Fundamentals on Chemistry: Acids and Bases Flashcards
Definition of an Arrhenius acid and base
An acid ionises in water to give H+ and anions
A base ionises in water to give OH- and cations
Spectator ions
Ions which do not partake in the reaction
Oxonium ion
H3O+ (sometimes called hydronium ion)
Definition of a Bronsted-Lowry acid and base
An acid donates H+ in a chemical reaction
A base accepts H+ a chemical reaction
Why is the Bronsted-Lowry definition an improvement on the Arrhenius definition?
Not all acid-base reactions happen in water
Definition of a Lewis acid and base
An acid accepts a pair of electrons
A base donates a pair of electrons
Why are all Bronsted-Lowry acids Lewis acids but not all Lewis acids and Bronsted-Lowry acids?
A Bronsted-Lowry acid donates a proton and in the process of this, accepts a pair of electrons therefore all Bronsted-Lowry acids are Lewis acids. However, not all acid-base reactions involve an H+ ion so not all Lewis acids are Bronsted-Lowry acids
What is a strong acid and a strong base
A strong acid is a powerful proton donor
A strong base is a powerful proton acceptor
The strength of an acid and a base is their most fundamental property
What is Ka?
Ka is the acid dissociation constant. The greater the degree of dissociation, the larger the value of Ka
Ka values can range from very large and positive to very small and negative so pKa is used
What is pKa?
pKa = -log10(Ka)
When Ka is large, pKa is small/negative
When Ka is small, pKa is large/positive
Why do strong acids appear to have the same pKa in water and how can this be solved?
Strong acids fully dissociate in water meaning they will all show the same pKa. To solve this issue, a process called solvent levelling is used. This is where a solvent which is a poorer H+ acceptor so the difference in dissociation is noticeable
Amphoteric
When a molecule can act as an acid or a base. Sometimes called amphiprotic
How to calculate pH
pH = -log10[H3O+]
The pH of water
At 298K, the pH of water is 7.0, pH can vary with temperature. The higher the temperature, the lower the pH
Assumptions of weak acids
1) [H3O+] = [A-]
- expect in extremely dilute solution
- in pure water [H3O+] = 10^-7 mol dm-3
2) There is very little dissociation of the weak acid
What is the degree of dissociation?
The ratio of dissociated ions to the starting compound
Acid strength of the hydrogen halides?
HI > HBr > HCl»_space; F
The acid strength depends on the position of equilibrium when reacted with water
Bond dissociation energy order D(H-X)
HF > HCl > HBr > HI
How can the bond hydrogen halides equilibrium with water be pulled to the right
X- can hydrogen bond with the oxonium ion which contributes positively and pulls the equilibrium position to the right
What happens when X- = F-
The hydrogen bonding is very strong that it ‘masks’ the acidity of the oxonium ions
While HI, HBr, and HCl all have low pKa values (meaning they are acidic), HF has a pKa value of 3.2 meaning it is a weak acid
There is an effect from the strength of the H-X bond but a larger effect from the hydrogen bonding
What is an oxo acid?
An oxo acid is one which contains an O-H group and will have a double bond between oxygen and another
element
Many of these oxo acids have more than one ionisable hydrogen
The general form for an oxo acid is HmEOn
How many equilibriums does each basic molecule have?
Monobasic - one equilibrium
Dibasic - two equilibriums
Tribasic - three equilibriums
It is harder to remove H+ from an anion due to the electrostatic attraction
What happens to the value of pKa as n-m increases
The value of pKa will decrease as the value of n-m increases
How many resonance forms would NOOH have?
- There are 2 resonance forms
- n-m = 1
- Negative charge delocalised over 2 oxygen atoms
Which is more stable HClO3 or HClO4?
HClO4 is more stable as it has more resonance forms meaning the negative charge is spread over more electronegative atoms than in HClO3
Bell’s Rule
pKa = 8-5(n-m)
This is an empirical equation
Superacids and superbases
Superacids are acids with very low pKa values
Superbases are bases with very high pKa values, and are almost all derived from reactions with alkali metals
Buffer solutions
A buffer solutions resists change in pH on addition of small amounts of acid and base
Consists of either:
- Weak acid and salt of its conjugate base
- Weak base and salt of its conjugate acid
Buffer capacity
The amount of acid or base which can be added to a buffer solution before it no longer resists change in pH
Henderson-Hasselback equation
pKa = pH - log10[A-]/[HA]
Derived from:
-log10(Ka) = -log10[H3O+] - log10[A-]/[HA]
Why should the pKa of a solution be close to the pH
Using the Henderson-Hasselback equation, when [A-] = [HA] then [A-]/[HA] = 1 so log10[A-]/[HA] = 0. Therefore pH = pKa + 0 so pKa will be relatively close to the pH of a solution
Amino acids
- Amino acids are the building blocks of proteins
- Proteins are chains of many amino acids arranged in 3D
- In the human body, there are 20 known amino acids all with the same structure
- Never found in a neutral form –> always at least one charge on the molecule but there may be no overall charge as it may be zwitterionic
What is a zwitterion?
A zwitterion is a molecule which is neutral and contains an equal number of positive and negative charges within it
Isoelectric point
The pH at which an amino acid will form a zwitterion. The symbol for isoelectric point is pI
