Chemistry Flashcards
(188 cards)
1
Q
What do photons do in high frequency?
A
can transfer greater amounts
of energy than photons in low frequency radiation.
2
Q
What happens when energy is transferred to atoms?
A
electrons within the atoms may be promoted to higher energy levels
3
Q
Why are there a series of lines at discrete wavelengths in an emission spectrum?
A
There are a series of lengths at discrete wavelengths because there is a series of different electron transitions between different energy levels.
4
Q
What happens during absorption spectroscopy?
A
electromagnetic radiation is directed at an atomised sample. Radiation is absorbed as electrons are promoted to higher energy levels.
5
Q
What happens during emission spectroscopy?
A
- In emission spectroscopy, high temperatures are used to excite the electrons within atoms.
- As the electrons drop to lower energy levels, photons are emitted.
- An emission spectrum of a sample is produced by measuring the
intensity of light emitted at different wavelengths.
6
Q
In atomic spectroscopy what does the concentration of an element in a sample refer to?
A
related to the intensity of light emitted or absorbed
7
Q
How many electrons can a orbital hold?
A
Orbitals can hold a maximum of two electrons.
8
Q
What are the four types of orbitals?
A
s, p, d and f
9
Q
What is quanta?
A
Electrons within atoms have fixed amounts of energy
10
Q
What is the principle quantum number n?
A
Indicates the main energy level
for an electron and is related to the size of the orbital
11
Q
What is the angular momentum quantum number l ?
A
the angular momentum quantum number l
determines the shape
of the subshell and can have values from zero to
n − 1
12
Q
What is the magnetic quantum number ml?
A
the magnetic quantum number
ml determines the orientation of the orbital and can have values between
−l and +l
13
Q
What is the spin magnetic quantum number?
A
the spin magnetic quantum number ms determines the direction of spin and can have values of +1/2 and -1/2
14
Q
What is the aufbau principle?
A
electrons fill orbitals in order of increasing
energy
15
Q
What is Hund’s rule?
A
when degenerate orbitals are available, electrons
fill each singly, keeping their spins parallel before spin pairing starts
16
Q
What is the Pauli exclusion principle?
A
no two electrons in one atom can have the same set of four quantum numbers, therefore, no orbital can hold more than two electrons and these two electrons must have opposite spins
17
Q
What is degenerate?
A
Orbitals are of equal energy
18
Q
What electronic configurations are the most stable resulting in higher ionisation energy?
A
There is a special stability associated with half-filled and full
subshells. The more stable the electronic configuration, the higher
the ionisation energy
19
Q
What charge are electrons and what do they do?
A
are negatively charged and repel each other. They are
arranged to minimise repulsion and maximise separation
20
Q
What is the order of electron pair repulsion?
A
non-bonding pair/non-bonding pair>non-bonding pair/bonding pair>bonding pair/bonding pair
21
Q
What are transition metals?
A
The d-block transition metals are metals with an incomplete d subshell in at least one of their ions.
22
Q
What happens in transition metals when electrons are lost?
A
When atoms from the first row of the transition elements form ions, it is the 4s electrons that are lost first rather than the 3d electrons.
23
Q
What is oxidation?
A
Oxidation can be defined as an increase in oxidation number.
24
Q
What is reduction?
A
Reduction can be considered as a decrease in oxidation number.
25
What are metal compounds with high oxidation states?
oxidising agents,
26
What are metal compounds with low oxidation states?
metals in low oxidation
states are often reducing agents.
27
What are ligands?
negative ions or molecules with non-bonding pairs of
electrons that they donate to the central metal atom or ion, forming
dative covalent bonds.
28
What is monodentate?
A ligand donates one pair of electrons
29
What is bidentate?
A ligand donates two pairs of electrons
30
What is the coordination number?
The total number of bonds from the ligands to the central transition metal is known as the coordination number.
31
How do d-d orbitals split?
Splitting of d orbitals to higher and lower energies occurs when the electrons present in approaching ligands cause the electrons in the orbitals lying along the axes to be repelled.
32
How is colour produced in d-d transitions?
- Light is absorbed when electrons 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.
- Electrons transition to higher energy levels when energy
corresponding to the ultraviolet or visible regions of the
electromagnetic spectrum is absorbed.
