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: C2H5OH C2H4 + H2O doesn’t

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 1010 collisions per second so lack of collisions is not the factor.

• Why don’t 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 rxn’s 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 (don’t 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, Ea(f) and the reverse activation energy, Ea(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