Chapter 20 - Acids, bases, and PH

Question 1

What is the important chemical component in an acid?

Answer 1

Hydorgen

Question 2

Briefly describe the arrhenius model of acids and bases

Answer 2

• Acids dissociate and release H+ ions in aqeous solution (e.g. water)
• Alkalis (soluble base) dissociate and release OH- ions in aqueous solution
• H+ ions are neutralised by OH- ions to form water. H+(aq) + OH-(aq) -> H2O(l)

Question 3

What is a Bronsted-lowry acid?

Answer 3

Species that donates a proton

Question 4

What is a Bronsted-Lowry base?

Answer 4

species that accepts a proton

Question 5

What is a conjugate acid-base pair?

Answer 5

contains two species that can be interconverted by transfer of a proton

Question 6

In the dissociation of HCl to H+ and Cl- , identify the conjugate acid and base

Answer 6

HCl releases a proton, so therefore is a conjugate acid

Cl- accepts a proton, so therefore is the conjugate base

Question 7

Give the acid base pairs in this equilibria:

HCl(aq) + OH-(aq) H2O(l) +Cl-(aq)

Answer 7

HCl - donates H+ - Acid 1

OH - accepts H+ - base 2

H20 - donates H+ - acid 2

Cl - accepts H+ - base 1

Question 8

When water is the base, what’s formed?

Answer 8

H3O+ - hydronium ion

Question 9

What do the terms monobasic, dibasic and tribasic refer to?

Answer 9

total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction

Question 10

How can ionic equations be simplified?

Answer 10

Cancel the spectator ions (don’t react)

Question 11

What’s the word equation for reaction of an acid and a metal?

Answer 11

Acid + metal -> salt +hydrogen

Question 12

What’s the word equation for reaction of acids and a carbonate?

Answer 12

acid + carbonate -> salt + water + CO2

Question 13

What’s the word equation for reaction of acids with a base?

Answer 13

acid + base -> salt + water

Question 14

What’s the word equation for reaction of an acids with an alkali?

Answer 14

acid + alkali -> salt + water

Question 15

What is the relationship between PH and concentration of H+?

Answer 15

Low value of [H+(aq)] = high PH

high value of [H+(aq)] = low PH

Question 16

What;s the equation for working out PH from [H+]?

Answer 16

PH = -log[H+(aq)]

Question 17

How do you work out [H+(aq)] using PH?

Answer 17

[H+(aq)] = 10^-PH

Question 18

What change in [H+] does 1 PH number increase give

Answer 18

x10 difference

Question 19

How is the PH calculated for a strong acid?

Answer 19

Assume it fully dissociates

Question 20

How is the new PH calculated for a strong acid on dilution?

Answer 20

Work out change in concentration of HA and therefore [H+]

[H+] x original vol/ new total vol

or

C1V1 = C2V2

then put back into PH = -log[H+]

Question 21

What is Ka?

Answer 21

• Acid dissociation constant
• standardised at 25ºC

Question 22

How do you work out Ka?

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

Answer 22

Ka = [H+(aq)] [A-(aq)]/ [HA(aq)]

Question 23

What is pKa value formula? (used to make Ka data more comparible)

Answer 23

pKa = -logKa

Question 24

How do you get Ka from pKa?

Answer 24

Ka = 10^-pKa

Question 25

Why are pKa values used and not Ka?

Answer 25

Much more manageable than Ka. Easier to compare relative acidic strengths using pKa strengths using pKa values than Ka values

Question 26

What does a lower pKa and larger Ka value tell us?

Answer 26

Stronger acid

Question 27

What does a lower Ka value and larger pKa value tell us?

Answer 27

Weaker acid

Question 28

What information is provided by Ka values?

Answer 28

Strength of acid/ extent of dissociation

Question 29

What is Kw?

Answer 29

Ionic product of water

Ions in water (H+ and OH-) multiplied together

Question 30

What’s the equation for Kw?

Answer 30

Kw = [H+(aq)] [OH-(aq)]

Question 31

What is the value of Kw?

Answer 31

1 x 10^-14 mol²dm^-6

Question 32

What are monobasic and dibasic bases?

Answer 32

• Monobasic bases are bases that release one OH- ion e.g. NaOH, KOH, NH₃
• Dibasic bases are bases that release two OH- ions e.g. Ba(OH)₂, Ca(OH)₂

Question 33

How do you find PH of a strong base?

Answer 33

[OH-] = [Base]

[H+] = Kw/[OH-]

PH = -log[H+]

Question 34

How do you calculate PH of reaction between strong acid and a strong base?

Answer 34

1. ) Calculate H+ moles
2. ) Calculate OH- moles
3. ) Calculate XS H+ or OH- (larger mol - small mol)
4. ) Calculate XS [H+] or [OH]
5. ) Calculate PH

Question 35

What are the 2 assumptions made about weak acids?

Answer 35

1. ) [H+]eqm = [A-]eqm
2. ) [HA] ≈ [HA]initial

[HA] at equilibrium virtually same as it was before any of it dissociated as so little dissociates

Question 36

How do you calculate [H+] of weak acids using Ka?

Answer 36

[H+]² = Ka x [HA]

[H+] = (Ka x [HA])^1/2

Question 37

How do you calculate PH of weak acids?

Answer 37

Ka = [H+]²/[HA]

[H+] = (Ka x [HA])^1/2

PH = -log[H+]

Question 38

How do you calculate PH for the solution formed from reaction between weak acid and a strong base?

Answer 38

For every mole of OH- added, one mole of HA is used up and one mole of A- is formed.

1. ) calculate moles HA (if it is still HA and not H+ - weak acids)
2. ) Calculate OH- moles
3. ) Calculate XS HA or OH-

IF XS is HA:

4. ) Calculate HA mol left + A- formed
5. ) Calculate [HA] left over + [A] formed
6. ) Use Ka to find [H+]
7. ) Find PH

If XS OH-:

4. ) Calculate [OH-]
5. ) Use Kw to find [H+]
6. ) Find PH

Question 39

How is the PH of a diluted solution of a weak acid?

Answer 39

Find concentration of weak acid when diluted:

using C1V1 = C2V2

Calculate [H⁺] :

[H⁺]² = Ka x [diluted weak acid]

PH = -log[H⁺]

Question 40

What’s the relation between [H⁺] and [OH^-] in pure water

Answer 40

They’re the same