Re-Cap of Gen Chem II

Hey everyone,

Per usual, this email will focus on General Chemistry II, but first, your homework for Verbal Reasoning II.

Your Required Homework before Verbal Reasoning II is as follows:

• Verbal Reasoning Review Notes:  Chapters 4-5
• Writing Sample Review Notes:  Chapters 2-5

For those of you who would like some additional Verbal Reasoning review before class, I recommend the following:

• Verbal Reasoning Foundation Review Unit 2

General Chemistry II Helpful Hints:

• The Kaplan Question Strategy – “Stop, Think, Predict, Match” – is a simple but elegant way to make sure that you systematically solve every problem and answer every question in the most efficient and effective way.  Sure, you could arrive at the answer in a number of ways, but why waste time with haphazard approaches?  Thinking and predicting are especially important in light of the fact that there are three wrong answers for every one correct and credited answer.
• Remember that while chemical kinetics and thermodynamics are very different studies (the one assesses rates of reactions while the other measures changes in energies), there are ways in which they “intersect.”  For example, K_eq = K_f/K_r  where K_f and K_r stand for the rate constants for the forward and reverse reactions, respectively.  At K_eq, the forward rate and the reverse rate are equal and thus rate_f/rate_r = 1.
• Conceptually, K_eq = K_sp = K_a = K_b; and Q = I.P.; all “K’s” are temperature-dependent, as is molar solubility.  The common ion effect reduces molar solubility (the value of “x” when you’re solving these questions), NOT K_sp.

Here are some other key takeaways from our General Chemistry II lesson:

• Chemical Kinetics
• This has been a topic that traditionally is tested often because of the many different types of questions that can be asked.  The OWQ, our coverage of this topic in Passage I, and the associated topical test does a nice job of covering the wide range of ways it can be tested.  To break it down further, here’s a review of the basic concepts:
• Distinguish and identify catalysts and intermediates.
• Rate Law expression (rate = k[A]^x[B]^y):  the rate of a reaction depends upon the bottleneck – that is, the slowest step in the mechanism.  The reactants in the slowest step will appear in the rate law, thus if the rate law is experimentally determined we can use that information to derive a plausible reaction mechanism (as the student did in the last paragraph of Passage I).
• Be able to determine the rate law expression from experimental data; to make sure you’ve got this skill down determine the rate law for given the following data below:

[A]	[B]	Rate of product formation
2	2	10
2	4	20
4	6	120

• Determination of the limiting reagent:  we did not formally review stoichiometry concepts in General Chemistry I or II, but you are expected to be quite familiar with how to balance a chemical reaction and answer questions related to yields (i.e. percent yield, theoretical yield, etc).  If you want more practice, check out the Stoichiometry topical test!

• Thermochemistry
• The application of thermodynamics to chemical reactions is called thermochemistry.  Thermodynamics is primarily concerned with heat transfer into or out of a system (recall the first law equation ΔU = Q - W).  Heat transfer can result from a chemical or physical process.  Calorimetry is how we measure heat transfer, thus allowing us to determine enthalpy and energy changes (more on calorimetry below).
• Understand difference between state functions (enthalpy, entropy, Gibbs free energy, internal energy) and process/path functions (work & heat).  Note that Hess’ Law can be applied to any state function since all are path-independent.
• Enthalpy:  know the relevant terminology (endothermic, exothermic, heat of formation, bond enthalpy) and look at diagram on page 204 to understand Hess’ Law and how it re-affirms that enthalpy is a state function.
• Entropy:  the second law of thermodynamics has many interpretations, but the main point is that the entropy of the universe continually increases, for while reversible process have a net change in entropy of zero, irreversible process have a positive net change in entropy.
• Reversible processes:  freezing of a puddle of water.  Freezing decreases the entropy of the water, but the heat energy given off by that process increases the entropy of the environment, thus net change in entropy of universe is zero.
• Irreversible process:  drop a glass cup on floor and it shatters into many pieces.  Increase in entropy of the system does not occur at expense of environment.
• Gibbs Free Energy: best way to conceptually understand this formula is to look at question 10 (passage 2). 