33
What is the equilibrium constant?
characterises the equilibrium
composition of the reaction mixture
34
In equilibrium constants what happens to solids and liquids?
taken as constant and given a value of 1 in the equilibrium
expression
35
What does the position of the equilibrium constant depend on?
reaction temperature and is independent of concentration and/or
pressure
36
What happens in endothermic reactions when the temperature is increased?
causes an increase
in K and the yield of the product is increased.
37
What happens in exothermic reactions when the temperature is increased?
causes a decrease in
K and the yield of the product is decreased.
38
What is homogeneous catalysis?
Homogeneous catalysts are in the same state as the reactants
39
What is heterogeneous catalysis cause?
The formation of activated complexes
40
Why can transition metals act as catalysts?
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
41
What is heterogeneous catalysis?
Heterogeneous catalysts are in a different state to the reactants.
42
What happens during homogeneous catalysis?
changing
oxidation states with the formation of intermediate complexes.
43
What does amphoteric mean?
can react as an acid and a base
44
at 25oC what is the Kw value?
1 x 10^-14
45
What is an acid?
A proton donor
46
What is a base?
A proton acceptor
47
What is a conjugate acid?
Formed by the gain of a proton
48
What is a conjugate base?
Formed by the loss of a proton
49
What are strong acids and bases?
Completely dissociate into ions in aqueous solution
50
What are strong acids and bases?
Completely dissociate into ions in aqueous solutionW
51
What are weak acids and bases?
Only partially dissociate into ions in aqueous solution
52
What dissolves in water to form a neutral solution?
A soluble salt of a strong acid and a strong base
53
What dissolves in water to form an alkaline solution?
soluble salt of a weak acid and a strong base
54
What dissolves in water to form an acidic solution?
strong acid and a weak base dissolves in water to
produce an acidic solution.
55
What is a buffer solution?
one in which the pH remains approximately
constant when small amounts of acid, base or water are added
56
What does an acidic buffer consist of?
consists of a solution of a weak acid and one of its salts
made from a strong base.
57
What does an basic buffer consist of?
A basic buffer consists of a solution of a weak base and one of its
salts
58
What is the colour of the indicator determined by?
ratio of (HIn) to (In^-)
59
When does the theoretical point at which the colour change occurs?
when (H3O^+) = KIn
60
What is the standard enthalpy of formation?
Is the enthalpy change when one mole of a substance is formed from its elements in their standard states.
61
What is the standard state of a substance is its most stable state?
1 atmosphere and at a specified temperature, usually
taken as 298 K.
62
What is entropy?
The entropy (S) of a system is a measure of the degree of disorder of
the system
63
What is the relationship between disorder and entropy?
The greater the degree of disorder, the greater the entropy.
64
What do solids have?
Solids have low disorder
65
What do gases have?
gases have high disorder
66
What is the relationship between entropy and temperature?
Entropy increases as temperature increases.
67
When is there rapid increases in entropy?
rapid increase in entropy at the melting point of a
substance and an even more rapid and larger change in entropy at
the boiling point.
68
What does the second law of thermodynamics state?
states that the total entropy of a reaction system and its surroundings always increases for a
spontaneous process
69
What does heat energy released into the environment do to the entropy?
Increases the entropy of the surorundings
70
What does heat energy absorbed by the environment do?
decreases the entropy of the surroundings
71
What is the thrid law of thermodynamics?
The third law of thermodynamics states that the entropy of a perfect
crystal at 0 K is zero
72
What is the standard entropy of a substance?
The standard entropy of a substance is the entropy value for the
substance in its standard state.
73
What does it mean is the change in free energy between reactants and products is negative?
The reaction is said to be feasible.
74
What is a feasible reaction?
A feasible reaction is one that tends towards the products rather than
the reactants
75
What temperatures will a reaction be feasible?
estimated by considering the range of values of T for which ∆°G < 0.
76
What is free energy at equilibrium?
∆ = G 0 .
77
When will a reverse reaction proceed spontaneously?
until the composition
is reached where ∆ = G 0 .
78
What does it mean if the enthalpy is less than zero and the entropy is greater than zero?
Spontaneous at all T
79
What does it mean if the enthalpy is less than zero and the entropy is less than zero?
spontaneous at low T (when T x entropy is small)
80
What does it mean if the enthalpy is greater than zero and the entropy is greater than zero?
Spontaneous at high T (when T x entropy is large)
81
What does it mean if the enthalpy is greater than zero and the entropy is less than zero?
