Heat of Reaction, Reaction Rates, & LeChatelier's Principle

Heat of Reaction, Reaction Rates, & LeChatelier's Principle

Enthalpy of Reaction

  • Definition: Enthalpy change (denoted as ( \Delta H )) represents the heat content of a system at constant pressure.
  • Endothermic Reactions:
    • Occur when ( \Delta H > 0 ) (reaction absorbs heat).
    • Thermochemical equation example: ( A + B + ext{heat} \rightarrow C ) ( \Delta H = +x \, kJ )
  • Exothermic Reactions:
    • Occur when ( \Delta H < 0 ) (reaction releases heat).
    • Thermochemical equation example: ( A + B \rightarrow C + ext{heat} ) ( \Delta H = -x \, kJ )

Potential Energy Graph

  • Components of the Graph:
    • Activation Energy (Ea): Energy required to start a reaction.
    • Identified as the peak of the graph before the reactants convert to products.
    • Bonds Broken: Energy is required to break the bonds of reactants.
    • Bonds Formed: Energy is released when new bonds are formed in products.
    • ( \Delta H ): The difference in energy between products and reactants; represents the overall energy change during the reaction.
Example of Potential Energy Graph:
  • The graph should illustrate:
    • The initial energy level of reactants,
    • The peak energy (activation energy), and
    • The final energy level of products, indicating whether it is higher (endothermic) or lower (exothermic).

Collision Theory

  • Definition: A theory that explains how chemical reactions occur and why reaction rates differ.
  • Important Points:
    • Molecular Collisions: For a reaction to occur, reactant particles must collide with enough energy and proper orientation.
    • Factors Affecting Reaction Rate:
    • Concentration: Increased concentration results in more collisions, speeding up reaction rate.
    • Temperature: Higher temperatures lead to higher energy collisions, increasing reaction rate.
    • Surface Area: More surface area allows more collisions to occur, speeding up the reaction.

LeChatelier's Principle

  • Definition: States that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change.
Stress Factors and Their Effects:

  1. Concentration Changes:

    • Increasing the concentration of reactants shifts the equilibrium to the right, producing more products.
    • Increasing the concentration of products shifts the equilibrium to the left, producing more reactants.

  2. Temperature Changes:

    • Increasing temperature shifts the equilibrium in the direction that absorbs heat (endothermic reaction).
    • Decreasing temperature shifts the equilibrium in the direction that releases heat (exothermic reaction).

  3. Pressure Changes (for gas reactions):

    • Increasing pressure will shift the equilibrium towards the side with fewer moles of gas.
    • Decreasing pressure will shift it towards the side with more moles of gas.

Conclusion

  • Understanding these concepts allows us to predict how various factors affect the rates of reactions and the position of chemical equilibria.