PH and indicators Flashcards
Define PH
Ph = -log10[H+]
or
Ph= -log10[ H3+O]
What is the ph scale
a scale that runs from 0 to 14 that indicted its level of acidity or alkalinity
Give three limitations of the pH scale
The pH is inly useful/valid in the 0-14 range
The pH scale only applies to aqueous solutions
The pH scale does not work with very concentrated solutions
Arrhenious definition for an acid
dissociates in water to produce H+ or H3O+ ions
Arrhenious definition for a stong base
dissociated fully in water to produce OH- ions
Arrhenious definition for a weak base
dissociates only slightly in water to produce OH- ions
Arrhenious definition for monobasic acid
each molecule dissociated to produce one H+ ions in solution
Arrhenious definition for dibasic acid
each molecule dissociates to produce two H+ ion in solution
Arrhenious definition for tribasic acid
each molecule dissociated to produce three H+ ions in solution
Arrhenious definition for a base
dissociated in water to produce OH- ions
Arrhenious definition for strong base
dissociates fully in water to produce OH- ions
Arrhenious definition for weak base
dissociates only slightly in water to produce OH-
Arrhenious definition for monobasic base
each molecule dissociated to produce one OH- ion in solution
Arrhenious definition for a dibasic base
each molecule dissociated to produce two OH- ions in solution
Arrhenious definition for tribasic base
each molecule dissociated to produce three OH- ions in solution
Two substances with the same pH
have the same [H+]
Have the same pOH
Have the same [OH-]
do acids still have an [OH-]?
yes they do… just very low - if asked for the [OH-] of an acid, treat ‘as if’ it is a base
Do Bases still have H+?
yes they do…just very low - if asked for the [H+] of an acid, treat it ‘as if’ it is an acid
what are acid base inicators
substances which change colour according to the ph of the solution in which they are placed
name a acid base indicator which change colour at 3-5 , 5-8 , 8-10
Methyl orange 3-5 yellow(base) to red(acid)
Litmus 5-8 Blue(base)to red(acid)
Phenolphthalein 8-10 Pink(base) to colourless (Acid)
what is the ph of an indicator
indicators themselves are weak acids or weak bases and dissociate in solution
HIn is an indicator that is itself a weak acid,Write a balanced equation to represent the dissociation of HIn in water/solution
Hln + H2O ⇌ ln- + H3O+
HIn is an indicator that is itself a weak acid
Place the indicator in an acid
concentration of H3O+ increases
the equilibrium shiftfs to decrease concentration of H3O+ to the left to use it up
Reverse reaction favoured
colour A (left hand side) due to increased concentration of Hln
HIn is an indicator that is itself a weak acid.
Place the indicator in a base
concentration of OH- increases, they will react with the H3O- ions to form H2O
concentration of H3O+ decreases
equilibrium shiftfd to increase concentration of H3O to produce more of it.
Foward reaction favoured to the right.
Colour B forms (RHS) to increase the concetration of ln-
XOH is an indicator that is itself a weak base.
Write a balanced equation to represent the dissociation of XOH in water / solution
XOH ⇌ X+ + OH-
XOH is an indicator that is itself a weak base.
Place the indicator in an acid
Concentration of H+ increases - they will react with OH- ions to from H2O. Concentration of OH- decreases.
Equilibrium shifts to increase concentration of OH- to produce more of it
Foward reaction favored to the right
Colour B forms due to increased concentration of OH- (RHS)
XOH is an indicator that is itself a weak base.
Place the indicator in an base
Concentration of OH- increases
Equilibrium shifts to decrease conc of OH-, to the left to use it up. The reverse reaction is favoured, Colour A forms due to increased conc of XOH
What is a pH titration
An acid base titration where a pH meter is placed in the acid in a beaker.
ALL OF THE BASE is added.Changes in PH are recorded
A pH curve is plotted
The Endpoint can be found from the vertical part of the graph. Endpoint if the volume of base required for neutralisation.
In pH titrations,why is a beaker used instead of a concial flask
a beaker is more suitable to place the ph meter probe and stir
In pH titrations,Why is the acid in a beaker and base in a burette
Allows us to start with a low ph value and work upwords
In pH titrations,Why is a indicator not required
Ph is directly measured by pH meter
Decribe a pH titration between a strong acid and a strong base, and list a potential indicator and explain
End point between 3 -11,
The vertical part of the titration is the volume of base required for neutralsation.
Methyl orange or Phenohphtalein
the vertial part of the titration- end point between pH 3-11
Methyl orange or Phenolphtalein changes colour according to the vertical part of the graph
Decribe a pH titration between a stong acid and a weak base and list a potential indicator and explain
the end point is between ph 3-7
a potential indicator is methyl orange. The vertical part of the titration is the volume of base required for neutralsation.Shown by the vertical part.
The end point is between3-7, Methyl orange changed colour according to the vertical part of the graph
Decribe a pH titration between a weak acid and a strong base and a potential indicator, explain
the end point is between ph 7-11.
potetial indicator is phenophothalein.The vertical part of the titration is the volume of base required for neutralsation. as shown by the vertical graph- the end point occours between a PH of 7-11. Phenophthalein changes colour coinciding with the vertical part of the graph
What is the formula for Vb?
Ma x Va / na =Mb x Vb/ nB
Write a equation for the dissociation of water
2H2O ⇌ H3O+ + OH-
what is the only thing that affects Kw
temperture
Is the self ionisation of water endothermic or exothermic, explain
As the temperture increases, Kw increases.
Equilibrium shifts to decrease temperture in the endothermic direction. Kw: the foward reaction if favoured.The foward reaction if endothermic, the self ionisation of water is endothermic
what is unique about pure water
[H3O+]=[OH-]
