Chapter 4: Chemical Reactions and Chemical Quantities – Study Notes

Chemical Reactions

Definition and Characteristics

Chemical reactions are processes in which substances (reactants) undergo chemical changes to form new substances (products). These changes involve the breaking and forming of chemical bonds.

• Chemical bonds must be broken and/or formed during a reaction.

• At least one new substance is produced as a result of the reaction.

• Chemical equations are used to represent the identities and quantities of substances involved in a reaction.

Chemical Equations

Structure and Interpretation

Chemical equations describe the proportions of substances participating in a chemical reaction. They provide a concise way to represent chemical changes using symbols and formulas.

• Reactants: Initial substances consumed in the reaction.

• Products: Final substances formed as a result of the reaction.

• Example: $\mathrm{SO_3(g) + H_2O(g) \rightarrow H_2SO_4(l)}$

States of Matter and Reaction Conditions

• States of matter are indicated in parentheses:

  • Gas: (g)

  • Liquid: (l)

  • Solid: (s)

  • Aqueous: (aq) – dissolved in water

• Reaction conditions may be specified above or below the reaction arrow:

  • High temperature: $\Delta$

  • Pressure, catalysts, etc.: written with the arrow

Interpretation of Chemical Equations

Mass and Mole Relationships

Balanced chemical equations provide quantitative relationships between reactants and products, which can be interpreted at the molecular, molar, and mass levels.

• Molecular interpretation: Number of molecules involved (e.g., 2 molecules H2 + 1 molecule O2 → 2 molecules H2O).

• Molar interpretation: Number of moles involved (e.g., 2 mol H2 + 1 mol O2 → 2 mol H2O).

• Mass interpretation: Masses of substances involved, calculated using molar masses (e.g., $(2 \times 2.02)\,\mathrm{g\ H_2} + 32.00\,\mathrm{g\ O_2} \rightarrow (2 \times 18.0)\,\mathrm{g\ H_2O}$).

Balanced Chemical Equations

Law of Conservation of Mass

Chemical equations must be balanced to reflect the Law of Conservation of Mass, which states that matter is neither created nor destroyed in a chemical reaction.

• The total mass of each element on the reactant side must equal the total mass of each element on the product side.

• The total number of atoms of each element must be the same on both sides of the equation.

• The total charge must also be balanced if ions are involved.

Rules for Balancing Equations

Guidelines and Common Mistakes

• Balance equations by adjusting coefficients (numbers in front of formulas), not subscripts.

• Never introduce extraneous chemical species to balance the equation.

• Never change the chemical formula (subscripts) of a substance to achieve balance.

• Balance elements that occur in only one compound on each side first.

• Balance free elements (e.g., O2) last.

• Balance unchanged polyatomic ions as groups if they appear on both sides.

• Fractional coefficients are acceptable during balancing and can be cleared by multiplying all coefficients by a common factor at the end.