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