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Chemistry > Acids and bases Module 5 > Flashcards

Acids and bases Module 5 Flashcards

1
Q

What are acids?

A

Proton donors
Release H+ ions when mixed with water in form of hydroxonium ions
HA + H2O = H3O+ + A-

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2
Q

What are bases?

A

Proton acceptors
When mixed with water taken hydrogen ions from water molecules
B + H2O = BH+ + OH-

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3
Q

What do you call acids in terms of how many protons they can release?

A

Monobasic, HCl
Dibasic H2SO4
Tribasic, H3PO4

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4
Q

Describe the equilibrium set up by an acid reacting with a base in terms of conjugate pairs?

A

HA(acid) + B(base) ⇋ BH+(acid) + A-(base)

In forward reaction HA acts as acid as donates a proton to B
In reverse reaction A- acts as base to accept proton from BH+
HA and A- are a conjugate pairs, HA being conjugate acid and A- being conjugate base
B and BH+ are a conjugate pairs, BH+ being the conjugate acid, and B being conjugate base

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5
Q

What are conjugate pairs?

A

Species linked by the transfer of a proton

Always on opposite sides of the equation

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6
Q

Water is a special case, what happens if you react it with an acid?

A

Forms a conjugate acid (H3O)+

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7
Q

Water is a special case, what do you get if you react it with a base?

A

Forms a conjugate base (OH-)

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8
Q

What do you get if react an acid with a reactive metal?

A

Hydrogen gas

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9
Q

What do you get if react an acid with carbonates?

A

CO2 and water

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10
Q

What are Alkalis?

A

Type of base which releases OH- ions in water

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11
Q

What do you get if react an acid with an alkali?

A

Water

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12
Q

What’s a metal oxide?

A

An insoluble base

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13
Q

What do you get if react an acid with a metal oxide?

A

Water

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14
Q

Formula for pH if you know it’s H+ concentration?

A

pH = -log10 (H+)

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15
Q

If you know the pH of solution how do you find it’s H+ concentration?

A

(H+) = 10^-pH

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16
Q

What’s an important rule for monobasic acids or bases?

A

H+ concentration = Concentration of acid

OH- concentration = Concentration of base

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17
Q

Equation for water dissociating

A

H2O ⇋ H+ + OH-

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18
Q

How do you find the ionic product of water Kw?

A

Kw = (H+)(OH-)

If water is pure then ratio between them is 1:1, so Kw is the same as (H+)^2

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19
Q

For a weak aqueous acid what equilibrium do you get?

A

HA ⇋ H+ + A-

20
Q

What’s important about a weak acid?

A

Don’t fully ionise in solution so H+ conc isn’t the same as acid concentration

Also it’s concentration is the same at start and at equillibrium

21
Q

Formula for Ka the acid dissociation constant?

A

Ka = ((H+) x (A-)) / (HA)

Or Ka = (H+)^2 / (HA)

22
Q

Units for Ka?

A

mol dm^-3

23
Q

Formula to find pKa?

A

pKa = -log10 Ka

24
Q

Formula to find Ka when you know pKa?

A

Ka = 10^-pKa

25
Q

What’s a buffer?

A

A solution that minimises changes in pH when small amounts of acid or base are added

26
Q

What do acidic buffers contain?

A

A weak acid and it’s conjugate base

27
Q

What are the 2 ways to form an acidic base?

A

Mix a weak acid with the salt of it’s conjugate base

Mix an excess of weak acid with a strong alkali

28
Q

What is the equilibrium for a weak acid and it’s conjugate base?

A

HA ⇋ H+(aq) + A-

Eg CH3COOH(aq) ⇋ H+(aq) + CH3COO-(aq)

29
Q

How do acidic buffers resist changes in pH?

A

If you add a small amount of acid the H+ ion increases,, meaning the position of equilibrium shifts to the left reducing H+ ion concentration. pH doesn’t change much

If Alkali is added OH- ion concentration increases, they react with H+ to form water, removing H+ ions from solution. Causing equilibrium to shift right so more CH3COOH dissociates, H+ ion concentration increases, pH doesn’t change much

30
Q

What are the 2 buffer systems that occur in our blood?

A

H2CO3 ⇋ H+ + HCO3-

H2CO3 ⇋ H2O + CO2

31
Q

How are levels of H2CO3 controlled in our blood?

A

Respiration, when we breah out CO2 it’s concentration reduces so less H2CO3 formed

32
Q

How are levels of HCO3- controlled?

A

The kidneys, with the excess being excreted in urine

33
Q

How to calculate the pH of a buffer solution?

A

Assume that initial concentration of salt is the same the equilibrium value of A-
HA initial concentration is the same as it’s equilibrium concentration
Use and rearrange the Ka formula if necessary

34
Q

What can you use to measure the pH of a solution?

A

pH meter

35
Q

How do you calibrate a pH probe?

A

Put in distilled water and adjust reading to 7

Do the same with pH 4 solution and pH 10 solution rinsing it with distilled water each time

36
Q

What do titrations allow you to do?

A

Find out exactly how much alkali is needed to neutralise and acid

37
Q

How to draw strong acid / strong base curve?

A

Starts near 0, ends near 14

vertical part in centre

38
Q

How to draw strong acid/ weak base

A

Starts near 0 ends near 10

Vertical part 3/4 on x axis

39
Q

How do you draw a pH plot?

A

pH on y axis

volume of alkali added on x axis

40
Q

How to draw weak acid/strong base?

A

Starts near 5 ends near 14

Vertical part 1/4 way down x axis

41
Q

How to draw weak acid/ weak base?

A

Starts near 5 ends round 10

No vertical part it’s S shaped

42
Q

What’s the vertical part of a pH plot?

A

The equivalence point

The point where the acid is neutralised and (OH-)=(H+)

43
Q

How do you chose which indicator to use for a titration?

A

The pH range of the indicator must lie completely within the vertical part of the pH plot

44
Q

Describe features f Methyl orange indicator?

A

Colour at low pH is red
Colour at high pH is yellow
Approx pH of colour change is between 3 and 4.5
So useful for strong acid/ strong base and strong acid/ weak base

45
Q

Describe phenolphthallein?

A

Colour at low pH is colourless
Colour at high pH is pink
Approx pH of colour change is 8 to 10
So useful for strong acid/ strong base and weak acid/ strong base

46
Q

Why can indicators be thought as weak acids?

A

They have differently coloured conjugate pairs, so as H+ or OH- concs change the equilibrium positions also do
phenolphthallein-H (colourless) ⇋ phenolphthallein-(pink) +H+
Methyl orange-H (red) ⇋ Methyl orange- (yellow) + H+

Chemistry (26 decks)

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