Reactions

• Chemical reaction: chemical change; reorganization of the atoms in one or more substances.
  • Bonds and broken and new bonds form.
  • Atoms are neither created no destroyed (mass is conserved).

Equation Symbols & Interpretations

<aside> 🥼 Chemical equation: representation/summary of a chemical reaction with reactants on the left wide of an arrow and products on the right side.

• e.g. $CH_{4(g)}+2O_{2(g)} \rarr CO_{2(g)}+H_2O_{(g)}$. The reactants are methane ($CH_4$) and oxygen ($O_2$). The products are carbon dioxide ($CO_2$) and water ($H_2O$). </aside>

• An arrow pointing to the products (→) separates the reactants and products.

• The reactants and products can be in any order as long as they are on the correct side of the arrow.

• All atoms present in the reactants must be accounted for among the products (balanced chemical equation).

• Info in a chemical equation:

  • The nature of the reactants and the products: specify the compounds (identified by experiment) involved in the reaction and the physical states of the reactants and products in next to the symbol.
    • Solid → (s).
    • Liquid → (l).
    • Gas → (g).
    • Dissolved in water (in aqueous solution) → (aq).
  • The relative numbers of reactants and products: indicated by coefficients in the balanced equation.
    • Coefficients can be interpreted in several ways (e.g. number of molecules, moles of substance). Coefficients do not represent masses of molecules.
    • Coefficients determined by knowing that the same number of each type of atom must occur on both sides of the equation due to the law of conservation of mass.

• Example reaction: hydrochloric acid in aqueous solution is added to solid sodium hydrogen carbonate. The products are carbon dioxide gas, liquid water, and sodium chloride.

  • Molecular equation: $HCl_{(aq)} + NaHCO_{3(s)} \rarr CO_{2(g)} + H_2O_{(l)} + NaCl_{(aq)}$.

Balancing Equations

• Balanced chemical equation: chemical equation that follows the law of conservation of mass (atoms conserved).

<aside> 🥼 The formulas of the compounds (identities) must never be changed in balancing a chemical equations (cannot change subscripts in the formula; atoms cannot be added or subtracted from a formula).

</aside>

• Balance by inspection (systematic trial and error):
  1. Start with the most complicated molecule(s)/the compound with the most number of atoms.
     • Unbalanced chemical equation: $C_2H_5OH_{(l)}+O_{2(g)} \rarr CO_{2(g)} + H_2O_{(g)}$.
     • The most complicated atom is ethanol ($C_2H_5OH$).
     • Determine what coefficients are necessary so that the same number of each type of atoms appear on both reactant and product sides without changing chemical formulas.
       • Carbon atoms: ethanol has 2 carbon atoms. Put a 2 as the coefficient for carbon dioxide for 2 carbon atoms on the products side.
         • $C_2H_5OH_{(l)}+O_{2(g)} \rarr 2CO_{2(g)} + H_2O_{(g)}$.
       • Hydrogen atoms: ethanol has 6 hydrogen atoms (5+1). Put a 3 in front of water for 6 hydrogen atoms (3x2) on the product side.
         • $C_2H_5OH_{(l)}+O_{2(g)} \rarr 2CO_{2(g)} + 3H_2O_{(g)}$.
       • Oxygen atoms: the products side has 7 oxygen atoms in total (2x2 = 4 from carbon dioxide; 3x1 = 3 from water). Ethanol has 1 oxygen atom. Put a 3 in front of oxygen for 7 oxygen atoms on the reactant side (1 from ethanol, 3x2 = 6 from oxygen).
         • $C_2H_5OH_{(l)}+3O_{2(g)} \rarr 2CO_{2(g)} + 3H_2O_{(g)}$.

1. Check if all atoms are balanced.
   • $C_2H_5OH_{(l)}+3O_{2(g)} \rarr 2CO_{2(g)} + 3H_2O_{(g)}$.
     • Reactants: 2 carbon atoms, 6 hydrogen atoms, 7 oxygen atoms.
     • Products: 2 carbon atoms, 6 hydrogen atoms, 7 oxygen atoms.
   • If fractional coefficients are needed to balance the equation, multiply the entire equation by the denominator for whole-number coefficients.
     • e.g. $2NH_{3(g)} + \frac{5}{2}O_{2(g)} \rarr 2NO_{(g)} + 3H_2O_{(g)}$ is multiplied by 2 and becomes $4NH_{3(g)} + 5O_{2(g)} \rarr 4NO_{(g)} + 6H_2O_{(g)}$.

Water as a Solvent*

Strong/Weak Electrolytes*

Strong Electrolytes*

• Soluble ionic compounds (see solubility rules).
• Strong acids:
  • Hydrochloric acid ($HCl$).
  • Hydrobromic acid ($HBr$).
  • Hydroiodic acid ($HI$).
  • Nitric acid ($HNO_3$).
  • Perchloric acid ($HClO_4$).
  • Chloric acid ($HClO_3$).
  • *Sulfuric acid ($H_2SO_4$).
    • Dissociates into hydrogen ion ($H^+$) and hydrogen sulfate ion (${HSO_4}^-$).