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MCAT > Acids and Bases > Flashcards

Acids and Bases Flashcards

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MCAT flashcards
1
Q

Arrhenius base and acid

A

base - produces OH in aq

acid - produces H in aq

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2
Q

Bronsted Lowry

A

acid - donates proton, H

Base- accept H

This is not limited to aq! NH3, Cl- are BL bases because they can accept an H, but they do not produce OH so they are not Arrhenius

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3
Q

conjugate acid base pair

A

related by transfer of H

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4
Q

Lewis acid and base

A

acid - e pair acceptor think H+ would love some electrons

base - e pair donator OH- get rid of them

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5
Q

BCl3, AlCl3 are what kind of acids?

A

Lewis…they can accept electrons, but have no H to donate (BL)

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6
Q

pH =

pOH =

A

pH = -log[H]

pOH = -log[OH]

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7
Q

What is the water dissociation constant, Kw?

What is the ph of water?

A

H20 –> H+ + OH-

Kw = [H][OH] = 10^14

pH is 7 neutral! dissociates evenly into H and OH

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8
Q

if [H] = .003 or 10^-3 ph=?

A

ph = 3

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9
Q

if Kb = 10^7 pkb = ?

A

pkb = 7

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10
Q

what is the scientific notation of Ka or Kb?

A

n x 10^m

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11
Q

logKa =

A

pka

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12
Q

what is the notation for pKa?

A

pKa = -m + logn

Log n will be between 0 and 1. the larger n is, the closer logn is to 1

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13
Q

Convert Ka to pKa

Ka = 1.8 x 10^-5

A

pKa = 5 - log(1.8)

log1.8 is closer to 0 than 1, so pKa will be closer to 5 than 4 = 4.7

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14
Q

what’s important about the dissociation of strong acids and bases in aq?

A

they completely dissociate

arrow ONLY points right

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15
Q

So when you put acid or base into water, you would think the dissociation of water would contribute to pH. But really, you can neglect it when the conc of acid or base is greater than 10^-7 M

A

got it?

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16
Q

HCl is acidic but at 10^-8 M, why is it’s ph=8?

A

because in solution with water, the dissociation of water gives a ton of H, makes it acidic!

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17
Q

what makes an acid or base weak?

A

only partially dissolves in water…so the equation has arrows pointing both ways, not just to right

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18
Q

Ka =

A

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

For weak acid HA,
HA + H2O –> H3O + A-

It’s really just like Keq, its [products]/[reactants]

NB! To find Ka or Kb, the rxn must be with water

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19
Q

smaller Ka = weaker or stronger acid?

A

weaker!

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20
Q

how does a conj base form?

A

when the acid loses a H

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21
Q

how does a conj acid form?

A

when the base gains an H

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22
Q

what does KaxKb = ?

A

Kw!! Write it out, it all does cancel out when Ka and Kb are conj acids and bases

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23
Q

For conj pairs, if Ka is small, is Kb big or small?

A

big….and vice versa

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24
Q

Essentially, what is Ka or Kb?

A

value at eq!

25
Q

CH3COOH –> H + CH3COO-

Ka = 1.8E-5
[CH3COOH] = 2

Find [H]

A

Ka = [CH3COO-][H]/[CH3COOH]

Ka = [x][x]/[CH3COOH-x] (1:1)

can assume that 2-x ~ 2 bc weak acid

solve for x

26
Q

what happens when you react acid and base?

The reverse?

A

get a salt and water

reverse is salt and water –> acid and base

27
Q

What are products and ph for following?

strong acid +strong base
strong acid + weak base
weak acid + strong base
weak acid + weak base

A

= salt and water pH=7
= salt pH7
= salt pH depends on conc.

28
Q

what is polyvalence?

A

when an acid releases 2 acid equivalents upon dissociation

Eg H2SO4 releases 2 H+

29
Q

2M of H3PO4, what is the Normality?

A

2x3 = 6N

30
Q

H2SO4 mm = 98g/mol

how many grams would you need to release 1 acid equiv?

A

49g

31
Q

what is an amphoteric or amphiprotic species?

Example?

A

can act as an acid or base depending on environ
Eg. Water

H2O can donate an H –> OH-
H2O can accept H –>H3O

32
Q

what is the goal of a titration and how do you carry it out?