Non-spontaneous at all (G>0)
82
What does the rate of a reaction depend on?
concentrations of the reactants.
83
What are orders of reaction used to relate?
relate the rate of a reaction to the
reacting species.
84
What does it mean if changing the concentration of reactant A has no effect on the rate of reaction?
, then the reaction is zero order with respect to A.
85
What does it mean if doubling the concentrations of a reactant A doubles the rate of the reaction?
then the reaction is first order with respect to A
86
How can first order rate be expressed?
rate = k [A] where k is the rate constant and [A] is the
concentration of reactant A in mol l^-1
87
What does it mean if doubling the concentration of reactant A increases the rate of reaction by fourfold?
then the reaction is second order with respect to A
88
How can a second order reaction be expressed?
rate = k(A)^2
89
What is the overall order of the reaction?
the sum of the powers to which the
concentrations of the reactants are raised in the rate equation.
90
What is a mechanism?
Reactions usually occur by a series of steps
91
What is the rate determining step?
The rate of reaction is dependent on the slowest step, which is called
the ‘rate determining step’.
92
What are molecular orbitals?
Molecular orbitals form when atomic orbitals combine. The combination of two atomic orbitals results in the
formation of a bonding molecular orbital and an antibonding orbital.
The bonding molecular orbital encompasses both nuclei.
93
What happens to molecular orbitals when non-polar covalent bonding occurs?
the bonding molecular orbital is symmetrical about the midpoint between two atoms
94
What happens in polar covalent bonding in molecular orbitals?
result from bonding molecular orbitals that are asymmetric
about the midpoint between two atoms. The atom with the greater
value for electronegativity has the greater share of the bonding
electrons.
95
What happens in ionic bonding in molecular compounds?
Ionic compounds are an extreme case of asymmetry, with
the bonding molecular orbitals being almost entirely located around
just one atom, resulting in the formation of ions.
96
How are sigma bonds formed?
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond are called sigma (σ ) molecular orbitals or sigma bonds.
97
How do pi bonds form?
Molecular orbitals that form by side-on overlap of parallel atomic
orbitals that lie perpendicular to the axis of the covalent bond are
called pi (π ) molecular orbitals or pi bonds.
98
What is hybridisation?
Hybridisation is the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals. These hybrid orbitals are degenerate.
99
When is there sp^3 hybridisation?
when there is single bonds
100
When is there sp^2 hybridisation?
When there is double bonds
101
When is there sp hybridisation?
When there is triple bonds
102
How do electrons fill molecular orbitals?
Electrons fill bonding molecular
orbitals, leaving higher energy antibonding orbitals unfilled
103
What is the HOMO?
highest bonding molecular orbital containing electrons is called the
highest occupied molecular orbital (HOMO)
104
What is the LUMO?
The lowest antibonding
molecular orbital is called the lowest unoccupied molecular orbital
(LUMO).
105
What does absorption of electromagnetic energy do to electrons in molecular orbitals?
Absorption of electromagnetic energy can cause electrons to be
promoted from HOMO to LUMO
106
What are chromophores?
A chromophore is a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum
107
When do chromophores exist?
a conjugated system
108
What happens if there is a high degree of conjugation?
The smaller the energy gap
between HOMO and LUMO therefore less energy required to promote electrons
109
What are isomers?
Molecules that have the same molecular formula but different
structural formulae are called isomers.
110
What are stereoisomers?
Stereoisomers occur when the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer
111
What are the different types of isomers?
geometric and optical.
112
What are the properties of geometric isomers?
♦ can occur when there is restricted rotation around a carbon-carbon double bond or a carbon-carbon single bond in a cyclic compound
♦ must have two different groups attached to each of the carbon atoms that make up the bond with restricted rotation
♦ can be labelled cis or trans according to whether the substituent groups are on the same side (cis) or on different sides (trans) of the bond with restricted rotation
♦ have differences in physical properties, such as melting point and boiling point
♦ can have differences in chemical properties
113
What are the features of optical isomers (8)?
♦ occur in compounds in which four different groups are arranged
tetrahedrally around a central carbon atom (chiral carbon or chiral
centre)
♦ are asymmetric
♦ are non-superimposable mirror images of each other
♦ can be described as enantiomers
♦ have identical physical properties, except for their effect on planepolarised light
♦ have identical chemical properties, except when in a chiral
environment such as that found in biological systems (only one
optical isomer is usually present)
♦ rotate plane-polarised light by the same amount but in opposite
directions and so are optically active
♦ when mixed in equal amounts are optically inactive because them rotational effect of the plane-polarised light cancels out — this is called a racemic mixture
114
What is elemental microanalysis?
