Comprehension Questions
(Include sketches and undertake any numerical calculations that assist in your discussion. All questions are worth 16 points).
Attempt all questions.
Question 1
Consider a reaction that takes place at 298 K in which two Cl atoms (radius 0.15 nm) approach each other with the possibility of forming a Cl2 molecule. Note a Cl-Cl bond has a strength of 6 x 10-19 J. Discuss the factors that affect the outcome of this interaction. Include the following in your discussion:
(i) How the potential energy, kinetic energy, and total energy change along the trajectory as the atoms approach starting from a large separation to the point where they collide.
(ii) Discuss the different possible outcomes of this collision process and the factors that determine these outcomes. Please be very specific.
(iii) Is there an energy barrier to the formation of Cl2. If not, why?
(iv) How it is possible to overcome a barrier to reaction (whether present in this case or not). Please discuss in terms of your response and any sketches used in (i) above.
Question 2
Archaeological records clearly show the sudden emergence and then the dominance of hot-blooded mammals over cold-blooded reptiles. Discuss the implications of the switch from cold to hot blooded species from the standpoint of thermodynamics.
How might this have provided an evolutionary advantage to mammals. Are there disadvantages? Are there any implications for extra-terrestrial life?
Question 3
(i) What do you understand by the concept of state function in thermodynamics?
Give examples of quantities that are state functions and contrast their properties with related thermodynamic quantities that are not state functions.
(ii) Distinguish between reversible and non-reversible (irreversible) processes in thermodynamics, giving examples in each case.
(iii) Consider the case of entropy S for which changes ΔS during a thermodynamic process can be expressed in two different ways (i) heat adsorbed, and (ii) changes in the number of available microstates. With the help of examples, discuss the applicability of the descriptions (i) and (ii) to both reversible and non- reversible thermodynamic processes.
(iv) Can you think of an example of an irreversible process that can be modelled as a series of reversible processes. Explain why this might be helpful in describing thermodynamic processes.
Question 4
Consider a liquid droplet in air that rests at the bottom of a cylinder that is enclosed by a frictionless piston.
In stage 1, the piston containing the droplet is placed in a constant heat reservoir (temperature T1) and the size of the drop is observed to decrease while the piston moves upwards.
In stage 2, the piston stops moving when it reaches a height H at which point a small quantity of the liquid droplet remains.
In stage 3, the temperature of the reservoir is changed to T2 and the piston moves downwards so that its final height is ½H.
Discuss these observations from a thermodynamic perspective. Identify the system and the surroundings. Indicate the sign of the changes (+/-) of relevant system quantities (q, w, U, H, G, S etc) during stages 1, 2 and 3. What can you say about the reservoir temperatures T1 and T2 - how they compare to each other and the temperature of the ambient in stage 1 prior to placing the cylinder in the heat reservoir?
Question 5 (carefully read the entire question before you start)
(i) Describe in detail how you would go about setting up experiments to determine the rate law for a reaction that involve two reactants, which can be any molecules of your choosing.
(ii) Demonstrate your approach by providing a numerical example, showing concentrations and reaction rates as appropriate. (Yes, I want a table with real numbers and correct units!!)
(iii) Determine the rate law and the order of the reaction. What is the rate constant? (please specify the units).
(iv) Write down a reaction mechanism for your reaction that includes a rate determining step and the presence of a single reaction intermediate. Show that the mechanism you propose is consistent with the overall stoichiometry of the reaction in (i).
(v) Show the mechanism you propose is consistent with the rate law you specified in (iii). What is the molecularity of the rate determining step. Express the observed rate constant in term of the rate constants associated with individual steps in your mechanism.
(vi) What experiments and analyses would you perform. to determine the activation energy for this reaction. For your reaction explain how the measured activation energy depends on the different reaction steps in your mechanism.