Acids And Bases Flashcards
What does an acid and base reaction involve?
Transfer of H+ ions
What is a bronsted Lowry acid?
A substance that can donate a proton
Proton donor
What is a bronsted Lowry base?
A substance that can accept a proton
Proton acceptor
What is a conjugate acid base pair?
A set of two species which become each other either by the gain or loss of a proton
In an acid and base reaction what are the spectator ions?
Ions which are not directly involved in the acid or base reaction.
For the reaction identify which are the conjugate acid and base pairs
HNO2 +H2O NO2- + H3O+
HNO2 acid 1 to NO2- base 1
H2O base 2 to H3O+ acid 2
What are mono, Di and Tri and poly basic acids?
Monobasic acid:
Each molecule can release 1 proton
Dibasic acids:
Each molecule can release 2 protons
Tribasic acids:
Each molecule can release 3 protons
Polybasic acids (Di/Tri) release their protons in multiple steps
What is a neutralisation reaction?
When an acid and base react and give a salt and water
Strong acid + strong base
What happens when an acid reacts with a carbonate?
Forms a salt, water and carbon dioxide
What happens when an acid reacts with a base?
Forms a salt and water
What is an alkali?
A base which dissolved in water forming OH- ions
What happens when an acid reacts with a metal?
Forms a salt and hydrogen gas
Redox reactions
How do you calculate H+ concentration and pH?
pH = -log([H+])
H+ = 10^-pH
What is a strong acid?
An acid which can fully dissociate in water
HA ——> H+ + A-
Therefore the [acid] = [H+]
What is a weak acid?
An acid which only partially dissociates in water and are therefore in equilibrium
We can only compare the strengths of weak acids
How do you calculate the pH of a strong acid?
Since [acid] = [H+]
We can easily find pH from
-Log(H+)
What is Ka?
How does it relate to strong and weak acids?
Acid dissociation constant
Ka = [H+] [A-] / [HA]
Strong acids - high Ka value
Weak acids - low Ka value
What is the strength of an acid?
How much an acid will dissociate when it is dissolved
Strength of weak acid is how far its equilibrium lies to the right
Give some examples of strong acids
HCL NHO3 H2SO4 HBr HI HClO4
How do you find the pH of a weak acid?
Use Ka
Must assume the all the protons in the solution come from the acid dissociation
Therefore [H+] = [A-] =[H+]^2
Ka = [H+]^2 / [HA]
[H+] = root( Ka X [HA] )
What is the relationship between PKa and Ka?
PKa = -log(Ka)
Ka = 10^-PKa
Strong acids - low PKa values
Weak acids - high PKa values
Why is water amphoteric?
Water can both accept and donate protons and therefore act as an acid and a base
What is the ionic product of water?
Kw = [H+] [OH-]
At 298K pure water has the same amount of [H+ ions as OH- ions and so
[H+] = [OH-]
What is Kw at 298K (25 degrees)?
Kw = 1x10^-14
=[H+]^2
Therefore pH =7 (neutral)
How do you determine the strength of a base?
Base strength is determine by its ability to dissociate and form OH- ions
A strong base will fully dissociate
Give some examples of strong bases
Hydroxides
Group 1 and 2 metals
NaOH ——> [Na+] + [OH-]
Give an example of a weak base
Ammonia
NH3 + H2O ——> NH4+ + OH-
How do you find the pH of a strong base?
Use the Kw expression
Assume that [OH-] =[base] (undissociated)
[H+] = Kw / [OH-]
What is a buffer?
A solution that minimises a change in pH by the addition of a small volume of acid or base.
They DO NOT STOP changes just minimise them.
What is an acidic buffer?
A mix of a weak acid and its conjugate base and salt.
Buffer solution = weak acid + it’s salt
CH3COOH + CH3COO-Na+
Acid partially dissociates into ethanoate ions and H+ ions
Eg. CH3COOH H+ + CH3COO-
Salt dissociates completely producing ethanoate and sodium ions
Eg. CH3COO-Na+ ——> CH3COO- + Na+
What happens when acid is added to an acidic buffer?
CH3COOH CH3COO- + H+
Adding extra acid shifts equilibrium to the left to remove the H+ added and maintain the pH
What happens when alkali is added to an acidic buffer?
CH3COOH CH3COO- + H+
Equilibrium shifts right to replenish the H+ ions which have been used up in neutralising the alkali added and therefore maintain the pH
What is a basic buffer?
Basic buffers resist a change in pH and maintain a pH above 7.
They are made from a mixture of a weak base and its salt
Eg. Ammonia and ammonium chloride
What are some naturally occurring buffers?
Buffers appear in biological systems as pH needs to be controlled
Carbonic acid and Hydrogen carbonate ions act as a buffer in the human body.
How do you calculate the pH of a buffer?
Use Ka (not H+ ^2) Assuming that [HA] (undissociated) = [HA] (equilibrium)
And
Assuming that [salt] = [A-] (equilibrium)
H+ = [HA] X Ka / [A-]
How can you find the concentration of an acid or a base experimentally?
Using acid-base titrations .
How do you do an acid base titration?
Usually add a know concentration of acid or base to a known volume of the other species in order to find the concentration of that species.
Using a pH meter enables us to follow the pH as we add the acid or base.
What is the equivalence point?
The point where all the acid or base in the know volume has reacted with the other
Moles of base = moles of acid
What is the end point?
The point at which an indicator changes colour due to there being equal numbers of HIn and In- ions.
What is an acid base indicator?
Usually a weak acid
And is used to determine the end point of a reaction by a distinguishable colour change.
How does and acid base indicator work?
HIn H+ + In-
The indicator and its conjugate base have different colours in solution
Eg. Bromothylmol blue
Yellow blue
Adding H+ shifts eqm left and solution becomes yellow
Removing H+ shifts eqm right and solution becomes blue.
How do you choose a suitable indicator?
Colour change must be sudden not gradual
End point must be near equivalence point
What are the key areas to consider on a titration curve?
Consider adding a base to an acid
- Slight increase as you add the base but the acid is in excess and so little change occurs
- Sharp increase when we have used up all the acid and small amounts of the base make a big change
- The base is now in excess so adding more of the base makes little difference
Explain why water remains neutral when it’s pH is less than 7?
There is greater dissociation at higher temperatures so the pH is reduced however the concentration of [H]+ and [OH-] remain equal so no change in neutrality.
Describe the steps in finding pH values for a pH curve when an alkali is neutralised by an acid?
Add the acid to the alkali 1cm3 at a time.
mix the solution ensuring it is equally reacting
record the pH with a pH probe or data logger
continue until the pH is constant.