Elemental microanalysis is used to determine the masses of C, H, O, S and N in a sample of an organic compound in order to determine its
empirical formula.
115
What does the empirical formula show?
An empirical formula shows the simplest ratio of the elements in a molecule
116
How can elemental microanalysis be determined from?
♦ combustion product masses
♦ percentage product by mass
117
What is mass spectrometry?
Mass spectrometry can be used to determine the accurate gram formula mass (GFM) and structural features of an organic compound
118
What happens in mass spectrometry?
- a small sample of an organic compound is
bombarded by high-energy electrons.
- This removes electrons from
the organic molecule generating positively charged molecular ions known as parent ions
- These molecular ions then break into smaller positively charged ion fragments
119
What does the mass-to-charge ratio determine?
the GFM of the molecular ion, and so a molecular formula can be
determined using the empirical formula
120
What is infrared spectroscopy?
is used to identify certain functional groups in an organic compound.
121
What happens when infrared is absorbed by organic compounds?
bonds within the molecule vibrate (stretch and bend). The wavelengths of infrared radiation that are absorbed depend on the type of atoms that make up the bond and the strength of the bond.
122
What happens during infrared spectroscopy?
- Infrared radiation is passed through a sample of the organic compound and then into a detector that measures the intensity of the transmitted radiation at different
wavelengths
- The absorbance of infrared radiation is measured in
wavenumbers, the reciprocal of wavelength, in units of cm-1
123
What is proton nuclear magnetic resonance?
can give information about:
- the different chemical environments of hydrogen atoms in an organic molecule
- about how many hydrogen atoms there are in each of these environments
124
What happens during NMR?
- 1H nuclei behave like tiny magnets and in a strong magnetic field
some align with the field (lower energy), whilst the rest align against it (higher energy)
- Absorption of radiation in the radio frequency region
of the electromagnetic spectrum causes the 1 H nuclei to ‘flip’ from the lower to the higher energy alignment.
- As they fall back from the
higher to the lower energy alignment the emitted radiation is detected
and plotted on a spectrum.
125
In NMR what is the chemical shift?
is related to the environment of the 1H atom and is measured in parts per million (ppm).
126
What does the area under the peak refer to in NMR?
The area under the peak is related to the number of 1
H atoms in that
environment and is often given by an integration curve on a
spectrum
127
What is the height of the integration curve in NMR proportional to?
The height of an integration curve is proportional to the
number of 1H atoms in that environment, and so a ratio of 1H atoms in
each environment can be determined.
128
What is Tetramethylsilane (TMS)
The standard reference substance used in 1
H NMR spectroscopy is
tetramethylsilane (TMS), which is assigned a chemical shift value
equal to zero
129
What is high resolution NMR?
uses higher radio frequencies than those used in low-resolution and provides a clearer specta.
130
What is high resolution NMR?
uses higher radio frequencies than those used in low-resolution and provides a detailed spectra.
131
What happens in high-resolution NMR?
- an interaction with 1
H atoms on neighbouring carbon atoms can result in the splitting of peaks into
multiplets
- The number of 1
H atoms on neighbouring carbon atoms
will determine the number of peaks within a multiplet and can be determined using the n+1 rule, where n is the number of 1
H atoms on
the neighbouring carbon atom.
132
What are drugs?
Drugs are substances that alter the biochemical processes in the
body.
133
What do medicines typically contain?
contains the drug plus other ingredients such as
fillers to add bulk or sweeteners to improve the taste.
134
How do drugs work?
Drugs generally work by binding to specific protein molecules. These protein molecules can be found on the surface of a cell (receptor) or can be specific enzyme molecules within a cell.
135
What do agonist drugs do?
An agonist mimics the natural compound and binds to the
receptor molecules to produce a response similar to the natural
active compound.
136
What do antagonist drugs do?
An antagonist prevents the natural compound from binding to the
receptor, and so blocks the natural response from occurring.
