The ratio of the average mass of an atom to the mass of C-12 isotope
It is the average mass of the element taking into account all its isotopes and their relative abundance, compared to one atom of C-12
What are the assumptions made in back titration?
all the (x) reacted
the only compound of the (tablet/thing) was (x)
there were no basic impurities in the (x) that would react with (y)
How can you determine who all the hydrated salt has been converted into anhydrous form? [1]
by repeating the process of heating and weighing until a constant mass is obtained (for accurate results it will be necessary to cool the sample to room temperature before each weighing)
State some assumptions in back titrations and other problems
no side reactions occur that generate other products
all of the (PbSO3) formed precipitates out as a solid and can be weighed
Describe difference between P4 and 4P [1]
P4 is a molecule comprising of 4 phosphorus atoms
4P is four phosphorus atoms
Atomic structure
Differentiate between a continuous spectrum and a line spectrum
continuous spectrum: radiation spread over all frequencies
line spectrum: radiation absorbed/emitted at certain/specific frequencies
Outline how the spectra of light from stars can be used to detect the presence of carbon [1]
Presence of dark/absorption lines to corresponding to those found for carbon
Missing wavelength/frequencies correspond to carbonβs spectrum
What do isotopes have in common?
same atomic number
same chemical properties because same number of electrons
(different physical properties because different number of neutrons)
Explain how the successive ionization energies in sodium are related to the electron configuration
the first ionization energy is the lowest as it corresponds to removing the electron in the valence level that is furthest from the nucleus and experiences the least electrostatic attraction (there is also some repulsion from other electrons)
the IE of the second electron is much larger as it involves removing an electron that is in an energy level closer to the nucleus and experiences a greater electrostatic attraction
as successive electrons are removed within the level, IEs increase due to the greater effective nuclear charge experienced by these electrons
the large increase in the 9th and 10th IEs is due the the 10th electron being removed removed from the energy level closest to to the nucleus, where it will experience significant electrostatic attraction to the nucleus
the 11th electron has the highest IE as it is also removed from the energy level closest to the nucleus and experiences the greatest electrostatic attraction to the nucleus (itβs the final electron is only experiences attraction to nucleus and no repulsion to other electrons, as the 10th one has already been removed)
Explain why noble gases show the highest first ionization energies respective to their periods
the outer energy level is full
there is an increased charge on the on the nucleus
they have the greatest attraction for electrons and the effective nuclear charge is the highest for their electrons
Explain why the relative atomic mass of argon is greater than that of potassium, even through the atomic number of potassium is greater than that of argon
argon has a greater number of heavier isotopes
argon has a greater number of neutrons
(potassium has more protons than argon, so it has a higher atomic number; argon has a higher atomic mass as there is a higher abundance of heavier isotopes that contain more neutrons than potassium)
Describe the bonding within the carbon monoxide molecule
triple covalent bond molecule
one electron pair donated by oxygen to carbon atom (coordinate covalent compound)
The graph of ionization energies of aluminium is shown. Explain why there is a large increase between the 9th and 10th ionization energies
the 9th IE corresponds to the following: Al8+βββAl9+β +β eβ
from Al8+ the electron is removed from the 2p orbital
from Al9+ the electron is removed from the 2s orbital
electrons in a 2s orbital are of a lower energy, and closer to the nucleus and experience a stronger force of electrostatic attraction and so are more difficult to remove
This question is about the hydrogen spectrum
Describe the appearance of the visible emission spectrum of hydrogen [2]
Line spectrum
Lines converge at high energy/frequency
Explain how this spectrum is related to the electron energy levels in a hydrogen atom [3]
Electron transition from higher to lower/second energy levels
Each transition causes emission of light of specific wavelength/energy
Each transition is related to energy difference
Energy levels in hydrogen atom are closer/converge at higher energy
Periodicity
Explain why noble gases have the highest first ionization energies
the electrons experience the greatest effective nuclear charge
so the valence electrons experience the greatest electrostatic force of attraction to the nucleus
Explain why chlorine has a smaller atomic radius than sodium
valence electrons of both are σ σ in third shell so they experience similar shielding effects
chlorine has 17 p and sodium has 11 p so valence electrons of chlorine will experience a greater attraction to the nucleus and be held closer to nuclear so smaller atomic radius
The effective nuclear charge of chlorine is higher as it has more protons, so the electrons in the outer energy level are pulled closer by the electrostatic attraction between the nucleus and the electrons
Explain why S2β has a greater ionic radius that Clβ
