Stoichiometry

Law of Conservation of Mass

The Law of Conservation of Mass states that matter is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products. This principle is fundamental to all chemical equations and stoichiometric calculations.

• Key Point: The number and type of atoms remain constant before and after a reaction.

• Example: Combustion of methane:

Chemical Equations: Anatomy and Balancing

Chemical equations represent the reactants and products involved in a reaction, including their physical states and proportions. Balancing equations ensures the conservation of atoms for each element.

• Reactants: Substances consumed, written on the left side.

• Products: Substances formed, written on the right side.

• States: Indicated by (s), (l), (g), (aq).

• Coefficients: Numbers placed before compounds to balance the equation.

• Subscripts: Indicate the number of atoms in a molecule; coefficients indicate the number of molecules.

Balancing Chemical Equations: Stepwise Method

Balancing equations involves adjusting coefficients to ensure equal numbers of each atom on both sides. The process is systematic:

1. Write the skeletal equation.

2. Balance atoms in complex substances first.

3. Balance free elements last.

4. If fractions appear, multiply all coefficients to clear them.

5. Check atom counts for balance.

Balancing Equations with Polyatomic Ions

When polyatomic ions appear unchanged on both sides, balance them as units. Start with cations, then anions, and check the final balance.

• Example:

Types of Chemical Reactions

Chemical reactions are classified by the changes in reactants and products:

• Combination: Two or more substances form one product.

• Decomposition: One substance breaks into two or more products.

• Combustion: Rapid reaction with oxygen, producing a flame.

• Double Displacement: Exchange of ions between compounds, often forming a precipitate.

Precipitation Reactions and Solubility

Precipitation reactions occur when two solutions combine to form an insoluble product (precipitate). Solubility rules help predict which compounds will precipitate.

• Example:

• Molecular, Ionic, Net Ionic Equations: Show compounds, ions, and only the reacting species, respectively.

Acid-Base Reactions and Neutralization

Acid-base reactions involve the transfer of a proton (H+) from the acid to the base, resulting in the formation of water and a salt.

• Arrhenius Definition: Acids increase H+ concentration; bases increase OH- concentration.

• Brønsted-Lowry Definition: Acids are proton donors; bases are proton acceptors.

• Strong Acids: HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4

• Strong Bases: Alkali metal hydroxides, Ca(OH)2, Sr(OH)2, Ba(OH)2

• Net Ionic Equation:

Redox Reactions

Oxidation and Reduction

Redox reactions involve the transfer of electrons between species. Oxidation is the loss of electrons, reduction is the gain of electrons.

• Oxidizing Agent: Causes oxidation, is itself reduced.

• Reducing Agent: Causes reduction, is itself oxidized.

• All combustion reactions are redox reactions.

Assigning Oxidation Numbers

• Elements in elemental form: 0

• Monatomic ions: Same as charge

• Oxygen: Usually -2, -1 in peroxides

• Hydrogen: +1 with nonmetals, -1 with metals

• Fluorine: Always -1

• Sum in neutral compound: 0; sum in polyatomic ion: equals ion charge

Balancing Redox Equations: Half-Reaction Method

The half-reaction method separates oxidation and reduction, balances each, and combines them for the overall equation.

1. Assign oxidation numbers.

2. Write oxidation and reduction half-reactions.

3. Balance elements, then O (with H2O), H (with H+), and charge (with electrons).

4. Multiply to equalize electrons.

5. Add, subtracting species on both sides.

6. Check for mass and charge balance.

Stoichiometric Calculations

Formula Weight and Molecular Weight

Formula weight is the sum of atomic weights in a chemical formula, typically for ionic compounds. Molecular weight is the sum for molecules.

• Example: CaCl2:

• Example: C2H6:

Percent Composition

Percent composition shows the mass percentage of each element in a compound.

• Formula:

• Example: Ethane (C2H6):

Mole Concept and Avogadro's Number

The mole is a counting unit in chemistry, defined as particles (Avogadro's number). Molar mass connects the molecular scale to the laboratory scale.

• 1 mole of C-12: 6.022 x 1023 atoms = 12 g

• Molar mass: Mass of 1 mole of a substance (g/mol)

Solution Concentration: Molarity

Molarity (M) is the number of moles of solute per liter of solution. It is used to describe solution concentration and calculate the amount of dissolved ions.

• Formula:

• Example: 1.0 M CaCl2 yields 1.0 M Ca2+ and 2.0 M Cl-

Empirical and Molecular Formulas

The empirical formula is the simplest whole-number ratio of elements in a compound. The molecular formula is a multiple of the empirical formula, determined by molar mass.

• Empirical formula can be calculated from percent composition or combustion analysis.

• Combustion analysis determines C and H from CO2 and H2O produced; O is found by difference.

Stoichiometric Calculations: Mass-Mole Relationships

Balanced equations provide mole ratios for reactants and products. Calculations often involve converting mass to moles, using mole ratios, and converting back to mass.

• Example:

• Start with mass of A, convert to moles, use coefficients to find moles of B, convert to mass.

Limiting Reactant, Theoretical Yield, and Percent Yield

The limiting reactant is the reactant that is consumed first, limiting the amount of product formed. The theoretical yield is the maximum possible product, and percent yield compares actual to theoretical yield.

• Percent Yield Formula:

Appendix: Solubility Chart

The solubility chart provides information on the solubility of common salts at 25°C, useful for predicting precipitation reactions.

Summary

This study guide covers the essential concepts of stoichiometry, chemical equations, reaction types, solution concentration, empirical and molecular formulas, redox reactions, and limiting reactants. Mastery of these topics is fundamental for success in General Chemistry.