MCAT Chemistry Review

MCAT Chemistry Review Chapter 1 AMU=weight of proton and neutron averaged but about the same so 1 amu= 1 proton or 1 neutron Atomic weight=grams/mole, a mole is 6.02 × 102 3 particles. Atomic weight is given in grams/ mole and one mole of C is 12 grams. Angular Momentum of an electron is given by L=(Nh)/2pie; N is energy level. Balmer series is electrons going from excited levels down to the second energy level. Lyman is going back to the fundamental. The f subshell contains 14 electrons. Hund’s rule-only pair after all orbitals are half full Unpaired Electrons in degenerate orbitals are said to have parallel spins. Chapter 2 Periods are rows, groups are family’s aka columns Removing an electron, ionization energy is always endothermic The second ionization energy is always harder because larger Z* Electron affinity is always exothermic and the more electronegative the greater the energy release will be. Metals are generally shiny, except mercury, high MPS and densities, can deform without breaking known as malleability, can be drawn into wires known as ductility. Metalloids aka semi-metals have both types of properties. Include Boron, Silicon, germanium, arsenic, and down diagonal Alkali metals have low densities, and low Z eff Chapter 3 Up to Li, Be and B often do not follow octet rule Coordinate covalent bonds are when an adduct is formed. Ionic bonds have a high melting point because create a lattice. Covalent bonds tend to form weak intermolecular interactions and have lower melting and boiling points. Dipole moment is given by the equation, u=qr where u is vector representing dipole moment and q is charge and r is the distance. Lest E.N. atom not H generally occupies center of bond. Electronic geometry describes the spatial arrangement of all pairs of electrons while molecular geometry is only for bonding pairs of electrons. So when asked molecular geometry we get things like trigonal pyramidal while electronic geometry would be tetrahedral. Azimuthal quantum number is l and describes subshells, whether S or p etc. Intermolecular forces like H bonds are only 10% the strength of covalent bonds. Dispersion forces or London forces are due to random fluctuations of electrons and are momentary. They are the weakest and are the result of induced doodles that change moment to t moment. Bigger atoms have stronger London forces because they are easier to polarize. Noble gases can liquefy as a result. Dipole-dipole is relevant for molecules with permanent dipoles in solid or liquid phase. Higher M and B Ps Bond length decreases as move to the right because more E.N. and smaller atomic radius. Van Der Walls forces are dispersion plus dipole-dipole interactions. Chapter 4 Ionic bonds have formula weight because no true molecules because crystal lattice. Formula weight like MW is based on formula. MW has unit g/mol. Equivalence is based on how much we need. SO if we need one mole of protons we need only a half a mole of ℎ2 𝑠𝑜4 . We can also figure out the gram equivalent weight by GEW=Molar mass/n, where n is the number of whatever we are using that the compound contains. So for 𝐻2 𝑆𝑜4 n would be two, and the gram equivalent weight would be half the molar mass. So similarly if we know how much mass is used and the gram equivalency we can calculate the equivalence. by Mass of compound/GEW. Normality is equal to n(Molarity). Empirical formula is the simplest whole number ration of the elements in the compound. % composition= mass of atom in empirical formula/mass of all atoms in formula. A combination reaction: 𝐴 + 𝐵 → 𝐶 Decomposition Reactions are the reverse Single Displacement Reactions are 𝐴 + 𝐵𝐶 → 𝐴𝐶 + 𝐵 (often redox reactions) Double displacement ... e.g. Neutralization reactions acid and base produce salt and water. Net Ionic Equation-spectator ions are often present and do not include them in net ionic equations. Pretend that the ionic compounds already dissociated. Stochiometric coefficients are placed in front of compounds and indicate number of moles. When balancing equations start with least represented and move on to most. percent yield=actual/theoretical times 100 Chapter 5 In a reaction mechanism the product of step 1 is an intermediate and can only be proven by kinetic experiments. The rate determining step is the slowest step. The 𝑟𝑎𝑡𝑒 = 𝑑𝑒𝑙𝑡𝑎 − [𝐴]/(𝑎(𝑑𝑒𝑙𝑡𝑎 𝑡)) ± [𝐵]/(𝑏(𝑑𝑒𝑙𝑡𝑎 𝑡) = [𝐶]/(𝑐(𝑑𝑒𝑙𝑡𝑎 𝑡)) for the reaction 𝑎𝐴 + 𝑏𝐵 → 𝑐𝐶 the unit is Molarity/Second For irreversible reactions the rate = 𝑘[𝐴]𝑥 [𝐵]𝑦 ; this is known as the rate law. X and Y are called the order of the reactions. The overall order is the sum. They can be fractions but probably will not be. Order is not stoichiometry tbd experimentally. There are two