Unit 4: Comprehensive Summary

Chemical reactions involve changes in chemical potential energy, known as enthalpy (H). The overall heat exchanged at constant pressure is the enthalpy change ($\Delta H$).

• Exothermic reactions release heat, have a negative $\Delta H$, and the products are more stable than the reactants.
• Endothermic reactions absorb heat, have a positive $\Delta H$, and the products are less stable than the reactants.
• Molar enthalpy changes (e.g., of formation, combustion, neutralization) are standardized to one mole of a substance.
• Calorimetry is used to measure heat changes, based on the formula $q = mc\Delta T$.

The rate of a reaction is the change in concentration per unit time. Collision Theory states that for a reaction to occur, particles must collide with the correct orientation and with energy equal to or greater than the activation energy ($E_a$).

The rate of reaction is affected by several factors that influence the frequency of effective collisions:

• Concentration/Pressure: Higher concentration/pressure increases collision frequency.
• Temperature: Increases both collision frequency and, more significantly, the energy of collisions, so more particles overcome the $E_a$.
• Surface Area: Increasing the surface area of a solid reactant increases collision frequency.
• Catalyst: Provides an alternative pathway with a lower activation energy, increasing the rate of both forward and reverse reactions.

Reversible reactions can proceed in both forward and reverse directions. In a closed system, they reach dynamic equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction, and macroscopic properties are constant.

The position of equilibrium is described by the equilibrium constant ($K_c$), which relates the concentrations of products to reactants at equilibrium. A large $K_c$ means products are favored, while a small $K_c$ means reactants are favored.

Le Chatelier's Principle states that if a change is applied to a system at equilibrium, the system will shift to counteract the change. This allows prediction of the effects of changing concentration, pressure, and temperature.