12. The ionisation of water and Kw

Question 1

the ionic product of water (Kw)

Answer 1

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

at 25 C Kw = 1.00 x 10 to the power of -14 mol2dm-6

Question 2

how do you get Kw from water

Answer 2

water exists at equilibrium
ionisation of water- H2O(l) H+(aq) + OH-(aq)
equilibrium lies to left. only tiny amount of water dissociated.
Kc=[H+(aq)] + [OH-(aq)] / [H2O(l)]
dissociation of H20 is so small considered constant like Kc
rearrange to combine two constants
Kc x [H2O(l)] = [H+(aq)] x [OH-(aq)] simplifies to
Kw= [H+(aq)] x [OH-(aq)]

Question 3

units for kw

Answer 3

moldm-3 x moldm-3 = mol2dm-6

Question 4

why does Kw = 1.00 x 10-14 mol2dm-6 at 25C

Answer 4

at 25C PH of water 7
[H+(aq)] = 10 to the power of -7 moldm-3 (00000000.1)
when pure water ionises same amount of OH- and H+ ions
this mean conc of OH- is also 10 to the power of -7 moldm-3
Kw= [H+(aq)] [OH-(aq)] = (1.0 x 10-7) x (1.0 x 10-7)= 10-14 mol2 dm-6

Question 5

constant Kw changes only if

Answer 5

temperature is altered

Question 6

significance of Kw

Answer 6

controls balance of [OH-] and [H+] in aq solutions

Question 7

all Aq solutions contain H+(aq) and OH-(aq) ions

1. in water and neutral solutions
2. in acidic solutions
3. in alkaline solutions

Answer 7

1. [H+(aq)] = [OH-(aq)]
2. [H+(aq)] > [OH-(aq)] (more H+)
3. [H+(aq)] < [OH-(aq)] (more OH-)

Question 8

how H+ and OH- are related at differnt PH values (25C)

Answer 8

indicies always add to -14

Question 9

1. common strong bases

2. strong bases are

Answer 9

1.NaOH
KOH
Ca(OH)2
2.fully dissociated in solution

Question 10

common weak base

Answer 10

NH3 ammonia

Question 11

how temperature affects Kw

Answer 11

the dissociation of water is endothermic
H2O H+ + OH- △H+ve
increase in temperature (favours endothermic direction) products increase = Kw increases