Chemistry MegaGuide

Admin Joined : 19 Sep 2008 Posts : 63

Elements
- Protons and neutrons have the same mass
- Mass Number - A - protons & neutrons (top)
- Good approximation of mass of an atom
- Atomic Number - Z - protons (bottom)
- Atomic Weight/Molar Mass - 1 amu = 1 g/mol , 6.022 x 1023 amu = 1 gram

Empirical Formula - whole number ratio of elements to eachother
Molecular Formula - exact number of elemental atoms in a molecule

Energy is always required to break a bond. Energy is NOT released during bond breaking.

Periodic Table
- Period - Horizontal row
- Group/Family - vertical column, elements in the same family have similar chemical properties such as same number of bonds or similarly charged ions
- Alkali Metals, Alkaline Earth Metals, Halogens, Noble Gases
- Representative/main-group elements and transition metals

Metals
- large atoms that tend to lose electrons to form cations or positive oxidation states
- Ductility, malleability, thermal and electrical conductivity, luster
- All metals except mercury exist as solids at room temperature

Non-metals - form anions
- Lower melting points than metals

Group 1A (Alkali Metals)
- Low densities, low melting points
- Highly reactive, especially with nonmetals to form ionic ions

Group 2A (Alkali Earth Metals)
- Harder/denser, melt at higher temps than Group 1A(Alkali metals)

Group 5A
- Can form 3 covalent bonds
- Except for nitrogen all other group 5A can form 5 covalent bonds using d orbitals
- Nitrogen forms strong pi bonds to make double and triple bonds

Group 6A (Chalcogens)
- Oxygen is the 2nd most electronegative element
- Oxygen is divalent and can form strong pi bonds

Group 7A (halogens)
- Highly reactive
- Fluorine always has an oxidation state of -1 meaning it can't make more than one bond

Halogens -Room Temperature State
Fluorine, Chlorine -Diatomic gases
Bromine -Diatomic liquid
iodine -Diatomic solid

- Small atoms have less room to stabilize charge by spreading it out making them bond more strongly to water resulting in greater heats of hydration
- The p orbitals on atoms that are too big don't overlap well so they don't bond (2nd row is fine, 3rd is not)

Transition Metals
- Lose electrons from their s subshell first and then from their d subshell
- Try to even out their d orbitals so they each have the same number of electrons

Atomic Size
- Cations are smaller than neutral counterparts because the loss of an electron decreases shielding
- Anions are larger than neutral counterparts
- Elements with the same number of electrons tend to get smaller as there are more protons
Z-Effective Nuclear Charge
- Nuclear charge Z minus the average number of electrons between the nucleus and the electron in question
- Increases left to right and top to bottom
- Atomic radius decreases left to right, increases top to bottom

Ionization Energy
- Increases left to right and bottom to top
- Second ionization energy > first ionization energy (not a contradiction of previous statement)

Electronegativity
- Tendency of an atom to attract an electron in a bond with another atom
- Increases left to right and bottom to top
- Undefined for noble gases(same with electron affinity)

Electron Affinity
- Willingness of an atom to accept an additional electron
- Increases from left to right and bottom to top

Metallic Character
- Increases from right to left and top to bottom

Types of Reactions
i. Combination: A + B -> C
ii. Decomposition: C -> A + B
iii. Single Displacement: A + BC -> AB + C
iv. Double Displacement: AB + CD -> AC + B

Crystals
- Ionic Crystals - consist of oppositely charged ions held together by electrostatic forces
- Molecular Crystals - composed of individual molecules held together by intermolecular bonds

Admin Joined : 19 Sep 2008 Posts : 63

Nomenclature

Ionic Compounds
- Add -ic to end of cation with greater positive charge and -ous to ion with less positive charge
- Monoatomic anions and simple polyatomic anions are given the suffix -ide
- Polyatomic anions with multiple oxygens end with the suffix -ite (less oxygens) or -ate (more oxygens)
- To name an ionic compound put the cation name in front of the anion name

Acids
- If the name of the anion ends in -ide the acid starts with hydro- and ends with -ic
- If the acid is an oxyacid the ending -ic is used for the species wit more oxygens and -ous for the species with less oxygens
Binary Molecular Compounds
- The name begins with the name of the element that is farthest to the left and lowest in the periodic table

Admin Joined : 19 Sep 2008 Posts : 63

Quantum

Principle Quantum Number (n)
- Transition metals - lag one shell behind the period
- Representative elements - given by the period in the periodic table

