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OCR A A-Level Inorganic Chemistry > Chapter 20 > Flashcards

Chapter 20 Flashcards

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1
Q

According to the Arrhenius model, what are acids and bases?

A
  • Acids: any substance that adds H+ ions to water
  • Bases: any substance that adds OH- ions to water
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2
Q

What do H+ ions often exist as, and why? Which form tends to be used in equations, and why?

A
  • Hydronium ions
  • H3O+
  • Protons can’t exist on their own
  • H+, as it is simpler to use
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3
Q

What is inaccurate about Arrhenius’ model?

A
  • It only describes acids and bases that are in solution
  • Acids and bases were only described in terms of H+ and OH- ions, which doesn’t include all of them (e.g. ammonia)
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4
Q

What replaced the Arrhenius model?

A
  • The Brønsted-Lowry model
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5
Q

How are acids and bases defined in the Brønsted-Lowry model?

A
  • In terms of their reactions with each other; a proton is transferred from the acid to the base
  • Acids: proton donors
  • Bases: proton acceptors
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6
Q

How is the dissociation of weak and strong acids shown?

A
  • Weak acid <-> H+ + anion
  • Strong acid -> H+ + anion (as equilibrium is so far to the right)
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7
Q

What type of acid is HF?

A
  • A weak acid
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8
Q

Give the 2 versions of the equations for the dissociation of water.

A
  • H2O <-> H+ + OH-
  • H2O + H2O <-> H3O+ + OH-
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9
Q

In the reaction of a strong acid and a weak acid, which acts as a Brønsted-Lowry acid and which acts as a Brønsted-Lowry base?

A
  • The strong acid is the Brønsted-Lowry acid
  • The weak acid is the Brønsted-Lowry base
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10
Q

What happens to a Brønsted-Lowry base in a reaction?

A
  • It accepts a proton to form its conjugate acid
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11
Q

What happens to a Brønsted-Lowry acid in a reaction?

A
  • It donates a proton to form its conjugate base
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12
Q

What are an acid and its conjugate base (and alternatively a base and its conjugate acid) called together?

A
  • A conjugate acid-base pair (or just a conjugate pair)
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13
Q

What role do conjugate pairs have in acid-base equilibria?

A
  • According to the Brønsted-Lowry model, acid-base equilibria always contain 2 conjugate pairs
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14
Q

What are monobasic, dibasic and tribasic acids?

A
  • Monobasic acid: an acid where 1 hydrogen ion can be replaced per molecule in an acid-base reaction
  • Dibasic acid: an acid where 2 hydrogen ions can be replaced per molecule in an acid-base reaction
  • Tribasic acid: an acid where 3 hydrogen ions can be replaced per molecule in an acid-base reaction
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15
Q

Which terms can be used instead of monobasic, dibasic and tribasic?

A
  • Monoprotic
  • Diprotic
  • Triprotic
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16
Q

What needs to be remembered when writing ionic equations?

A
  • Only aqueous compounds (not even solid ionic compounds) can be split into their ions (since ions can only be aqueous)
17
Q

How does the pH scale work, and why?

A
  • The pH scale is actually a logarithmic scale used to quantify [H+] more easily
  • Each number (x) on the pH scale actually represents [H+] of 1x10^-x moldm^-3
  • Log base 10 was used to remove the powers of 10, and the negatives were then removed (which is why a low value actually means high acidity)
  • Although the pH scale goes from 0 to 14, it is possible to have values outside the range- however, they are so dangerous that it isn’t common to use them
18
Q

How do you calculate the concentration of hydrogen ions from pH values? How is this rearranged to calculate pH from the concentration of hydrogen ions?

A
  • [H+] = 10^-pH
  • pH = - log [H+]
19
Q

What is the difference in concentration between solutions with a difference in pH of 1?

A
  • The one with the lower pH has a concentration 10 times larger
20
Q

Why is the pH scale in terms of [H+]?

A
  • At the time of its creation, the Arrhenius model was followed
21
Q

How can you calculate the pH of a strong, monobasic acid if given its concentration?

A
  • Since the ratio of the acid and the hydrogen ions is 1:1 in the equation of their dissociation, they have the same concentration
  • You can put this value into the equation to find the pH of a solution
22
Q

How can you calculate the pH of a strong, monobasic acid if it is diluted?

A
  • You can use the concentration of the acid and the starting volume to find the number of moles
  • Since the number of moles stayed the same but the volume changed, you find the new value for [H+], and use this to find the pH
23
Q

How can you calculate the pH of a solution after a strong, monobasic acid is reacted with a base (solid and aqueous)?

A
  • If the base is solid:
  • you can assume the volume stays the same, since the volume of water produced would be negligible
  • you use stoichiometry to find the number of excess moles of acid (some are used up forming salt and water)
  • using the new number of moles, you can calculate [H+], and therefore the pH
  • If the base is aqueous:
  • the volume changes (it is the volume of the acid and base added together)
  • you calculate the number of moles in the same way as when the base was solid
  • you find the concentration from the new values for number of moles and volume (the volume of water is still ignored), and use this to find the pH
24
Q

What is Ka?

A
  • The acid dissociation constant
25
Q

How is Ka calculated? What does this mean for it?

A
  • It is Kc for the dissociation of a weak, monobasic acid
  • Its units are always moldm^-3
26
Q

What do values of Ka tell us about weak acids?

A
  • The higher the Ka value, the stronger the acid (since equilibrium is further to the right)
27
Q

What issue do Ka values have, and what has been done about this?

A
  • Just like values of [H+], Ka values can be very small, and therefore hard to compare
  • Just as [H+] has pH, Ka has pKa
28
Q

How do you convert between Ka and pKa?

A
  • pKa= -logKa
  • Ka= 10^-pKa
29
Q

What do values of pKa tell us?

A
  • The larger the value of pKa, the weaker an acid is
30
Q

What 2 approximations can be made when finding Ka for weak acids, and why can each be made?

A
  • The concentration of the acid stays mostly constant, as the dissociation of weak acids is small
  • There is an equal concentration of protons and anions (the production of H+ ions from the dissoication of water is negligible)
31
Q

How can Ka be rewritten?

A
  • Ka= [H+]^2/ [HA] (start)
32
Q

What limitation is there to using approximations to find Ka for weak acids?

A
  • In ‘stronger’ weak acids, [H+] becomes significant, so you can no longer assume that the concentration of the acid at the start and at equilibrium is equivalent
33
Q

What is Kw?

A
  • The ionic product of water
34
Q

Where does Kw come from?

A
  • Water can be treated as a weak acid as it dissociates slightly
  • Ka= [H+][OH-] / [H2O]
  • As water is mainly undissociated, [H2O] is a constant
  • Kw= Ka × [H2O]
35
Q

What is the formula for Kw, and therefore its units?

A
  • Kw = [H+][OH-]
  • mol^2 dm^-6
36
Q

Even if water doesn’t have a pH of 7, how do we know that it’s always neutral?

A
  • [H+] = [OH-]
37
Q

What is the other form of the formula for Kw when dealing with water, and why does it work?

A
  • Kw = [H+]^2
  • [OH-] = [H+]
  • Water is neutral
38
Q

What can change the value of Kw?

A
  • Temperature
39
Q

How is the pH of strong bases calculated?

A
  • In monoacidic bases like NaOH, their concentration is equal to the concentration of hydroxide ions (in the equation of their dissociation, they’re in a 1:1 ratio)
  • You then use the equation for Kw to calculate [H+], and therefore the pH

OCR A A-Level Inorganic Chemistry (14 decks)

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