A

React known V of known conc with known V of unknown conc

To determine the M of an acid or base

33
Q

what is the eq pt?

A

of acid equivalents = #of base equivs

34
Q

is the eq pt of strong acid and base always at pH=7?

A

not always but usually

35
Q

at what pt does the color indicator change colors?

A

the end pt NOT the eq pt, but these are usually close enough

36
Q

what is the equation to find V needed to reach end pt?

A

VaNa = VbNb

where a is acid
b is base
N is normality

37
Q

what is a buffer solution?

A

mixture of a weak acid and its salt ( the conj base+cation)
or vice versa
Eg. CH3COOH and CH3COO-Na+
NH3 and NH4CL-

38
Q

why are buffer solutions useful?

A

they resist pH changes when small amounts of acids or bases are added

39
Q

Henderson Hasselbalch Equation

A

estimates the pH or pOH of solution for weak acid or base buffer:

pH = pKa + log[conj base]/[weak acid]

pOH = pKb + log [conj acid]/[weak base]

NB! when [acid or base]=[conj], pH=pKa (work it out)

40
Q

For buffer, when does [species] = [conj]?

Ie when does pH = pKa

A

halfway to eq pt! aka at pka

41
Q

diprotic acid will have how many eq pts?

How can you see an eq pt on the graph?

A

2!

rapid change in pH

42
Q

what are the flat regions of the titration graph?

A

buffer regions where the pKa values are

43
Q

how can you calculate Ka from Kb and vice versa?

A

Kb = Kq/Ka

44
Q

high Ka is strong or weak acid?

A

strong acid

45
Q

for strong acid:
Ka is high or low?
pka is high or low?

A

Ka is high
pKa is low

pKa = -log Ka

46
Q

When you see pka or pkb think:

-logKa and -log Kb

p means -log

A

ok?

47
Q

pka +pkb =

A

14!

48
Q

pka is kind of like where the acid wants to be for pH, it has used up all the H. So if pka =2 for an acid, at any pH over 2, there are OH around and the substance will lose an H to OH

At pH1. there are more H around and the substance will grab them to grab as many H as it can (as it is able, aka it’s pka)

A

same with pkb

So an acid with pka of 1 or 2 is really strong, you can’t possibly get any lower…

base with high pka is very basic

49
Q

Pka is _______ related to pH

Ka is _________related to pH

A

directly

inversely

50
Q

pH tells you about a solution

A

pka tells you about the species, where it wants to be, what it is capable of

and for pH, conc matters

51
Q

so when you have equal amounts of conj base and conj acid, the pH is equal to…

A

pKa! it’s at equilibrium

52
Q

what influence do buffers have on titration curves?

A

flat part of the curve
also known as the inlfection pt
where pH = pka (or 14-pkb)
The area around this pt is where the solution has buffering capacity

53
Q

what is the difference between buffer and neutralization?

A

buffering capacity is when the conc of acid and conj base are equal. This occurs at pH = pKa. They are both acting to resist change in pH. The acid donates a H, the base picks it up.

54
Q

what is the equivalence pt?

A

where pH changes drastically

This is where the titrant has essentially sopped up all the opposing species

Eg if you’re titrating an acid with NaOH, it’s the point where OH has sopped up all the H from the acid, so pH changes drastically

for diprotic, H2So4, the first eq pt is where the first H is all sopped up. The 2nd eq pt is where the 2nd H has been sopped up

55
Q

REMEMBER! There is no Ka or Kb for a strong acid or base. Why?

A

Because there is no equilibrium for the equation. A strong acid and base go to completion!

56
Q

when [weak acid] = [conj base] =

A

pka = half eq pt!

57
Q

where is Ka on titration curve?

A

It’s not really there! You can find pKa, then find Ka from there.

58
Q

Henderson-Hasselbalch

A

pH = pKa + log [conj base]/[weak acid]

pOH = pKb + log [conj acid]/[weak base]

59
Q

does pka depend on the conc of acid?

A

no, it’s an intrinsic property of the acid itself, makes no difference how much acid is present, the pka of HCl will never change

MCAT (14 decks)

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