Azimuthal Quantum Number (L)
- s, p, d, and f
- L = n-1

Magnetic Quantum Number (m)
- From -L to L
- Gives the precise orbital of the subshell

Spin Quantum Number
- -1/2 to 1/2

- λ = h / mv

- Aufbau Principle - each new proton added a new electron is added
- Hund's Rule - electrons will not fill any orbital in the same subshell until all orbitals in that subshell contain at least one electron

Admin Joined : 19 Sep 2008 Posts : 63

Gases, Kinetics

- STP Conditions - 0°C and 1atm, 22.4L/mol
- Mean Free Path - distance travelled by a gas between collisions

- Kinetics deals with rate of reaction as it moves towards equilibrium, thermodynamics deals with balance of reactants and products after they have achieved equilibrium

Kinetic Molecular Theory
- Gas molecules exert no forces other than repulsive forces due to collisions
- Gas molecules make completely elastic collisions
- Average kinetic energy of gas molecules is directly proportional to temperature of gas

- PV = nRT is a state equation. When gases do work by expanding the pressure drops to compensate for the volume increase but it also drops to compensate for the kinetic energy loss since the nRT side drops as well

- K.E. = (3/2)RT

- v1/v2 = sqrt(m2/m1). Find K.E. from temperature, plug into this formula to find relative velocities from relative mass

- Effusion - spreading of a gas from very high pressure to very low pressure through a "pinhole"
- Effusion Rate 1 / Effusion Rate 2 = sqrt( M2/M1)

- Diffusion - is the spreading of one gas into another gas or into empty space
- Also approximated by above law

- PV=nRT Adjustments
- Real gases deviate from ideal behavior when their molecules are close together
- High pressure pushes gas molecules together
- Low temperature causes gas molecules to settle near eachother
- [ P + a(n/V)2](V-nb) = nRT
- b is a measure of volume actually occupied
- a is strength of intermolecular attractions
- A and B generally increase with molecular mass and complexity of gas
- Vreal > Videal because gases have volume
- Preal < Pideal because gases attract eachother

Admin Joined : 19 Sep 2008 Posts : 63

Reaction Rate

- k=zpeEa/RT
• P = steric factor
• Z = fraction of collisions having the effective spatial orientation p
- Increasing the rate is NOT a statement about the equilibrium
- Rateforward = kf(A)a(b)b
• overall order = a + b
- If looking at a table…when concentration is doubled reaction rate quadruples making the order = 2
- -[Reactants]/xt = [Products]/yt where x and y are constants
- Elementary reactions - if the MCAT tells you a reaction is elementary then you can use the coefficients as the order…otherwise? No.

Order Equation of graph necessary to make a straight line
[A]0 [A], slope = -kf
[A]1 ln[A], slope = -kf
[A]2 1/[A], slope = kf
[A]3 1/2[A]2, slope = kf

Two Step Reactions
• If second reaction is rate limiting then [reactants] for the second step depends on the rate constant of the first step
1.) NO + Br2 -> NOBr2 (Fast Step)
2.) NOBr2 + NO -> 2NOBr2 (Slow Step)
- k1[NO][Br2] = k-1[NOBr2]
- .:. k1/k-1[NO][Br2] = [NOBr2]
- Since the rate is set by the slow step k2[NOBR2][NO] = [NOBr]
- Making k2k1/k-1[NO]2[Br2]

Catalysts
• A catalyst may lower the activation energy or increase the steric factor 'p'
• Heterogeneous Catalyst - in a different phase than the reactants and products
• The binding of a catalyst is almost always exothermic and the rate of catalysis depends upon the strength of the bond between the reactant and the catalyst
- Too weak and not enough adsorption
- Too strong and too much energy is required to remove the reactant

Effects of Solvent on Rate
• the solvent bonds may stabilize an intermediate
• Degree of solvation affects k
• Rate constant is a function of solvent and temperature
• Solvent bonds must be broken before

Admin Joined : 19 Sep 2008 Posts : 63

- Equilibrium -
• The point where the forward rate is equal to the reverse rate
• Equilibrium is the point of greatest entropy
• Equilibrium is a dynamic process

- Law of Mass Action
• The law of mass action is good for all chemical equations, including non-elementary equations
- This means use the coefficients as the exponents regardless of molecularity(? pp39)
• The equilibrium constant K = [Products]coeffiecients / [Reactants]coefficients
- If you look closely this means K = kf/kr BUT only for elementary equations
- K depends only upon temperature
- Equilibrium constant for a series of reactions is equal to the product of the equilibrium constants for each step
- Do NOT confuse equilibrium constant with equilibrium
- Don't include solids or pure liquids

