Module 5 Flashcards
How would a... - 0 order reaction - 1 order reaction - 2 order reaction ...look on a concentration-time graph?
- 0th would be a descending straight line
- 1st would be a downwards curve, with a constant half-life
- 2nd would be a steeper downwards curve, with a changing half-life
How can you calculate the rate constant, k, from a 1st order concentration-time graph?
ln(2) divided by (half-life)
How would a... - 0 order reaction - 1 order reaction - 2 order reaction ...look on a rate-concentration graph, and how could you calculate k from it?
- 0th would be a horizontal straight line where k=r
- 1st would be a diagonal straight line (line y=x) where k=gradient
- 2nd would be a steep upwards curve where k would equal the gradient of the rate against concentration squared
How can you calculate an initial rate from a graph or from a ‘clock reaction’?
take the tangent to the curve at t=0 or (for clock reactions) the rate is proportional to ‘1/t (but this is only accurate for about the first 15% of the reaction)
What is a mole fraction of a gas?
Proportion by volume
How do you calculate partial pressure, p?
mole fraction x total pressure
What condition changing will change Kp and Kc?
Temperature changes
What is a Bronsted-Lowry acid?
Proton donor
What is a Bronsted-Lowry base?
Proton acceptor
How do you calculate pH?
pH = -log([H+])
What is the difference between strong and weak acids?
Weak acids only partially dissociate
Which has a larger Ka, strong acid or the weak acid?
Stronger acids have larger Ka values (and therefore smaller pKa values)
When calculating the pH of a weak acid what approximations are made to simplify the expression?
1) [H+]eqm = [A-]eqm
2) since weak acids only partially dissociate, the equilibrium (eqm) of the dissociation will lie very far to the left, as a result it can be assumed that [HA]eqm = [HA]start
What is Kw (define it), and what is its value at 298K?
Kw is the ionic product of water, which is 1x10*-14 at 298K
What is a buffer system?
A system in a mixture that minimises a pH change, composed of a weak acid and its conjugate base
How can a buffer be prepared?
Mix a weak acid with its conjugate base salt OR partially neutralise a weak acid with a stong base
What is true when [HA] = [A-], consider the nature of the Ka equation?
Ka = [H+] and therefore pKa = pH
What is a pH indicator?
It is a weak acid that has a different colour to its conjugate base
Why might a weak acid + strong base pH titration curve look different to others?
Becuase, when the base is initially added a buffer, will be set up, trying to oppose the change in pH and therefore putting a ‘kink’ in the curve
Define lattice enthalpy
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions
Define enthalpy change of formation
The enthalpy change that accompanies the formation of one mole of a compound from its elements
Define first ionisation energy
The enthalpy change required to remove one electron from each atom in a mole of gaseous atoms to produce one mole of gaseous (1+) ions
Define first electron affinity
The enthalpy change when one mole of electrons is added to one mole of atoms forming one mole of gaseous (1-) ions
Define enthalpy change of solution
The enthalpy change that takes place when one mole of a solute dissolves in a solvent
Define enthalpy change of hydration
The enthalpy change that accompanies the dissolving of gaseous ions into water to form one mole of aqueous ions
Which factors affect lattice enthalpy, and how?
As ionic radius increases lattice enthalpy becomes less negative
As ionic charge increases lattice enthalpy becomes more negative
Which factors affect hydration enthalpy and how?
As ionic radius increases hydration enthalpy becomes less negative
As ionic charge increases hydration enthalpy becomes more negative
What would be the sign of ΔS in a reaction where the liquid reactants become gaseous products?
positive (+) since the system is becoming more disordered
What is the Gibbs’ free energy equation?
ΔG = ΔH - TΔS (for T in Kelvin)
What makes a reaction ‘feasible’?
satisfying ΔG < 0 (ΔG being negative)
What limitations are there to predicting feasibility?
Rate may be very slow
Activation energy may be very high
If both species in a half cell are aqueous ions (for example Fe2+ and Fe3+) what electrode is used?
Platinum, Pt
In an operating cell, what happens to the electrode with the more reactive metal (negative electrode)?
It loses electrons (is oxidised)
Apart from the wire (and voltmeter) what else connects the two half cells?
A salt bridge (usually potassium nitrate)
How do you calculate cell potential?
electrode potential of the positive electrode minus the electrode potential of the negative electrode
What are the limitations of predicting feasibility for cell potentials?
No indication of activation energy or rate
Concentrations are unlikely standard
Actual conditions are unlikely standard
What are the differences between primary and secondary cells?
Primary cells aren’t rechargeable and are alkaline based, secondary cells are rechargeable and are acid based
How do lithium-ion cells work?
Lithium ions move from a graphite-lithium electrode across a polymer membrane to a cobalt oxide electrode, losing an electron as they do so to carry charge around a circuit
In a (hydrogen) fuel cell, fuel and oxygen move into the cell and water moves out, what are the differences between an alkali hydrogen fuel cell and an acid one?
In an alkali fuel cell, the water leaves on the same end that the hydrogen comes in (the anode) and hydroxide ions diffuse across to it from the cathode. In an acid one, the water leaves at the cathode (where the oxygen comes in) and hydrogen ions diffuse across to it.
What is different about the 4s subshell? List the 3d subshell electron configurations of the first row of d-block elements
It fills and empties first
1,2,3,5,5,6,7,8,10,10 (Notice how it skips? Chromium would rather 3d5 than 3d4, 4s1 and Copper would rather 3d10 than 3d9, 4s1)
What are transition elements? Which (top row) d-block elements aren’t transition elements?
Transition elements:
- d-block elements that form at least one ion with a partially filled d orbital
- form compounds in which it has variable oxidation states
- form coloured compounds
- act as catalysts
What is a ligand?
Molecule or ion that can donate a pair of electrons to a central metal ion to form a coordinate bond
What is a complex ion’s coordination number?
The number of coordinate bonds attached to the central metal ion
Which compounds form tetrahedral complex ions?
Cobalt and Copper
Which compounds form square planar complex ions?
Platinum, Palladium and Gold
Define stereoisomerism
When molecules have the same structural formula but with a different arrangement in space
How can you distinguish between optical isomers of a (bidentate ligand) complex ion?
They will appear different colours in plane polarised light
What happens when ammonia is added to pale blue hexa-aqua-copper ions?
First a pale blue precipitate forms but then it dissolves again in excess ammonia to form a dark blue solution of [Cu(NH3)4(H20)2]
What happens when chloride ions are added to pale blue hexa-aqua-copper ions?
A yellow solution forms of [CuCl4] notice how it is only 4, to replace 6 H20 molecules - this is because chlorine is ‘bigger’!
What happens when ammonia is added to violet hexa-aqua-chromium ions?
A purple solution of hexa-ammonia-chromium is formed
What forms when sodium hydroxide is added to a transition metal ion?
A precipitate