sulfide has 16 p and chloride has 17 p
sulfideβs valence electrons in outer energy level experience less attraction from the nucleus and will be held less closely, resulting in a larger ionic radius
Explain why there are relatively large differences between the 4th and 5th, and between the 10th and 11th ionization energies
4th electron removed from 3d and 5th electron removed from 3p
10th electron removed from 3p and 11th electron removed from 3s
Electrons removed from lower energy level (closer the nucleus) are attracted more strongly
Electrons closer to the nucleus also have a greater effective nuclear charge
Explain why Ni2+ is green but Sc3+ is colorless
Color is due to partially filled d sub-level
d sub-level is split
Ni2+ has incomplete 3d sub-level but Sc3+ has no 3d electrons
Electrons move from lower to higher sub-levels when they absorb energy/light
Ni2+ appears green because it absorbs red
Explain what is meant by a transition metal [1]
Incomplete d sub-level in its compounds/ions
Explain why melting points of
Group 1 elements decrease down the group
metallic bond strength decreases down the group as delocalized valence electrons are in energy levels more distant from the nucleus
higher distance between nucleus and delocalized electrons so less bond strength
Group 17 increase down the group
exist as diatomic molecules attracted by weak London forces
they increase down the group as the atoms become larger and contain more electrons so higher melting points (bigger electron cloud increases the LDF as there is an increased probability of temporary dipoles forming)
Chemical bonding and structure
Given two white solids, find out which one is an ionic compound
test the mp: ionic solids usually have high melting points
test the solubility: ionic compounds usually dissolve in water but not hexane
test the conductivity: ionic compounds in aq solutions are good conductors, as are ionic compounds when they are molten
Why are double bonds shorter and stronger than single bonds?
multiple bonds have a greater number of shared electrons and so have a stronger force of electrostatic attraction between the bonded nuclei
there is a greater pulling power on the nuclei, bringing them closer together, resulting in bonds that are shorter and stronger than single bonds
What are the main ideas of the VSEPR theory?
total no. of electron domains around the central atom determines the geometrical arrangement of the electron domains
shape of the molecule is determined by angles between the bonded atoms
repulsion applies to electron domains
non-bonding pairs (lone pairs) have a higher concentration of charge than bonding pairs because they are not shared between two atoms, and so cause slightly more repulsive than bonding pairs
Describe the bonding in metals [2]
electrostatic attraction
between a lattice of metal positive ions/cations AND a sea of delocalized electrons
Suggest why the mp of vanadium is higher than that of titanium [1]
vanadium has a smaller ionic radius leading to a stronger metallic bonding
Explain why an alloy is harder than the pure metal
foreign atoms/ions distort the regular arrangement of atoms/ions or are of a different size to the pure atoms/ions
this prevents layers from sliding over each other
Compare the structure and bonding in carbon dioxide and silicon dioxide [3]
Structure: CO2 molecular and SiO2 three dimensional/giant covalent
CO2 linear and SiO2 tetrahedral
Intramolecular bonding: covalent bonds in CO2 and SiO2
Double bonds in CO2 and single bonds in SiO2
Predict and explain whether NH3 and BF3 are polar molecules
BF3 is nonpolar as no net dipole; in BF3 bond polarities cancel each other out/symmetrical distribution of charge
NH3 is polar as there is net dipole movement; NβH bond polarities do not cancel each other out/unsymmetrical distribution of charge (there is a lone pair, making it tetrahedral)
Describe the nature of ionic bonding [1]
Electrostatic attraction AND oppositely charged ions
Explain resonance [1]
More than one possible position for a multiple/Ο bond
Explain why x is more volatile than y (template question) [2]
Hydrogen bonding in y
Dipole-dipole forces in y are more polar
C has greater molar mass/more dispersion/London/instantaneous induced dipole-induced dipole forces/van der Waal forces;
Energetics and thermochemistry
Comment on the relative values of the first and second electron affinity values of (oxygen) as given in the data booklet [2]
The first electron affinity corresponds to the attraction of an outer electron into the outer energy level of oxygen, which is an exothermic as heat is released
The second electron affinity corresponds to a negatively charged oxide ion accepting an additional outer electron into an energy level despite the mutual repulsion between the negatively charged species, which is an endothermic process
Describe assumptions made using this expression (graph: temperature vs. time) to calculate heat changes
all the heat released by the reaction is transferred to the solution and there is no heat loss to surroundings
the specific heat capacity of the solution is equal to that of water
the density of the solution is 1.00g/cm3 (same as water)
the temperature changes occur uniformly thought the solution
the polystyrene cup does not absorb any heat (it has negative heat capacity)
the excess zinc powder does not absorb any heat (it has negligible heat capacity)
The heat graph is extrapolated to calculate the maximum temperature allowing for heat loss; state the assumption made.