137
What does the structural fragment of a drug molecule allow?
allows it to form interactions with a receptor binding site or to an enzyme active site normally consists of different functional groups correctly orientated with respect to each other
138
What is Stoichiometry?
the study of mole relationships involved in chemical
reactions
139
What is is the percentage by mass?
the mass of solute made up to 100 cm3 of
solution.
140
What is percentage by volume?
Percentage by volume is the number of cm3 of solute made up to 100
cm3 of solution
141
What is ppm?
parts per million and refers to 1 mg per kg or
1 mg per litre.
142
How is the percentage yield reduced?
♦ mass transfer or mechanical losses
♦ purification of product
♦ side reactions
♦ equilibrium position
143
What is weighing accurately approximately?
This means the exact mass should be known and should be close to the true value stated.
144
What is the process for weighing by difference?
1. Measure the mass of the empty weigh boat
2. Measure out the solid in the weigh boat
3. Take note of the mass of the weigh boat + solid and use this to work out how much solid has been measured out (mass of weigh boat + solid - mass of empty weigh boat)
4. Transfer solid to beaker
5. Reweigh the empty weigh boat to calculate total solid transferred to beaker.
145
What is the process of heating to constant mass?
— heating a substance
— allowing to cool in a desiccator to prevent absorption of water
— weighing
— repeating the steps of heating, cooling and weighing until no further changes in mass are observed
146
What is gravimetric analysis?
Gravimetric analysis is used to determine the mass of an element or
compound in a substance
147
What must happen to the substance in order to use gravimetric analysis?
The substance is converted into another substance of known
chemical composition, which can be readily isolated and purified. The conversion can occur either through precipitation or volatilisation
148
What is the precipitation conversion?
In precipitation conversion the substance undergoes a precipitation reaction.
- The precipitate is separated from the filtrate and the
- filtrate tested to ensure the reaction has gone to completion.
- The precipitate is washed, dried to constant mass and then weighed.
149
What is the volatilisation conversion?
In volatilisation conversion the substance is heated and any volatile products (often water) are evaporated. The substance is heated to constant mass and the final mass recorded.
150
What is a standard solution?
A solution of accurately known concentration is known as a standard solution
151
How can a standard solution be prepared?
♦ weighing a primary standard accurately
♦ dissolving in a small volume of solvent (usually deionised or
distilled water) in a beaker
♦ transferring the solution and rinsings into a volumetric flask
♦ making up to the graduation mark with solvent
♦ stoppering and inverting
152
What is a different method of making a standard solution?
Standard solutions can also be prepared by accurate dilution by
pipetting an appropriate volume of a standard solution into a
volumetric flask, making up to the graduation mark with solvent,
stoppering and inverting
153
What are the properties of a primary standard?
♦ be available in a high state of purity
♦ be stable when solid and in solution
♦ be soluble
♦ have a reasonably high GFM
154
Why is sodium hydroxide not a primary standard?
- It has a relatively low GFM,
- is unstable as a solid (absorbs moisture) and unstable as a
solution.
- Sodium hydroxide solution must be standardised before being used in volumetric analysis.
155
What are some examples of primary standards?
♦ sodium carbonate,
♦ hydrated oxalic acid,
♦ potassium hydrogen phthalate,
♦ silver nitrate,
♦ potassium iodate,
♦ potassium dichromate,
156
What are redox titrations?
redox titrations based on reactions between oxidising and
reducing agents
157
What are complexometric titrations?
based on reactions in which complexes
are formed — EDTA is an important complexometric reagent and
can be used to determine the concentration of metal ions in
solution
158
What are back titrations?
back titrations used to find the number of moles of a substance by reacting it with an excess volume of a reactant of known
concentration.
159
What is colorimetry?
uses the relationship between colour intensity of a
solution and the concentration of the coloured species present.
160
What does a colorimeter measure?
A colorimeter or a spectrophotometer is used to measure the
absorbance of light of a series of standard solutions, and this data is
used to plot a calibration graph.
161
What is distillation?
Distillation is used for identification and purification of organic
compounds.
162
What can distillation determine?
The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm its identity
163
What can distillation be used for?
to purify a compound by separating it from less volatile substances in the mixture.
164
What is heating by reflux?
Heating under reflux allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
165
What is required to carry out heating under reflux?
the reaction mixture is placed
in a round-bottomed flask with anti-bumping granules and the flask is
fitted with a condenser. The flask is then heated using an appropriate
source of heat.
166
What is vacuum filtration?
