Enthalpy Changes in Chemical Reactions Lesson #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… 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.

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

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

Enthalpy Vs Rxn Proceeding

Exothermic Reactions H 2 + S ---> H 2 S  H = - 20 KJ negative  H means exothermic H 2 + S ---> H 2 S +

Endothermic Reactions CH 3 OH  C(s) + 2H 2 (g) + ½ O 2 (g)  H = KJ positive  H means endothermic CH 3 OH +  C(s) + 2H 2 (g) + ½ O 2 (g)

Provincial Exam Questions

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…

KE Vs Number of Molecules

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.

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.

Activation Energy Graph

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

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.

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