MCAT Chemistry


The smallest representative unit of an element that reatins all the characteristics of that element.


The center of an atom containing the neutrons and protons.


No charge


Mass of about 1 dalton


Relative charge of +1


Mass of about 1 dalton


Relative charge of -1


Mass of about 0.0005 daltons

Pauli exclusion principle

No two electrons in an atom may have exactly the same set of four quantum numbers

Periodic Law
The properties of the elements are periodic functions of their atomic numbers.
Electron Affinity

The energy given off when a neutral atom in the gas phase picks up an electron to form a negatively charged ion

Ideal gas law

PV = nRT


P = Pressure in atmospher or torr

V = Volume (Liters L)

T = Absolute temperature (Kelvins K)

n = moles of the gas

R = universal gas constant

Graham’s Law
The rate of diffusion is inversely proportional to the square root of the



Dipole forces

  1. Intermolecular forces that exist between polar molecules . Active only when the molecules are close together. The strengths of intermolecular attractions increase when polarity increases.

Hydrogen bonds

A chemical bond in which a hydrogen atom of one molecule is attracted to an electronegative atom, especially a nitrogen, oxygen, or flourine atom, usually of another molecule

London dispersion forces


Van der Waals dispersion forces

A weak attractive force between atoms or nonpolar molecules caused by an instantaneous dipole moment of one atom or molecule that induces a similar temporary dipole moment in adjacent atoms or molecules

Solid –> Liquid
Liquid –> Solid
Vaporization or Boiling
Liquid –> Gas

Gas –> Liquid

Triple point
The temperature and pressure at which all three phases (gas, liquid & solid) may coexist.
Bronsted-Lowry acid

A substance that donates a proton to an aqueous solution

Lewis acid
A substance that accepts an electron pair
Bronsted-Lowry base
A substance that accepts a proton.
Lewis base

A substance that donates an electron pair.


Moles per liter


(Grams / molecular weight) / liters of solution





moles of solute per kilograms of solvent
equivalents of sloute per liters of solution
Grams per equivalent weight

-log [H+


A measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. The pH scale commonly in use ranges from 0 to 14.


A solution that contains a weak acid or a weak base and a salt of that weak acid or weak base.

Number of equivalents in neutralization

Lacid X Nacid = Lbase X Nbase
First Law of Thermodynamics
Energy cannot be created or destroyed.
Second Law of Thermodynamics

The spontaneous flow of heat is always unidirectional from the higher to lower temperature.

Third Law of Thermodynamics
The entropy of all pure crystalline solids mayb be taken as zero at the abolute zero of temperature.
Exothermic Reaction

A negative ΔH


Heat is evolved into the surroundings


Endothermic Reaction




Endergonic Reaction

A positive ΔH 


A reaction that requires additional heata from the surroundings


The degree of disorder of a system.


Unrecoverable energy.




A thermodynamic function of a system equivalent to the sum of intrernally energy of a system plus its volume multiplied by the pressure exerted on it by the environment.



Gibbs free Energy



The total amount of energy that is used up or released by a reaction.
Rate controlling step

The slowest step of a reaction that controls or determines the rate of the overall (usually multistep) reaction expressed as:


Rate= k[A]a[B]b


Where A & B are reactants and a & b are concentrations.

Activation energy



The minimum amount of energy necessary to transform reactants –> products


Energy difference between the starting energy and the top of the potential energy trajectory


A biological catalyst that substantially increases the rate of a reaction by allowing the reaction to follow a lower energy pathway that does not involve attaining the full energy of activation.


not used up in the reaction itself.

Equilibrium constant



In a reversible reaction this is expressed as:





Where C & D are products and A & B are reactants

Le Chatelier’s Principle

Principle that states that when a system is at equiliburium it will shift to relieve any stress placed on it.


ex: if too many reactants are added the system will shift to increase the amount of products to balance out the system

Specific gravity

The weight of an object in air divided by the loss of weight when weighed in water

Mass (m)

Volume (v)

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