Exam 3 Flashcards
(T/F) All molecules in a gas have the same speed.
False, they do NOT have the same speed.
What is the relationship between gamma and temperature?
inverse relationship
When does effusion occur?
when the diameter of the hole is smaller than the mean free path (lambda)
What is diffusion?
transport of matter due to concentration gradient
What is Fick’s first law? Write it out.
flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient; write out Dineli’s notes, bottom of page 4
What is flux?
amount of matter passing through unit area in unit time
What is thermal transport?
transport of energy down a temperature gradient
What is the energy of each molecule of a monatomic gas?
3/2kBT
What did Arrhenius say in 1889? (2)
Van’t Hoff equation for temperature dependence of the equilibrium constant is (dlnKc/dT) = (deltaH/RT^2); law of mass action relates the equilibrium constant to the ratio of the forward and backward rate constants kc = k1/k-1
What is the physical interpretation of the e^(-Ea/RT) term in the Arrhenius equation?
fraction of reactant with enough energy to react
What is the idea behind the Lindemann mechanism?
unimolecular reactions proceed because the reactant molecule is energized by a partner molecule in collision; concentration of energized reactant molecule is very small so invoke steady-state approximation
What are the k1/k-1/k2/k-2/k3/k-3 term names in chain reactions?
k1 = initiation, k-1 = termination, k2 = propagation, k-2 = inhibition, k3 = propagation, k-3 = inhibition
Define physisorption.
attractive forces between substrate and adsorbate arise from van der Waals interactions.
Define chemisorption.
adsorbate bound to surface by covalent or ionic forces
Define adsorption isotherm.
plot of surface coverage as a function of gas pressure at constant temperature
What happens in a temperature jump experiment?
temperature of the equilibrium reaction mixture is
suddenly changed at constant pressure; following change in temperature, chemical system responds by relaxing to a new equilibrium state that corresponds
to new temperature
What does the value tau in the temperature jump experiment represent?
amount of time it takes to decay to 1/e of initial value
How do we calculate tau, given k1, k-1, and the equilibrium concentrations of A and B at the new temperature?
write out Dineli’s Exam 4 Notes, Page 50
Write out the Arrhenius expression for a rate constant.
write out Dineli’s Exam 4 Notes, Page 51
What is the Transition State Theory? (2)
rate of reaction is the number of activated complexes passing per second over the top of the potential energy barrier; reactant is the activated complex (AB#) in equilibrium with each other (invoking steady state)
What are the three main differences between the Arrhenius theory and Transition State theory?
TST uses concept of free energy, but Arrhenius does not; arbitrary factor of A in Arrhenius expression (result of integration) but specific factor in TST; similar activation energy term in both expressions
Write out the chemical equations for the Lindemann mechanism.
write out Dineli’s Exam 4 Notes, Page 52
What is the general idea behind the steady state approximation?
the concentration of the energized molecule in the Lindemann mechanism is very small, and changes to it can be ignored
In the Langmuir adsorption model, what does theta represent?
the fraction of surface sites occupied by an absorbate
Write out the equation for theta in the Langmuir adsorption model. Identify each component.
write out Dineli’s Exam 4 Notes, Page 54
What is the primary difference between the Langmuir and BET models?
Langmuir — monolayer formation; BET — multilayer formation; BET tends to take longer to completely cover the surface since multilayers start to form before the monolayer is covered
