Module 5 Flashcards
Rate of Reaction, Acid and Bases, Entropy, Enthalpy, Redox, Electrodes, Transition Elements
What is Kc?
A quantitative measure of the proportion of reactant in an equilibrium mixture. Large Kc means the equilibrium lies to the right with a high yield of products
What is a homogenous equilibrium?
Contains species that all have the same state
What is a heterogenous equilibrium?
Contains species that have different states
What is Kp?
A equilibrium constant for gaseous equilibriums as it is more convenient to use partial pressures
What is Lattice Enthalpy?
The enthalpy change when one mole of an ionic compound is formed from its gaseous ions
Is bond forming endothermic or exothermic?
Exothermic
2 factors that affect lattice enthalpy?
Ionic size (radius) - smaller ions attract more strongly than larger ions
Ionic charge - ions of greater charge attract more strongly than ions with a smaller charge
What is standard enthalpy change of formation?
The enthalpy change when one mole of compound is formed from its elements in their standard states under standard conditions
What is standard enthalpy of atomisation?
The enthalpy change when one mole of GASEOUS atoms is formed from its element in its standard state under standard conditions
What is first ionisation energy?
The enthalpy change when one electron is removed from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
What is second ionisation energy
The enthalpy change when one electron is removed from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
What is first electron affinity?
The enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
When is second electron affinity
The enthalpy change when one electron is added to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
What is enthalpy change of solution?
The enthalpy change that takes place when one mole of a compound (solute) is completely dissolved in water under standard conditions
What two processes take place when an ionic compound dissolves into water?
1) The ionic lattice dissociates into its gaseous ions (endothermic)
2) The gaseous ions are hydrated with H2O molecules (exothermic, +ve ions attract slightly negative O, -ve ions attract slightly positive H)
What is standard enthalpy change of hydration?
The enthalpy that takes place when one mole of isolated gaseous ions is dissolved into water forming one mole of aqueous ions under standard conditions
What are the standard conditions?
101kPa and 298K
273K is what in degrees
1
How to work out how many electrons are removed from a mass of an element
Take the grams of the element (mass)
divide the mass by the Mr
Times the answer to the above by Avogadro’s constant (in formula book)
Name and bond angle:
2 bond pairs
0 lone pairs
Linear, 180
Name and bond angle:
2 bond pairs
1 lone pair
Non-linear, 117.5
Name and bond angle:
2 bond pairs
2 lone pairs
Non-linear, 104.5
Name and bond angle:
3 bond pairs
0 lone pairs
Trigonal Planar, 120
Name and bond angle:
3 bond pairs
1 lone pair
Pyramidal, 107
Name and bond angle:
4 bond pairs
0 lone pairs
Tetrahedral, 109.5
Name and bond angle:
6 bond pairs
0 lone pairs
Octahedral, 90
What is entropy?
A measure of the dispersal of energy in a system. The entropy is greater when the system is more disordered. It is always positive. It increases with temperature
What is standard entropy change of reaction?
The entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactant and products being in their standard states
What sign does Gibbs (free energy change) need to be for a reaction to be feasible?
Negative
Limitations of predictions made for feasibility
-Some reactions have a negative Gibbs but still do not seem to actually take place at the expected temperature
-Feasibility predictions do not take into account the reactions kinetics (rate of reaction)
What is the difference between NH3 and PH3?
NH3 has hydrogen bonding which is a stronger intermolecular force than PH3 has
NaSO4 or Na2SO4?
Na2SO4
How to work out enthalpy of formation?
Products-Reactants
How to work out k from half lives
ln2/half life average
If there are two weak acids, which acid becomes the acid in the acid-base pair
The acid with the higher Ka value
Ka=
Ka=[H+]^2/[HA]
pH of a buffer
pKa + log([A-]/[HA])
Kw=
[H+][OH-] at 298K
If you have pH how can you find the [OH-]?
14-pH = p0H
10^-pOH = [OH-]
Acid is?
Acid is a proton donor
Base is?
Base is a proton acceptor
How is a choice of indicator linked to the pH curve?
Must change colour within the equivalence point
What is a buffer solution?
A mixture that minimises pH change on addition of small amounts of an acid or base
If the oxidation number is odd, eg (III) or (VII), how many oxygen in the compound and what is the charge
Oxygen = oxidation number + 1 / 2
Charge = -1
If the oxidation number is even, eg (IV) or (VI), how many oxygen in the compound and what is the charge
Oxygen= oxidation number + 2 / 2
Charge = -2
Is the oxidising agent in the flask or burette in a redox titration?
Burette
What is the colour change of oxidation in a manganate (VII) redox titration?
pink to colourless
What colour does starch indicator turn in the presence of iodine?
Blue-black
Aqueous iodine is this colour
Brown
Colour change at end point of titration of iodine and thiosulfate? (starch indicator added)
Blue-black to colourless
Are the moles of titrated I2 the same as the “liberated”/redox product I2?
