Chapter 4: Chemical Reactions and Chemical Quantities – Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Chemical Reactions and Chemical Quantities

Introduction to Chemical Reactions

Chemical reactions are processes in which substances (reactants) are transformed into new substances (products) through the breaking and forming of chemical bonds. These reactions are fundamental to chemistry and are represented using chemical equations.

Chemical reaction: A process where one or more substances are converted into different substances.
Combustion reaction: A type of reaction where a substance reacts with oxygen to form oxygen-containing compounds and releases heat.

Chemical Equations

Chemical equations provide a shorthand way to describe chemical reactions, indicating the formulas, states, and relative quantities of reactants and products.

States of matter: Indicated in equations as (g) for gas, (l) for liquid, (s) for solid, and (aq) for aqueous solution.
Balancing equations: Ensures the law of conservation of mass is obeyed by having equal numbers of each type of atom on both sides of the equation.

Example: Combustion of Methane

The combustion of methane demonstrates the need to balance chemical equations:

Unbalanced:
Balance O and H atoms to obey conservation of mass.

Counting oxygen atoms in the unbalanced methane combustion equation Counting hydrogen atoms in the unbalanced methane combustion equation

Balanced equation:

Balanced methane combustion equation with molecular models

Steps for Balancing Chemical Equations

Balancing equations involves systematic steps to ensure atom conservation:

Write the skeletal equation with correct formulas.
Balance atoms in complex substances first, then pure elements.
Balance free elements last by adjusting coefficients.
If fractional coefficients are present, multiply all coefficients to clear fractions.
Check that all atoms are balanced on both sides.

Balancing oxygen in cobalt(III) oxide and carbon reaction Balancing cobalt and carbon in the equation Table showing balanced atoms for the equation

Example: Combustion of Butane

Unbalanced:
Balance C, then H, then O.

Balancing carbon and hydrogen in butane combustion Balancing oxygen in butane combustion $Clearing fractions in butane combustion equation$ Table showing balanced atoms for butane combustion

Balancing Equations with Polyatomic Ions

When polyatomic ions appear unchanged on both sides of the equation, balance them as units.

Balance cations first, then anions, then check all atoms/ions.

Balancing Sr2+ and Li+ ions in a reaction Balancing PO4 3- and Cl- ions in a reaction Table showing balanced ions for the equation

Stoichiometry: Quantitative Relationships in Chemical Reactions

Reaction Stoichiometry

Stoichiometry is the study of the quantitative relationships between reactants and products in a chemical reaction, based on the balanced equation.

Stoichiometric coefficients indicate the relative number of moles of each substance.
These ratios are used as conversion factors in calculations.

Mole-to-Mole Conversions

Use the coefficients from the balanced equation to convert between moles of reactants and products.

Example:
Stoichiometric ratio:

Mass-to-Mass Conversions

To relate the mass of one substance to the mass of another, follow these steps:

Convert mass of A to moles of A.
Use the stoichiometric ratio to convert to moles of B.
Convert moles of B to mass of B.

General mass-to-mass stoichiometry flowchart Example: mass-to-mass conversion for octane combustion Calculation for sulfuric acid formation

Limiting Reactant, Theoretical Yield, and Percent Yield

Limiting Reactant

The limiting reactant is the reactant that is completely consumed first, thus limiting the amount of product formed.

Identify by calculating the amount of product each reactant can produce; the smallest amount determines the limiting reactant.

Limiting reactant in a pizza analogy

Theoretical Yield

The theoretical yield is the maximum amount of product that can be formed from the limiting reactant, as predicted by stoichiometry.

Percent Yield

Percent yield measures the efficiency of a reaction, comparing the actual yield to the theoretical yield:

Percent yield calculation using pizza analogy

Example: Limiting Reactant in Methane Combustion

Balanced equation for methane combustion Limiting reactant calculation for methane combustion

Example: Limiting Reactant and Percent Yield in Magnesium Oxide Formation

Limiting reactant calculation for MgO formation Percent yield calculation for MgO formation

Example: Limiting Reactant and Theoretical Yield in Ammonia Synthesis

Limiting reactant calculation for ammonia synthesis

Example: Limiting Reactant and Theoretical Yield in Titanium Production

Limiting reactant calculation for titanium production

Types of Chemical Reactions

Combustion Reactions

Combustion reactions involve a substance reacting with oxygen to form one or more oxygen-containing compounds, often releasing heat and light.

General form: (for hydrocarbons)

Alkali Metal Reactions

Alkali metals react vigorously with nonmetals and water:

With nonmetals: Form ionic compounds (metal halides).
With water: Form hydroxide ions and hydrogen gas.

Reactions of alkali metals with water

Halogen Reactions

Halogens react with metals to form metal halides, with hydrogen to form hydrogen halides, and with each other to form interhalogen compounds.

Test tubes containing chlorine, bromine, and iodine

Summary Table: Key Stoichiometric Relationships

Concept	Definition	Key Equation
Limiting Reactant	Reactant that determines the maximum amount of product	Calculate product for each reactant; smallest is limiting
Theoretical Yield	Maximum possible amount of product	Based on limiting reactant
Percent Yield	Efficiency of reaction