MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Compounds and Stoichiometry
Concept Summary

Molecules and Moles

· Compounds are substances composed of two or more elements in a fixed proportion.

· Molecular weight is the mass (in amu) of the constituent atoms in a compound as indicated by the molecular formula.

· Molar mass is the mass of one mole (Avogadro’s number or 6.022 × 10²³ particles) of a compound; usually measured in grams per mole.

· Gram equivalent weight is a measure of the mass of a substance that can donate one equivalent of the species of interest.

· Normality is the ratio of equivalents per liter; it is related to molarity by multiplying the molarity by the number of equivalents present per mole of compound.

· Equivalents are moles of the species of interest; equivalents are most often seen in acid—base chemistry (hydrogen ions or hydroxide ions) and oxidation—reduction reactions (moles of electrons or other ions).

Representation of Compounds

· The law of constant composition states that any pure sample of a compound will contain the same elements in the same mass ratio.

· The empirical formula is the smallest whole-number ratio of the elements in a compound.

· The molecular formula is either the same as or a multiple of the empirical formula; it gives the exact number of atoms of each element in a compound.

· To calculate percent composition by mass, determine the mass of the individual element and divide by the molar mass of the compound.

Types of Chemical Reactions

· Combination reactions occur when two or more reactants combine to form one product.

· Decomposition reactions occur when one reactant is chemically broken down into two or more products.

· Combustion reactions occur when a fuel and an oxidant (typically oxygen) react, forming the products water and carbon dioxide (if the fuel is a hydrocarbon).

· Displacement reactions occur when one or more atoms or ions of one compound are replaced with one or more atoms or ions of another compound.

o Single-displacement reactions occur when an ion of one compound is replaced with another element.

o Double-displacement reactions occur when elements from two different compounds trade places with each other to form two new compounds.

· Neutralization reactions are those in which an acid reacts with a base to form a salt (and, usually, water).

Balancing Chemical Equations

· Chemical equations must be balanced to perform stoichiometric calculations.

· Balanced equations are determined using the following steps in order:

o Balancing the least common atoms.

o Balancing the more common atoms (usually hydrogen and oxygen).

o Balancing charge, if necessary.

Applications of Stoichiometry

· Balanced equations can be used to determine the limiting reagent, which is the reactant that will be consumed first in a chemical reaction.

· The other reactants present are termed excess reagents.

· Theoretical yield is the amount of product generated if all of the limiting reactant is consumed with no side reactions.

· Actual yield is typically lower than theoretical yield.

· Percent yield is calculated by dividing actual yield by theoretical yield and converting to a percentage.

Ions

· Like organic chemistry, ions in general chemistry have a system of nomenclature:

o Roman numerals are used for nonrepresentative elements to denote ionic charge.

o —ous endings can also be used to indicate lesser charge, while —ic endings indicate greater charge.

o All monatomic anions end in —ide.

o Oxyanions are given a suffix indicating how oxidized the central atom is. Those that contain a lesser amount of oxygen are given the suffix —ite, and those with a greater amount are given the suffix —ate.

o Oxyanion series with more than two members are given an additional level of nomenclature. The species with the fewest oxygens is given the prefix hypo—, and the species with the most oxygens is given the prefix per—.

o Polyatomic ions containing hydrogen denote the number of hydrogens using hydrogen or bi— to denote one, or dihydrogen to denote two.

· Ionic charges are predictable by group number and type of element (metal or nonmetal) for representative elements, but are generally unpredictable for nonrepresentative elements.

o Metals form positively charged cations based on group number.

o Nonmetals form negatively charged anions based on the number of electrons needed to achieve an octet.

· Electrolytes contain equivalents of ions from molecules that dissociate in solution. The strength of an electrolyte depends on its degree of dissociation or solvation.