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Physical chemistry 2 > Acid, Bases And Kw > Flashcards

Acid, Bases And Kw Flashcards

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1
Q

What is an acid and what is a base

A

Acid: releases protons
Base: accepts protons

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2
Q

What are Brønsted-Lowry acids?

A

• Proton donors that release hydrogen ions (H+) when they are mixed with water

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3
Q

What is the end product of hydrogen ions released in water

A

• You never get H+ ions by themselves in water, they’re always combined with H2O to form hydroxonium ions, H3O+

• HA(aq) + H2O(l) -> H3O+(aq) + A-(aq)

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4
Q

What are Brønsted-Lowry bases

A

• Proton acceptors
• In solution, they grab hydrogen ions from water molecules
• B(aq) + H2O(l) -> BH+(aq) + OH-(aq)

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5
Q

What difference between Strong acids and weak acids and give examples with equations

A

• Stong acids: dissociate almost completely in water-nearly all the H+ ions will be released
• Hydrochloric acid is a strong acid
• HCl -> H+ + Cl-

• Weak acids: dissociate only very slightly in water so only small numbers of H+ are formed
• Ethanoic acid or citric acid
• An equilibrium which lies well over to the left
• CH3COOH equilibrium CH3COO-(aq) + H+(aq)

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6
Q

What is the difference between strong bases and weak bases. give examples and equations

A

• Strong bases ionise almost completely in water
• Sodium hydroxide (NaOH)
• NaOH -> Na+ + OH-

• Weak bases only slightly ionise in water
• Ammonia (NH3)
• Just like weak acids, the equilibrium lies well to the left
• NH3 + H2O equilibrium NH4+ + OH-

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7
Q

What is transferred when Acids and Bases React

A

Protons

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8
Q

How do acids get rid of their protons and give an example

A

• They can only get rid of them if there’s a base to accept them
• HA(aq) + B(aq) equilibrium BH+(aq) + A-(aq)
• It’s an equilibrium

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9
Q

Note

A

The equilibrium’s far to the left for the weak acids, and far to the right for strong acids

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10
Q

What ions does water dissociate into? And give equations

A

• Hydroxonium and hydroxide ions
• H2O + H2O equilibrium H3O+ + OH-
• H2O equilibrium H+ + OH-
• Equilibrium law can be applied to calculate the equilibrium constant

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11
Q

What are the effects of water dissociating slightly

A

• There’s always so much water compared to the amount of H+ and OH- ions that the concentration of water is considered to have a constant value, you constant
• Called the ionic product of water and given the symbol Kw
• Kw= Kc x [H2O] = [H+][OH-]
• Kw= [H+][OH-]
• The units for Kw mol2dm-6

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12
Q

What condition affects the value of Kw

A

Temperature

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13
Q

Note

A

In pure water, there is always one H+ ion for each OH- ion. if you’re dealing with pure water, then you can say that Kw=[H+]2

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14
Q

What is the purpose of pH scale

A

The measure of the hydrogen ion concentration

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15
Q

Why is a logarithmic scale used to measure the concentration of hydrogen ion

A

The concentration of hydrogen ions in a solution can vary enormously

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16
Q

Give the formula for calculations pH

A

• pH= -log10[H+]
• [H+] is the concentration of hydrogen ions in a solution, measured in mol dm-3

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17
Q

Describe the pH scale

A

The pH scale normally goes from 0 (very acidic) to 14 (very basic). 0H is regarded as being neutral

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18
Q

How can you calculate the pH from hydrogen concentration?

A

If you know the hydrogen concentration of a solution, you can calculate the pH using this formula:
• pH= -log10[H+]

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19
Q

How can you calculate the hydrogen ion concentration from pH

A

[H+] = 10^-pH

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20
Q

Describe strong monoprotic acids

A

• Such acids such as hydrochloric acid and nitric acid ionise fully in solution
• Monoprotic means that each molecule of acid will release one proton when it dissociates
• This means one mole of acid produces one mole of hydrogen ions.
• So the H+ concentration is the same as the acid concentration

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21
Q

Describe Strong diprotic acids

A

• A strong diprotic acid releases 2 protons when it dissociates
• Produces 2mol of hydrogen ions for each mole of acid
• Sulfuric acid is an example of a strong diprotic acid

22
Q

Formula for molar concentration

A

11.22 g dm-3/ Mr

23
Q

What is the formula for acid dissociation constant

A

Ka

24
Q

Why is the Ka constant used to find pH

A

Weak acids and bases only dissociate slightly in an aqueous solution, so the H+ is not the same acid the acid concentration

25
Q

Give the equation for weak acid, HA

A

HA (aq) equilibrium H+(aq) + A-(aq)

26
Q

Give the equation for Ka

A
  • Ka= [H+] [A-]/ [HA]
  • Ka= [H+]2/[HA]
27
Q

What are the units for Ka

A

mol dm-3

28
Q

What is Ka dependent on?

