LC Chemistry- PH and Indicators

3 limitations of the ph scale

-doesn't distinguish between strong acid and weak acid

-pH values outside the range of 0-14 cant be reliably calculated

-only applies to diluted solutions

ways ph can be measured (3)

universal indicator paper, universal indicator solution and ph meter

ka

dissociation constant of an acid

weak acid indicator equation

Hln(colour x) = [H+] + ln-(colour y)

weak acid indicator in acidic solution

shifts equilibrium from right to left and colour x predominates

weak acid indicator in basic solution

shifting the equilibrium from left to right and colour y predominates

weak base indicator equation

lnOH (colour x) = X+(colour y) + OH-

weak base indicator in acidic solution

equilibrium shifts to the right and colour y predominates

weak base indicator in basic solution

equilibrium shifts to left and colour x predominates

strong acid vs strong base example

NaOH vs HCl

strong acid vs weak base example

HCL vs Na2CO3

strong base vs weak acid example

NaOH vs CH3COOH

weak acid vs weak base example

CH3COOH vs NH3

Equation for the self ionisation of water

H2O <-> H+ + OH-

Give expression for the ionic product of water

Kw = [H+][OH-]

-concentrations of [H+] and [OH-] in moles/l

Give Kw of water at 25 deg C

1x10^-14

What happens to the value of Kw if HCl is added to water?

The value of Kw is a constant and cannot change

If [H+] concentration increases the [OH-] concentration must decrease so that the two values multiplied together give 1x10^-14

in other words this gives an acidic solution

Define pH

-log10[H+]

Equation for the general equation of an acid undergoing dissociation

(HA) <-> (H+) + (A-)

Define the full dissociation constant of an acid

Ka = [H+][A-]/[HA]

Give an equilibrium reaction for the formation of the hydronium ion

(dissociation of acid in water)

HA + H2O <-> H3O+ + A-

The larger the value of Ka, the ____ (higher or lower)the concentration of H3O+ and the ___ (stronger or weaker) the acid

1) higher

2) stronger

The ____ the acid, the weaker its conjugate base.

The ____ the acid, the stronger its conjugate base.

1) stronger

2) weaker

Formula to determine the pH of a weak acid

[H+] = sq root of (Ka x Macid)

Formula to determine the pOH of a weak acid

[OH-] = sq root of (Kb x Mbase)

Define an acid base indicator

a substance that changes colour according to the pH of the solution in which it is placed

Why is very little indicator added to a titration reaction?

the indicator is a weak acid or base and its concentration makes negligible contribution to the H+/OH- concentration

Define the range of an indicator

the pH interval over which there is a clear change of colour for that indicator

Methyl orange:

-approx. pH range

-acid colour

-base colour

3-5

red

yellow

Litmus:

-approx. pH range

-acid colour

-base colour

1)5-8

2)red

3)blue

Phenolphthalein:

-approx. pH range

-acid colour

-base colour

1)8-10

2)colourless

3)pink

Name indicator(s) suitable for titrating a strong acid against a strong base

Why can these be used?

1)methyl orange

2)litmus

3)phenolphthalein

They have one colour at the bottom (around pH 3) of the vertical part and another at the top (around pH 10) of the vertical part

therefore they can successfully identify the end point

Name indicator(s) suitable for titrating a strong acid against a weak base

For what pH values is the curve vertical?

Why can these be used?

1)methyl orange

Curve is vertical between 3-7 approx

It has one colour at pH 3 and another at pH 7 ie top and bottom of vertical part of the graph

it is the only indicator whos range lies within the steep vertical part of the graph

Name indicator(s) suitable for titrating a weak acid against a strong base

Why can this be used?

1)phenolphthalein

colour change occurs between pH 7 and 10

which coincides with the sharp rise/vertical part of the graph

Why can an indicator not be used when titrating a weak acid against a weak base?

there is no sudden jump in pH value at any stage therefore it is not possible to detect the end point using an indicator

Why does H2SO4 have a lower pH than HCl?

H2SO4 is diprotic and 1 molecule of H2SO4 produces 2 H+ ions

HCl is monoprotic and 1 molecule of HCl produces 1 H+ ion

Which of the following conjugate bases is stronger?

A- (of a weak acid) or HSO4- (of a strong acid)

A-

1)weak acids exit in undissociated form in water

2)A- is more likely to accept a proton