• Equilibrium
• Understand difference between kinetics & equilibrium:
• Kinetics tells us about the rate of a reaction, while thermodynamics tells us what equilibrium will look like.  That said, keep in mind that the equilibrium expression is derived from the rate laws of the forward and reverse reactions of each step of the overall reaction.
• Remember that exponents of the concentrations of the reactants and products are equal to their stoichiometric coefficients in the equilibrium expression, not necessarily in the kinetic rate law.
• Understand the relationship between Q and K_eq.  The equations on page 209 are restatements of this concept.
• Remember that the degree sign means standard conditions, which means that T = 298 K, P = 1 atm, and concentrations are 1 M!
• Le Chatelier’s Principle:  know the three ways to stress a system out of equilibrium, and be able to predict how the system will respond to each type of stress.
• System #1 on page 211 introduced the problem solving approach for many questions related to equilibrium expressions.  Review our approach to this question: how did we define the variable x? Where did we assume it was negligible?  Why? 
• Solution Equilibria
• K_sp is not any different than K_eq – we use different terminology but the principles discussed above still apply.
• Know the terminology (ion product, unsaturated, saturated, supersaturated, molar solubility, common ion effect, solubility product constant).
• Understand the relationship between the common ion effect, molar solubility, ion product, and K_sp.
• Know how to use the K_sp expression to solve for molar solubility or solubility product constant.
• How do you modify the expression when a common ion is present?

• Dissociate vs. Dissolve
• On this page we discussed the dissociation of crystalline solids.  We didn’t use the word dissolve, because dissolving a solid in solution results in a breakdown of INTERmolecular forces between molecules.  The molecule remains intact as it dissolves into solution.  For example, when glucose is dissolved in water the atoms within a glucose molecule remain covalently bonded to each other.  In contrast, a molecule of NaCl will dissociate in solution because the ionic bonds that hold the atoms in a lattice structure will be solvated.  
• The factors that determine whether or not any given crystal salt will dissociate are its lattice energy (i.e. the electrostatic attractive forces holding the salt together), solvation energy (i.e. strength of attraction b/w solvent and dissociated ions), and entropy (solute form is more disordered that crystalline form).

• Electrolytes and Conductivity (we didn’t have time for this in class, but you’re expected to know this info):
• In aqueous solutions, electrical conductivity depends upon the presence of ions in solution.  The movement of these ions in response to an electric field is what makes up current.  Note that pure water does not conduct an electrical current well since the concentrations of OH^- and H₃O⁺ are very low.
• A strong electrolyte is a solute that dissociates completely into its constituent ions (e.g. NaCl, KI, HCl) in solution.  A weak electrolyte hydrolyzes incompletely in solution (e.g. acetic acid, ammonia, etc.).  Non-electrolytes do not ionize at all (e.g. O₂, N₂,noble gases, sugar, etc.).

• Solubility rules
• Direct questions are rare, but knowledge of these rules can help you understand passages.  In other words, memorizing these rules will not really help you.  Knowing the general trends can help.  At the least know that all salts of alkali metals, nitrates, and ammonium ions are water soluble.

