Acids and bases Flashcards
What is a neutralisation reaction?
When an acid reacts with a base
Acid + metal —>
salt + hydrogen
Acid + metal oxide —>
salt + water
Acid + metal hydroxide —>
salt + water
Acid + metal carbonate —>
salt + CO2 + water
Bronsted Lowry definition of an acid
proton donor
Bronsted Lowry definition of a base
proton acceptor
Define pH
-log10[H+]
How to work out concentration of H+ ions using pH
10^-pH
What does a smaller pH indicate about the concentration of H+ ions
The smaller the pH, the greater the concentration of H+ ions
What is a strong acid?
A strong acid fully dissociates into its ions
Position of the equilibrium of strong acid dissociation
Equilibrium lies entirely to the right hand side
What is a weak acid?
A weak acid only slightly dissociates into its ions
Position of the equilibrium of weak acid dissociation
Equilibrium lies entirely to the left hand side
What is Ka
[products] / [reactants]
Ka of a weak acid
[H+]2 / [HA]
H+ ions of weak acid
square root of Ka x [HA]
pH of a strong acid
-log[H+]
Ka conversion into pKa
pKa = -log10Ka
pKa conversion into Ka
Ka = 10^- pKa
How does a stronger acid affect the value of pKa
The stronger the acid, the lower the pH and the lower the pKa value
Position of equilibrium of water
To the left
What does the position of the equilibrium of water indicate about [water]
Position of equilibrium is very far to the left so [H2O] is effectively constant
What is Kw?
Ionic product of water
Value of Kw at 298K
1 x 10^-14
Kw expression for water
Kw = [H+][OH-]
Why is pure water always neutral
[H+] = [OH-]
How to work out [H+] of pure water
[H+]2 = Kw
Position of equilibrium of strong bases
Equilibrium lies entirely to the right hand side
How to work out [H+] of strong bases
[H+] = Kw / [OH-]
What is a buffer?
A solution which can resist changes in pH when a small amount of acid or base is added
What is an acidic buffer made of?
A weak acid and a soluble salt of that acid. Maintains a pH below 7
What is a basic buffer made of?
A weak base and a soluble salt of that base. Maintains a pH above 7
How to work out [H+] of a buffer
[H+] = (Ka x [HA]) / [A-]
How to work out pH of a buffer made by neutralisation?
ICE box
use end moles in equation
What is half neutralisation?
Enough base has been added to neutralise exactly half of the acid present
At half neutralisation point what does Ka equal?
Ka = [H+]
At half neutralisation point what does pKa equal?
pH
What is a standard solution?
Exact concentration is known
What are the practical steps you need to take to produce a pH titration curve?
Add 25cm3 of acid into a 250ml beaker
Measure the initial pH of the acid solution in the beaker using a pH meter and record
Add 1cm3 of base from the burette
Swirl the mixture
Measure the new pH and record
Continue adding base until it is in excess
Add in smaller intervals when approaching end point
Why is it important to calibrate pH meter?
After storage it may not give an accurate reading
How to calibrate pH meter?
Place the pH meter in a solution of known pH and then adjust the meter accordingly
What is the equivalence point?
When exactly enough acid has been added to neutralise the base
What is the end point?
Exact volume of acid or base which needs to be added to cause an indicator to change colour
How to choose a suitable indicator?
Changes colour somewhere on the vertical section of the pH titration curve
When does phenolphthalein change colour and what colour is phenolphthalein in acid and alkali? (3)
At a pH of about 10
Acid: colourless
Alkali: red
When does methyl orange change colour and what colour is methyl orange in acid and alkali? (3)
At a pH of about 4.5
Acids: red
Alkali: yellow
What is the half-neutralisation point?
Enough base has been added to neutralise exactly half of the acid
A student has an unknown weak acid of unknown concentration and a bottle of 0.5 mol dm-3 NaOH.
Explain, using brief practical details, how you would determine the Ka of the weak acid. [6]
Measure 25 cm3 of acid into a beaker/conical flask using volumetric pipette.
Use a pH probe to measure the initial pH
Add 1cm3 NaOH from a burette swirl and record the pH
Repeat until the NaOH is in excess
Plot a graph of pH against vol NaOH added in cm3
Use the vertical section to find the volume of NaOH needed for neutralisation
The half neutralisation point is half the volume of neutralisation.
Read off the pH at the half neutralisation point.
At half neutralisation ka = [H+]/pKa=pH
Ka = 10-pH
Why is a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration? (1)
Concentration of hydrogen ions in aqueous solution covers a very wide range
What are the properties of a good indicator? (3)
Sharp colour change - no more than one drop of acid/alkali needed for colour change
End point must be the same as the equivalence point, or titration gives wrong answer
Distinct colour change so it is obvious when end point has been reached
What products are buffers found in? (2)
Shampoos
Detergents
