w2 Flashcards
Arrhenius Equation relates to
energy, collision frequency, temperature, orientation
Arrhenius Equation
k = rate constant = Ae ^-Ea/RT
Frequency factor (units L mol -1 s-1)
- combines collision frequency and probability of correct orientation
- specific to each reaction
- temp dependent
fraction of molecules with minimum energy for reaction
always less than 1, changes significantly with temp
Effect of catalysts on RR
provides a mechanistic pathway with a lower activation energy, can be regenerated, very effective, specific, accelerates reactions.
Homogeneous catalysis: interconversion of but-2-ene without catalysts. Ea: 264 kJ mol-1
- large activation energy
- because C=C must be broken
- low frequency
- requires high temps
mechanism of iodine-catalysed isomerization of cis-but-2-ene (learn the circle)
- converted to trans-but-2-ene with iodine
- emphasizes the catalytic cycle
- rate = [cis-C4H8][I2]^1/2
heterogeneous catalysis: ethylene hydrogenation
- H2 molecules collide with platinum surface and form Pt-H bones, weakening H-H bones and leaves H bound to surface
- C2C4 also attaches to surface through bones from each C atom to a Pt by breaking the C=C bond in each molecule
- Rxn with adjacent H atoms first leads to formation of C2H5 and then makes ethene C2H6
- New C-H bonds are formed as Pt-C bonds are weakened and C2H6 molecules leaves the surface
- In their place, other C2H4
and H2 molecules can bond to the surface and undergo reaction.
Rxn mechanisms
- sequence of bond-making
and bond-breaking steps
that occurs during the
conversion of reactants to
products. - can’t be proved but shown through experiments
sequence of steps: rxn of Br2 (g) and NO (g)
Nitric oxide is a free radical, i.e., it has an unpaired electron. It is sometimes
denoted by a dot in its formula, i.e., ·NO. (Ozone depletion). - unlikely to have a singe step reaction
Br2 molecules and NO molecules combine to produce molecules of an
intermediate species, Br2NO.
Br2NO then reacts with another NO molecule to form two molecules of the
reaction product.
Elementary steps
Each step.
The equation for an elementary step describes particular collisions:
1. which particles collide,
2. which bonds are broken or formed,
3. and which particles are formed.
Molecularity of an elementary step
the number of molecules, ions, or atoms that collide and undergo change.
– A unimolecular step involves reaction of only one particle,
– a bimolecular step involves two particles,
– termolecularstep involves three particles (atoms, molecules or ions).
Rate equations for elementary steps
Determined directly from stoichiometric equations
1. A -> product, unimolecular, Rate = k[A]
2. A + B -> product, bimolecular, Rate = k[A][B]
3. A+A -> product, bimolecular, rate = k[A]^2
4. 2A + B ->product, termolecular, rate = k[A]^2[B]
not expected to have the same value
What controls the rate at which products are produced
the slowest elementary step (rate limiting step
Nucleophilic substitution rxns
involves the substitution of one electron-rich nucleophile (such as a chloride or a hydroxide ion) in molecules of a substance (referred to as the substrate) for an atom, group of atoms, or an ion, called the leaving group.
A good nucleophile…
needs to be able to donate an electron pair to an electron deficient (electrophilic) site on the substrate. The leaving group must accept an electron pair.
