genchem

• (redox) 4 ways to oxidize:1. attach an oxygen2. increase the number of bonds to oxygen3. remove hydrogen4. remove electrons
• (redox) 4 ways to reduce:1. remove an oxygen2. reduce the number of bonds to oxygen3. add hydrogen4. add electrons
• A lipid is any substance that is insoluble in water and soluble in alcohol / ether / choroform.
• what is molecular weight, what is formula weight, and what’s the difference?Both are the sum of the atomic weights of the constituent atoms in AMU“Formula wt” typically refers to ionic compounds, where there is no actual ‘molecule’ (since they’re crystals).


  AMU:  1/12 the mass of an atom of carbon 12If a question asks for formula weight, exclude covalent compounds, those have molecular weight, not formula weight.
• For a chemical equations to be "balanced"": the total number of atoms of each species must be the same on both sides of the equation"
• How to balance chemical equations:Easy ones: start with the most complicated reactant or product, from there determine other coeffs.Harder ones:  place unknown coeffs in front of each reactant/product, write one equation for each element using product of coeff and subscript.


  [Note -- for multiple choice questions may be easier to just check each answer and exclude the ones that don't balance, i.e. where the total atoms of each element are not the same on both sides.]Example from below: C8H18 + O2 → CO2 + H2O aC8H18 + bO2 → cCO2 + dH2OOne equation for each element: C:  8a = c H:  18a = 2d O:  2b = 2c + d Solve by substitution: a = 2,  b = 25, c = 16, d= 18
• What does "limiting reagent"" refer to?: The reactant, if any, that is present in a quantity that will be used up before any of the other reactants are used up.How to identify a limiting reagent?: convert the given masses or volumes to moles, then divide each by the stoichiometric coefficient, then whichever reactant comes out to the smallest number is the limiting reagent."


  In a rxn where the amounts of reactants don’t match the stoichiometric coeffs, one reactant must run out first. That is the limiting reagent. It must be a reactant, not a product (duh). How: convert the given masses or vols to moles, divide each by the stoich coeff, whichever is smallest is the limiting reagent.
• What are the Group 1A elements?: Li Na K Rb Cs Fr What is this group called?: alkaline metals
• What are the group 2A elements?: Be Mg Ca Sr Ba Ra What is this group called?:  alkaline earth metals
• What are the oxygen family elements?: O S Se Te PoWhat is this group called?: chalcogens
• What are the period 2 elements?: Li Be B C N O F Ne
• What are the period 3 elements in order: Na Mg Al Si P S Cl Ar
• What are the inert gases in order: He Ne Ar Kr Xe Rn
• What are the halogens (list in order): F Cl Br I
• What is order of electronegativity of elements more electronegative than H: FONClBrISCH


  Thus F is most electronegative element.
• The oxidation number of H in a molecule is:+1 if bonded to carbon or to any atom other than a metal -1 if bonded to any metal
• What are the rules for finding oxidation states?:Default: free elements are 0Sum: must equal net charge (thus for monatomic ion ox no. = charge)Metals: never negative; in compounds, grp 1 is +1, grp 2 is +2F: always -1H: +1 unless bonded to a metal, in which case -1 due to rule for metals aboveO: always -2 except in peroxide (H2O2) is -1 (H => +1 takes precedence) or O2 which is 0.Halogens: -1Oxygen family: -2C: can be anything from -4 to +4 depending on what is bound to it


  [In case of conflict the lower numbered rule takes precedence]Default: 0 (thus for free elements, ox no. is 0 -- e.g. N2, P4, S8, He -- all 0.Rules: 1.      The default state of all elements is 0 2.      The sum of the oxidation states in any molecule or ion must equal the net charge 3.      Group 1 metals are +1, Group 2 metals are +2. Metals NEVER have a negative ox state. 4.      Fluorine is -1 5.      Hydrogen is -1 if bonded to a metal, +1 if bonded to anything else. 6.      Oxygen is -2 except in peroxides (e.g. H2O2) it’s -1 (as required by rule 5) 7.      Halogens are -1, oxygen family elements are -28.      Oxygen family: -2------------------Organics: H bonded to C is +1. C can be -4 to +4, determined by whatever it is bonded to.See https://courses.lumenlearning.com/suny-potsdam-organicchemistry2/chapter/19-1-definition-of-oxidation-state-for-carbon/   "
• What is the oxidation state of each of the atoms in each of the following molecules: a) NO3−   O:each -2     N:+5 b) HNO2   H:+1 O:both -2    N:+3 c) O2         O: each 0 d) SF4       F: each -1   S:+4 e) Fe3O4   O: -2 each  Fe: +8/3 each


