Acids, Bases and pH (20) Flashcards
Arrhenius model
This suggests that acids release H+ ions and alkalis release OH- ions when dissociated and this combines in a neutralisation to form water. An alkali is just a soluble base.
Bronsted-Lowry acid
This is a proton donor
Bronsted-Lowry base
This is a proton acceptor
Conjugate acid-base pair
The best example of this is seeing their dissociation of HCl into H+ and Cl-. The conjugate acid base pair are the HCl and Cl-. This shows two species can be interconverted by the transfer of a proton
Conjugate acid in example
This would be the HCl because it’s releasing a proton in the forward reaction to form its conjugate base.
Conjugate base in example
This would be Cl- because it is accepting a proton in the backward reaction to become it’s conjugate acid.
Examples of conjugate acid-base pairs
HCl H+ and Cl-
HNO3 H+ and NO3-
H2SO4 H+ and HSO4-
CH3COOH H+ and CH3COO-
Two conjugate acid-base pairs in the same equation
When you combine the dissociation of HCl and the neutralisation equation, there are two conjugate acid-base pairs. HCl and Cl- are one pair and OH- and H2O are the other pair. In the forward reaction the HCl is the acid as it releases a proton and the OH- is base as it accepts a proton. In the reverse reaction, the H2O is acid and the Cl- is base.
Hydronium Ion
Normally in an aqueous solution, dissociation is the movement of a proton from an acid to base and doesn’t work unless water is present. But in this case, water is acting as the base and accepting the H+ ion.
Here the first conjugate acid base pair is the HCl and Cl- and the second pair is water and the hydronium ion. The hydronium ion is the active ingredient in any aqueous acid so you could write any neutralisation with H3O+ instead of H+.
Monobasic, Dibasic and Tribasic
This refers to the number of hydrogen ions in the acids that will be replaced by a metal ion to form a salt. You can look at the number of hydrogen to get an idea
What is HCl
Monobasic - 1 H+
What is CH3COOH
Monobasic - 1 H+
What is H2CO3 (carbonic acid)
Dibasic 2 H+
What is H3BO3 (boric acid)
Tribasic 3 H+
What does pH measure?
It measures the concentration of hydrogen ions. 10^-1 (1) and 10^14 (14) are used to show the 14 pH groups
pH less than 7 shows increasing acidity
pH of 7 is neutral
pH more than 7 shows increasing alkalinity
How to measure the pH?
You can use a pH indicator which will give you the most accurate measure or you can use universal indicator and pH indicator paper and observe the colour. Red is acid and alkali is purple
Mathematical relationship of pH in terms of H+
pH = -log [H+]
Mathematical relationship of H+ in terms of pH
[H+] = 10^-pH
How many more H+ ions are there in pH 1 than pH 2?
There are 10 times more ions and which means there’s a big difference between each pH.
How to find the pH of a strong acid.
A strong acid will fully dissociate and the concentration of H+ ions is the same as the concentration of the acid. Always round the pH to 2 d.p.
What is Ka?
This is the equilibrium constant for the dissociation of an acid. It’s written in the same way as Kc.
The relationship between pKa and Ka
pKa = -logKa
The relationship between Ka and pKa
Ka = 10^-pKa
Strong acid pKa and Ka values
The stronger the acid, the larger the Ka values and the smaller the pKa value
Weak acid pKa and Ka value
The weaker the acid, the smaller the Ka value and the bigger the pKa value.
How to find the pH of a weak acid.
A weak acid will only partially dissociate so won’t have the same concentration of ions as there are in the acid. It depends on the Ka values and equilibrium concentrations.
Approximation 1
[ H+] = [ A- ]
This will only work if the pH is less than 6 and is not a very weak acid or dilute solution
Approximation 2
[ HA equil ] = [ HA start ] - [ H+ ]
This means that [ HA start] = [ HA equil]
It only works for small Ka values and is not justified for stronger weak acids.
Formula for concentration of H+ ions in weak acid
[ H+ ] = square root Ka x [ HA start]
Ionisation of water
Water molecules can combine together and act as an acid and a base to form a hydronium ion and a hydroxide. This then makes the normal neutralisation equation
Kw
This is the ionic product of water. It sets a neutral point on the pH scale. In water it will be 1x10-14
Kw = [ H+ ] [ OH- ]
What pH does water have?
7
How do you know when a solution is acidic?
The [ H+ ] > [ OH- ]
How do you know when a solution is alkali?
The [ H+ ] < [ OH- ]
How do you work out the conc of H+ and OH- ions if you know the pH of water?
The powers should add up to -14. The conc of H+ will be the actual pH.
Strong alkali
A strong alkali is a soluble base that releases OH- ions. It fully dissociates into a metal ion and an OH- ion.
NaOH -> Na+ and OH-
How do you work out the pH of a strong base?
The conc of the base = conc of the OH- ions.
Then do the Kw calculation and find the conc of H+ ions which you can use to work out the pH.
