Question

You are studying the kinetics of the reaction: (this is due tomorrow!!)

               H₂(g) + F₂(g) → 2HF(g)

You are attempting to determine a mechanism for the reaction. You run 2 separate reaction trials keeping one reactant at a much higher pressure than the other reactant. The lower pressure reactant begins at 1.000 atm. OOPS! You forget to record which reactant was at the higher pressure. The data you collected for the first trial are:

Pressure of HF(atm)       Time(min)

0               0

0.300               30.0

0.600               65.8  

0.900               110.4

1.200               169.1

1.500               255.9

When you ran the second trial( in which the higher pressure reactant was much higher than the first trial) you determine the that the value of the pseudo-rate constants are equal. Another student in your lab is studying the same reaction. She has found that the reaction is 40 times faster at 55^oC than at 35^oC. You also know from the energy level diagram that the mechanism has 3 steps and the first step has the highest activation energy. You look up the bond energies of the reactants and find H-H = 432 kJ/mol ; F-F = 154 kJ/mol; H-F = 565 kJ/mol

a- Sketch a qualitative energy level diagram for the mechanism

- Identify E_a , position of activated complexes and position of intermediates on your graph (Please solve for Ea, so its easier for me to understans which reaction is rate controlling)

b- Develop a reasonable mechanism for the reaction

c- Which reactant was limiting in the trials (aka the low pressure reactant) (please identify the actual reactant and show explanation, so I can understand)

Answer 1

1.

From the given bond enthalpies we can determine the enthalpy of reaction (exothermic/endothermic). Thus we can draw the energy level diagram.

Since the reaction is exothermic, energy of products is less than energy of reactants.

Also,it is given in the question that for energy level diagram the mechanism has 3 steps and the first step has the highest activation energy.

With this information let us draw the energy level diagram is shown above. (Note: the heights of the second and third humps must be lower than the first-step activation energy).

Since it is three step reaction, in each step there will be a intermediate (shown in green mark). The first intermediate is the activated complex, because it is mentioned in the question that it has the highest acivation energy.

Activation energy of the reaction can be determined when rate conatnts of reaction are given different reaction temperatures.

But we can say that rate determining step is the first step becuase it has the highest activation energy.

2.

over all reaction is

3. From the question it is clear that low pressure reactant is limiting and from mechanisam we can confirm that F₂ is the limiting reactant.

The slowest step in the reaction is called rate detemining step. Over all rate of the reaction depepnds on the rate determining step. So F₂ is the limiting reactant.