Chapter 7: Chemical Reactions – Structured Study Notes

Introduction to Chemical Reactions

Overview of 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 changes are fundamental to chemistry and are observed in everyday phenomena.

• Gas-Evolution Reactions: Occur when reactions in liquids produce a gas, such as the classic grade school volcano experiment (baking soda and vinegar).

• Combustion Reactions: Involve hydrocarbons (e.g., octane in gasoline) reacting with oxygen to produce carbon dioxide, water, and heat. These are a type of oxidation-reduction reaction where electrons are transferred.

• Precipitation Reactions: Occur when substances in solution react to form a solid (precipitate), such as the formation of soap scum in hard water.

Examples and Applications

• Grade School Volcano: Sodium bicarbonate reacts with acetic acid to produce carbon dioxide gas, water, and sodium acetate.

• Laundry Detergent: Detergents contain substances that react with calcium and magnesium ions in hard water to form solids, removing these ions from the water.

Evidence of a Chemical Reaction

Molecular and Macroscopic Changes

Chemical reactions can be identified by changes at the molecular level, but we often rely on observable macroscopic changes.

• Formation of Compounds: Atoms combine to form new molecules.

• Decomposition: Original molecules break down into simpler substances.

• Atom Exchange: Atoms in molecules swap places with atoms in other molecules.

Observable Evidence

1. Color Change

2. Formation of a Solid in a previously clear solution

3. Formation of a Gas when a substance is added to a solution

4. Emission of Light

5. Emission or Absorption of Heat

Not a Chemical Reaction

• Physical changes, such as boiling water, may produce bubbles and gas but do not involve a chemical change. Both water and steam are composed of H2O molecules.

The Chemical Equation

Representing Chemical Reactions

Chemical equations are used to represent chemical reactions. The reactants are listed on the left, and the products on the right.

• Example: $CH_4 + O_2 ightarrow CO_2 + H_2O$

• The state of each reactant or product is indicated in parentheses: (g) for gas, (l) for liquid, (s) for solid, (aq) for aqueous solution.

Abbreviation	State
(g)	gas
(l)	liquid
(s)	solid
(aq)	aqueous (water solution)

Balancing Chemical Equations

• Atoms must be conserved; the number of each type of atom on both sides of the equation must be equal.

• Balance equations by adjusting coefficients (not subscripts).

• Example: $CH_4(g) + 2O_2(g) ightarrow CO_2(g) + 2H_2O(g)$

Reactants	Products
1 C atom	1 C atom
4 H atoms	4 H atoms
4 O atoms	4 O atoms

Steps to Write Balanced Equations

1. Write correct chemical formulas for reactants and products.

2. Balance elements that occur in only one compound on each side first (metals before nonmetals).

3. Balance free elements last by adjusting their coefficients.

4. If fractions appear, multiply the entire equation to obtain whole numbers.

5. Check that the equation is balanced by counting atoms on both sides.

Example: Aluminum and Sulfuric Acid

• Unbalanced: $Al(s) + H_2SO_4(aq) ightarrow Al_2(SO_4)_3(aq) + H_2(g)$

• Balance polyatomic ions as units.

• Balanced: $2Al(s) + 3H_2SO_4(aq) ightarrow Al_2(SO_4)_3(aq) + 3H_2(g)$

Reactants	Products
2 Al atoms	2 Al atoms
6 H atoms	6 H atoms
3 S atoms	3 S atoms
12 O atoms	12 O atoms

Aqueous Solutions and Solubility

Terminology

• Soluble: A compound that dissolves in a particular liquid.

• Insoluble: A compound that does not dissolve in the liquid.

• Aqueous Solution: A homogeneous mixture of a substance with water.

• Dissociation: Ionic compounds usually separate into their component ions when dissolved in water.

Electrolytes and Conductivity

• Strong Electrolytes: Substances that completely dissociate into ions in solution (e.g., NaCl, AgNO3).

• Non-Conductors: Pure water does not conduct electricity because it does not contain ions.

• Strong Electrolyte Solutions: Solutions of soluble ionic compounds conduct electricity due to the presence of ions.

Solubility Examples

• NaCl(aq): Contains only Na+ and Cl- ions.

• AgNO3(aq): Contains Ag+ and NO3- ions; polyatomic ions dissolve as intact units.

• AgCl(s): Remains as a solid in water, does not dissociate, and does not conduct electricity.

Summary Table: States of Matter in Chemical Equations

Abbreviation	State
(g)	Gas
(l)	Liquid
(s)	Solid
(aq)	Aqueous (water solution)

Key Takeaways

• Chemical reactions are identified by changes in composition and observable evidence such as color change, gas formation, and precipitation.

• Chemical equations must be balanced to reflect the conservation of atoms.

• Aqueous solutions can conduct electricity if they contain dissolved ions (strong electrolytes).

• Solubility rules help predict whether a compound will dissolve in water and whether a precipitation reaction will occur.

Additional info: These notes are based on textbook slides for Chapter 7 of "Introductory Chemistry" and are suitable for college-level Introduction to Chemistry students.