EL

Exam Notes

Adding Half Equations for Redox Reactions

  • Concept is similar to adding half equations, half reactions for redox, and adding equations together when doing equilibria.
  • Anything on the same side adds up; anything on opposite sides cancels out (like canceling electrons in redox reactions).
  • The final reaction is provided as the target.
  • Reactions may need to be flipped to align with the target reaction.

Exothermic and Endothermic Reactions

  • Exothermic reaction in the forward direction implies the reverse direction is endothermic.
  • Exothermic (exo) has a negative \Delta H, while endothermic (endo) becomes positive when reversed.
  • Flipping the reaction changes the sign of \Delta H (positive becomes negative or vice versa).
  • Based upon the equation: Products - Reactants = \Delta H
    • If Products - Reactants is negative, products are smaller than reactants.
    • Flipping it makes Reactants - Products, resulting in a positive value.
  • Relates to equilibrium: products over reactants for K value, which is inverse of K when equation flips.

Coefficients

  • Coefficients in this context do not become exponents (unlike equilibrium calculations).
  • Coefficients are multiplied by the \Delta H value. If you double everything in a reaction, then you double the \Delta H value (energy for the reaction).
  • Doubling coefficients multiplies the heaps of formation, effectively doubling the number.
  • Focus on multiply and divide rather than square and root.

Flipping Equations

  • When flipping an equation, change the sign of \Delta H.

Example 1: Simple Reaction

  • The C2H6 needs to be on the right side, so flip the second reaction.
  • Flipping makes \Delta H positive: +1560 instead of -1560.
  • Calculate the final \Delta H by adding the individual \Delta H values together.

Example 2: More Complex Reaction

  • Check if components are on the correct sides first, then adjust numbers.
  • Flip the second equation to get NH3 on the left side, making \Delta H +91.8.
  • Multiply the first equation by two, doubling its \Delta H (to 361).
  • Multiply the bottom equation by three to get six waters.
  • Cancel out components (2 and 2, 6H2 and 6H2, etc).
  • Add up the three modified \Delta H values to get the final answer.

Hess's Law Problems

  • Hess' Law problems and thermochemical equations problems involve:
    • Calculating energy production based on grams of substance (like problem #7).

Textbook Problem 16

  • Given: one gram releases a certain number of kilojoules.
  • Given: 7.4 grams of ethanol.
  • Have conversion factor between grams of ethanol and kilojoules; solve for it like here (48.04).
    248 o four
  • Know the delta t of both the water and the calorimeter.
  • Calorimeter of the bomb hole is C.