- Partial Pressure Equilibrium Constant
• Kp = K(RT)∆n
• K is concentration equilibrium constant
• Kp is partial pressure equilibrium constant, for gas reactions you can use this equation to find K
• n is the sum of the coefficients of the products minus the sum of the coefficients of the reactants
• Any two or more single reactions or series of reactions resulting in the same products from identical reactants must have the same equilibrium constant for a given temperature
• MUST be able to work with partial pressure equilibrium constants for the MCAT so learn it…I know you don't understand it yet

- Reaction Quotient
• Q = Productscoefficients/Reactantscoefficients
• Since products always move towards equilibrium Q will always move towards K
• If Q=K reaction is at equilibrium
• If Q>K then the [products] is greater than it should be and the reaction will shift leftward
• If Q<K then the [products] is less than it should be and the reaction will shift rightward

- Le Chatelier's Principle
• When a system at equilibrium is stressed the system will shift in a direction to reduce that stress
• Three types of stress
i. Addition or removal of product or reactant
ii. Changing pressure
iii. Heating or cooling the system
- If a product of a reaction is heat adding heat will shift the equilibrium to more reactants…not counterintuitive once you realize that heat is produced when bonds are made…and bonds are broken by adding heat
• Molarity Effect
- If you concentrate a solution the equilibrium will shift to the side with less moles…

Admin Joined : 19 Sep 2008 Posts : 63

Thermodynamics

System ΔEnergy ΔMass
Open Yes Yes
Closed Yes No
Isolated No No

Extensive Properties - change with amount
Intensive Properties - do not change with amount
State Functions - state properties describe the state of a system. The change in a state property going from one state to another is the same regardless of the process via which the system changed
Path functions - opposite of state functions
• i.e. work, heat

Work
• w=PΔV under constant pressure
• If force is constant pressure is constant by P=F/A
• Work can be different going from one state to another…if pressure changes too then there is a different amount of work done
• If we have a piston on its side and add heat so that it expands(but keep the gas the same temperature) it does work on the surroundings
• The temperature of the gas stays the same meaning pressure decreases as volume expands. This means that the pressure is decreasing so the force is not constant
 Force can't be constant if temperature is constant
• Second Law of Thermodynamics
• Heat can not be completely changed into work in a cyclical process
• Efficiency = 1 - Tc/Th
K.E.avg= (3/2)kT

Enthalpy - ΔH = ΔU + PΔV
• Two systems may have the same amount of internal energy but if they are at different pressures they have a different capacity to perform work…hence enthalpy was invented
• Is a state function
• An ideal gas enthalpy depends only on temperature
• Is an extensive property
• At constant pressure change in enthalpy is equal to heat
• An element at 25C and 1atm is arbitrarily assigned an enthalpy value of 0J/mol
• Standard enthalpy of formation - ΔH°f
• Change in enthalpy that creates one mole of that product from its raw elements
• For a reaction involving no change in pressure ΔH=q
• ΔH°reaction = ΔHf°products - ΔHf°reactants
• ΔH°reaction > 0 the reaction is endothermic (counterintuitive?)
• ΔH°reaction < 0 the reaction is exothermic
• Hess' law says when you add reactions you can add their enthalpies

Entropy (Joules/Kelvin)
- is a state function
-is an extensive property(increases with number, volume, and temperature)
-a reaction can be unfavorable in enthalpy and proceed…but it can't be unfavorable with entropy and succeed
-reactions at equilibrium have achieved maximum universal entropy
-if a reaction increases the number of gaseous molecules then the reaction has positive entropy (nature likes more molecules)
-ΔS = dqrev/T (defined by change in heat per kelvin in a reversible process)

Gibbs Free Energy(G)
• ΔG = ΔH - TΔS
• Must minimize ΔG to achieve equilibrium
• A negative delta G value (-ΔG) corresponds to a spontaneous process
• This equation is only good for constant temperature reactions
• Is an extensive property
• Is a state function
• Represents the maximum Non-PV work available from a reaction
• An isolated system can change its gibbs free energy

