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Chemistry A Level > 1.12 Acids and Bases > Flashcards

1.12 Acids and Bases Flashcards

1
Q

What is Arrhenius’ theory for what acids and bases are?

A

-Acids are substances that produce hydrogen ions in solution, and Bases are substances that produce hydroxide ions in solution

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2
Q

What is a hydroxonium ion?

A

Scientists discovered that H+ cannot exist independently in aqueous solution as it has no electrons. Instead, H+ associates with water and forms the hydroxonium ion (H3O+)

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3
Q

What are Brønsted-Lowry’s definitions for what Acids and Bases are?

A

-An acid is a substance which donates protons in a reaction/ is a proton donor. A base is a substance which accepts protons in a reaction/is a proton acceptor

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4
Q

What are Lewis acids?

A

Lone pair acceptors

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5
Q

What are Lewis bases?

A

Lone pair donators

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6
Q

Compare Arrhenius’ and Brønsted-Lowry’s definitions for acids and bases

A

HCl(aq) + NH3(aq) <=> NH4+(aq) + Cl-(aq)

HCl(g) + NH3(g) <=> NH4+(g) + Cl-(g)

In both these reactions HCl has donated H+ and NH3 has accepted H+. In both the gaseous phase and aqueous solution they are brønsted-Lowry reactions, whereas only the first is an Arrhenius reaction, meaning that the BL theory is more applicable to a wider range of reactions.

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7
Q

What is a conjugate acid?

A

A conjugate acid is the species produced when a base gains a proton

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8
Q

What is a conjugate base?

A

A conjugate base is the species produced when an acid loses a proton

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9
Q

Explain what conjugate pairs are (using a reaction as an example)

A

HA + H20 <=> H3O+ + A-
In the forward reaction, HA is an acid and water a base. However in the backwards reaction the H3O+ is the acid as it donates a proton, and A- the base as it accepts a proton. If you consider HA to be the acid then HA to be the acid then A- is its conjugate base. If you consider A- to be the base then HA is it’s conjugate acid

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10
Q

Give some characteristics of strong acids

A

Very low pH, and fully ionise/dissociate

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11
Q

Who created the pH scale?

A

Søren Sørenson

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12
Q

How do you calculate the pH of an acid?

A

pH = -log10(concentration of H+)

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13
Q

How can you calculate the concentration of H+ using the pH?

A

10^-pH

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14
Q

What equation can be used to calculate the concentration of H+ after a dilution?

A

[H+] = ([old H+]xold volume) / new volume

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15
Q

What is Kw?

A

The ionic product of water

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16
Q

What is the value of Kw at 298K?

A

1.0x10^-14

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17
Q

How can we calculate the concentration of [H+] in a base?

A

By dividing Kw by the concentration of [OH-] to give the concentration of [H+]

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18
Q

How can we work out the pH of a mixture of strong bases and strong acids?

A

1) Calculate the moles of H+ and OH-
2) Excess moles - Limiting moles
3) calculate the concentration from the remaining moles
4) use this concentration to calculate the pH

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19
Q

What is the acid dissociation constant (Ka)?

A

The quantifiable measurement of the ability of a weak acid to dissociate.
Calculating using the equilibrium HA+H2O<=>A-+H3O+
and the equation Ka= ([H+]x[A-])/[HA]

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20
Q

If a Ka value is high, is the acid stronger or weaker?

A

Stronger

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21
Q

What are the units for Ka?

A

Moldm^-3

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22
Q

What is the relationship between the concentration of Ethanoic acid at equilibrium and the initial concentration?

A

[CH3COOH]equ=[CH3COOH]initial

23
Q

For calculations, what can we assume about the concentrations of H+ and CH3COO-?

A

that [H+]=[CH3COO-]
Meaning that we can calculate Ka as
Ka= ([H+]^2)/[CH3COOH]initial

24
Q

How do you calculate pKa?

A

pKa=-log10(Ka)

25
Q

How can you calculate Ka (from pKa)

A

Ka= 10^-pKa

26
Q

The larger the pKa, the stronger or weaker the acid?