cases where this is not true, if the reaction mechanism is a single step and balanced overall reaction is reflective of the netire chemical process than stoichiometry is enough. OR if the rate determining step is indicated. If the rate determining step requires the use of intermediate than the rate for the intermediate must also be determined. The law of mass action which gives you Keq tells you where the equilibrium position lies but the rate tells you how quickly the reaction will get to equilibria. K in the equation is not actually a constant but is dependent on activation energy and temperature, so for a reaction at specific temp it is constant. A fool proof method for determining x and y from the chart is saying doubling the concentration of b has resulted in a ___ of the production rate of the product. Than just plug the numbers in to solve for K. Zero order is where the concentration of reactants is irrelevant and the rate=k. K has the units of M/s. Only changing the temperature of adding a catalyst will change the rate of the reaction. A catalyst works by lowering energy of activation and increasing the value of K. First order is rate= 𝐾[𝐴], k has the units of 𝑆 −1 . Radioactive decay is like this and has the equation [A at time T]=[A at initial time]𝑒 (−𝑘𝑡) 𝑘 = [1/𝑀𝑆] Mixed order reactions rely on the reactions whose order changes over time. The collision theory of chemical kinetics states that the rate of a reaction is proportional to the amount of collisions per second between the reacting molecules. The product is formed when correctly oriented with enough energy. Only a fraction of the molecules have the KE to surpass the activation energy or energy barrier. So rate=fz where f is fraction that have enough energy and orientation and Z is number of collisions. When have enough energy and collide they form a transition state, and old bonds weaken and new bond start to form, they can either dissociate into the products or revert to reactions without any additional energy. They cannot be isolated. In a reaction profile the enthalpy change is important. The change in enthalpy is given by change of PE. The concentration, temperature, or even medium (whether solid or gas; or whether polar or nonpolar solvent), and catalysts effect the reaction rate. Thea is changed for forwards and backwards reaction and do not affect equilibrium. Only make spontaneous reactions go quicker to equilibria. Reversible reactions have a point of dynamic equilibrium. Equilibrium is reached when the systems entropy is at a maximum and the gabs free energy is at a minimum. law of mass action: 𝑘𝑒𝑞 = 𝐶 𝑐 𝐷𝑑 /𝐴𝑎 𝐵𝑏 . 𝑘𝑒𝑞 = 𝑘𝑓𝑜𝑟𝑤𝑎𝑟𝑑/𝑘𝑟𝑒𝑣𝑒𝑟𝑠𝑒 = 𝑘𝑐 for solutions we use the term kp and it in terms of pressure. IF there are more than one step in the reaction then we multiply the quail of each step and 𝐾𝑐 = 𝑘1𝑘2/(𝑘 − 1(𝑘 − 2)) Qc is the value at any given time before equip. Pure solids and liquids have a value of 1 in law of mass action equation. The larger K eq the more complete a reaction will be. In the opposite direction keq is 1/K Changes in concentration, pressure or volume in the case of gases, to side with less moles of gas, change in temp (specifically because K rather than concentrations depend on temp), think about release of heat in terms of exothermic and endothermic. Be careful with rate law not giving stochiometric values Chapter 6 Isolated vs closed system-in isolated the system cannot exchange energy or matter while in closed it can exchange energy not matter. When a system undergoes a process there is some change, isothermal is when temp is constant, adiabatic is when no heat is changed between the system and environment, and isobaric is when the pressure of the system is constant. Spontaneous changes are when no external energy needed to drive a reaction. They may not go to completion and may be slow this is based on rate law and law of mass action. So, an adiabatic process can be heated by changing pressure. State functions describe the system in an equilibrium state but not how the system got there. Temperature, pressure volume density internal energy, enthalpy, entropy and Gibbs are state. Work and heat are not. Standard conditions are 25 C and 1 atm. Standard temp and pressure or ST are 0 C and 1 ATM. The standard state is the most stable form of a compound in the standard conditions, for water its liquid, for carbon its solid graphite. Standard enthalpy’s, entropy and Gibbs get the little dot above them. A large amount of cool water can have greater heat than a small amount of hot water because q=mc delta t First law of thermodynamics says that change in internal energy=amount of heat transferred-the amount of work done by the system. There are constant