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A-level Chemistry > Acids And Bases > Flashcards

Acids And Bases Flashcards

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1
Q

What does an acid and base reaction involve?

A

Transfer of H+ ions

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2
Q

What is a bronsted Lowry acid?

A

A substance that can donate a proton

Proton donor

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3
Q

What is a bronsted Lowry base?

A

A substance that can accept a proton

Proton acceptor

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4
Q

What is a conjugate acid base pair?

A

A set of two species which become each other either by the gain or loss of a proton

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5
Q

In an acid and base reaction what are the spectator ions?

A

Ions which are not directly involved in the acid or base reaction.

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6
Q

For the reaction identify which are the conjugate acid and base pairs

HNO2 +H2O NO2- + H3O+

A

HNO2 acid 1 to NO2- base 1

H2O base 2 to H3O+ acid 2

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7
Q

What are mono, Di and Tri and poly basic acids?

A

Monobasic acid:
Each molecule can release 1 proton

Dibasic acids:
Each molecule can release 2 protons

Tribasic acids:
Each molecule can release 3 protons

Polybasic acids (Di/Tri) release their protons in multiple steps

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8
Q

What is a neutralisation reaction?

A

When an acid and base react and give a salt and water

Strong acid + strong base

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9
Q

What happens when an acid reacts with a carbonate?

A

Forms a salt, water and carbon dioxide

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10
Q

What happens when an acid reacts with a base?

A

Forms a salt and water

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11
Q

What is an alkali?

A

A base which dissolved in water forming OH- ions

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12
Q

What happens when an acid reacts with a metal?

A

Forms a salt and hydrogen gas

Redox reactions

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13
Q

How do you calculate H+ concentration and pH?

A

pH = -log([H+])

H+ = 10^-pH

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14
Q

What is a strong acid?

A

An acid which can fully dissociate in water

HA ——> H+ + A-

Therefore the [acid] = [H+]

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15
Q

What is a weak acid?

A

An acid which only partially dissociates in water and are therefore in equilibrium

We can only compare the strengths of weak acids

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16
Q

How do you calculate the pH of a strong acid?

A

Since [acid] = [H+]

We can easily find pH from
-Log(H+)

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17
Q

What is Ka?

How does it relate to strong and weak acids?

A

Acid dissociation constant

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

Strong acids - high Ka value

Weak acids - low Ka value

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18
Q

What is the strength of an acid?

A

How much an acid will dissociate when it is dissolved

Strength of weak acid is how far its equilibrium lies to the right

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19
Q

Give some examples of strong acids

A 
HCL
NHO3
H2SO4 
HBr
HI
HClO4
20
Q

How do you find the pH of a weak acid?

A

Use Ka
Must assume the all the protons in the solution come from the acid dissociation

Therefore [H+] = [A-] =[H+]^2

Ka = [H+]^2 / [HA]

[H+] = root( Ka X [HA] )

21
Q

What is the relationship between PKa and Ka?

A

PKa = -log(Ka)

Ka = 10^-PKa

Strong acids - low PKa values

Weak acids - high PKa values

22
Q

Why is water amphoteric?

A

Water can both accept and donate protons and therefore act as an acid and a base

23
Q

What is the ionic product of water?

A

Kw = [H+] [OH-]

At 298K pure water has the same amount of [H+ ions as OH- ions and so

[H+] = [OH-]

24
Q

What is Kw at 298K (25 degrees)?

A

Kw = 1x10^-14

=[H+]^2

Therefore pH =7 (neutral)

25
Q

How do you determine the strength of a base?

A

Base strength is determine by its ability to dissociate and form OH- ions

A strong base will fully dissociate

26
Q

Give some examples of strong bases

A

Hydroxides
Group 1 and 2 metals

NaOH ——> [Na+] + [OH-]

27
Q

Give an example of a weak base

A

Ammonia

NH3 + H2O ——> NH4+ + OH-

28
Q

How do you find the pH of a strong base?

A

Use the Kw expression
Assume that [OH-] =[base] (undissociated)

[H+] = Kw / [OH-]

29
Q

What is a buffer?

A

A solution that minimises a change in pH by the addition of a small volume of acid or base.

They DO NOT STOP changes just minimise them.

30
Q

What is an acidic buffer?

A

A mix of a weak acid and its conjugate base and salt.

Buffer solution = weak acid + it’s salt
CH3COOH + CH3COO-Na+

Acid partially dissociates into ethanoate ions and H+ ions
Eg. CH3COOH H+ + CH3COO-

Salt dissociates completely producing ethanoate and sodium ions
Eg. CH3COO-Na+ ——> CH3COO- + Na+

31
Q

What happens when acid is added to an acidic buffer?

A

CH3COOH CH3COO- + H+

Adding extra acid shifts equilibrium to the left to remove the H+ added and maintain the pH

32
Q

What happens when alkali is added to an acidic buffer?

A

CH3COOH CH3COO- + H+

Equilibrium shifts right to replenish the H+ ions which have been used up in neutralising the alkali added and therefore maintain the pH

33
Q

What is a basic buffer?

A

Basic buffers resist a change in pH and maintain a pH above 7.

They are made from a mixture of a weak base and its salt

Eg. Ammonia and ammonium chloride

34
Q

What are some naturally occurring buffers?

A

Buffers appear in biological systems as pH needs to be controlled

Carbonic acid and Hydrogen carbonate ions act as a buffer in the human body.

35
Q

How do you calculate the pH of a buffer?

A 
Use Ka (not H+ ^2)
Assuming that [HA] (undissociated) = [HA] (equilibrium)

And

Assuming that [salt] = [A-] (equilibrium)

H+ = [HA] X Ka / [A-]

36
Q

How can you find the concentration of an acid or a base experimentally?

A

Using acid-base titrations .

37
Q

How do you do an acid base titration?

A

Usually add a know concentration of acid or base to a known volume of the other species in order to find the concentration of that species.

Using a pH meter enables us to follow the pH as we add the acid or base.

38
Q

What is the equivalence point?

A

The point where all the acid or base in the know volume has reacted with the other

Moles of base = moles of acid

39
Q

What is the end point?

A

The point at which an indicator changes colour due to there being equal numbers of HIn and In- ions.

40
Q

What is an acid base indicator?

A

Usually a weak acid

And is used to determine the end point of a reaction by a distinguishable colour change.

41
Q

How does and acid base indicator work?

A

HIn H+ + In-

The indicator and its conjugate base have different colours in solution

Eg. Bromothylmol blue
Yellow blue

Adding H+ shifts eqm left and solution becomes yellow

Removing H+ shifts eqm right and solution becomes blue.

42
Q

How do you choose a suitable indicator?

A

Colour change must be sudden not gradual

End point must be near equivalence point

43
Q

What are the key areas to consider on a titration curve?

Consider adding a base to an acid

A
  1. Slight increase as you add the base but the acid is in excess and so little change occurs
  2. Sharp increase when we have used up all the acid and small amounts of the base make a big change
  3. The base is now in excess so adding more of the base makes little difference
44
Q

Explain why water remains neutral when it’s pH is less than 7?

A

There is greater dissociation at higher temperatures so the pH is reduced however the concentration of [H]+ and [OH-] remain equal so no change in neutrality.

45
Q

Describe the steps in finding pH values for a pH curve when an alkali is neutralised by an acid?

A

Add the acid to the alkali 1cm3 at a time.
mix the solution ensuring it is equally reacting
record the pH with a pH probe or data logger
continue until the pH is constant.

A-level Chemistry (33 decks)

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