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Enthalpy Changes in Chemical Reactions #4

Enthalpy Changes in Chemical Reactions #4. Enthalpy. Enthalpy (H) The “heat content” of a substance It is the total KE and PE of a substance at constant pressure KE + PE = constant (enthalpy). Kinetic & Potential Energy. KE: it is the energy of MOTION …

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Enthalpy Changes in Chemical Reactions #4

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  1. Enthalpy Changes in Chemical Reactions #4

  2. Enthalpy Enthalpy (H) • The “heat content” of a substance • It is the total KE and PE of a substance at constant pressure • KE + PE = constant (enthalpy)

  3. Kinetic & Potential Energy • KE: it is the energy of MOTION… ex; atoms and electrons moving within the molecules, or in the entire system (reaction) • PE: energy existing due to position and the sum of all attractions/repulsions between particles. PE is directly related to BOND energies.

  4. Bond energy • Bond energy is the amount of energy required to break/form a bond between two atoms. Ex:

  5. Enthalpy changes • Chemists interested in enthalpy changes (H ) • H = H products- H reactants

  6. Enthalpy Vs Rxn Proceeding

  7. Exothermic Reactions • H2 + S ---> H2S H = - 20 KJ • negative H means exothermic • H2 + S ---> H2S +

  8. Endothermic Reactions • CH3OH C(s) + 2H2(g) + ½ O2(g) H = + 201 KJ • positive H means endothermic • CH3OH +  C(s) + 2H2(g) + ½ O2(g)

  9. Provincial Exam Questions

  10. Kinetic Energy Distributions • In general, molecules at R.T and pressure undergo about 10^10 collisions/second! Yet, there are only so many successful collisions…

  11. KE Vs Number of Molecules

  12. KE Distributions • Increased Temperature= increased number of molecules that have enough energy to react/cross the activation energy barrier successfully. Yes, collisions would also increase, but only 1% more collisions for every 10C increase in Temp.

  13. Activation Energy • The minimum potential energy needed in a collision before a reaction can take place. • It can also be defined as the minimum energy colliding particles must have in order to have a “successful” collision.

  14. Activation Energy Graph

  15. At the start of a rxn, most energy is stored as kinetic energy. • As molecules approach each other, electrons on the outer shells start to repel one another; slowing down and causing an increase in PE

  16. If enough PE is gained, bonds can be broken/made and we form an activated complex. • After the rxn, the new products start repelling each other, moving away, increasing speed and KE, therefore decreasing PE.

  17. Try solving the questions on those pages p.12 p.16 p.19 and p.20

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