Vacuum filtration involves carrying out a filtration under
reduced pressure and provides a faster means of separating a
precipitate from a filtrate. A Büchner, Hirsch or sintered glass funnel
can be used during vacuum filtration.
167
What is the process of recrystallisation?
♦ dissolving an impure solid gently in a minimum volume of a hot
solvent
♦ hot filtration of the resulting mixture to remove any insoluble
impurities
♦ cooling the filtrate slowly to allow crystals of the pure compound to form, leaving soluble impurities dissolved in the solvent
♦ filtering, washing and drying the pure crystals
168
What is recrystallisation used for?
to purify an impure solid
169
How is the solvent used for recrystallisation chosen?
chosen so that the compound
being purified is completely soluble at high temperatures and only
sparingly soluble at lower temperatures.
170
What is solvent extraction?
Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which the solute is soluble.
171
What happens during solvent extraction?
- When carrying out a solvent extraction, the two immiscible solvents
form two layers in the separating funnel.
- The solute dissolves in both solvents and an equilibrium establishes between the two layers.
- The ratio of solute dissolved in each layer is determined by the equilibrium constant, K .
- The lower layer is run off into a container and the upper layer is poured into a second container.
- This process is repeated to maximise the quantity of solute extracted
172
When is the quantity of solute extracted is greater?
if a number of extractions
using smaller volumes of solvent are carried out rather than a single
extraction using a large volume of solvent.
173
What must a solvent be?
♦ immiscible with the liquid mixture or solution (usually water)
♦ one in which the solute is more soluble in than the liquid mixture or solution (usually water)
♦ volatile to allow the solute to be obtained by evaporation of the solvent
♦ unreactive with the solute
174
What is melting point?
The melting point of a
substance is the temperature range over which the solid first starts to
melt, to when all of the solid has melted
175
What does melting point analysis do?
The identity of a pure compound can be confirmed by melting point analysis and a comparison of the experimentally determined melting point with a literature or known melting point value.
176
How does melting point produce a range?
The presence of impurities in the compound lowers the melting point and broadens its melting temperature range due to the disruption in intermolecular bonding in
the crystal lattice.
177
How does mixed melting point identify if the substance is pure?
Determination of a mixed melting point involves mixing a small
quantity of the product with some of the pure compound and
determining the melting point. The melting point value and the range
of the melting temperature can be used to determine if the product
and the pure compound are the same substance
178
What is chromatography?
Chromatography is a technique used to separate
the components present within a mixture.
179
What is thin layer chromatography (TLC?
Thin-layer chromatography (TLC) uses a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic.
180
How can Rf values be calculated?
Rf = distance travelled by sample/distance travelled by the solvent
181
What will the Rf value be under the same conditions?
Under the same conditions (temperature, solvent, and saturation levels) a compound always has the same Rf value (within experimental error).
182
How can the identity of a compound be confirmed (chromatography)?
♦ comparing the experimentally determined Rf values with a
literature or known value determined under the same conditions
♦ making a direct comparison on a TLC plate between the
compound being tested and the pure substance — a co-spot
could be used
183
What does TLC assess?
TLC is used to assess the purity of substances. A pure substance, when spotted and developed on a TLC plate, should appear as a single spot (some impurities may not be visible by TLC analysis). The presence of more than one spot shows that impurities are present.
184
How does chromatography separate substances?
Chromatography separates substances by making use of differences in their polarity or molecular
size
185
What is the inductive effect?
The alkyl groups have a positive inductive effect. They are electron donating and push electrons towards the positive charge on the carbon which helps to stabilise it. This means that tertiary carbocation are the most stable with primary cations being the least stable.
186
What is the steric effect?
In SN2 mechanisms, the nucleophile attacks from the side opposite the halogen. Tertiary haloalkanes will resist this attack as it is sterically hindered. Three bulky alkyl groups prevent the access to the positive carbon.
187
When is a back titration helpful
A back titration is useful when trying to work out the quantity of substance in a solid with a low solubility
188
What is the process of TLC?
- A small sample of the mixture being tested is spotted onto the base (pencil)
line of the chromatogram.
- A solvent dissolves the compounds in the spot and carries the compounds up the chromatogram.
- How far the compounds are carried depends on how soluble the compounds are in the chosen solvent and how well they adhere to the plate.
- A developing agent or ultraviolet light is normally required to visualise
the spots on the chromatogram.
