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89 Cards in this Set
- Front
- Back
What are covalent bonds? |
Arise from the sharing of electrons between two atoms |
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What are coordinate bonds? |
Where bothe electrons come from one atom |
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How do electrons reach the configuration of the nearest noble gas? |
Through bonding - filling all the available orbitals in the outer shell |
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What is the octet rule?
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When the atom is in the main group, this means that it has eight electrons around it (1xs + 3xp orbitals) |
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What is the 18-electron rule? |
When the atom is a transition metal, it has 18 electrons around it (1xs + 3xp +5xd orbitals) |
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What is meant by paramagnetic? |
Where molecules with unpaired electrons are attracted to the poles of a magnet |
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What is meant by diamagnet? |
Molecules with no unpaired electrons are repelled from the poles of a magnet |
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What is the issue with the Lewis theory? |
Not a complete story Evident by the presence of unpaired electrons in bonded molecules |
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What is the first step general strategy for Lewis structure? |
Calculated the total number of electrons for the molecule by adding up the valence electrons |
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What is the second step general strategy for Lewis structure?
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Draw out the skeletal structure |
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What is the third step general strategy for Lewis structure?
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Distribute electrons to atoms surrounding |
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What is the fourth step general strategy for Lewis structure?
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Place remaining electrons on central atom |
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What is the five step general strategy for Lewis structure?
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If there are less than 8 on the central atom, redistribute one or more from external atoms to form new bonds |
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What is resonance? |
The property of having a molecular structure which cannot adequately be represented by a single structural formula but is a composite of two or more structures of higher energy
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What is the first exception to the octet rule? |
If the total number of valence electrons is odd then one atom cannot satisfy the octet rule (e.g. NO) |
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What is the second exception to the octet rule? |
In electron deficient molecules, the central atom can have fewer than 8 electrons |
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What is the third exception to the octet rule? |
Elements in the 3rd period can have more than 8 electrons - expanding their octet |
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What is the formula for formal charge? |
Formal charge = V - L - 1/2B V= number of valence electrons is a free atom L= the number of lone pair electrons B= the number of electrons in bonds |
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How to choose the right Lewis Structure? |
If more than one structure is possible, choose the one with the lowest formal charges |
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What is the Valence Shell Electron Pair Repulsion Theory? |
VSEPR Based on the idea that pairs of electron will repel each other The optimal geometry about the central atom depends on the number of electron pairs surrounding it |
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How can we calculate the number of bonding electrons around a central atom? |
By looking at the lewis structure of the molecule |
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What is the method to count electrons for VSEPR predictions? |
Identify central atom + peripheral atoms Count no. of valence electrons of central atom Count no. of electrons donated to central atom by ligands Determine total no. of electron pairs around central atom This gives VSEPR predicted geometry for electron distribution Distinguish lps + bbs Distribute the lp + bp to obtain a molecular geometry |
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What is the shape with 4 pairs - all bonding? |
Tetrahedral |
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What is the shape for four pairs - 3 bp? |
Trigonal pyramidal |
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What is the shape for four pairs = 2 bp? |
bent |
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What is the order of repulsion strength? |
lp-lp > lp-bp > bp-bp |
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What causes subtle changes to bond angles? |
lps occupy more space than bps Double/triple bond occupy more space than a single bond |
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What's the shape for 5 bonds? |
trigonal bipyramindal |
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What's the shape of 5 pairs - 4 bps? |
seesaw |
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What are all transitions accompanied by? |
A change in energy (enthalpy) |
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What is enthalpy related to in reactions? |
Changes in bonding |
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What is homolytic bond cleavage (fission)? |
When a bond breaks, bond electrons are shared equally between bonding atoms |
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What is heterolytic bond cleavage (fission)? |
When a bond breaks, bond electrons are not shared equally between bonding atoms, resulting in ion formation |
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What are bond energies? |
Average enthalpy for breaking a bond in the gas phase |
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What causes bond energies to increase? |
Increase in no. of bonds (double > single) Increases across a period |
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What causes bond energies to decrease? |
Decrease down a column |
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What can bond energies be used for? |
Estimating enthalpies of a reaction |
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What do ionic compounds contain? |
Cations (pos) + Anions (neg) |
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What are some simple ionic compounds? |
Na+ + Cl- -> NaCl Mg^2+ + 2Cl- -> MgCl2 2Al^3+ + 3O^2- -> Al2O3 |
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What are some complex ionic compounds? |
Ca^2+ + CO3^2- -> CaCO3 Ho^3+ + PO4^3- -> HoPO4 |
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What is meant by isoelectronic? |
Having the same number of electrons or same electron structure |
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Name two ions that are isoelectronic with Li+ |
Be^2+ B^3+ |
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What is ionisation energy? |
The energy required to remove one electron from an atom |
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Where on the periodic table has the highest affinity for electrons? |
The right-hand side |
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What is Delta-Hf? |
