Ph and indicators
Base dissociation constant
(B+)(OH-)/(BOH)
Ph titration
1. Base in burette acid in beaker, start with low ph 2. Stir with ph sensor(measures h+ ion conc) 4. Graph ph vs time using data logger 5. No indicator
Limitations
1. Limited to ph in 0-14 range 2. Doesn't work with v conc solns as complete dissociation doesn't always occur(can't predict) 3. Limited to aqueous solutions
Why is ph always 7
1. Same number of h+ as Oh- ions as for every h+ formed when a h20 dissociates an oh- is also formed
Ionic product of water
1by10tothe-7
The self-ionisation of water
2H2O -> h3O+ + OH-
Strong acid strong base
3 to 10 Large jump cause by addition of one drop- sharp Any indicator that's one colour at 3 and another at 10
Methyl orange
3 to 5 Acid- red Base- yellow
Strong acid weak base
3 to 7 Methyl orange- range 3 to 7 -litmus doesn't change till 8 -phenol range above
Litmus
5 to 8 Acid- red Base- blue
Weak acid strong base
7 to 10 Phenol- range lies within vertical part
Phenolphthalein
8 to 10 Acid- colourless Base- pink
Strong acid
A good proton donor eg hcl h2so4 hno3 - almost completely dissociates
Universal indicator
A mixture of many
A weak acid
A poor proton donor only slightly dissociates
Acid base indicator
A substance that changes colour according to the ph of the solution in which it is placed
The ph scale
Acidic solution-less than 7 Neutral solution-equal to 7 Alkaline soln- greater than 7
Calculating ph strong acid strong base
Assumed to have completely dissociated in water (see notes)
When they're a weak acid
Colour of dissociated molecule is different to that of the dissociated molecule
Structure of indicators
Contain lots of benzene rings
Ph of weak acids and weak bases
Depends on value of dissociation constant and solutions concentration (see notes)
Why different ranges
Each indicator has their own dissociation constant
When h+ ions added
Eq to left to absorb (acidic)
When oh- ions added
Eq to right as h+ combines with oh- to make water so more h+ must be made
Strong base
Good proton acceptor eg. Naoh and koh
Acidic solution
H+ greater than oh-
Basic solution
H+ less than Oh-
Strong acid
Has a weak conjugate base
Acid dissociation constant
Ka = (H+)(A)/(HA) (H20 doesn't appear but use if asked for kc)
Kw
Kc(h2o) = (H+)(OH-)
Weak base indicators acid added
Oh- and h+ form water so shifts to right
Weak acid weak base
Ph rises gradually so not possible to detect end point using indicator as no sudden jump- not used to find concs
Graph name
Ph titration curve
Weak base
Poor proton acceptor
Base added
Shifts to left to absorb excess oh-
Weak acid
Strong conjugate base
Dependent on
Temp- increases with increasing temp as ph falls
Values of Ka
The larger the value the higher is the concentration of h3o+ and the stronger the acid
The ph of a solution
The negative logarithm to the base 10 of the hydrogen ion conventration measured in moles per litre
Range of an indicator
The ph interval over which there is a clear change of colour for that indicator
Ie
The stronger the acid the weaker the conjugate base The weaker the acid the stronger the conjugate base
Measuring ph
Universal indicator Ph sensor and data logger sensitive to h+ ion conc eg. Hospitals
Meaning
Variation in h2o v small in comparison to its size The ionic product of water
Ion concentration
Very low so equilibrium lies much to the left
