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OCR Chemistry A > 5.1.3 - Acids and Bases > Flashcards

5.1.3 - Acids and Bases Flashcards

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1
Q

Bronsted - Lowry Acid

A

Proton donor

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2
Q

Bronsted - Lowry Base

A

Proton acceptor

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3
Q

How are acids related to bases

A

Acid —> proton + conjugate base

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4
Q

How are bases related to acids

A

Base + proton –> conjugate acid

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5
Q

pH

A

Conversion of hydrogen ion conc
pH = - log [H^+]
[H^+] = 10 ^-pH

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6
Q

For strong monoprotic acids [H^+] =

A

[HA] - acid fully dissociates

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7
Q

What does water dissociate to give

A

H^+ and OH^-

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8
Q

Kw

A

[H^+][OH^-]

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9
Q

Kw at 25 degrees

A

1.0 * 10^-14

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10
Q

In neutral solutions at 25 degrees [H^-] =

A

[OH^-]

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11
Q

How can an aq solution of an acid contain OH-

A

Water dissociates

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12
Q

When is a solution alkaline

A

[OH^-] > [H^+]

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13
Q

Finding pH of strong bases using Kw

A

Rearrange Kw

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14
Q

Finding [OH^-] using pKw

A 
Kw = [H+][OH-]
pKw = pH + pOH
14 = pH + pOH
14-pH = pOH
[OH-] = 10^-pOH
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15
Q

Assumptions w/ Ka

A

Little dissociation [HA] eqm = [HA] undis. Less valid for stronger weak acids w Ka > 10-2

[H2O] is constant so [H+]=[A-]. Less valid for v weak acids and dilute sol

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16
Q

Ka

A

Dissociation constant for a weak acid

Bigger the value, sronger the acid

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17
Q

Finding [H+] from Ka

A 
Ka = [H+][A-]/ [HA]
[H+] = [A-]
Ka = [H+]^2/ [HA]
[H+]^2 = Ka [HA]
Square root answer
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18
Q

Dilution of strong base

A

[OH-] in orig.
[OH-] in diluted ([OH] * orig vol/total vol)
Find [H+] using Kw

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19
Q

Reaction between strong acid and strong base

A 
Calculate MOLES H+ (x2 if dibasic)
Calculate MOLES  OH-
Calculate XS MOLES 
Calculate XS [H+] or XS [OH-] (using total vol.)
If XS [OH] use Kw to find XS [H+]
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20
Q

Example of weak base

A

NH3

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21
Q

Buffers

A

A solution that minimises pH changes in addition of SMALL amounts of acid or base

22
Q

Components of a buffer solution

A

Weak acid and its conjugate base

23
Q

How to make buffer solutions

A

Mixing weak acid and its conjugate base (ethanoic acid and sodium ethanoate)
By partially neutralising a solution of the weak acid to give a mixture of the acid and its salt (React NaOH with an excess of the weak acid)

24
Q

What does a buffer solution look like

A

HA only partially dissociates and the salt completely dissociates

25
Q

How do buffers resist change to pH if an acid is added

A

HA H+ and A-

[H+] increases
Eqm shifts to left as A- reacts w/ H+
Removes added H+
[H+] and pH are restored

26
Q

How do buffers resist change to pH if an alkali is added

A

Ha H+ and A-

[OH- ] increases
OH- reacts w/ H+
HA dissociates to form more H+
Eqm shifts to right to restore pH

27
Q

HA and conjuate base buffer (solution)

A

Find moles of HA and A-
[HA] = moles of HA/ Vol 1+2
[A-] = moles of conjugate base / Vol 1+2
Use Ka to find [H+]

28
Q

HA and conjugate base buffer (solution and solid)

A

Find moles of HA and A-
[HA] start = [HA] end
[A-] = moles of conjugate base/Vol HA

29
Q

HA and base buffer (solution)

A

Find moles of HA and B
nHA end = nHA start - nNaOH
[HA] = n/Vol of base
[A-] = [NaOH]

30
Q

Weak acid and strong base

A

Calculate moles HA
Calculate moles OH
Find XS HA or OH-

If XS HA, use HA + OH- –> A- + H2O to find nHA and nA- after
Find [HA] and [A-] using total vol
Find [H+]

If XS OH- use Kw to find [H+]

31
Q

Acid Base Indicators

A

HI –> H+ + In-

Weak acid (Ka)
Unionised is diff colour to anionic form
32
Q

Acid base indicators in acid

A

In acid [H+] inc

Eqm moves to left to give colour A

33
Q

Acid base indicators in bases

A

In alkali [OH-] inc
Reacts w/ water
Eqm moves to right

34
Q

End point of a titre

A 
Ka = [H+]
pKa = pH
35
Q

End point of titration

A

[HA] and [A-] are the same
Colour is a mix between HA form and A- form
pKa = pH

36
Q

Indicator for SA and SB

A

Phenolphthalein
Litmus
Methyl orange

37
Q

Indicators for SA and WB

A

Methyl orange

38
Q

Indicator for WA and SB

A

Phenolphthalein

39
Q

Indicator for WA and WB

A

None suitable because no rapid change in pH during titration and indicator needs a sharp pH rise to change colour quickly

40
Q

Finding pH of Ba(OH)2 from conc.

A

Times [OH] by 2

41
Q

Finding conc. of Ba(OH)2 from pH

A

Half [OH-]

42
Q

Finding pH when base is added to a buffer sol

A 
Find [HA] and [A-]
Use Ka to find [H+] --> pH 
Find moles of B
Find moles of HA (using c*vol. of buffer sol.)
Find excess HA or B

If HA:
Find [HA] using total vol (base and buffer)
Use Ka to find [H+] –> pH

43
Q

% dissociation of HA

A

[H+]/[HA] * 100

44
Q

Experimentally determining the value of Ka

A

Preparing a standard solution of the weak acid of known concentration
Measuring the pH of the standard solution using a pH meter

45
Q

Most effective buffers

A

[HA] = [A-]
pH is same as pKa for HA
Range is approx around two pH values of pKa

46
Q

Calculating conc ratio of HCO3-/H2CO3

A

[HCO3-]/[H2CO3] = Ka/[H+]

47
Q

Using a pH meter

A

Using a grad. pipette, add a measure volume of acid to a conical flask
Place electrode into flask
Add aq base 1cm3 at a time
Swirl and record pH
Add base drop wise when a rapid increase occurs
Add base 1cm3 at a time until there’s an excess and pH is basic

48
Q

What does the equivalence point tell you

A

Exact volume of solution that reacts w the other

49
Q

Adding metal to buffers

A

Calculate moles of M
Calculate moles of HA: 1-(charge of M * nM)
Calculate moles of A-: 1 + (charge of M *nM)
[H+] = Ka * HA/A-

50
Q

Making buffers using HA and metal

A

Find moles of metal
Find nA-: charge of metal * nM
Find nHA: nHA - nA-
Find [HA] and [A-] using total vol.

OCR Chemistry A (16 decks)

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