1.12 Acids and Bases Flashcards
What is the Brønsted-Lowry classification of an acid?
Proton donor
What is the Brønsted-Lowry classification of a base?
Proton acceptor
What is a strong or weak acid?
Strong acids completely dissociate (ionise)
Weak acids partially dissociate (ionise)
What are some strong and weak acids?
Strong: Sulfuric acid Hydrochloric acid Nitric acid Phosphoric acid
Weak:
Carboxylic acid
What are some strong and weak bases?
Strong:
Hydroxide
Weak:
Ammonia
Amines
What is a conjugate acid/base?
Formed from an acid/base
Conjugate base - species formed when an acid lost a proton
Conjugate acid - species formed when a base gained a proton
What word is significant for acids and bases?
Amphoteric
A species can act as an acid or a base
What is significant about water?
It is amphoteric and self protonating (always contains some H+ ions and OH-)
It is weakly dissociated
What are the types of acids that fully dissociate?
Monoprotic - for each acid molecule = 1H+ ion
Diprotic - for each acid molecule = 2H+ ion
How do you know what type of ‘protic’ a base is?
Monoprotic - takes only 1H+ ions to neutralise the 1OH-
Diprotic - takes 2H+ ions to neutralise the 2OH-
What is the equation for pH?
pH = -log[H+]
pH is always 2 dp!!!!!
What is the equation for H+ concentration?
[H+] = 10 ^-pH
What is Kw?
Another constant that incorporates Kc and [H2O]
Determined from pure water
What are some Kw equations?
Kw = [H+][OH-]
Or
Kw = [H+]^2
So
[H+] = Kw/[OH-]
What is Kw?
Kw = 1x10^ -14
What do you have to remember working out pH or H+ conc?
If the acid/base is monoprotic or diprotic!
You might need to multiply or divide by 2 or 4 depending on the direction of the dissociation (so use the equation as well - or make one)
How do we work out partial neutralisation calculations?
Find the moles of the acid and base
Find which is in excess (consider the ‘protic’ - multiply by 2? Before decision
Which ever is in excess take one from the other
Excess moles / total volume = conc of excess
If acid - find the pH
If base - use Kw to find [H+] and the find the pH
In Q’s look for ‘solution’
How do you decide which is an acid/base and or conjugate?
Use the definitions
Look in the correct direction of the reaction for that compound
What is significant about weak acids?
They only weakly dissociate in aqueous solutions
How is a weak acid equation represented?
HA ⇌ H+ + A-
H - the hydrogen ion
A - the undissociated negative ion in the weak acid
What is Ka?
The weak acid dissociation constant
Ka = [H+] [A-]
—————
[HA]
Units = mol dm-3
What is the relationship between Ka and equilibrium?
The greater value of Ka
The equilibrium is further to the right
The greater the dissociation
The greater the weak acid
Therefore a lower pH (more H+ ions)
How do you work out H+ using Ka?
[H+] = square root of Ka x [HA]
What are the equations to move between Ka and pKa?
pKa = -log Ka
Ka = 10 ^-pKa
What is the relationship between pKa and equilibrium?
The lower the value of pKa
The stronger the acid
Equilibrium lies to the right side
Due to Ka increasing
When are indictors used?
Indicators need to change colour in the vertical point on the graph
What is significant about the vertical point of an acid/base graph?
H+ moles = OH- moles
It contains the equivalence point
What happens to the graph with a weak acid and a weak base?
There is no vertical point of the graph
As the pH change at equivalence takes place gradually
An indicator can’t be used as they need a sudden change
Where is the equivalence point with: Strong acid & strong base Weak acid & strong base Strong acid & weak base Weak acid & weak base
Strong acid & strong base - pH 7
Weak acid & strong base - above pH 7
Strong acid & weak base - below pH 7
Weak acid & weak base - pH 7
What is significant about indicators?
They are weak acids - they will weakly dissociate
We need to control the extra [H+] ions being added
What do you need to make sure with colour change of indicators?
You write two colours as it’s a colour change!
What is meant by the strength of an acid?
A measure of how readily an acid dissociates
What happens if an acid reacts with an acid?
The strongest acid is the proton donor (loses a H+)
The weaker acid gains that proton
What is half equivalence?
To determine the Ka of a weak acid when titrated with a strong base
1) record the volume of OH- required to produce equivalence (neutralisation)
2) repeat but add exactly half the volume of OH- required (half equivalence)
3) use a pH meter (or pH curve) to find the pH of the solution produced in (2) this pH can determine the Ka of the acid
What is a buffer solution?
It’s pH is resistant to change on the addition of relatively small quantities if an acid or a base, or by dilution
What equation do we use to work out Ka or H+ of a buffer solution?
Ka = [H+] [A-]
—————
[HA]
[H+] = Ka x [HA]
—————
[A-]
What does a buffer solution do?
A buffer solution artificially maintains the A- to allow the equilibrium to move to the left or right to maintain the H+
What is an acidic buffer made from?
pH < 7
Made from a weak acid
And
Na+ or K+ salt eg sodium ethanoate
What is an alkaline buffer made from?
pH > 7
Made from a weak base eg ammonia
And
It’s chloride eg ammonium chloride
What are the two types of buffer calculations?
1) weak acid + salt of the weak acid
2) weak acid + strong base
How to do the calculation of a buffer of a weak acid and salt of the weak acid?
[H+] = Ka x (moles of HA / total volume)
All divided by
(Moles of salt / total volume)
pH = -log[H+]
How are the moles of A- worked out in a weak acid + salt of the weak acid calculation?
Moles of salt = moles of A-
How do you calculate the new pH when you add something else to a buffer?
Work out the moles of the H+ or OH- added
Use HA ⇌ H+ + A- to establish the direction of the equilibrium depending on what you add
+ or - the moles being added from the previous moles of HA and A-
Use the new moles to work out [H+] and don’t forget the addition of the volume for total volume
How to do the calculation of a buffer of a weak acid and strong base?
Calculate the moles of both Write HA + OH- ⇌ A- + H2O Use this to do equilibrium moles To work out HA do HA - OH- moles OH goes to 0 and A- and H2O is the initial OH- moles Sub these moles into: [H+] = Ka x (moles of HA / total volume) All divided by (Moles of salt / total volume)
pH = -log[H+]
