This is the AP Chemistry tips.  

Unit 1 Atomic Structure Properties

Any two gases at the same temperature have the same KE but NOT the same velocities.   KE = ½ mv².  Large mass slow moving gas, low mass faster moving gas.

Larger = size, volume. Heavier = mass. More = amount (like moles). Don't mix them up!

Atoms/elements/ions/molecules/compounds don’t “like” or “want” anything. Give the scientific reason why things happen.

Don't confuse atom and ion or molecule and formula unit

Mass spectrometer readings are all about isotopes! (For CB anyways)

Coulomb's law F = K q q /r2, in short larger charges differences and shorter distances experience greater forces

PES, photoelectron spectroscopy,  is all about coulomb's law and e- config (for CB)

When explaining trends, don't talk about placement on the table. Talk about things in the atoms like nuclear pull and distance.

Frequency measured in hertz (Hz), remember to convert wavelength to meters.  1 nm = 1 x 10-9  m

Periodic trends are not an explanation, instead discuss effective nuclear charge and distance (Coulomb's law)

Molar mass is grams per mol.  Experimental questions often ask you to calculate this, M = m / n

Unit 2 Molecular and Ionic Compound Structure and Properties

Lone pairs on the central atom doesn't mean that the molecule is polar (think XeF4 and KrF2)

Use electron domains to help with determining shapes AND double bonds represent a single domain.

Always get the correct total number of electrons for Lewis Dot structures, know what can break the octet rule 

The size of the electron cloud matters for LDF (larger= more easily polarized= stronger IMF), not for dipole-dipole. 

Polarity is determined by the difference in electronegativity of the atoms in the bond (greater difference in EN= more polar= stronger IMF).

Nonpolar molecules can contain polar bonds.

The greater the difference in electronegativity between two atoms, the more polar the bond is.

Lattice energy MgO has a greater attraction than NaF, similar ionic radii, greater charge.  NaF > KCl same charge, smaller ionic radii

If the question says, "Draw ALL resonance structures," chances are there's more than one Lewis Structure.

Horizontal trends: they both have the same energy level (subshell), but one of them has more protons (greater nuclear charge).

Vertical trends: if valence electrons are in an orbital with a higher n; this orbital is higher in energy and farther away from the nucleus

If the question asks you to compare 2 substances make sure you refer to TWO substances

When ions are created, electrons are removed from the valence shell first.  For transition metals, that is s not d.  Iron loses from 4s before 3d. Fe = [Ar]3d64s2, Fe2+ = [Ar]3d6

The most electronegative elements F > O > Cl = N  (Cl and N are approximately equal)

Covalent network solids are giant molecular structures!  Common examples are carbon as diamonds or graphite, silicon, and SiO2 (glass or sand)

Unit 3 Intermolecular Forces

LDF, London Dispersion Forces, depend on size and number of POLARIZABLE ELECTRON CLOUD(S), not mass!

Ideal gas conditions gas move randomly with no significant forces and the particle size is irrelevant.  This occurs at high temperature, and low pressure.

Two deviations occur.  If the real gas is smaller than an ideal gas would be, then gases are experiencing significant intermolecular forces.  If the real gas is larger than an ideal gas, the gas particles’ volume is too large to ignore.  

At low temperature or high pressure the gas is closer to not being a gas anymore, condensing.  That is why it isn’t ideal.

M1V1=M2V2 This is the dilution equation.  It is not on the formula sheet, but extremely useful in dilutions and neutralizations

R is 8.314 kPa L /mol K but kPa L = J! On the sheet it is J /mol K this is important to make units match, especially with ΔG = -R T lnK

The other R values are 0.08206 ATM L/ mol K or 62.36 TORR L /mol K, these are 8.314 converted to different pressures.

Do NOT use 22.4 L unless you are actually at STP, 273 K.  STP is NOT standard state. Standard state is 298 K

Standard State is 298 K, STP is 273 K.  They rhyme!

Sometimes the data tells you something that is different than what you predicted (i.e. CCl4 has stronger IMFs than HCl) GO WITH THE DATA!