pressure and constant volume calorimeters to measure changes in heat. A pressure calorimeter is just a coffee cup and the pressure is atmospheric pressure. The bomb calorimeter is a constant volume, so because W=p change in v and change in v =o the change in internal energy is the change in heat in the steel and oxygen which is equal to the change in heat of the surroundings which is the water. TO solve for the standard heat of a reaction we take the sum of the enthalpy of the products the sum of the enthalpy of the reactants. The enthalpy change for a phase change is heat of vaporization. Hess's law works for any state function including entropy and Gibbs or even bond dissociation energies particularly for gas phase. Breaking bonds is always endothermic, while forming is exothermic. The change in enthalpy sis given by sum of bonds broken plus sum of bonds formed which = total energy absorbed - total energy released. Just keep in mind what is endothermic and what is exothermic. Combustion reactions can occur in the absence of O2 but with F2 or in the case of HCl with H2 The second law of thermodynamics-energy spontaneously disperses from being localized to spread out. The change in entropy is = Q reversible/temperature. Work must be done to concentrate energy. Thermodynamics deals with if it will happen kinetics is how fast. The standard free energy change can be derived from the equilibrium constant. The greater the value of K the more positive the natural log the more negative change in G will be and the more spontaneous the reaction. SO change in G=rtlnK but if were not at equilibrium because the reaction started we get the equation rtln(q/K) because we have change in G - change in G at standard conditions. So if Q/Keq is greater than one the reaction will not be spontaneous but if its smaller than one it will e spontaneous and negative. Review: Adiabatic process refer to volume changes in a closed system without experimentally significant losses or gains of heat. The temperature is not constant during an adiabatic process. Melting is isothermal but there is heat flow so it is not adiabatic. Adiabatic and isothermal are opposites Longer hydrocarbons release more heat. IF keq is smaller than 1 the reverse reaction is preferred. I struggle with the difference between adiabatic and isothermal Enthalpy’s is not part of laws of thermodynamics. Chapter 7 1 ATM=101 kPa The assumptions of the Kinetic Molecular theory are based on ideal gases: 1) particles whose volumes are negligible. 2) No intermolecular attraction or repulsion 3) random motion 4) elastic collisions meaning that there is conservation of kinetic energy and momentum 5) average KE is proportional to temp => KE=3/(2KT), K is Boltzmann constant the root mean square speed urms=sqrt(3RT/(MM)) Molecular mass must be in Kg Movement of gas molecules through a medium is known as diffusion Avogadro worked with constant P and T; Boyle worked with constant n and T; Charles law states that when n/P constant one mole of gas at stp is 22.4 L Daltons law of partial pressure: Is that pressure is the sum of partial pressures. A gases cannot be considered ideal if there is too much pressure, temperature decreases lead to intermolecular forces becoming more significant. larger and more polar molecules correct for the effects with the Vander wails equation of state. Chapter 8 Water is an exception in that in solid phase it is less dense. Solids can be crystalline or amorphous. The crystalline can be metallic or ionic. Unit Cells are made up of either simple cubic, which are rare because just at vertices and not densely packed, body centered cube and face centered. Face just has a bunch more in the cube. They are both for ionic and metallic solids. Liquids and gases are known as fluids because they do not resist shearing forces. Fusion=melting At points of equilibria like the triple point the Gibbs free energy of gas=solid=liquid. All slopes are typically positive, but for water on phase diagram one is negative. The lines on a phase diagram are called lines of equilibrium or phase boundaries. Colligative properties are when physical properties of a solution depend upon concentrations of dissolved particles regardless of identity. Raoul’s law is also for solvents but similar to Daltons. For that reason, there is boiling point elevation and freezing point depression. the change is equal to a constant times molality times i which is the vent Hoff factor. Chapter 9 All solutions (homogeneous) are mixtures but not vice versa. A solute it dissolved in a solvent. Solvation=dissolution; when the new interactions are stronger its exothermic and follow le hotelier. (example Co2 in H2O) Entropy can also effect