A

The weaker

27
Q

What are indicators?

A

Weak acids which have a different colour to their conjugate base

28
Q

At what range of pH is Methyl Orange red?

A

At a pH of less than 3.2

29
Q

At what range of pH is Methyl Orange yellow?

A

At a pH above 4.4

30
Q

At what range of pH is Phenolphthalein colourless?

A

At a pH less than 8.2

31
Q

At what range of pH is Phenolphthalein pink?

A

At a pH above 10.0

32
Q

What is pH range for a colour change for methyl orange?

A

3.2-4.4

33
Q

What is pH range for a colour change for phenolphthalein?

A

8.2-10.0

34
Q

What is the equivalence point?

A

When the moles of acid = moles of alkali

35
Q

What indicators can you use for a strong acid+strong base reaction?

A

Methyl orange or phenolphthalein

36
Q

What indicators can you use for a strong acid+weak base reaction?

A

Methyl orange

37
Q

What indicators can you use for a weak acid+strong base reaction?

A

Phenolphthalein

38
Q

How do you calculate the pHs of a strong acid, strong base, weak acid or weak base for a pH curve?

A

– Strong acid calculate the [H+] and then the pH
– Strong base calculate the [OH-] convert to [H+] then calculate the pH
– Weak acid calculate the [H+] = √Ka x [HA] and then calculate the pH
– Weak base assume = pH 11-12 ish

39
Q

What is a buffer solution?

A

A solution which resists small changes in pH when small amounfs of acid or alkali are added or dilution occurs.

40
Q

What does an acid buffer consist of?

A

A weak acid and one of its salts.
OR
A weak acid and a strong alkali

41
Q

What does a basic buffer consist of?

A

A weak alkali and one of its salts.
OR
A weak alkali and a strong acid

42
Q

What are the equilibria formed in an acidic buffer?

A

CH3COOH <=> CH3COO- + H+

A huge excess of CH3COOH and CH3COO-

43
Q

What does an acid react with if added to an acidic buffer?

A

It reacts with CH3COO- to form CH3COOH, however the ratio of Acid:salt remains practically the same as there are such large concentrations of each compared to H+

44
Q

What does a base react with if added to an acidic buffer?

A

It reacts with any H+ present (forming water), and force the equilibria to shift to the right to replace any H+, increasing the conc of CH3COO-

45
Q

What does an acid react with if added to an basic buffer?

A

It reacts with OH- to form H2O, decreasing conc of weak base and increasing conc of salt. However these concentrations are so large that the ratio remains practically the same and pH remains the same.

46
Q

What does a base react with if added to a basic buffer?

A

It reacts with the salt to form the base increasing conc of weak base and decreasing conc of salt. However these concentrations are so large that the ratio remains practically the same and pH remains the same.

47
Q

Give the equation used to find the pH of an acidic buffer?

A

-log10(Kax[HA] / [A-]) = pH

48
Q

At half equivalence, what is equal to Ka?

A

[H+]

49
Q

At half equivalence, what is equal to pKa?

A

pH

50
Q

Give some uses of buffer solutions

A
  • Buffers are used to calibrate pH meters
  • Enzymes are easily denatured by changes in pH so buffer solutions prevent this
  • Shampoo contains acidic buffers as alkaline conditions make hair dull and rough
  • pH is maintained between 7.35 and 7.45 in blood
51
Q

Give the equation to calculate the [H+] in a buffer solution.

A

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

52
Q

Give the equilibria for an acid reacting with hydroxide.

A

HA + OH- <=> A- + H2O

53
Q

Why does calibrating a pH meter before it is used make it more accurate?

A

Because over time the meter fails to give accurate readings.

54
Q

How do you obtain a pH curve for a titration.

A
  • Measure pH of the acid
  • Add alkali in known small portions
  • Stir mixture
  • Measure pH
  • Repeat until alkali is in excess
  • Add in smaller increments near endpoint

Chemistry A Level (35 decks)

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