Larger and more electron cloud(s) = more London Dispersion = More polarizable

Stronger IMFs higher Boiling Point, lower vapor pressure

H-H is not a hydrogen bond.  Hydrogen bonds are IMF between molecules that have the atoms nitrogen, oxygen or fluorine bonded to hydrogen to make a dipole.  

The actual hydrogen bond is the intermolecular force between these molecules

Mention ALL of the IMFs that each substance experiences. Do NOT say "like dissolves like" when justifying substances dissolving. 

To justify dissolving substances say polar molecules are more attracted to other polar molecules, so they can pull it apart, Making it dissolve.

Nonpolar particles are attracted to other nonpolar (LD), this is how you have a chance for them to dissolve, but they are not as attractive to polar molecules as other polar molecules so they separate (oil and water).  This is why polar doesn’t dissolve nonpolar substances.

Changing phases involves breaking IMF’s not bonds

Unit 4 Chemical Reactions

10 mL of a 1 M weak acid and 10 mL of a 1 M strong acid would require the SAME amount of 1 M NaOH to reach the equivalence point.  Equivalence point is when the moles analyte = moles titrant

Adding water to the analyte (what you are testing in titration) won’t change the number of moles of analyte present.

OIL RIG or LEO goes GER for redox reactions

Oxidation numbers are the theoretical charge on all atoms in a compound if you imagine all bonds are ionic.

That is to say if you can determine the charge of iron in FeO and Fe2O3 (2+ and 3+ respectively because O is 2-) then you can figure out the oxidations states of atoms in CO2 (C=+4, O=-2)

The weird oxidation state of oxygen is peroxide.  H2O2 (H= +1, O = -1)

The oxidation state of free atoms or diatomic elements is 0 when not bonded to anything else

If an atom increases in oxidation state from reactant to product it is oxidized  If it decreases, then it was reduced.

All equations must be balanced for atoms AND charge!

Net Ionic Equations dissociate all aqueous strong electrolytes and eliminate spectator ions.

Unit 5 Kinetics 

High activation energy ---slow reaction rate.  Low activation energy ---fast reaction rate.  Catalysts give a new pathway with a lower activation energy.

Kinetics straight line graphs; zero order: [A] vs. t,  first order: ln [A] vs. t,  second order: 1/[A] vs. t

Raising the temperature does NOT affect ΔH or the activation energy.  It may help you to achieve the activation energy, but doesn’t change it

Increasing temperature increases both the forward and the reverse reaction rates, it just increases the endothermic direction reaction’s rate more.

Reactants require effective collisions between molecules of sufficient energy to overcome the activation energy to start the reaction

Specific parts of the reactant molecules must collide at the proper orientation with sufficient force (activation energy) for the reaction to occur!

Due to this, elementary steps must be unimolecular (one particle) or bimolecular (2 particles).  Getting anymore than that to collide at the proper orientation is very unlikely to occur.  

Rate k has units, equilibrium K does not.  rate = M/s, use the order to figure our k’s units.  For 0 order = M/s, for 1st order = 1/s, for 2nd order = 1/Ms

Don’t forget your units for the rate law constant, k.  Rate is M/time.  The constant makes the units match in the equation, rate = k [A]n

If the data shows that the half-life is constant over time, it is first-order kinetics for that substance.

Unit 6 Thermodynamics

The sign of ΔH is always from the perspective of the system.  Negative means it is losing energy to the surroundings.  It is exothermic.  Positive means it is taking energy from the surroundings, it is endothermic.

Remember the above is talking about chemical energy and heat energy.  Losing chemical energy to make heat energy will warm the surrounding (exothermic), gaining heat energy from the surroundings for chemical energy will make it colder (endothermic).

BOND ENERGY is reactants minus products, everything else ( ΔG ΔH ΔS) is products minus reactants.

Bonds breaking is endothermic ΔH is + (reactants).  Bonds forming Exothermic ΔH is – (products).  Overall delta H is the net sum of this.

