MCAT General Chemistry Review - Steven A. Leduc 2015


MCAT G-Chem Formula Sheet

Stoichiometry

Avogadro’s number: NA = 6.02 × 1023

# moles = Image

% composition by mass of X = Image × 100%

Mole fraction: Xs = Image

Molarity : M = Image

Nuclear and Atomic Chemistry

NA amu (u)= 1 gram

Ephoton = hf = hc

electron energy: En = Image for any 1-electron (Bohr) atom

Z = # of protons = atomic number, N = # of neutrons

A = Z + N = mass number

Image

Bonding and Intermolecular Forces

formal charge:

FC = V − (Image B + L)


V = (# of valence e s)


B = (# of bonding e s)


L = (# of lone-pair e s)

VSEPR Theory

Image

intermolecular forces (D=dipole, I=induced, i=instantaneous):

ion—ion > ion—D > H-bonds > D—D > D—ID > iD—ID (London dispersion)

Periodic Trends

Image

electronegativity of some common atoms:

F > O > (N ≈ Cl) > Br > (I ≈ S ≈ C) > H

Thermodynamics

T (in K) = T°C + 273, 1 cal ≈ 4.2 J, q= heat

q = mcT = CT (if no phase changes)

q = nHphase change (∆T = 0 during phase change)

enthalpy change: ∆H = heat of rxn at const P

H < 0 Image exothermic, ∆H > 0 Image endothermic

standard state: 1 M, 25°C, 1 atm

H°rxn = ΣnH°f,products − ΣnH°f,reactants

Laws of Thermodynamics (E = energy, S = entropy):

1) Euniverse is constant. ∆Esystem = QW.

2) Spontaneous rxn ImageSuniverse > 0

3) S = 0 for pure crystal at T = 0 K

Gibbs Free Energy: ∆G = ∆HTS [const.T]

G < 0 Image forward reaction is spontaneous

G = 0 Image at equilibrium

G > 0 Image reverse reaction is spontaneous

G° = −RT ln K ≈ −2.3RT log K

Gases

STP:

T =0 °C = 273 K, P = 1 atm = 760 torr = 760 mmHg

Avogadro’s Law: Vn

Vat STP = n(22.4 L)

Boyle’s Law: V ∝ 1/P (at constant T)

Charles’s Law: VT (at constant P)

Combined: P1V1 / T1 = P2V2 / T2

Ideal-Gas Law: PV = nRT

van der Waals: Image

Dalton’s law of partial pressures: Ppot = Σ Pi

Graham’s law of effusion:

Image

Kinetics

Concentration rate =

Image

Reaction rate =

Image

Rate law for rate-determining step:

rate = k[reactant1]coeff1

Arrhenius equation: k = Ae−Ea/RT

Equilibrium

For generic balanced reaction aA + bB Image cC + dD,

Image

(gas rxns use partial pressures in Kp expression)

Keq is a constant at a given temperature

Keq < 1Image equilibrium favors reactants

Keq > 1 Image equilibrium favors products

Reaction quotient: Q = Image

Le Châtelier’s Principle

Q < Keq Image rxn proceeds forward

Q = Keq Image rxn at equilibrium

Q > Keq Image rxn proceeds in reverse

Acids and Bases

pH = −log[H+] = −log[H3O+]

pOH = −log[OH]

Kw = [H+][OH] = 1 × 10−14 at 25 °C

pH + pOH = 14 at 25 °C

Ka = Image

pKa = − log Ka

Kb = Image

pKb = − log Kb

KaKb = Kw = ion-product constant for water

Henderson-Hasselbalch equations:

Image

acid-base neutralization:

a × [A] × VA = b × [B] × VB

Redox and Electrochemistry

Rules for determining oxidation state (OS):*

1) OS of pure element = 0

2) sum of OS’s = 0 in neutral molecule

sum of OS’s = charge on ion

3) Group 1 metals: OS = +1

Group 2 metals: OS = +2

4) OS of F = −1

5) OS of H = +1

6) OS of O = −2

7) OS of halogens = −1 of O family = −2

If one rule contradicts another, rule higher in list takes precedence.

F = faraday = 96,500 C/mol e

G = −nFEcell

Ecell > 0 Image spontaneous

Ecell < 0 Image reverse rxn is spontaneous

Nernst equation: E = E° −Imagelog Q

Faraday’s Law of Electrolysis:

The amount of chemical charge is proportional to the amount of electricity that flows through the cell.

* These rules work 99 percent of the time.