Acids And Bases Flashcards
What is the Brønsted-Lowry definition of
a) an acid
b) a base
a) A proton donor
b) A proton acceptor
What does monoprotic mean
It donates 1 H+ per molecule
What does diprotic mean
It donates 2 H+ ions per molecule
What do strong acids and bases do
Completely dissociate
What equilibrium is set up when a weak acid dissolves in water
HA + H2O (equilibrium) H3O+ + A-
Or
HA (equilibrium) H+ + A-
Give the equation of NH3 (weak base) dissolving in H2O
NH3 + H2O (equilibrium) NH4+ + OH-
Or
NH3 + H+ (equilibrium) NH4
The weaker the base the ____ it dissociates and the more the equilibrium lies to the _____
Less
Left
Does water act as an acid or a base? What is this called
Both
Amphoteric (acts as both an acid and base)
Definition of pH
pH= -log[H+]
As the concentration of H+ increases, what happens to the pH
pH decreases
To what precision is pH given to
2 d.p.
Equation to convert pH to [H+]
[H+]=10^-pH
When converting from H2SO4 to H+ do you divide or multiply by 2
Multiply
What does Kw=
[H+][OH-]
What is Kw at 298K/ 25 degrees Celsius
1x10^-14 mol^2dm^-6
Why is [H2O] not included in the equation for Kw
Water is only weakly dissociated so equilibrium lies far over to the left and it’s concentration can be taken as constant
In pure water what must [H+]=
Therefore…?
[OH-]
Kw= [H+]^2
Is the dissociation of water exothermic?
No it is endothermic
Explain why pure water at 50 degrees celsius is not acidic despite being at a pH of 6.63
[H+]=[OH-]
Why does Kw increase with temperature
Equilibrium shifts to the right to decrease temperature
Degree of dissociation increases
Kw increases
pH increases
Define:
Monoprotic base
Diprotic base
Can accept 1 proton
Can accept 2 protons
Are hydroxides strong bases?
Yes
3 steps for calculating pH of diluted acid
1: calculate moles of H+
2: concert to concentration by dividing by new volume
3: pH=-log[H+]
Steps for working out pH of a strong base
1: calculate miles of OH-
2: convert to concentration by dividing by new volume
3: [H+]=Kw/[OH-]
4: pH=-log[H+]
How do you calculate pH of a mixture of acid and base
- calculate moles of H+
- calculate moles of OH-
- work out what is in XS and convert to concentration by dividing by total volume
If H+ is in XS: pH=[H+]
If OH- is in XS: [H+]=Kw/[OH-]
pH=-log[H+]
Why must a pH meter be calibrated before use?
They do not give an accurate reading over time
How to calibrate a pH meter
Place meter in buffer solution of known pH and plot a calibration curve
Eg rinse probe with deionised water, dry with a paper towel and place in buffer
Wait for reading to steady then set meter to known pH of buffer
What is a buffer solution
Maintains same pH despite the addition of small amounts of acid, base, or water
Method for drawing a pH curve
Measure initial pH
Add solution from burette in small portions
Stir mixture
Record pH after each addition
Add solution dropwise as you near the end point
After end point add solution 1cm3 at a time until in XS
What container do you use for a titration to find a pH curve
A beaker - NOT a conical flask
So both probe and burette can fit
What is the equivalence point
When sufficient base has been added to neutralise the acid (or vice versa)
Describe a pH curve of a:
Strong Base added to a strong acid
Strong base added to a weak acid
Very low to very high
Low to very high
Describe the pH curve of a:
Weak base added to a strong acid
A weak base added to a weak acid
Very low to quite high
Quite low to quite high
How to use pH curve to find concentration of acid or base
Use volume at end point (vertical line) to find concentration of substance added
What does pKa equal
pH
What is the half neutralisation point
Half way between the zero and equivalence point
What does Ka equal
[H+][A-]
————-
[HA]
Why can Ka be simplified to Ka=[H+] at half neutralisation point
Half the acid has been neutralised so [HA]=[A-]
How to tell if it’s a half neutralisation point calculation
If one reactant has half the volume of the other but equal concentration
Ie if one has half the moles of the other
What is an acid-base indicator
A water soluble, weak organic acid, whose acid form (HA) and base form (A-) have different colours
Why can only 2-3 drops of an acid-base indicator be added to a solution
It is acidic so it will affect pH otherwise
What happened when the indicator is placed in acidic solution
The extra H+ ions will push the equilibrium to the LHS so the colour of HA will be seen
What happened when the indicator is placed in alkaline solution
The extra OH- ions will react with the H+ ions, pushing the equilibrium to the RHS and so the colour of A- will be seen
What properties must a suitable indicator have
Colour change must be sharp rather than gradual at end point
End point of the titration given by the indicator must be the same as the equivalence point
Indicator should give a distinct colour change
When will change point for an indicator be
When [HA]=[A-] and neither colour dominates
At this point [H+]=Ka
pH at which the colour of the indicator changes =
pKa of the indicator
How do you calculate Ka from a titration pH curve?
From the curve, read off the volume at equivalence
Divide this volume by 2 to find volume at half equivalence
Read pH at the half equivalence volume - this will be your pKa value
Calculate the 10^(-pKa) to find Ka
What is the molar ratio of a carbonate to a monoprotic acid?
1:2
Define buffer solution
A solution that resists change in pH when small amounts of acid, base or water are added
What can Ka be useful for?
To confirm an acid’s identity
The weaker the acid, the _____ it dissociates and the more the equilibrium lies to the ____
How about bases?
Less
LHS
Same for bases
Does water strongly dissociate?
What does this mean for the equilibrium?
No- weakly dissociated
Lies far to the left and conc of water is taken as constant
What is the ionic product of water
Kw
Kc when water is taken as constant - [H+][OH-]
How does temperature affect Kw?
Why?
Kw increases with temperature
It involves breaking covalent bonds (inputting energy) and is an endothermic process
What are the two types of acidic buffer?
Weak acid and one of its salts
Weak acid and strong acid (salt is made as a product of the reaction)
What is sodium ethanoate used for in an acidic buffer?
Acts as a source of conjugate base, CH3COO-
What is added to an alkaline buffer containing NH3
NH4+ Cl-
What happens in the equilibrium of blood when
An Acid
A Base
Is added
a) extra H+ shift equilibrium to left to remove H+
b) extra OH- remove H+ by reacting to form water. Equilibrium shifts left to replace H+
Give example of buffers in everyday life
Why are buffers needed in them
Detergent
Shampoo
Wrong pH could damage fabric or skin and hair
For acidic what must you say after explains buffer action?
The ratio of
[HA]
————
[A-]
Remains constant
How many moles of Fe2+ react with one chromate ion?
6
Is there any point rearranging equations before putting in numbers?
Yes - you get a mark for it
Why is the gradient of a PH curve initially near 0?
Buffer formed (excess weak acid and strong base)
Describe pH curve of strong base to weak acid
Initial ‘ramp up’
Near horizontal until endpoint where curve is vertical and then levels out
What must you dip the probe in to calibrate it?
Buffer solutions (pH 4, pH 7 and pH10)
Can you swirl when doing investigate pH
No - mixture in a beaker so must stir
How to work out original [HX] from incomplete pH curve
At equivalence point
n OH- = n HX
So divide volume added at equivalence point by original volume of HX