Breaking attractions (bonds, IMFs, nucleus-valence e-) REQUIRES energy (endothermic) forming bonds releases energy (exothermic)

Heat energy is q, q = m c  ΔT.   Enthalpy,  ΔH, is  ΔH = q / mol reaction

Unit 7 Equilibrium

Reverse the reaction, the new K is the reciprocal of the old K.  Multiply the reaction by 2, the new K is the old K squared. 

A reaction with a large K is said to be product-favored, which is NOT the same as saying "the reaction shifts toward the right." 

If there's no stress imposed on the system, there's no LeChatelier’s involved.

Finding Q determines which direction a reaction will proceed to re-establish equilibrium.  If K > Q to the right, if K < Q to the left.  The greater than or less than sign is an arrow.

If you have a reaction with a HUGE K, assume the reaction goes to completion, treat it like a normal stoichiometry or limiting reactant problem.

 Inert gases and catalysts DO NOT shift an equilibrium!

If x = the molar solubility of an ionic solid (in mol/L).  Ksp for AgCl = (x)(x) = x2  Ksp for PbCl2 = (x)(2x)2 = 4x3

Q, reaction quotient, uses the same equation as K, equilibrium constant.  Q is where your concentrations are now, K is at equilibrium

The only thing that changes the value of K for an equilibrium is changing the temperature.

LeChatelier’s principle is not an explanation in itself.  A shift to reduce a stress (K v Q) is.

Beware the pressure equilibrium expression, Kp.  Don’t use [A], that is for concentration!!  Kp uses PA

Unit 8 Acids and Bases

pH = pKa at half equivalence point, with a strong acid weak base or weak acid strong base titration. This makes the best buffer because [HA] = [A-].

Diluting a salt solution increases the percent ionization.  

Percent ionization of HA = [A-] / [HA]  x 100

Larger Ka = more ionizable or dissociated 

Acid-base reactions proceed from the strong side to the weak side. Use the magnitude of K to determine which way the reaction is favored.

MV in M x L =moles, M x mL= millimoles.  Millimoles works just fine a lot of the time.

HX versus HY...If HX is a stronger acid than HY, then Y- is the stronger base than X-.

 If you complete an equilibrium calculation and find "x", you might be not done! x may not be the answer the question is asking.  Read the question.

A buffer is a weak acid and its conjugate base or WB/CA. The acidic species in a buffer neutralizes added base, basic species neutralizes an acid.

Buffers work because the concentration of weak acid/conjugate base is so much greater than the concentration of H+.  Because of this additional H+ OH- will react with weak acid/conjugate base

HF is a weak acid, all other halogens make strong acids.  Just remember,  “get the F out”

Strong acid means that the conjugate base won’t accept H+.  Weak acid means the conjugate base acts as a weak base.

Unit 9 Applications of Thermodynamics

If your answer is ΔH, ΔS, ΔG, or Eo, make sure your answer includes the proper SIGN and the proper UNITS. 

Thermodynamically favorable:  ΔG < 0, E > 0, K > 1

At equilibrium, ΔG=0, E=0. Both essentially indicate how far away from equilibrium the system is.

Thermodynamics tells you IF it will happen. Kinetics tells you how FAST.

ΔH and ΔG of formation for an element is always zero, however elements have absolute ΔS that is not 0

Gibbs free energy is used to determine spontaneity.  It does so by combining entropy and enthalpy, not just entropy of the system.  ΔG = ΔH  - T ΔS 

Units of ΔH and ΔS are often different, kJ/mol reaction and J/mol reaction K.  Make sure the units agree before plugging into ΔG equation ΔG = ΔH  - T ΔS

ΔG = -RT lnK and ΔG = -n F E both give you values in J

The more (+) standard reduction potential, E, is the reduction.  The less (+) standard reduction is the oxidation reaction.  

Oxidation occurs at the anode, reduction at the cathode.  AN OX, RED CAT

Electroplating shortcut....m = MM I t/nF  or t = mnF/ MM I

When adding two half-reactions, the electrons must cancel out.  Reversing the rxn, changes the sign.  Doubling it does NOT double voltage

Never multiply electric potentials  (cell, reduction, or oxidation).