  Rules:1.      The default state of all elements is 02.      The sum of the oxidation states in any molecule or ion must equal the net charge3.      Group 1 metals are +1, Group 2 metals are +2. Metals NEVER have a negative ox state.4.      Fluorine is -15.      Hydrogen is -1 if bonded to a metal, +1 if bonded to anything else.6.      Oxygen is -2 except in peroxides (e.g. H2O2) it’s -1 (as required by rule 5)7.      Halogens are -1, oxygen family elements are -28.      Oxygen family: -2a) By Rule 6, the oxidation state of O is –2; therefore, by Rule 2, the oxidation state of N must be +5 b) By Rule 5, the oxidation state of H is +1, and by Rule 6, O has an oxidation state of –2. Therefore, by Rule 2, N must have an oxidation state of +3 in this molecule. c) By Rule 1 (which is higher in the list than Rule 5 and thus takes precedence), each O atom in O2 has an oxidation state of 0. d) By Rule 4, F has an oxidation state of –1. So, by Rule 2, S has an oxidation state of +4. e) By Rule 6, O has an oxidation state of –2. So, by Rule 2, Fe has an oxidation state of +8/3. (Notice that oxidation states do not have to be whole numbers.)
• What is the empirical formula of benzene?:  CH


  Benzene is C6H6, so reducing to lowest common multiple gives CH."
• Volume of 1 mole of any ideal gas at STP:  22.4 L
• The conductivity of solid ionic compounds is very poor because these form a rigid lattice in which ions cannot move.


  Terrible – rigid lattice, ions can’t move     In spite of the fact that ionic compounds are composed of ions, solid ionic compounds tend to be poor conductors of electricity because the charged particles are rigidly set in place by the lattice arrangement of the crystalline solid. In aqueous solutions, however, the lattice arrangement is disrupted by the ion–dipole interactions between the ionic components and the water molecules. The cations and anions are now free to move, and as a result, the solution of ions is able to conduct electricity."
• How is atomic weight different from atomic mass number?:Atomic weight is the weighted average of the masses of naturally occurring isotopes in AMU. Hence typically not a whole number.Atomic mass number: sum of the number of protons and neutrons (thus a whole number, and refers to a specific isotope)
• When does gamma decay typically occur?: after alpha or beta decay, to return nucleus to ground state What part of the atom does it come from?: the NUCLEUS – this is not an electron transition What is effect on atomic number and mass of the decaying atom? No effect on atomic number or mass


  [Although a gamma particle is a photon, it comes from the nucleus, not from an electron energy level transition. It does not change the number of protons or neutrons, so no change in atomic mass or atomic number -- it is merely a transition of the nucleus to a more stable state.]
• What is the energy of a photon of frequency f?: E = hf Constant involved and its value?: h is Planck’s const, about 6.6 E-34 J s
• What is a Bohr atom?: An atom/ion that has only one electron, e.g.   H,   He+,  Li2+ What is formula for electron energy in Bohr atom?: E = -13.6 eV / n2


  Where n is the energy level = 1, 2, .... Note this is a negative energy which gets smaller in magnitude with increasing n, going to 0 at n = ∞
• How to find electron orbital configuration of an ion:Anions: same electron configuration as for the element with the same number of electrons.Cations: remove electrons from the HIGHEST n subshell.