dissolution, it increases with dissolution (NaCl is endothermic in water but because of entropy dissolves, the more heated). When a solution is saturated adding more solute precipitates. Concentrated is less than saturated. Some solutes are only minimally soluble (sparingly soluble salts). All salts of alkali metals or ammonium ions or nitrates (No3) are water soluble. The halogens are soluble except when bonded to Ag (silver), Pb and Hg Sulfates are soluble except with Ca, Sr, Ba and Pb All metal oxides are insoluble except alkali metals and CaO, SrO, and BaO Hydroxides are insoluble unless they are with alkali metals or Ca, Sr and Ba All carbonates phosphates sulfides and sulfites are insoluble unless with ammonium or alkali metals. Iron (II) simples means Fe2+ Ferrous is Fe2+ and Ferric is Fe3+ the suffix ic is always higher in charge. Monatomic anions drop become ide, hydride, fluoride is Foxyanions are when they have more than one O, the one with more Os is ate and less is ite. Nitrite is No2- while nitrate is NO3If there are four options with Os they use hypochlorite, chlorite, chlorate and perchlorate which are ClO- to ClO4If you can add more than one hydrogen its just hydrogen carbonate or bicarbonate, and then dihydrogen phosphate for H2PO4Percept composition by mass: mass of solvent/mass of total solution times 100 Mole fraction is moles of solute/moles of total solution molality-is moles of solvent over kgs of solvent A very convenient equation to remember is MiVi=MfVf for determining how to dilute a concentration properly. AgCl only dissolves slightly Higher pressure favors dissolution of gas solutes. So Ksp is larger for gases at higher pressures. Ksp is to Keq as Q is to I.P. (Ion product) as same is to same Saturation is when I.P=Ksp When figuring out the concentrations given Ksp, and 𝑃𝑏𝐵𝑟2 → 𝑃𝑏 + 2𝐵𝑟 then 𝐾𝑠𝑝 = [𝑥][2𝑥]2 Review: Raoul’s law unlike ideal gas law holds only when the attraction between the molecules of the different components of the mixture is equal to the attraction between the molecules of one component in its pure state. This is the condition for an ideal solution. When solutions are very attracted to one another they have an even lower vapor pressure than predicted by Raoul and when really unattracted have a higher vapor pressure. Henry's law is that the solubility of gas is directly proportional to the partial pressure above the gas p=kc where k is a constant and c is the concentration. Chapter 10 Arrhenius said bases dissociate into OH- and acids to H+; Bro Lowry- acids are hydrogen donors and bases are acceptors; Lewis adduct formation For Arrhenius acids take the anion and add hydro---ic acid so fluoride (F-) becomes hydrofluoric acid For oxcy acids if the anion ended in item then it becomes oust acid while ate become ic acid. Nitrous acid is HNO2 while Nitric acid is HNO3 note they are both -1. Amphoteric species is one which can act like an acid or base depending one environment. It can also auto-ionize. Water forms a hydronium ion and a hydroxide ion. 𝐾𝑤 = 10−14 at 298 K p just means taking the -log of something. − log(3 × 10−4 ) = 4 − 𝑎 small amount because 3 is closer to 1 than 10 and log of 1 is 0 and log of 10 is 1. Strong acids include-HCL, HBR, HI, H2So4, HNO3 HCLO4(perchloric acid) Strong bases include hydroxides When strong base Ka=[H3O][OH] When weak base ka=[H3O][A-]/[HA] The smaller Ka or Kb the weaker the acid or base, respectively. 1.0 is the cut off for weak acids or bases. non ionic weak bases will always be amines. 𝐾𝑎 × 𝐾𝑏 = 10−14; for conjugate acid and base, so the two are inversely related. A salt can be hydrolyzed with water and form an acid and base. for non hydroxides the cation of the salt formed can react with water and reform the weak base. so 𝐻𝐶𝑙 + 𝑁𝐻3 − 𝑁𝐻4 → 𝐶𝑙 − then the 𝑁𝐻4 + + 𝐻2𝑂 → 𝑁𝐻3 + 𝐻3𝑂 Conjugates of weeks are stronger than conjugates of strong. Titrations involve a titrant of unknown concentration and a titrant of known concentration. We know the volume of each. equivalence point is when the number of acid equivalents equals the number of basic equivalents. Again important equation is 𝑁𝑎𝑉𝐴 = 𝑁𝑏𝑉𝑏 for remembering Titrations can be done with a pH meter or an indicator which changes color at the end point because first it reacts with the necessarily stronger acid or base. Almost never do weak/weak titration because curve not sharp. Titration of a solution of acid with base means the acid is present and we add the base. For weak/strong the early part of the titration is arleady on an incline but equivalence is even sharper. Buffers are weak A or B with its salt pH=pKa =log(conj base/weak acid) Chapter 11 Oxidation for a free element is zero but for monatomic its equal to the