• Heat & Phase Changes
• Heat v Temperature
• Getting sloppy with terminology can lead to misinterpretations and lead you down the wrong path, and these two often get confused so let’s try to clarify the difference between these two.
• Heat = amount of kinetic energy transferred from one object to another as the result of a temperature difference between them.  Rapidly moving molecules in hotter object collide with more slowly moving molecules in colder object, resulting in transfer of KE to slower-moving object causing it to speed up, i.e. think of it as a microscopic transfer of energy.
• Temperature = macroscopic property of a system that is proportional to the average kinetic energy of all the molecules in that system. 
• Temperature is a property of a system, but heat is a process function that tells the energy transfer that occurs into or out of a system.  In other words, a system does not contain heat.  A system contains energy (proportional to temperature) and heat is energy in transit.
• Types of Heat Transfer
• Conduction:  transfer of heat through direct contact via collisions with neighboring molecules
• Convection:  movement of heat through a fluid (liquid or gas) medium. 
• Think of a cold object (raw steak) and hot object (electric coils of convection oven).  The medium is air.  Air particles collide with the surface of the hot object (electric coils), result in the transfer of heat from the coils to the air particles.  The air particles rise up and collide with the cold object (raw steak), transferring heat energy to the food.  The air particles that collide with the steak get cooled back down.  At the end of the day, we’ve transferred heat energy from the electric coils to the steak via air.  Yay, cooked steak!  Boo, raw steak (easy ticket to food poisoning!).
• By the way, note that via Charles’ Law we know that the volume of the air particle will drop, and since the mass remains constant that means their density increases, which means they will sink back to the bottom where they’ll get re-heated by the electric coils.  In other words, the concept of hot air rising is a bit of a misnomer because hot air doesn’t really “rise”: what happens is that hot air has lower density than cold air so the cold air sinks down this making it look like hot air rises.  Next time you want to get your super nerd face on make sure to point that out to someone.
• Radiation: transfer of energy by electromagnetic waves. 
• No medium required because electromagnetic waves can propagate through a vacuum (ex:  Sun warming Earth!)
• Calorimetry (measuring heat transfer)
• There are two types of calorimeters: constant-pressure and constant-volume.
• Constant-pressure calorimeter: in this case, Q = ΔH
• Constant-volume calorimeter: in this case, Q = ΔH = ΔU (think of 1^st law of thermodynamics, ΔU = Q – W)
• Review the example provided in the Review Notes of a constant-volume bomb calorimeter to understand how it can be applied to problems
• Phase Changes & Phase Equilibria: Know both equations for Q and understand heating curves
• Vapor Pressure & Boiling Point
• Key relationship to know:  boiling point of a solution occurs when the temperature of the solution has raised the vapor pressure above the solution to be equal to the atmospheric pressure. 
• Vapor pressure of a solution depends upon the temperature of the solution (proportional relationship, i.e. higher temperature means higher vapor pressure) and its heat of vaporization (inverse relationship, i.e. higher heat of vaporization means lower vapor pressure).
• Vapor pressure is independent of the shape and volume of a container (pressure is force per unit area), but the rate of vaporization does increase with surface area because molecules can only escape from the surface.  In other words, if we put the same solution in two different containers the one with the greater surface area will evaporate quicker but the vapor pressure above both will be the same.
• Phase Diagrams:
• The most popular phase diagram is a Pressure vs. Temperature curve; be familiar with understanding how to read this curve.
• Know the terminology (triple point, critical point, freezing, melting/fusion, evaporation, condensation, sublimation, deposition).
• How is the phase diagram for water different?  Why?  Is this the meaning of life?
• Raoult’s Law uses another type of phase diagram (see your Review Notes) so be ready for other types of phase diagrams.

To reinforce all the content we covered in General Chemistry II, complete the following topical tests:

• Stoichiometry Test 1
• Comments:  These questions do a nice job of testing your stoichiometric abilities.  Granted, many of these questions involve a ton of math, and the AAMC is unlikely to test you directly on stoichiometry the way this test does…but some of the passages and questions may indirectly test your ability to determine stoichiometrically balanced reactions.  Once you get through this test, you should be able to handle any stoichiometry question.
• Kinetics & Equilibrium Test 1
• Comments:  Passage I is just as likely to show up in the Biological Sciences section, and you’ll probably find its question set refreshingly straightforward.  On the other hand, passage II has a lot of icky math and it directly tests your ability to apply log rules.  This passage is a throwback to the way the MCAT used to be written, but it is worth your time to see if you can understand all of the concepts.  In other words, after reading the question stem, are you at a complete loss as to how to solve the question, or do you understand how to get started? 
• Thermodynamics & Thermochemistry Test 1
• Comments:  Many of the questions here directly test your math skills, but there’s enough conceptual questions in both passages to make it worth your while; furthermore, the AAMC hasn’t sworn off mathematical questions.  As always, focus on the concept behind each question.

That’s all for now.  See you at Verbal II!