that the temperature decreases at a uniform rate (so linear extrapolation is appropriate)
The experiment is repeated and another value of enthalpy is obtained. Comment on these differences
Heat to the atmosphere/surroundings
The specific heat capacity is take of pure water, even though it might not be
The density of the solution taken to be that of pure water
Uncertainty of thermometer is so high so temperature change is unreliable
Not all solid dissolved
Why does the reaction
Represent the bond enthalpy of HCL: HCl(g)βββH(g)β +β Cl(g)
bond enthalpy is the energy required to break 1 mole of a bond in gaseous molecule averaged over similar compounds
the bond breaks with each atom taking on the bonded electron and no charged species being formed
this equation shows the HβCl bond being broken to give H(g) and Cl(g)
Represent the CβCl bond enthalpy in tetrachloromethane: CCL4βββCCl3(g)β +β Cl(g)
bond enthalpy is required to break 1 mole of a bond in a gaseous molecule averaged over similar compounds
therefore the equation representing a bond enthalpy of the CβCl bond should only involve the breaking of one CβCl bond
Explain why the data booklet value of enthalpy of combustion differs from
The theoretical value
the enthalpy change calculated from the bond enthalpies are averaged values obtained from similar compounds and are not specific to the compounds in the reactions
the tabulated bond enthalpies are also obtained from gaseous compounds but the combustion reactions conducted in the reactions uses liquid methanol
The lab calculated value
heat released from the reaction is lost to the surrounding air as well as going into heating the glass test-tube and is not all going into heating the water
complete combustion might not occur in the lab, which would decrease the amount of heat released by the reactions
The following reactions take place in the ozone layer by the absorption of UV light
State and explain, by reference to the bonding, which of the reactions required a shorter wavelength: I) O3βββO2β +β O.; and II) O2βββO.β +β O.
the bond order in O2β=β2; the bond order in O3β=β1.5, because of the two resonance structures (coordinate bond)
reaction II will require shorter wavelength as the bond in O2 has a higher bond order and is stronger than the bonds in O3
UV light of a higher energy (shorter wavelength) will be required to break the stronger bond in O2
Explain why ozone can be decomposed by light with a longer wavelength than the required to decompose oxygen
the oxygen double bond has a bond order of 2, stronger than the 1.5 bond order in ozone
so less energy is required to dissociate ozone than oxygen
longer wavelength radiation of lower energy is needed to dissociate ozone
State whether the the following reactions of the born-haber cycle are endothermic or exothermic
Β½ Cl2βββCl(g) and Li(s)βββLi(g)
bond dissociation and enthalpy of atomization
so both endothermic
Cl(g)β +β eββββClβ(g) and Li(g)βββLi+(g)β +β eβ
electron affinity is exothermic
ionization energy is endothermic
Li+(g)β +β Clβ(g)βββLiCl(s)
reverse lattice enthalpy is exothermic
(otherwise, lattice enthalpy is endothermic)
Β½ Cl2(g)βββCl(g) and Cl(g)βββCl(g)β +β eββββClβ(g)
bond dissociation is endothermic
electron affinity is exothermic
This is a question about the born-haber cycle
The theoretically calculated value for lattice enthalpy of magnesium chloride is different from the experimental value explain
theoretical values are calculated assuming that bonding in MgCl2 is ionic in nature; however, the bond between Mg and Cl also has some covalent character
this results in the experimental atomic enthalpy being higher than the theoretical lattice enthalpy
Explain why magnesium oxide has a higher lattice enthalpy than magnesium chloride
lattice enthalpy increases in magnitude with increasing charge of the ions in salt and with decreasing ionic radius
for MgO, the ionic attraction is between 2+ and 2β ions, so this will result in a higher lattice enthalpy for MgCl2 where the attraction is between 2+ and 2β ions
the O2β ions also has a smaller ionic radius than Clβ so this results in a higher lattice enthalpy of MgO