A

Ka is dependent on the temperature of a particular acid

29
Q

Give the formula for pKa

A

pKa= -log10 Ka

30
Q

Give the formula for Ka

A

Ka= 10-pKa

31
Q

The smaller the pKa…..

A

The stronger the acid

32
Q

How do we use titration to find the concentration of an acid or base

A
  • Add a standard solution of acid to measured quantity of base or vice versa
  • Pipettes and burettes are used for accurate measurement
  • An indicator/ pH meter is added to show you exactly when its neutralized by the acid
33
Q

What does the vertical on a pH curve indicate

A

It is here that all the acid is neutralized

34
Q

When is methyl orange used as an indicator

A
  • For a strong acid and weak base
35
Q

When is phenolphthalein used as an indicator

A
  • For weak acid and strong base
36
Q

What indicator is used for a weak acid and weak base

A

Neither methyl orange and phenolphthalein

37
Q

What indicator is used for a strong acid and strong acid

A

Both methyl orange and and phenolphthalein

38
Q

What is a buffer

A

A buffer is a solution that resists changes in pH when small amounts of acid or base are added or when its diluted

39
Q

Give two types of buffers

A
  • Acid buffers
  • Base buffers
40
Q

What are acidic buffers made from?

A

A weak acid and one of its salts

41
Q

Describe this reaction:
CH3COOH(aq) equilibrium H+(aq) + CH3COO-(aq)

A
  • Ethanoic acid dissociates slightly
  • Salt fully dissociates
  • undissociated ethanoic acid molecules
  • lots of ethanoate ions from the salt
42
Q

CH3COOH(aq) equilibrium H+(aq) + CH3COO-(aq), Describe the effect of adding an acidic buffer on adding small amounts of acid

A
  • If you add small amounts of acid, the H+ concentration concentration
  • H+ ions react with CH3COO- to form CH3COOH
  • Equilibrium shifts to the left reducing H+ concentration
  • So pH doesn’t change
43
Q

Describe the effect of adding an acidic buffer on adding small amounts of base

A
  • Small amount of base, OH- concentration increases.
  • OH- ions react with H+ ions to form water
  • Removing H+ ions from the solution, causes CH3COOH to dissociate to form H+ ions
  • Equilibrium shifts to the right
  • The H+ concentration increases so the pH does not change
44
Q

What are basic buffer made form?

A

Weak base and one of its salt

45
Q

Describe the effect of adding a basic buffer on adding small amounts of a base
e.g. NH4Cl(aq) -> NH4+(aq) + Cl-(aq)
NH3(aq) + H2O(aq) equilibrium NH4+(aq) + OH-(aq)

A
  • If a small amount of base is added, the OH- concentration increases
  • Solution is more basic
  • OH- ions will react with the NH4+ ions to form NH3 and H2O
  • Equilibrium will shift to the left
  • Removing OH- ions and stopping pH from changing much
46
Q

Describe the effect of adding a basic buffer on adding small amounts of an acid
e.g. NH4Cl(aq) -> NH4+(aq) + Cl-(aq)
NH3(aq) + H2O(aq) equilibrium NH4+(aq) + OH-(aq)

A
  • Adding small amounts of an acid leads to H+ concentration increasing , solution becomes more basic
  • Some of the H+ ions react with OH- ions to make H2O
  • Equilibrium shifts to the right to replace the OH- ions that have been used up
  • Some of the H+ ions react with NH3
  • These will remove the extra H+ ions so the pH won’t change much
47
Q

What are the purposes of buffers

A
  • Shampoos, contains buffers to prevent human hair from becoming rough if exposed to alkaline conditions
  • Enzymes in the body
    And buffers in the body to keep the right pH
48
Q

What is a weak acid?

A

Acid partially or slightly dissociates in water to form H+ ions

49
Q

Suggest why the pH probe is washed with distilled water between each of the calibration measurements

A

• Different solutions must not contaminate each other
• To wash off residual solution/ substance
• To avoid missing end point

50
Q

Write an expression for the acid dissociation constant Ka for ethanoic acid.

A

ka= [CH3COO-] [H+]/ [CH3COOH]

51
Q

Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.

A
  • Cl is (more electronegative so) withdraws electrons
    OR negative inductive effect of Cl
  • Weakens O–H bond

Physical chemistry 2 (7 decks)

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