charge. The sum of oxidation numbers of all atoms in a molecule is equal to the charge of the atom. The oxidizing agent is reduced and the reducing agent is oxidized. To balance redox separate into two half reactions 𝑀𝑛𝑂4 − + 𝐼− → 𝐼2 + 𝑀𝑛2 + ====⇒ 𝐼−→ 𝐼2 and 𝑀𝑛𝑂4− → 𝑀𝑛2 + Then we balance, each equation adding water and protons to each side Then balance charge by adding electrons, then make sure same number of electrons on each side and then add up two halves. In electrochemical cells there are galvanic also known as voltaic, electrolytic and concentration cells. The anode is always where oxidation takes place and the cathode is where reduction takes place. The electrons will move from the anode to the cathode and current runs the other way. In galvanic cells the reactions are spontaneous, -G. There are two electrodes in separate compartments each referred to as a half cell. In the Daniell cell set up there is a slat bridge with an inert salt. The electrode with the higher reducing potential is reduced and the lower potential is oxidized, the oxidation allows for the flow of electrons which makes the current and allows the reduction to take place. Eventually, the excessive charge would provide a counter voltage but the salt bridge allows for negative anions diffuse into the oxidized electrode compartment and positive in the reduced compartment. The cell diagram starts with anode l anode solution ll cathode solution l cathode Electrolytic Cells- have a battery between the and end cathode and just one solution of molten NaCL or something. To form Na and Cl is not spontaneous so - charge goes to cathode and reduces the Na+ and positive charge goes to anode and oxidizes the Cl-. This produces sodium and chlorine. The Faraday Constant is the c charge of electron times Avogadro’s number, and represents the flow of a mole of electrons. Concentration Cells are two half cells where they both have the same compound but at different concentrations. The left will have more Cu2+ and so will be brought to equilibrium and current will flow until its at equilibrium. I suppose either Cu2+ can be reduced or Cu can be oxidized or both. Anodes attract anions. In electrolytic cells the species with the higher reduction potential is forced to be oxidized, hence it is not spontaneous and the anode will be oxidized and the higher reduction potential. EMF is given by Ecell= Ecathode-Eanode. E=reduction potential. EMF and Gibbs free energy: change in G= -nFEcell n is the number of moles of electrons exchanged, this can be under standard conditions. Then if its not at standard conditions we get the equation Ecell=E(standard)cell-RTln(Q)/nF We also have nFEstandard=RTlkKeq, So if K is less than 1 its nonspontaneous because its negative. An oxidizing agent is the species that is reduced. When we oxidize something with a positive reduction potential its value becomes negative. So it becomes reduced-oxidized. High Yield Problems: Alkenes melt at a higher melting point than alkanes and I think aromatic are even higher. Symmetry makes the melting point higher. I guess because of better bonds. Branching also lowers the melting point. Since cyclohexane is more symmetrical than hexane, cyclo has a higher melting point. Forces that stabilize a molecule more in teh solid state than in the liquid state will cause a molecule to have a higher melitng point. Molecular Weight seems irrelevent. For meling point look for heav molecules than polarity, than trends in symmetry. Remember dispersion forces are the only intermolecular force amongst non polar compounds. Remember when doing the rate law its whatever the reactant is multiplied by raised to the power that will give you whatever the product is multiplied by. Remember for equilibrium equations you can use the 𝑘[𝑎] = 𝑘 − 1[𝑏] Given a reaction profile we can figure out Keq using change in G=-RTln(keq) very important! There is no heat of formation for pure elements. Molar solubility is the amount the molar concentration, using that we can solve for Ksp When figuring out equivalence points its where the amount added has the same normal value as the part that exists, not based on pH. Also remember when calculating equivalence point, we must take into account the added volume from the titrand. For redox reactions start with the bare minimum of half reactions without electrons and just what is reduced and what is oxidized. For redox reactions determine how many electrons flow based on balanced equation. Change in G is the maximum amount of work that the galvanic cell can do because it represents change in potential energy. Power is work over time. Faradays Constant gives how many couplombs of charge make up one mole of electrons, so if we know current