Things I forget

Question 1

What is a back titration?

Answer 1

• 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

Question 2

When is a back titration useful?

Answer 2

when trying to work out the quantity of a substance in a solid with low solubility

Question 3

What must a primarystandard be?

Answer 3

• available in a high state of purity
• be stable when solid and in solution
• be soluble
• have a reasonable high GFM

Question 4

Give 6 examples of primary standards

Answer 4

sodium carbonate

hydrated oxalic acid

potassium hydrogen
phthalate

silver nitrate

potassium iodate

potassium dichromate

Question 5

Why is sodium hydroxide not a primary standard?

Answer 5

• has a relatively low GFM
• is unstable as a solid (absorbs moisture) and unstable as a solution

Question 6

How is percentage yield reduced?

Answer 6

• mass transfer or mechanical losses
• purification of product
• side reactions
• equilibrium position

Question 7

What does the unit ppm refer to?

Answer 7

1mg per kg or 1mg per liter

Question 8

What happens in absorption spectroscopy?

Answer 8

• Em radiation is detected at an atomised sample
• radiation is absorbed as electrons are promoted to higher energy levels

Question 9

How is an absorption spectra produced?

Answer 9

by measuring how the intensity of light varies with wavelength

Question 10

What happens in emission spectroscopy?

Answer 10

• high temperatures are used to excited electrons within atoms
• as the electrons drop to lower energy levels, photons are emitted

Question 11

How is an emission spectra produced?

Answer 11

by measuring the intensity of light emitted
at different wavelengths.

Question 12

What is the magnetic quantum number?

Answer 12

the magnetic quantum number (ml) determines the orientation of the orbital and can
have values between
-l and +l

Question 13

What is the aufbau principle?

Answer 13

electrons fill orbitals in order of increasing energy

Question 14

What is Hunds rule?

Answer 14

when degenerate orbitals are available, electrons fill each singly,
keeping their spins parallel before spin pairing starts

Question 15

2 electron pairs? (three atoms)

Answer 15

linear
180

Question 16

3 electron pairs (4 atoms)

Answer 16

trigonal planar
120

Question 17

4 electron pairs (5 atoms)

Answer 17

tetrahedral
109.5

Question 18

5 electron pairs (6 atoms)

Answer 18

trigonal bipyramidal
90, 120, 180

Question 19

6 electron pairs (7 atom)

Answer 19

octahedral
90

Question 20

Order of electron pair repulsion strength

Answer 20

non bonding pair/non bonding pair>non bonding/bonding>bonding/bonding

Question 21

formula for oxalatate

Answer 21

C2O4 2-

Question 22

formula for cyanide

Answer 22

CN -

Question 23

How can colours of many transition complexes be explained?

Answer 23

• 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

Question 24

How can heterogeneous catalysts be explained?

Answer 24

• 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

Question 25

How can homogeneous catalysts be explained?

Answer 25

in terms of changing oxidation states with the formation of intermediate complexes

Question 26

How does an acid buffer work?

Answer 26

- 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

Question 27

What is the third law of thermodynamics?

Answer 27

the entropy of a perfect crystal at 0 K is zero.

Question 28

What is the second law of thermodynamics?

Answer 28

the total entropy of a reaction system and its surroundings always increases for a spontaneous process.