Things I forget Flashcards
What is a back titration?
- used to find the number of moles of a substance by reacting it with an excess volume of reactant of known concentration
- the resulting mixture is then titrated to work out the number of moles of the reactant in excess
- from the initial number of moles of that reactant, the number of moles used in the reaction can be determined
- the initial number of moles of the substance being analysed can then be calculated
When is a back titration useful?
when trying to work out the quantity of a substance in a solid with low solubility
What must a primarystandard be?
- available in a high state of purity
- be stable when solid and in solution
- be soluble
- have a reasonable high GFM
Give 6 examples of primary standards
sodium carbonate
hydrated oxalic acid
potassium hydrogen
phthalate
silver nitrate
potassium iodate
potassium dichromate
Why is sodium hydroxide not a primary standard?
- has a relatively low GFM
- is unstable as a solid (absorbs moisture) and unstable as a solution
How is percentage yield reduced?
- mass transfer or mechanical losses
- purification of product
- side reactions
- equilibrium position
What does the unit ppm refer to?
1mg per kg or 1mg per liter
What happens in absorption spectroscopy?
- Em radiation is detected at an atomised sample
- radiation is absorbed as electrons are promoted to higher energy levels
How is an absorption spectra produced?
by measuring how the intensity of light varies with wavelength
What happens in emission spectroscopy?
- high temperatures are used to excited electrons within atoms
- as the electrons drop to lower energy levels, photons are emitted
How is an emission spectra produced?
by measuring the intensity of light emitted
at different wavelengths.
What is the magnetic quantum number?
the magnetic quantum number (ml) determines the orientation of the orbital and can
have values between
-l and +l
What is the aufbau principle?
electrons fill orbitals in order of increasing energy
What is Hunds rule?
when degenerate orbitals are available, electrons fill each singly,
keeping their spins parallel before spin pairing starts
2 electron pairs? (three atoms)
linear
180
3 electron pairs (4 atoms)
trigonal planar
120
4 electron pairs (5 atoms)
tetrahedral
109.5
5 electron pairs (6 atoms)
trigonal bipyramidal
90, 120, 180
6 electron pairs (7 atom)
octahedral
90
Order of electron pair repulsion strength
non bonding pair/non bonding pair>non bonding/bonding>bonding/bonding
formula for oxalatate
C2O4 2-
formula for cyanide
CN -
How can colours of many transition complexes be explained?
- in terms of d-d transitions
- light is absorbed when electron in a lower energy d orbital are promoted to a d orbital of higher energy
- if light of one colour is absorbed, then the complementary colour will be observed
How can heterogeneous catalysts be explained?
- in terms of the formation of activiated complexes and absorption of reactive molecules onto active sites
- the presence of unpaired d electrons or unfilled d orbitals is thought to allow activated complexes to form
- this can provide reaction pathways with lower activation energies compared to the uncatalysed reaction
How can homogeneous catalysts be explained?
in terms of changing oxidation states with the formation of intermediate complexes
How does an acid buffer work?
- weak acid provides hydrogen ions when these are removed by the addition of a small amount of base
- the salt of the weak acid provides the conjugate base, which can absorb excess hydrogen ions produced by the addition of a small amount of acid
What is the third law of thermodynamics?
the entropy of a perfect crystal at 0 K is zero.
What is the second law of thermodynamics?
the total entropy of a reaction system and its surroundings always increases for a spontaneous process.
