Chemical Reactions and Thermodynamics Notes

Four Ways to Speed Up a Chemical Reaction

• Increase Temperature: Elevating the temperature increases the kinetic energy of the molecules, leading to more frequent and effective collisions.

• Increase Surface Area: When solids are involved, breaking them into smaller pieces increases the surface area available for reaction, enhancing the rate of reaction.

• Increase Concentration: Increasing the concentration of the reactants increases the number of particles in a given volume, leading to a higher probability of collision between reactants.

• Add a Catalyst: Catalysts, such as enzymes, lower the activation energy required for a reaction to occur, facilitating a faster reaction without being consumed in the process.

Reaction Progress and Activation Energy

• Activation Energy: The minimum energy required for a chemical reaction to occur. An enzyme lowers this energy, allowing reactions to proceed more easily.

• Diagrams may illustrate the energy changes throughout the progress of a reaction, showing activation energy and the role of enzymes in reducing this threshold.

Effects of Pressure on Chemical Reactions

• Increase Pressure: Changing the pressure of a reaction system affects equilibrium; increasing pressure causes the reaction to shift toward the side with fewer moles of gas.

  • Example:

    • Reaction: 2A + B 2C + 3D

    • Result: Reaction shifts to the left (toward reactants) due to fewer moles on the left side (3 vs. 5).

• Decrease Pressure: Conversely, decreasing pressure shifts the equilibrium toward the side with more moles of gas.

  • Example:

    • Reaction: 2A + B
      ightleftharpoons 2C + 3D

    • Result: Reaction shifts to the right (toward products) due to more moles on the right side (5 vs. 3).

Thermodynamics of Reactions

• Exothermic Reactions: Reactions that release heat to the surroundings.

  • Example: Combustion, as in fire, where heat is a by-product.

• Endothermic Reactions: Reactions that absorb heat from their surroundings.

  • Example: Use of cold packs, which absorb heat and feel cold.

• Heat Transfer: Heat energy always flows from hot regions to cold regions, emphasizing the second law of thermodynamics.