Energy in a Reaction (cont.) (p.17)

Review Draw a graph of enthalpy vs. reaction proceeds for this rxn: R P + 60 kJ

Kinetic Energy Distributions (p.17) The rxn: C 2 H 5 OH C 2 H 4 + H 2 O doesnt really react at all at room temperature and very slowly at 200°C. Why is the rate so low at room temp.? –At room temperature and pressure molecules undergo about collisions per second so lack of collisions is not the factor. Why dont all molecules react once you reach a temp where some of the molecules can react? –Not all molecules in a system have the same kinetic energy

This is a normal KE distribution. Kinetic Energy Distributions (p.18t)

Temp and KE Relation (p.18m) As temp is increased, the average KE of the molecules is increased. At a higher temperature, more molecules have enough KE to pass the minimum energy required for a successful collision The main reason for increase in rate is b/c of HARDER COLLISIONS, not more frequent collisions.

Rule of Thumb (p.18b) In general a 10°C increase in temp results in a doubling of the rate (for a slow rxn). Compare the area under curves A and B. What would a temp change from 40°C to 60°C do to a slow rxns rate?

Activation Energies (p.20t) Since molecules have to overcome an energy requirement we can show the PE diagram as follows: The rxn occurs at point A. Molecules w/o enough energy to react, bounce off each other (dont get to the top and fall back down the hill).

What happens to molecules as they approach each other? –They slow down b/c their e- clouds repel. What happens to their KE and PE? –KE energy goes down (less speed) but PE goes up (energy stored in repulsive force) Activation Energies (p.20t)

Definitions (p.21t) The activated complex is the arrangement of atoms which occurs when the reactants are in the process of rearranging into products. Activation energy is the minimum PE required to change the reactants into the activated complex.

Possibilities (p.21m) When molecules approach each other there are 3 possible scenarios: –The KE is less than the required PE The molecules will bounce off each other (ineffective collision) –The KE is equal to the required PE The molecules have the potential to react. –The KE is more than enough to match the required PE A reaction will probably happen. A collision is effective if it results in a rxn.

Comparison of KE and PE (p.22t) Notice that as KE goes down, PE goes up and vice versa. The total energy in the system remains constant.

Geometry Considerations (p.22m) Two things are required for a successful collision: –Sufficient KE –Correct alignment (geometry) Example of correct alignment

Activation Energies (p.24) Some rxns can go both forward and backward (this is a major concept in Chem 12) which makes sense looking at a PE diagram. Note the forward activation energy, E a (f) and the reverse activation energy, E a (r). Cross off bottom third of p.24 and top half of p.25 but highlight Suggestion on p.25.

Homework Hebden #29, 32, 34-37, 39, 40, 43, 44. Quiz next class