an acid?

A

The charge of A in HA
The polarity of the HA bond
The strength of the HA bond

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22
Q

How does the charge of A in HA affect acid strength?

A

If a is positive, it will be stronger as the + charge of H and A repel, making proton donation easier.
If a is negative it will be weaker as the -ve charge of A and + H attract, making proton donation harder

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23
Q

How does the polarity of the HA bond affect acid strength?

A

If the A is more electronegative the delta + charge on H will be higher, making the transfer easier

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24
Q

How does the strength of the HA bond affect acid strength?

A

HA will be stronger if the HA bond is weak (and longer eg. for large elements)

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25
Q

Which is more important for determining acid strength between strength and polarity of bond?

A

Strength

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26
Q

What helps to determine acid strength with oxoacids?

A

Whether Hs are bonded to Os or to less electronegative atoms

The number of atoms bonded to the central atom

27
Q

How does Hs bonded to Os or other atoms affect acid strength of oxoacids?

A

If there are more Hs on Os, these have a larger delta plus value and so are more easily removed, making the acid stronger.
If there are fewer, they are less positive and harder to remove, making the acid weaker.

28
Q

How does the number of electronegative atoms on the central atoms affect acid strength of oxoacids?

A

More electronegative atoms on the central atom means that a negative charge formed when the H is removed can be shared between multiple centers, stabilizing the molecule and making it more likely to form. Therefore the acid is stronger. NB this can be extended to the rest of the molecule

29
Q

Are haloacids (CH2Cl COOH) stronger than regular acids and why?

A

Haloacids are stronger as they have more electronegative centers to share the negative charge with, making them more stable. (The more substituted, the stronger)

30
Q

What is Kw?

A

Kw= [H3O+][OH-], and is called the autoprotolysis of water. At room temperature it is 1x10-14, but this increases with an increase in temperature.

31
Q

What are the assumptions we can make when calculating the pH of a strong acid?

A

We can assume than [H3O+] = [HA] = [A-] as the acid fully dissociates, however this doesn’t work when the acid is very dilute. This also applies to solutions of strong bases.

32
Q

What assumptions can we make for calculating the pH of a weak acid?

A

We can assume that [H3O+] = [A-]

33
Q

What can’t we assume when calculating the pH of a weak acid?

A

We can’t assume that [HA] = starting conc [HA]. Instead, we must subtract the [H3O+] from the starting conc to get the conc at equilibrium.

34
Q

How do we calculate the pH of a weak acid salt?

A

All salts fully dissociate, so conc [NaA] = [A-]
The pH is greater than 7 as the acid salt produces OH- ions
We get [H3O+] from the pH
[HA] = [OH-] so we can use Kw to find thsi
[A-] = [NaA] - [HA]
Then calculate Ka

35
Q

What is the henderson-hasselbach equation?

A

pH = pKa + log([A-]/[HA])

36
Q

What is the henderson-hasselbach equation used for?

A

Buffer solutions or solutions containing a mix of weak acid/conjugate base

37
Q

What happens when the ratio of A- to HA is 1?

A

pH = pKa

38
Q

What is a titration?

A

A method of volumetric analysis used for determining the concentration of an unknown solution by letting it react with a solution of known concentration. We observe a physical property when moles of 1 = moles of other, usually via an indicator.

39
Q

What happens at the equivalence point?

A

Moles of A/a = moles of B/b

The pH changes very rapidly.

40
Q

What is the endpoint?

A

The point at which the indicator changes color

41
Q

When does the color of the indicator change most rapidly? (Using henderson-hasselbach equation of Kin)

A

It changes most rapidly when the ratio of HIn/In- is close to one, or when pKin = pH.

42
Q

How does pKin help us choose an indicator?

A

We want one with a pKin close to the equivalence pH (within 1 unit).

43
Q

What is an indicator?

A

A weak acid, for which the acid and the conjugate base have different colors (due to different extents of conjugation)

44
Q

How do you approximate equivalence pH for a weak acid/strong base titration?

A

pH = 7 + 0.5pKa + 0.5log[A- at eq point]

45
Q

Where is the buffer zone located on a titration curve?

A

Halfway between start and eq point

46
Q

What are some defining features of titration curves for polyprotic acids?

A

They have multiple eq points and buffer regions

47
Q

In a polyprotic titration curve, which of the proton losses are easiest?

A

Difficulty of proton loss increases after each previous loss, due to a less positive charge on A. This makes subsequent compounds weaker acids, so the eq for each subsequent reaction is less product favored.

48
Q

What species are present at eq points?

A

Only A-

49
Q

What species are present at buffer zones?

A

HA/A-

50
Q

What species are present at start point?

A

HA, H3O+

51
Q

What species are present after eq point?

A

A-, OH-.

52
Q

How do you know where to put different buffer points in a polyprotic titration curve?

A

All endpoints (and therefore buffer zones) are the same distance away from each other

53
Q

What is a buffer solution?

A

A solution whose pH changes less than that of pure water as acid or base is added. Typically they contain equal (or near equal) amounts of an acid/base and its conjugate

54
Q

Why doesn’t a buffer solution’s pH change quickly?

A

It has both proton acceptors and donators in it. In more concentrated solutions, there are even more of these acceptors/donors to mop up extra H3O+/OH-

55
Q

How do you decide what ratio to put acid/base in to create a buffer at a certain pH?

A

Use the henderson hassellbach equation-
pH = pKa + log(ratio)
pH - pKa = log (ratio)
10^(pH-pKa) = ratio

56
Q

How do you know how much of each acid/base to add to make a buffer of a set volume when you have a ratio?

A

Just multiply that ratio up to get the required volume

57
Q

Where do buffers exist in nature?

A

Blood and the sea

58
Q

Where do indicators exist naturally?

A

Flowers- eg. cornflowers and poppies have the acid/conjugate base dye. Their colors depend on the surrounding soil pH

59
Q

What is a zwitterion?

A

A species with both a +ve and -ve charge on it

60
Q

How can amino acids be separated?

A

The amino acids form zwitterions, bases and acids at unique pHs. At a particular pH, one will remain neutral (due to its zwitterion) while others will be acidic/basic.
Therefore if they are laid on a plate with +ve and -ve electrodes, they can be separated at different pHs as some will be attracted to the -ve electrode, some to +ve and some will not move. (NB this can also be applied to proteins)

61
Q

What is the isoelectric point in terms of amino acids/proteins?

A

The pH at which the majority o a species is a zwitterion.

62
Q

What does the isoelectric point of proteins depend on?

A

Their relative compositions of amino acids

63
Q

How does acid rain form?

A

The SO2 and SO3 produced commercially dissolves in water to give H2SO4, H2SO3 etc. This reduces the pH of rain, enabling it to destroy buildings, plants and ecosystems.

Chem 110 (6 decks)

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