Chemistry Lecture Notes: Kinetics, Equilibrium, and Thermodynamics

REACTION RATES AND COLLISION THEORY

• Collision Theory Requirements: For a reaction to occur, particles must collide with:     * Sufficient energy (Activation Energy).     * Proper orientation/arrangement.
• Factors Affecting Rates:     * Temperature: Higher temperature increases particle speed and collision frequency.     * Pressure (Gases Only): Higher pressure increases collision frequency.     * Surface Area: Increasing surface area (e.g., breaking solids into pieces) speeds up reactions.     * Concentration: Higher concentration leads to more collisions and faster rates.     * Nature of Reactants: Stronger or more complex bonds result in slower reactions.     * Catalyst: A substance that increases the reaction rate by lowering the Activation Energy ($E_a$) without shifting the equilibrium.

EQUILIBRIUM AND LE CHATELIER'S PRINCIPLE

• Conditions for Equilibrium:     * Must be a closed system.     * The reaction must be reversible.     * Rates of the forward and reverse reactions are equal.     * Concentrations of reactants and products remain constant.
• Le Chatelier's Principle: A system at equilibrium responds to a disturbance to reduce the stress.     * Concentration: Adding a substance shifts the reaction away from that side; removing a substance shifts it toward that side.     * Temperature: Increasing temperature shifts the reaction away from the side containing heat energy.     * Pressure (Gases Only): Increasing pressure shifts the equilibrium toward the side with fewer moles of gas; decreasing pressure shifts it toward the side with more moles of gas.

ENTHALPY AND POTENTIAL ENERGY

• Enthalpy ($H$): The total heat content of a system.
• Heat of Reaction ($\Delta H$): Calculated as $P.E._{\text{products}} - P.E._{\text{reactants}}$.
• Exothermic Reactions:     * System releases heat ($\Delta H$ is negative).     * Energy of products is lower than reactants.
• Endothermic Reactions:     * System gains heat ($\Delta H$ is positive).     * Energy of products is higher than reactants.
• Activation Energy ($E_a$): The energy required to go from reactants to the top of the potential energy hill.

ENTROPY AND SPONTANEITY

• Entropy ($S$): A measure of the disorder or randomness of a system.
• Factors Increasing Entropy:     * Phase changes from solid to liquid to gas.     * Increasing temperature.     * Increasing the number of product molecules/moles.
• Spontaneous Reactions: These occur naturally and favor the production of products while releasing free energy.
• Natural Tendency: Systems in nature tend to move toward lower energy (enthalpy) and higher disorder (entropy).

ANALYSIS OF REACTION: $2NO(g) \rightleftharpoons N_2(g) + O_2(g) + 21.6\,kcal/mol$

• Temperature Increase: Shifts reaction to the left (favors reactants).
• Removing $N_2(g)$: Shifts reaction to the right (favors products).
• Pressure Increase: No shift occurs because there are equal moles of gas on both sides ($2$ moles left, $1+1=2$ moles right).
• Adding Catalyst: No shift in equilibrium; speeds up both rates equally.
• Increasing $O_2$ Concentration: Adding $NO(g)$ will shift the reaction to the right, causing an increase in $O_2$.