D - Ions Flashcards
What is the equilibrium expression for reaction
aA + bB → cC + dD?
What are the units of K?
K = ([C]c[D]d)/([A]a[B]b)
Units of K are variable, but for the expression K has no units
What is the expression for reaction quotient Q for reaction
aA + bB → cC + dD
and how is it different to K?
Q = ([A]a[B]b)/([C]c[D]d)
Q gives the status of the reaction by mass balance.
Q can have many values whereas K only has one.
What happens when Q < K, Q > K and Q = K?
Q < K → the reaction will favour the products
ΔG = negative
A + B → C + D
Q > K → the reaction will favour the reactants
ΔG = positive
A + B ← C + D
Q = K → the system is at equilibrium
ΔG = 0
A + B ⇌ C + D
What is the Arrhenius definition of acids and bases?
Acid → increases the concentration of H3O+
Base → increases the concentration of OH-
How are pKa and pKb related to the strength of the acid or base?
Low pKa = equilibrium lies to the right = strong acid
Low pKb = equilibrium lies to the right = strong base
pKa and pKb are the pH at the equilibrium point; the lower the value of pKa/b, the sooner the species will dissociate
What are the equations for Ka and Kb?
HA(aq) + H2O(l) → A-(aq) + H3O+(aq)
Ka = ([A-][H3O+])/[HA]
BOH(aq) → B+(aq) + OH-(aq)
Kb = ([B+][OH-])/[BOH]
What is a buffer?
Buffers resist change in pH by providing a source and sink for protons.
The solution contains roughly equal amounts of a weak acid and its conjugate base.
The pH will be roughly maintained when small amounts of acid or base are added.
What are the steps involved in designing a specific buffer?
1 → Choose an acid + conjugate base that has its pKa ≈ pH needed
2 → Calculate the ratio of HA : A- using the Henderson-Hasselbalch equation.
3 → Check with pH meter and adjust if necessary.
What happens when a sparingly soluble salt dissolves in water, and what is the equilibrium expression?
An equilibrium is set up between the undissolved salt and the dissociated ions:
MX(s) → M+(aq) + X-(aq)
Ksp = [M+][X-]
(The reactant is not included because it is presumed to have a concentration of 1M)
MX2(s) → M+(aq) + 2X-(aq)
Ksp = [M+][X-]2
Ksp → solubility product
What happens when a sparingly soluble electrolyte is dissolved in one of its ions?
The solubility decreases a lot.
Eg. AgCl dissolving in 0.1M NaCl
As the concentration of Cl- is a lot higher than it would be on dissolving in water, the concentration of Ag+ must be lower in order to keep the value of Ksp constant.
What is the difference between voltage and current?
Voltage → the driving force of a reaction; gives the direction of a reaction
Current → gives the quantity of electricity
What is the generic form of an electrochemical cell (no Pt)?
(Red)(s) | (Ox)(l) ‖ (Ox)(l) | (Red)(s)
Left(Red) + Right(Ox) → Left(Ox) + Right(Red)
‖ = salt bridge
What is the generic form of an electrochemical cell with Pt, and why is Pt used?
Pt | (Red)(l) , (Ox)(l) ‖ (Ox)(l) , (Red)(l) | Pt
Left(Red) + Right(Ox) → Left(Ox) + Right(Red)
The Pt provides a solid surface for the reaction to occur on
‖ = salt bridge
How are electrode potentials measured?
They are all measured against a standard Hydrogen electrode, which is used as a reference because it has a voltage of 0V.
Electrode reaction: H+(aq) + e- → ½H2(g)
The H2 gas must be at standard pressure (1 atm) and the acid at standard concentration (1M).
Why might testing electrodes with biological applications involve using 2 standard electrodes?
The standard Hydrogen electrode will have a very low pH due to the concentration of H+ required.
There may be another electrode with the same reaction as the standard H2 electrode but at pH 7.0.
