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.