  Thus for example, Zn is [Ar]4s23d10 but  Zn+2 is NOT [Ar]4s23d8, it is [Ar]3d10 because 4s is higher energy than 3d.--------------------The way we designate electronic configurations for cations and anions is essentially similar to that for neutral atoms in their ground state. That is, we follow the three important rules: Aufbau's Principle, Pauli-exclusion principle, and Hund's Rule. The electronic configuration of cations is assigned by removing electrons first in the outermost p orbital, followed by the s orbital and finally the d orbitals (if any more electrons need to be removed). For instance, the ground state electronic configuration of calcium (Z=20) is 1s22s22p63s23p64s2. The calcium ion (Ca2+), however, has two electrons less. Hence, the electron configuration for Ca2+ is 1s22s22p63s23p6. Since we need to take away two electrons, we first remove electrons from the outermost shell (n=4). In this case, all the 4p subshells are empty; hence, we start by removing from the s orbital, which is the 4s orbital. The electron configuration for Ca2+ is the same as that for Argon, which has 18 electrons. Hence, we can say that both are isoelectronic, having the same of number of neutrons. The electronic configuration of anions is assigned by adding electrons according to Aufbau's building up principle. We add electrons to fill the outermost orbital that is occupied, and then add more electrons to the next higher orbital. The neutral atom chlorine (Z=17), for instance has 17 electrons. Therefore, its ground state electronic configuration can be written as 1s22s22p63s23p5. The chloride ion (Cl-), on the other hand, has an additional electron for a total of 18 electrons. Following Aufbau's principle, the electron occupies the partially filled 3p subshell first, making the 3p orbital completely filled. The electronic configuration for Cl- can, therefore, be designated as 1s22s22p63s23p6. Again, the electron configuration for the chloride ion is the same as that for Ca2+ and Argon. Hence, they are all isoelectronic to each other.The same rule will apply to transition metals when forming ions. You should note that the ns electrons are always lost before the (n-1)d when forming cations for transition metals. For example,the electron configuration for Zn: [Ar]4s23d10 the electron configuration for Zn+2: [Ar]3d10https://chem.libretexts.org/Courses/University_of_Arkansas_Little_Rock/Chem_1402:_General_Chemistry_1_(Kattoum)/Text/7:_The_Structure_of_Atoms_and_Periodic_Trends/7.4:_Electron_Configurations_of_Ions"
• What are the four quantum numbers and their allowed values:n – energy – 1, 2, …ℓ -- azimuthal – 0 … (n-1)mi  – magnetic -- -ℓ … +ℓms  – spin --  +1/2 and -1/2
• What quantum numbers do the s, p, d, and f subshells correspond to?:Answer: The azimuthal quantum number ℓ = 0, 1, 2, and 3 respectively.
• What subshell (s, p, d, or f) are the valence electrons of: Elements in groups 1A and 2A: only the s electrons in highest shell Elements in groups IIIA through VIIIA: the s and p electrons in highest shell Transition elements: the s and d electrons (note the d number will be one less than the s number)


  see https://m.wikihow.com/Find-Valence-Electrons
• What is main defining characteristic of metals?: valence electrons are loosely held and easily move through the material Where are metals in the periodic table?: basically everything to the left of the metalloids, which is everything in the s, d, and f blocks plus a few more like Pb.
• What are metalloids?: elements that have some properties of both metals and non-metalsWhich elements are they?: those to the right of the transition metals and left of the non-metals


  Metalloids are elements that have some properties of both metals and non-metals. They are usually semiconductors. B  Si  Ge  As  Sb  Te  Po are metalloids. "
• How does electronegativity differ in concept from electron affinity (two ways)?:1.  EA is a quantifiable number, an energy. EN is not.2. EN depends on what the atom is covalently bonded with.
• What is a conjugate base of a Lowry-Bronsted acid?: whatever is left over from the acid after dissociating its H+


  Whatever is left over from the acid after dissociating its H+’s A conjugate base is what is left over after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a species formed by the removal of a proton from an acid.[1] Because some acids are capable of releasing multiple protons, the conjugate base of an acid may itself be acidic. In summary, this can be represented as the following chemical reaction: Acid + Base ⇌ Conjugate Base + Conjugate Acidhttps://en.wikipedia.org/wiki/Conjugate_acid"
• Steps to determine lewis dot structure of a compound given a formula are:1. Count valence electrons, accounting for ionic charge if any.2. Place least electronegative atom in center, others around it3. Place two electrons in each bond.4. Complete the octets of the outer atoms by adding electrons in pairs.5. Place remaining electrons on central atom in pairs6. If central atom doesn't have octet, move a lone pair from an outer atom into a bond. Repeat until central atom has octet.7. In special cases, period 3 central atom can have more than octet (common with P).