Chemical kinetics
Explain why temperature increases the rate of reaction [2]
Greater proportion of molecules have energy higher than activation energy
Greater area under maxwell-boltzmann curve to the right of Ea
Greater frequency of collisions between molecules
More collisions per unit time/second
This is about rate equations
Explain how you would confirm the rate equation rate = k[NO]2[H2]
Test the effect on the reaction rate of varying concentration independently
Test the effect of varying [NO] on rate whilst keeping [H2] constant and vice versa
Test that doubling [NO] quadruples the rate
And soubling [H2] doubles rate
State, giving your reason, whether confirmation of the rate expression would prove that the mechanism given is correct. [1]
No, different mechanisms could give the same rate expression
Mechanisms can be divorced
Does not give information about what occurred after RDS
Equilibrium
Outline the characteristics of a homogeneous chemical system that is in a state of equilibrium
If the system is homogeneous all of the reactants and products are in the same phase
If it is in equilibrium the concentrations of the reactants and the products remain constant as the rates of the forward and reverse reactions are the same
Acids and bases
Distinguish between the terms strong acid and weak acid [2]
Strong acid: completely dissociated/ionized in solution/water
Weak acid: only partially dissociated/ionized in solution/water
Outline what is meant by amphiprotic with H2PO2- [2]
Can act as both Bronsted-Lowry acid and BL base, can accept and donate a hydrogen ion or proton
HPO22- and H3PO2
This question is about phosphorus
State the equation of P4O10 and water [1]
P4O10 + 6H2O β 4H3PO4
Suggest why oxides of phosphorus are not major contributors to acid deposition [1]
Phosphorus not commonly in fuels
No common pathways for phosphorus oxidised to enter the air
Amount of phosphorus-containing organic matter undergoing anaerobic decomposition is small
The levels of sulfur dioxide, a major contributor to acid deposition, can be minimized by either pre-combustion and post-combustion methods. Outline one technique of each method. [2]
Post-combustion:remove SO2 by neutralization with alkali/base
Redox processes
Define the term standard electrode potential, and state what the negative value means
It is the voltage obtained for a half-cell when it is connected to the standard hydrogen electrode
The negative value of SEP indicated that the electrons flow from the half cells to the hydrogen electrode, meaning that oxidation is occurring in the half-cell
Explain the function of the salt bridge in the electrochemical cells
Required to complete the electrical circuit
To allow the movement of ions into the half-cells to maintain the balance of charges as ions are formed and consumed by the redox reactions occurring
Why is aluminium preferred over iron in most uses? [1]
Aluminum is less dense and more malleable
More resistant to corrosion
Better conductor of heat
State the conditions necessary for the potential of the cell to calculus the standard electrode potential for the cell [1]
1.0 mol dm-3 and 298 K
Which equation represents the reduction process occurring in the standard hydrogen electrode?
2H+(aq) + 2e- β H2(g)
Which metal will be produced: Cd or Cr (EΓΈ is lower)
Cd2+ is a stronger oxidizing agent than H2O and will be displaced to produce Cd
Cr2+ is a weaker oxidizing agent than H2O and H2 will displace in preference to Cr
Describe the materials and conditions used in the standard hydrogen electrode. [5]
Platinum electrodes
1 mol dm-3 [H+(aq)]
H2 gas
1 atm (1.01 * 105 Pa)
298 K
This question is about
Organic chemistry
Describe a chemical test that could be used to distinguish between pent-1-ene and pentane.
Add bromine (water)
Pentane no change/stays brown and pent-1-ene decolorizes bromine (water)
OR add acidified KMnO4
Pentane no change/stays purple and pent-1-ene decolorizes acidified KMnO4
(This reaction is a sign of unsaturation)
Explain using equations, the following in the free-radical mechanism of the reaction of methane with chlorine: initiation, propagation, termination [4]