For a voltaic cell, discuss Q compared to Q at standard state, 1, to explain a shift to product or reactant.  Not LeChatelier's Principle, a cell at equilibrium is dead! 

If Q increases the reaction proceeds more towards the reactants, decreasing the voltage.  Note the overall reaction can still be going towards products, just not as much as if it were from standard state.

If Q decreases, then you have more reactants, and the reaction will proceed more towards the products increasing the voltage.

Nernst Equation  E = Eo – RT/(nF) ln Q.  If Q  > 1, Eo > E.  If Q < 1, E > Eo.  

Electrons travel in the wire from anode to cathode. Ions flow through the salt bridge: anions toward the anode, cations toward the cathode.

Loss of mass of electrode is due to atoms of electrode going into solution as ions, not the loss of electrons

Laboratory procedure 

Percent error = (observed- actual)/actual x100, is different from percent yield = actual yield /theoretical yield x 100, and mass percent = mass substance/ total mass x 100

Lab error for AP normally ask you to explain an increase or decrease in experimental values.

When explaining a lab based error, explain that a certain mistake would increase or decrease your calculated value and why.  Sample calculations are a valid justification.

Burets and volumetric flasks measure liquids more accurately than graduated cylinders or beakers. DON’T USE A BEAKER TO MEASURE ACCURATELY!

Burettes are read from the top down, not the bottom up like a graduated cylinder 

Burets and burettes ar the same thing

Always filter, rinse (wash), and dry a precipitate in gravimetric analysis

You can tell a precipitate is dry if the mass doesn’t change after heating

Clean up an acid spill with a carbonate or some other safe weak base, not an equally corrosive strong base

The proper way to use a burette or pipette is to rinse it first with your solution.  Rinsing with water would dilute the solution, increasing the amount that would be added

Volumetric flasks do not need to be rinsed with solution or dried!   Add your known chemical and fill to the line to with distilled water to make a solution.  Excess distilled water left in won’t matter if your next step is to fill with distilled water.

AP Chemistry Test taking skills 

Leave no multiple choice blank.  Leave no multiple choice blank!!  Guessing does NOT count against you, any blank problem is marked wrong!  You always have  25% chance of getting it right.

Avoid the use of the word "it" Refer to substances by name, so they know exactly what you meant to say. 

Redefining the meaning of a term is NOT a justification.

Your goal is not to get a 100%. ~80% is a 5. So don't feel bad that you don’t know everything.  A 3 is only ~40%

Spend more time on the problems you know rather than the problems you don't know

On the FRQs...Read every single word of each part of the FRQ very carefully before you start to write down your answer.

After you have finished your FRQ answer, read it back to yourself. Did you actually answer the question that was being asked? 

If part (a) looks too confusing to you, skip it and come back to it later. You can answer part b before part a. Label your answers clearly. 

RTQ ATQ Read the question. Answer the question…And my favorite, ATFQ, Answer the full question.

Don’t spend too long on a MC question. Efficiency is key. Never stare at a question.   Answer all questions  Use ONE of the following two strategies for MC

MCQ pacing-Answer 10 MCQs in the first 15 min, 20 MCQs in 30 min, 30 in 45 min, 40 in 60 min 50 in 75 min, don’t check every question, check every 10

OR 

Do the MC in two to three passes - 1st ones you know are quick, 2nd ones you know that take time, 3rd try the ones you didn't know.  

The 2-3 pass method is better for kids who struggle with time.  You put your time into the ones you know, but it wastes some time rereading questions.  If you don’t know which MC method to use, start with pacing, if you get behind switch to the 2-3 pass method.

FRQ pacing 20 min per long question (1-3) 10 minutes per short question (4-7).  That leaves 5 extra minutes.

If they ask you to make a choice (higher, lower, increase, decrease) make the choice and write this part down FIRST, justify your answer!

For free response questions don't write too much!  Just answer the question!