Nuclear and Kinetics (E3) Flashcards
catalyst
a substance that speeds up the rate of a chemical reaction but is not consumed during the course of the reaction
Where does a catalyst appear in a reaction mechanism? Is it a reactant, a product, or neither?
a catalyst will appear in the steps of a reaction mechanism, but not in the overall chemical reaction (it is not a reactant or product)
How does a catalyst speed up a reaction? What is the effect?
generally, alters the mechanism of the reaction in a substantial way so that the new barriers along the reaction coordinate are significantly lower. If there are multiple barriers, then the catalyst will lower the highest barrier significantly. Remember the rate of the reaction may only depend on the highest energy transition state (peak of barrier), so only the highest barrier really matters - at least initially? Also, remember barrier = activation energy
homogeneous catalysts
catalysts in the same phase as the chemical reactants
heterogeneous catalysts
catalysts and the chemical reactants are in different phases
What does it mean by “catalysts are selective?”
a catalyst doesn’t speed up all reactions, only a very particular reaction
How do catalysts work?
Catalysts are somewhat reactive, which allows catalysts to react with reactant molecules. Thus the reactant molecules can break bonds and form weak, temporary bonds with the catalyst. We don’t want catalysts to be too reactive because we don’t want permanent bonds to be formed.
Catalysts can also “hold” molecules in particular configurations while simultaneously weakening particular bonds. This allows the catalyst to essentially “help” the chemistry by arranging the reactants in favorable geometries while weakening bonds that need to break along the reaction coordinate.
Does a catalyst go through chemical changes during a reaction? What is the state of a catalyst at the end of a reaction? Is its final state different from its initial state?
A catalyst may go through chemical changes during the course of a chemical reaction, but it will always return to its original state. e.g. MnO2 catalyzes the decomposition of H2O2. MnO2 starts as a solid, separates into Mn2+ (aq) and O2 (g) and forms Mn(OH)2 during the steps of a mechanism, but at the final step, it returns to MnO2 (s).
How can a catalyst be recognized in a mechanism?
It will be in the same state when it is a reactant in the first step and a product in the final step, but it will not appear in the overall reaction. You must look at the elementary steps in the mechanism (check the first for reactant and last for products for the existence of a catalyst).
enzymes
biological catalysts - proteins that fold into particular conformations so that they can help speed up very particular chemical reactions
What particular reactant do enzymes with react to/bind to?
substrates
active site
physical location on the enzyme in which the substrate binds to
Write the generic reaction of a substrate with an enzyme. What does the reaction mean/imply?
substrate + enzyme complex —> product
the complex formed from the substrate and enzyme can be separated back to the original substrate and enzyme (substrate + enzyme complex is reversible), but once the complex becomes a product, the reaction is unlikely reversible
inhibitor
molecules that block the active site of the enzyme. these molecules mimic a substrate but don’t do any chemistry.
Are enzymes selective?
Yes, they can be very specific so that an enzyme will only catalyze a very specific reaction for a very specific molecule.
What controls the temperature dependence of the rate constant?
size of the energy barrier
How can you speed up the rate of a reaction?
increase temperature - increases the number of molecules with energy to overcome the barrier
lower energy barrier - introduce a catalyst
What is the most practical method of increasing the rate of a reaction? Why?
Introducing a catalyst to lower the energy barrier because increasing temperature could cause decomposition and some reactions would require maintaining a very high temperature in order to overcome the barrier, which would be impractical.
reaction coordinate
path that links reactant molecules and product molecules
What do the minima on the reaction coordinate represent?
intermediates
each step in a mechanism will have a ____ ____
transition state
You can identify a transition state on the reaction coordinate because…
It will have higher energy than either reactants or products and it will be between the minima. (It’s the peak of the energy barrier?)
activation energy
height of the barrier along the reaction path; measured in kJ/mol; the minimum kinetic energy that particles must possess for a chemical reaction to occur.
transition state theory attempts to explain…
reaction rates in terms of energetic differences between ground states and transition states
rate determining/limiting step
largest barrier on the reaction coordinate; simply put (not perfectly correct though): the rate of the overall reaction is determined by the rate of the slowest step in the mechanism (and preceding steps?)
transition state
highest energy between reactants/products for a single step reaction; top of the hump
transition state is also known as..
activated complex
what are some qualities of the transition state/activated complex?
- high energy species
- not stable
- very reactive
What is Arrhenius theory based upon?
temperature dependence of the rate constant (for kinetics)
What are the four factors that affect reaction rates? How are they related to reaction rates? i.e. directly/inversely proportional
- temperature, directly
- add catalyst, increase rate
- concentration (pressure), directly
- nature of reactants - how “chunky” is the mix. e.g. tiny shreds of wood very flammable, huge log w/ tiny fire nearly impossible to light;
aka surface area, directly proportional
Unlike thermodynamics, kinetics covers…
the “in between” and time factor. thermo was all about state functions. you didn’t care how you got the product. kinetics, you do care; hence the mechanism w/ all of its elementary steps and intermediates
alpha particle
fast moving He2+ nucleus (4 neutrons 2 protons)
beta particle
fast moving electron
gamma radiation
pure energy - true EM radiation, has no mass; photon
in nuclear reactions, you want to balance what on the reactants and products sides?
neutrons and protons
isotope
specific for each element; same number of protons but different number of neutrons
nuclear chemistry
changes in the nucleus of the atoms
- identity of element changes
- lots and lots of energy
nuclide
specific nucleus with a specific number of protons and number of neutrons
positron
anti-electron, beta w/ a plus, has no mass like an electron but has a +1 charge
nucleons
protons and/or neutrons
beta decay is commonly used for…
carbon dating
what is the most stable nuclei?
Fe (mass # = A = 56)
list the three nuclear particles starting from least penetration power to most penetration power
alpha, beta, gamma
the rate constant is proportional to…
how fast the reaction goes
How do you determine the rate law for…
a) an elementary step
b) an overall reaction
a) stoichiometry determines its rate law
b) must compare with empirical data
mechanism
steps involved in a chemical reaction; typically written as a series of elementary steps
unimolecular
one molecule reacting by itself
bimolecular
two molecules reacting
an example of a unimolecular reaction is
radioactive decay
intermediate
a chemical species involved in a mechanism that does not appear in the overall reaction; typically both a product and reactant in different elementary steps
