Ionic Flashcards
Why is it difficult to measure the equilibrium constant of strong electrolytes?
The solute is present entirely as ions
Why can the equilibrium constant be determined in weak electrolytes?
The solute is incompletely ionised in solution
What does the law of mass action predict at equilibrium for a weak electrolyte?
K = [C][D]/[A][B]
K value when equilibrium lies to the RHS
K greater than 1
K value when equilibrium lies to the LHS
K less than 1
[H+] value for acidic solutions
[H+] > 1x10^-7 mol L-1
[H+] value for alkaline solutions
[H+] < 1x10^-7 mol L-1
Why is the log of [H+] and [OH-] taken?
Molarities vary over many orders of magnitude
How is pH related to [H+]?
pH = -log10[H+]
What does 1 unit of pH indicate for molarity?
1 unit difference in pH = factor of 10 change in molarity
Why can water have a pH of less than 7?
Due to carbon dioxide in the atmosphere dissolving in water to give carbonic acid. Dissociation releases H+ ions and pH drops
What is Kw?
Kw = [H+][OH-] = 1x10^-14 mol2L-2
What is a Brønsted acid?
A proton donor
What is Ka?
The ionisation or dissociation constant for an acid
What does the value of Ka indicate?
If Ka is large, acid is strong
If Ka is small, acid is weak
What is Kb?
The ionisation or dissociation constant of a base
What does the value of Kb indicate?
If Kb is large, base is strong
If Kb is small, base is weak
What can pKa be used for?
Expressing strengths of acids and bases using the same term
How can pKa be determined using Ka?
pKa = -logKa
How can Kb be converted ot Ka?
Ka x Kb = Kw = 1x10-14
What does lower Pka indicate about the strength of an acid?
Lower value of pKa indicates a strong acid as the -log is taken
What does a lower pKa indicate about the strength of a base?
Lower value of pKa indicates a weak base
What is the pH equation using pKa and c for an acid?
pH = 1/2 (pKa - logC)
What equation is used to determine pH using pKa and c for a base?
pH = 1/2 (pKw + pKa + logC)
What is a reaction between an acid and a base called?
Neutralisation
What is a salt?
Any ionic compound formed by neutralisation of an acid with a base
What are properties of a salt?
Ionic
H2O soluble
High melting point
Crystalline solid
What is a cation?
A positively charged species
What is an anion?
A negatively charged species
What is hydrolysis?
The breaking of a bond by action of water
What characteristics does the conjugate base of a strong acid have?
Weak conjugate base
What is an amphiprotic salt?
A salt that can function as both a base and an acid
How can the pH of an amphiprotic salt be found?
pH = 1/2 (pKa1 + pKa2)
What are buffers?
A mixture of weak acids (or bases) and their salts
How do buffers work?
They have reserves of potential acidity or basicity and can nullify the tendency for pH changes (source or sink for protons)
How does a buffer of a weak acid and its conjugate base work?
The weak acid can transfer protons to a strong base and the conjugate base can accept protons from a strong acid
How does a buffer of a weak base and its conjugate acid work?
The weak base can accept protons from the strong acid and the conjugate acid can transfer protons to a strong base
At what pH do buffers work best?
pH = pKa of the acid or base from which they are made
What is used for a buffer for pH 7?
The 2nd ionisation of phosphoric acid H3O4 + NaOH
Henderson Hasselbach Equation for a weak acid and salt
pH = pKa + log([A-]/[HA])
pH = pKa + log ([SALT]/[ACID])
What is the buffer capacity?
β = the number of moles of strong monobasic acid (or monoacidic base) required to produce an increase or decrease of 1 unit on pH scale
What is the Buffer Capacity equation?
β = no of moles/change on pH
What is a high capacity buffer?
The amount of base present is about 10% or more of the amount of acid (otherwise the base gets used up quickly)
What is the effective range of a buffer?
pH = pKa - 1 and pH = pKa +1
So the effective range is ±1
What are the 3 main biological buffers?
Dissolved CO2, Dihydrogen phosphate (and its sodium salt) and protein macromolecules
What biological buffer is responsible for the most buffering action?
Dissolved CO2 to give carbonic acid and its sodium salt
What are zwitterions?
Amphiprotic molecules and in solution both functional groups ionise to form salt
What are amino acids?
Zwitterions
At what pH are amino acids ionised?
Amino acids are ionised at all pH values
Why does the body need to have specialised uptake of amino acids?
Amino acids can’t cross the biological membrane by passive diffusion
What is the isoelectric point of a protein or amino acid?
When the number of ionised COO- groups = NH3+ the pH = isoelectric point
What does pI represent?
The isoelectric point represents the minimum solubility of proteins
What is pI useful for?
Used in electrophoresis to separate mixtures of proteins as pI is the point at which migration of protein in an electric field is slowest
How can the percentage ionised of an acid be calculated?
F ionised = 1/ 1 + antilog(pKa -pH)
How can the percentage ionised of a base be calculated?
F ionised = 1/ 1 + antilog(pH -pKa)
If pH = pKa + 1 for an acidic drug, what is the % of drug ionised
90%
What functional groups make a drug acidic?
Carboxylic acids and phenol groups
What functional groups make a drug basic?
A nitrogen atom with a lone pair of electrons available for bonding
Why are amides neutral?
Carbon-Nitrogen bond has double bond character due to resonance
Why must basic and acidic drugs not be administered together?
A precipitation of less water soluble free base may occur
Why are sulfonamides weakly acidic?
Due to the powerful electron-withdrawing effect of the SO2 and the stabilisation of the anion by resonance
What is the equation for multiple extractions?
Wn = W (A/PS+A)^n