Admin Joined : 19 Sep 2008 Posts : 63

Heat

- There are two ways to transfer energy in a system: Heat(q) and Work
- Heat - movement of energy from hot to cold
1.) Conduction - thermal energy transfer via molecular collisions
- Q/t = kA(Th-Tc)/L
- k is thermal conductivity
- The rate of heat flow(Q/t) would be the same in all slabs even if they each had different lengths, thicknesses, and different thermal conductivities(this leads to the 5th bullet)
- The order of the slabs wouldn't matter either
- A higher conductivity results in a lower temperature difference across any slab of a given length
2.) Convection - thermal energy transfer via fluid movements
- Differences in pressure or density drive warm fluid in the direction of cooler fluid
3.) Radiation
- Rate at which an object radiates is its Power P
- P = σε(Te4 - To4)
- ε = emissivity of surface(between 0 and 1)

Admin Joined : 19 Sep 2008 Posts : 63

Solution - homogeneous mixture of two or more compounds in a single phase
- Solvent - compound which there is more of
- Solute - compound there is less of
- Types of solutions
- Ideal solutions - made from compounds with similar properties; compound can be interchanged within the solution without changing the spatial arrangement of the molecules or the intermolecular attractions
- Ideally dilute solution - solute molecules are separated so that they have no interaction with eachother; mole fraction of solvent is 1
- Nonideal solutions -

Colloid - a colloid is like a solution except the solute forms into larger particles
- i.e. aerosol, foam, emulsion
- Can be separated with a semipermeable membrane using dialysis

London Dispersion Forces - Nonpolar molecules are held together by weak bonds caused by instantaneous dipole moments
- The bonds of a nonpolar solute are too strong to be broken by the weak forces of a nonpolar solvent

Electrolyte - a compound which forms ions in aqueous solution
- Strong electrolytes create solutions which conduct electricity well and contain many ions

Name Formula
nitrite NO2-
nitrate NO3-
sulfite SO32-
sulfate SO42-
Hypochlorite ClO-
chlorite ClO2-
chlorate ClO3-
perchlorate ClO4-
carbonate CO32-
bicarbonate HCO3-
phosphate PO43-

Molarity - (mol/L) moles of the compound divided by the volume of the solution
Molality - (mol/kg) moles of solute divided by kilograms of solvent
Normality - measures the "number of equivalents" per liter of solution
- Depends on the type of reaction taking place in the solution
- An acid-base reaction with H2SO4 would be a 2 normal solution because two equivalents of protons

Solution Formation
- 3 steps to making a solution
- 1.) break bonds between solute molecules
- 2.) break bonds between solvent molecules
- 3.) formation of bonds between solvent and solute
- At constant pressure the enthalpy change of a reaction equals the heat
- For condensed phases not at high pressure(most MCAT solutions) the enthalpy change approximately equals the internal energy change( ΔH ~= ΔU)
- Heat of Hydration - the combined enthalpy of steps 2 and 3
- A solution with negative enthalpy will give off heat when it forms creating a stronger bond
- A solution with positive enthalpy will take in heat when it forms
- Positional entropy always increases in the formation of a solution so solution formation ALWAYS has positive entropy

Vapor Pressure
- Clausius-Clapeyron equation
- ln(Pv) = -(ΔHvap/R) * (1/T) + C
- ΔHvap is the heat of hydration
- Pv is vapor pressure
- WHAT IS R?
- Vapor pressure increases with temperature
- Vaporization is an endothermic process
- When vapor pressure equals local atmospheric pressure a compound boils
- Melting point - temperature at which the vapor pressures of the solid is equal to the vapor pressure of the liquid
- Above the melting point the liquid vapor pressure is greater than that of the solid
- Below the melting point the liquid vapor pressure is less than that of the solid
- Nonvolatile solute - when a solute with no vapor pressure is added to a solution it reduces that solutions vapor pressure by taking up room at the surface of the liquid reducing the area in which the liquid interacts with the vapor

Raoult's Law - Pv = χaPa
- For nonvolatile solutes - if 97% of the solution is solvent, the vapor pressure will be 97% of the vapor pressure of the pure solvent
- For volatile solutes - if 97% of the solution is solvent, then the vapor pressure will be 97% of the pure solvent PLUS 3% of the vapor pressure of the pure solute
- Vapor pressure if a nonideal solution will deviate from Raoult's law
- Negative heats of solution form stronger bonds so less molecules can break free so there is lower vapor pressure and there will be a negative deviation
- Positive heats of solution form weaker bonds so more molecules can break free so there is increased vapor pressure and there will be a positive deviation
- .:. a positive heat of solution leads to an increase in vapor pressure while a negative heat of solution leads to a decrease in vapor pressure