Topic 12-Acid/Base Quilibria Flashcards
What does an acid do to be classed as an acid?
Releases protons
What does a base do to be classed as a base?
Accepts protons
What determines the strength of an acid?
Strong acids dissociate (ionise) almost completely in water- nearly all H+ ions will be released.
How do you class a weak acid?
Weak acids only dissociate slightly in water so small numbers of H+ ions are formed. An equilibrium is set up which lies well over to the left.
Define a Bronsted Lowry Acid:
Substance that can DONATE a PROTON
Define a Bronsted Lowry Base:
Substance that can ACCEPT a PROTON
In an equation acids are linked to a ….. …… on the other side of the equation
Acids are linked to a conjugate base
Acid base reactions involve the ……… of …….
Acid base reactions involve the transfer of protons
Define the term pH
A figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acidic and higher values more alkaline.
Define Conjugate acid/base pair
Two species that differ by H + .
Give the equation for pH calculations
pH = -log [H+]
Where [H+] is the concentration of hydrogen ions in the solution
How do you know if an acid is STRONG?
It will completely dissociate
Finding [H+] from pH
[H+] = 1 x 10 ^(-pH)
Monoprotic?
Each mole of acid produces 1 mole of hydrogen ions.
Polyprotic?
Each molecule of a strong polyprotic acid releases 2 protons when it dissociates.
Ka is an example of an …… ……
equilibrium constant
Acids and Bases react in …… reactions
Neutralisation Reactions
Define a buffer
A solution that minimises changes in pH when a small amount of acid or base is added.
Acidic buffers are made from:
a weak acid + it’s salt
Basic buffers are made from:
a weak base + it’s salt
What do titration curves show?
The change in pH when a volume of an acid or a base is added
What is the equivalence point?
The point which the exact volume of base has been added to neutralise the acid or vice versa
How can you improve the accuracy of a pH metre?
Maintaining a constant temperature
What is the value of Kw at 298K
1.0 x10 ^-14
What physical factors affect the value Kw? How?
Temperature- if it increases the eq shifts to the right so Kw increases
pKw =
-logKw
What is a titration?
The addition of an acid/base of known conc to an unknown acid/base to determine the concentration. Indicator used to show point of neutralisation, as is pH metre
List equipment needed for a titration
Conical flask White tile Burette Acid/alk of known conc Pipette and filler unknown acid/alk Clamp stand
What generally happens to the pH of the solution around the equivalence point?
There is a large and rapid change in pH (except weak/weak titration)
3 properties of a good indicator?
Sharp colour change
Endpoint must be same as equivalence point
Distinct colour change
What colour is methyl orange in acid & alkali.
Red in acid
yellow in alkali
What colour is phenolphthalein in acid/alkali
Colourless in acid
pink/red in alkali
What is the half neutralisation point?
When volume = half the volume added at equivalence point
What do acidic buffer solutions contain in general terms?
A weak acid and a soluble salt of that acid that fully dissociates.
What do basic buffer solutions contain in general terms
A weak base and a soluble salt of that weak base.
what ion causes a solution to be alkaline
OH-
state assumptions that you make when calculating Ka/pH
all H+ comes from acid (none from water dissociation) If weak acid only small amount of it will have ionised so that we can approximate that [HA]eqm≈ [HA]original standard temp (298K)
give the typical range of a weak acid pH
typically pH 2.5 - 6.5.
For an acid HA, [H+] = [A-] we can rewrite the Ka expression replacing …. with …..
[H+] = [A-] so can subs in [H+]^2
Define a ‘neutral solution’
One where the concentration of OH- = H+
Phenolphthalein changes from ….. to …… within the pH range …….-…..
colourless to pink within pH 8.3-10
Methyl Orange changes from ….. to …… within the pH range …….-…..
red to yellow between pH’s 3.1-4.4
Which 2 indicators are suitable for strong acid/base titrations?
Methyl Orange (strong acid) and Phenolphthalein (strong base)
What assumptions do you make when making Ka calculations with buffers?
That the salt fully dissociates
Acid = (practically) un-ionised
