Enthalpy Changes in Chemical Reactions#4

EnthalpyEnthalpy (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

•H2 + S ---> H2S H = - 20 KJ •negative H means exothermic

•H2 + S ---> H2S +

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)

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