P- Acid-Base Equlibria

Question 1

State Brønsted–Lowry theory

Answer 1

• acid = proton donor
• base = proton acceptor

Question 2

How does an acid produce H+ ions in solution?

Answer 2

H+ reacts w/ H2O molecules by giving a proton

Question 3

Equation for reaction between a strong acid and water.

Answer 3

HA (g) + H2O (l) –> H3O+ (aq) + A- (aq)
- H3O = hydroxonium ion
- simpler ver: HA (aq) –> H+ (aq) + A- (aq)

Question 4

def + examples of strong acids

Answer 4

• completely ionise
• eg. HCl, H2SO4, HNO3, H3PO4

Question 5

def + examples of weak acids

Answer 5

• partially ionise
• eg. all organic acids (carboxylic) ethanoic CH3COOH, only inorganic weak acid- HF

Question 6

In a reaction, how much of a strong acid is turned into products?

Answer 6

100%

Question 7

Give an example of a monoprotic acid

Answer 7

HCl –aq–> H+ (aq) + Cl- (aq)
1 mole of HCl gives 1 mole of H+ –> monoprotic

• HNO3- strong monoprotic

Question 8

Give an example of a diprotic acid

Answer 8

H2SO4 (l) –aq–> 2H+ (aq) + SO4 2- (aq)
1 mole of H2SO4 gives 2 moles of H+

has a ↓ pH for the same conc as monoprotic ∵ gives ↑ moles of H+

Question 9

Give an example of a triprotic acid

Answer 9

H3PO4 –> 3H+ (aq) + PO4 3- (aq)
1 mole of H3PO4 gives 3 moles of H+

Question 10

Define pH.

Answer 10

pH = -log10 [H+]
- [ ]- conc in moldm^-3

p: -log10

ALWAYS GIVE TO 2dp

Question 11

Equation for reaction between a weak acid and water

Answer 11

HA (aq) ⇌ H+ (aq) + A- (aq)

Question 12

[H+] = ?

rearrange pH formula

Answer 12

[H+] = 10 ^-pH

Question 13

Give an equation for the dissociation of HF.

Answer 13

HF (g) ⇌ H+ (aq) + F- (aq)

Question 14

Calculating pH from [H+]:
What is the pH if [H+] = 1 x 10^ 4

Answer 14

4
1 x 10^4 –> 4

check both values are roughly the same

Question 15

Formula for [H+] of diluted acid.

Answer 15

[H+] = original [H+] x org. vol/ diluted vol (cm3)

Question 16

HA (l) ⇌ H+ (aq) + A- (aq)
Ka = ?

Answer 16

[H+] x [A-]
Ka = ——————–
……………..[HA]
- only for weak acids!!
- [H+] = [A-]
- ↓ Ka - weaker acid (↓pKa)

Question 17

pKa = ?

Answer 17

pKa = -log10 Ka
- Ka = 10^ -pKa

Question 18

What limits the range of pH scale?

Answer 18

solubility - saturated solution- x dissolve any more

Question 19

Is water an acid or a base?

Answer 19

Amphoteric- both

Question 20

What is the equation for water at equilibrium?

Answer 20

H2O (l) ⇌ H+ (aq) + OH- (aq)

Question 21

What is the conc of H+ at room temp?

Answer 21

1 x 10^ -7 moldm-3
pH 7.00

Question 22

Kw = ?

Answer 22

Kw = [H+][OH-]
1 x 10^ -14 mol2dm-6 at stp

stp = 298K, 100kPa

Question 23

pH value for neutral

Answer 23

varies w/ temp

Question 24

def for neutral

Answer 24

[H+] = [OH-]

Question 25

Is the dissociation endothermic or exo + why?

Answer 25

endo
- ↑ temp ↑ Kw – eq. shifted to RHS

Question 26

What is a strong base + give examples?

Answer 26

fully ionises in water
- eg. NaOH, KOH, Ca(OH)2, Ba(OH)2

Question 27

What is a weak base + give an example.

Answer 27

partially ionises in water
- eg. NH3 {NH3 (g) + H2O (l) ⇌ NH4 + (aq) + OH- (aq)}

Question 28

What are [H+] and [OH-] in strong bases?

Answer 28

they are different!
Kw still 1 x 10^-14

Question 29

How do you compare strengths of bases?

Answer 29

see how readily they produce OH- in solution-
1. contain OH- (eg. NaOH (s) –> Na+ (aq) + OH- (aq) )
2. take H+ from H2O (eg. CaO (s) + H2O (l) –> Ca2+ (aq) + 2OH- (aq) )

Question 30

Formulas for the half neutralisation of a weak acid

Answer 30

when half of HA molecules reacted w/ OH-:
- [HA] = [A-]
- Ka = [H+]
- pKa = pH

Question 31

What are acid buffers made of?

Answer 31

weak acid + salt of weak acid
eg. CH3COOH + CH3COO- (both at high conc.&raquo_space; [H+]

Question 32

How does an acidic buffer work?

Answer 32

when H+ (acid) added:
- CH3COO- + H+ –> CH3COOH
- H+ removed by CH3COO- –> x much change in [H+] —> pH stable

when OH- added:
- CH3COOH + OH- –> CH3COO- + H2O
- OH- removed by CH3COOH –> pH stable

Question 33

What is a basic buffer made of?

Answer 33

weak base + salt of weak base

Question 34

How does a basic buffer work?
eg. NH3 + NH4+

Answer 34

H+ added:
- NH3 + H+ –> NH4+
- H+ removed by NH3 –> [H+] barely changes –> pH stable

OH- added:
- OH- + NH4+ –> NH3 + H2O
- OH- removed by NH4+ –> pH stable

Question 35

What is the equivalence point?

Answer 35

when solutions are mixed in exactly the right proportions as in the equation

Question 36

What is the end point?

Answer 36

when indicator changes colour