Yes :)
More positive standard electrode potential means
greater tendency to gain electrons, and undergo reduction
Standard conditions of electrochemical cells
298K
1.00moldm-3
1 atm
If only aqueous/liquid in the half cell equation, what solid can be the electrode
Pt(s) / Platinum solid
If there is a gas in the half cell equation what is needed?
A gas cylinder is placed over the electrode and low enough to be submerged in the solution. The gas is pumped into the solution this way
What is between half cells
A salt bridge which completes the electrical circuit and allows the movement of ions between the two half cells
Standard hydrogen half cell
2H+(aq) + 2e- (reversable arrow) H2(g)
-needs H2(g) (at 1 atm) pumping into H+(aq) (at 1.00moldm-3)
-needs a solid platinum electrode
What is the standard electrode potential of a half cell?
The electromotive force/voltage of a half cell compared with a standard hydrogen half-cell, measured at 298K with solution concentrations of 1moldm-3 and a gas pressure of 1atm
Is the standard cell potential positive or negative?
Positive
What are primary cells?
Non-rechargeable
(provide electrical energy until the chemicals have reacted to such an extent that the voltage falls and the cell is ‘flat’
What are secondary cells?
Rechargeable
The reaction that provides electrical energy can be reversed during the recharging process
What is a fuel cell?
A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
Alkali Hydrogen-Oxygen fuel cell
Reduction: 1/2O2(g) + H2O(l) +2e- -> 2OH-
Oxidation: H2(g) + 2OH-(aq) -> 2H2O(l) + 2e-
Redox: H2(g) + 1/2O2(g) -> H2O(l)
Acid Hydrogen-Oxygen fuel cell
Reduction: 1/2O2(g) + 2H+(aq) + 2e- -> H20(l)
Oxidation: H2(g) -> 2H+(aq) + 2e-
Redox: H2(g) + 1/2O2(g) -> H2O(l)
On a concentration-rate graph, which order corresponds to which line:
-Straight across
-Straight diagonal increasing
-Increasing curve
-Straight across is order 0
-Straight diagonal increasing is 1st order
-Increasing curve is 2nd order
On a concentration-time graph, which order corresponds to which:
-Straight diagonal decreasing
-Decreasing curve with constant half lives
-Decreasing curve with non constant half lives
Straight diagonal decreasing is order 0
Decreasing curve with constant half lives is 1st order
Decreasing curve with non constant half lives is 2nd order
What is the rate determining step?
The slowest step in a multistep reaction
Does a catalyst effect Kc and Kp?
No.
It increases the rate of both the forward and reverse reactions by the same amount.
Does pressure affect Kp?
No
If the forward reaction is exothermic and temperature increases, what is the effect on Kp/c
The value decreases
If the forward reaction is endothermic and temperature increases, what is the effect on Kp/c
The value increases
Does concentration effect Kc?
No
What colour is Mn0₄¯?
Purple
What colour is Cr₂O₇²¯?
Orange
What colour is Cu²+?
Blue
What colour is Cu(OH)₂ and what state symbol should it have?
Blue(s) Precipitate
What colour is Fe²+?
Pale Green
What colour is Fe(OH)₂ and what state symbol should it have?
Green(s) Precipitate
What colour is Fe³+?
Yellow
What colour is Fe(OH)₃ and what state symbol should it have?
Rusty-Brown(s) Precipitate
What colour is Mn²+?
Pale Pink
What colour is Mn(OH)₂ and what state symbol should it have?
Light-Brown(s) Precipitate
What colour is Cr³+?
Violet (or Green)
What colour is Cr(OH)₃ and what state symbol should it have?
Grey-Green(s) Precipitate
How is a OH¯ ion produced for a precipitation reaction?
Either from aqueous sodium hydroxide, or aqueous ammonia (whereby the ammonia reacts with water to make an equilibrium with OH¯ and NH₄+
What do you make from reacting a large excess of NH₃(aq) and Cu²+?
Blue precipitate made first will dissolve to a deep blue solution. The complex ion formed contains 4NH₃ ligands and 2H₂O ligands
What do you make from reacting Cu²+ and large excess of concentrated HCl(aq)?
Blue solution will turn yellow. If HCl is added dropwise until excess a green solution will appear before the yellow. The complex ion formed has 4Cl¯ ligands and a 2¯ charge
What happens when a large excess of NaOH(aq) is reacted with Cr³+?
The grey-green precipitate will dissolve to make a green solution. The complex ion formed has 6OH¯ ligands and a charge of 3¯
What happens when an excess of NH₃(aq) reacts with Cr³+?
A purple solution is formed and the grey-green precipitate will dissolve. The complex ion formed had 6NH₃ ligands. 6 molecules of water are a biproduct of this reaction.
What three transition element ions don’t change colour when excess NaOH or excess NH₃ is added?
Fe²+, Fe³+ and Mn²+
