Chemical Energy and Reaction Fundamentals

Chemical Energy and Molecular Bonds

Chemical energy is the energy associated with chemical bonds and intermolecular attraction. The chemical energy of a set of reacting substances changes since bonds break and form new bonds, creating new products.

We call the energy exchanged in the form of heat between a system containing a chemical reaction and its surroundings the heat of reaction.

Exothermic and Endothermic Reactions

• Exothermic Reaction: The energy of the reactants is greater than that of the products and has a negative sign (e.g., combustion). These reactions release chemical energy via heat.
• Endothermic Reaction: The energy in the system grows. The exchange of energy via heat goes from the surroundings toward the system, so the heat of reaction is positive. The energy of the reactants is less than that of the products.

Thermochemical Equations

Thermochemical equations indicate not only the substances involved in a chemical reaction but also their state of matter and how much energy is involved. The value of the heat of reaction depends on:

1. The state of matter of the reactants and products.
2. The amount of substances in the reactions.

Acids, Bases, and Electrolytes

Some substances dissolve in water, releasing ions which water molecules surround. These dissolved ions make the aqueous solutions containing them electrical conductors; this is why these substances are called electrolytes.

According to Arrhenius theory, a substance is an acid if it is capable of releasing hydrogen ions (H⁺) when it dissolves in water, and it is a base if it liberates hydroxide ions (OH^-).

The pH Scale and Ion Concentration

We use the pH scale to determine whether a solution is acidic, basic, or neutral. We must keep in mind that:

• The concentration of hydronium ions (H₃O⁺) in pure water is 10^-7 mol/L.
• An excess of hydronium ions in water, due to a dissolved acid, will mean that there is a proton concentration greater than 10^-7 mol/L.
• If, however, a base is dissolved in water, an excess of hydroxide ions (OH^-) will result in a hydronium concentration below 10^-7 mol/L.

Oxidation Numbers and Rules

• Group 1: H, Li, Na, K, Rb, Cs, Fr (+1)
• Group 2: Be, Mg, Ca, Sr, Ba, Ra (+2)
• Transition Metals:
  • Cr (+2, +3, +6); Fe (+2, +3); Co (+2, +3)
  • Mn (+2, +3, +4, +6, +7); Ni (+2, +3)
  • Pd (+2, +4); Pt (+2, +4); Cu (+1, +2); Ag (+1)
  • Au (+1, +3); Zn (+2); Cd (+2); Hg (+1, +2)
• Group 13: B, Al, Ga, In, Tl (+3)
• Group 14: C (-4, +2, +4); Si, Sn, Pb (+2, +4); Ge (+4)
• Group 15: N, P, As, Sb (-3, +3, +5)
• Group 16: O (-2); S, Se, Te (-2, +2, +4, +6)
• Group 17: F (-1); Cl, Br, I (±1, +3, +5, +7)

Nomenclature Suffixes

• Hypo-ous: Group 17 (1), Group 16 (2)
• -ous: Group 17 (3), Group 16 (4), Group 15 (3), Group 14 (2)
• -ic: Group 17 (5), Group 16 (6), Group 15 (5), Group 14 (4), Group 13 (3)
• Per-ic: Group 17 (7)
• With oxygen, use only -ic/-ate and -ous/-ite.