Enthalpy of formation Energy required to form a compound from its constituent elements in their natural state (under stable conditions) |
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What is lattice energy? |
Change in energy when an ionic solid is changed into isolated ions in the gas phase This is very difficult to directly measure |
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What is Coulomb's law? |
Ions are held together by electrostatic forces |
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What can be used to determine lattice energies? |
Born-Haber cycle |
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Using the Born-Haber cycle for NaCl, determine the lattice energy |
1. Enthalpy of atomization of Na = +108 kJ mol^-1 2. 1/2 bond dissociation enthalpy of Cl2 +240/2 = +120kJmol-1 3. Ionisation enthalpy of Na = +496 kJ mol-1 4. Electron affinity of Cl = -349kJmol-1 5.Minus lattice energy of NaCl |
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When do electrostatic interactions increase? |
If Q+ or Q- increases If radius of cation or anion gets smaller This means lattice energies are largest for small, highly charges ions |
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What happens with energy in a covalent bond? |
On formation, Delta-H is positive, energy is required In dissociation, Delta-H is negative, energy os released |
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What holds ionic compounds together? |
Electrostatic interactions |
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What is lattice energy related to? |
Charge on the ions + their radii |
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How is lattice energy measured? |
Using Born-Haber cycles |
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Where is ion size the largest on the periodic table? |
Left on the period Down the column |
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What is the solubility of a salt dependent on? |
Lattice energy Energy of hydration |
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How are ions attracted to water molecules? |
Via an ion-dipole force |
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What do ionic solids experience? |
Strong attractive forcesAlso strong attractions between ions + water molecules
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When does hydration energy increase? |
When Q increases Radius decreases |
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Why is solubility often difficult to predict? |
Factors that give large lattice energies also give large hydration energies |
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What substances are soluble? |
Compounds of group 1are soluble Compounds of group 2 are less soluble Compounds of group 17(e.g. Cl-) are soluble Nitrates (NO3-) + most sulfates (e.g. SO4^3-) are soluble |
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What substances are insoluble? |
Most sulfides (S^2-) Most Carbonates (CO3^2-) Most Hydroxides (OH-) Oxides (O^2-) |
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What general rules govern solubility? |
Salts where ions are similar of size are less soluble than those where ions have different sizes |
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Why does this rule govern solubility? |
Lattice energy remains unchanged by hydration energy increases so, solubility increases |
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What are oxidation states? |
A way of keeping track of the valence electrons in a compound Essential in balancing oxidation-reduction |
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What is the oxidation state of an atom in its elemental form? |
Zero |
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What is the oxidation sate of a monoatomic ion? (e.g. Na+) |
The same value as the charge (+1 for Na+) |
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What are the rules for assigning oxidation states? |
1. F is always -1 (except F2, which is 0) 2. Oxidation state of other halogens is always -1, unless bonded to a halide above it 3. O is -2, unless bound to F or itself 4. H is +1, but -1 when bound to metals 5. Os of Group 1 is +1 in most cases Os of Group 2 is +2 in most cases |
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What is nomenclature? |
Choosing names for things |
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What are the alternate nomenclautres of Cr^6+ |
Cr^VI+ Cr^+VI Cr^VI Cr(VI) |
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Why do oxidation states need to be determined? |
Some elements have multiple possible oxidation states |
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What are the rules for assigning oxidation states based on? |
The principle that electrons in bonds between 2 atoms should be assigned to the more electronegative atom, so that both achieve closed shell electron configurations |
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What happens to electrons in partners that are equally electronegative? |
Electrons are shared equally |
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What if the number of bonded electrons is more than the valence electrons? |
Negative oxidation number |
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What if the number of bonded electrons is less than the valence electrons? |
Positive oxidation number |
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What are reduction reactions? |
The transfer of electrons from another species to the reduced species |
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What are oxidation reactions? |
The transfer of electrons from the oxidised substance to reduced |
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What are spectator ions? |
A species that remains the same on both side of an equation |
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Why are spectator ions cancelled out of an equation? |
To get a net ionic equation |
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What happens to the oxidation states in redox reaction? |
Os of the reducing agent will increase Os of the oxidising agent will decrease |
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What is OIL RIG? |
Oxidation is loss of electrons Reduction is gain of electrons |
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What are reducing agents? |
Donates electrons Is oxidised itself |
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What are oxidising agents? |
Accepts electrons is reduced itself |
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What are the metallic trends in the periodic table? |
Left is metallic Right is not metallic Metallic nature increases down a column |
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What is a trend in the first row of the periodic table? |
The first row of the periodic table is often very different from the other rows |
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What are traits of group 1 + 2 of the periodic table? |
Low ionisation energies Low electronegativities Large radii All metallic |
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What gives elements metallic propeties? |
Delocalised electrons to form sea because the ionisation energy is low - easy to remove electrons |
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What is a difference between group 1 + 2 elements? |
Group 2 elements are generally harder + less reactive than their group 1 counterpart This is due to the extra electron in the sea for group 2, so there is more glue sticking the metal atoms together |
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what is amphoterism? |
Ability to react with both acids + bases Evident in group 1 + 2 |