  For a good short slide set / tutorial on how to do this see https://www.slideshare.net/mrtangextrahelp/tang-05-lewis-dot-diagrams-52816557"
• The Lewis dot structure for SO2 is:  "

  How to get this using the Lewis dot rules:1. Put least electronegative atom in the middle (here S) and other atoms on outside2. Count the valence electrons. Here, two O's at 6 each and one S with 6 so 18.3. Place 2 electrons in each bond4. Complete the octets of the outer atoms by adding electrons in pairs5. Put remaining electrons on central atom in pairs. So far we used 16 electrons, two left6. If central atom does not have octet, move one or more lone pairs from outer atoms into a bond (again, in pairs). Here S doesn't have an octet, and in this structure the O's would have formal charge of -1 and S would have +2, we want the FC's to be as close to 0 as possible. So we can move two of the lone pair electrons from one of the O's into the bond, givingThis gives a resonance structure since, from symmetry, it doesn't matter which O the lone pair comes from.Note that now the S has a -1 formal charge and the single bonded O has -1. Therefore this molecule is polar. Also note that from VSEPR the molecular geometry is ""bent"" due to the lone pair on the S.Above figures are excerpted from a quite helpful slide show on Lewis structures at https://www.slideshare.net/mrtangextrahelp/tang-05-lewis-dot-diagrams-52816557-----------------Finally, one might wonder why this:isn't a valid structure. The reason is it has ten electrons around the central S, which isn't an octet. But wait -- S is a third period atom, shouldn't it be able to use an ""expanded"" octet, in effect using the d shell to hold the extra two electrons? For a discussion of this, see https://chemistry.stackexchange.com/questions/87057/lewis-structure-of-so2. In any case, the canonical Lewis structure for SO2 is the resonance structure shown above."
• What is the Lewis dot structure for H20?:"
• What is the Lewis dot structure for N2?:"
• In VSEPR theory what is the difference between orbital geometry and molecular geometry?:Orbital geometry refers to the geometry of the electron groups including lone pairs.Molecular geometry refers to only the actual atoms, ignoring any lone pairs.


  Thus for example for a molecule like PCl3 orbital geometry is tetrahedral but molecular geometry is trigonal pyramidal.
• Determine the orbital geometry and predict the shape of each of the following molecules or ions: H2O    -- orbital geometry: tetrahedral. Molecular shape: bent SO2   -- orbital geometry: trigonal planar. Molecular shape: bent NH4+   -- orbital geometry: tetrahedral. Molecular shape: tetrahedral PCl3     --  orbital geometry: tetrahedral. Molecular shape: trigonal pyramid CO32–   --  orbital geometry: trigonal planar. Molecular shape: trigonal planar"
• How to determine what hybrid orbitals are present?:1.      Count the total number of other atoms bonded to the atom PLUS lone pairs. This is the number of hybrid orbitals.2.      If total is     2:  Orbital is sp, molecule geometry is linear     3:  Orbital is sp2, molecule geometry is trigonal     4:  Orbital is sp3, molecule geometry is tetrahedral


  First note these are ORBITALS OF A MOLECULE, not a bond between atoms 1.      Count the total number of other atoms bonded to the atom PLUS lone pairs. This is the number of hybrid orbitals. 2.      If total is      2:  sp  -> linear      3: sp2 -> trigonal      4: sp3 -> tetrahedral 3.      NOTE: ALL ORBITALS affecting one atom are THE SAME – you can’t have one sp2 and one sp3 on the same atom.