first part Flashcards
types of bond breaking
heterolytic
homolytic
heterolytic bond breaking
when both electrons in a covalent bond go to one species.
2e- chemistry
ions form
homolytic bond breaking
when one electron goes to each species
1e- chemistry
radicals are formed
what is bond dissociation enthalpy
energy required to break a bond homolytically
the radical one
what does a high bde mean
its a strong bond
needs lots of energy to break it
poorly stabilised radicals
(lots of energy is needed bc the radicals dont want to be formed bc they will be poorly stabilised) 9hydrogen radical, methyl radical)
what does a low bde mean
little energy needed to break the bond homolytically
weak bond
well stabilised radicals ( tertiary structures, strong electrophilic radicals)
what is enthalpy change
heat given in or taken out by a reaction
we can estimate this using the bde values
breaking bonds require
energy
positive numbers
forming bonds dont require
energy
negative numbers
negative enthalpy value means the reaction is
exothermic
positive enthalpy value means the reaction is
endothermic
what is S
change in enthalpy
likes disorder
gas> solid
2 molecules>1 molecule.
higher S = higher entropy
snake in a tree
snakes like being in the leaves bc its more disordered than the trunk. larger s is more disordered as the leaves are higher off the ground than the bark
gibbs free energy equation
change in cG = cH -Tcs
kJmol-1 = kJmol-1 - K JK-1mol-1
what is an exergonic reaction
reaction where the products have less energy than the starting material
products are favoured at equilibrium
looks like exothermic reaction profile
what is an endergonic reaction
where the starting materials have lower energy than the products
starting materials are favoured at equilibrium
looks like endothermic reaction profile
in the endergonic and exergonic reaction profiles, what is the gibbs energy
height difference between the 2 lines of sm to p
if G has a negative value,
its an exergonic reaction
products are favoured
if G has a positive value,
its an endergonic reaction
sm are favoured
equilibrium constant equation
conc of products / conc of reactants
another change in gibbs equation kJmol-1
(- gas constant x temp) x ln(equilibrium constant)
what does the value of gibbs free energy tell us
if sm or p are favoured at equilibrium
doesnt tell us about the rate.
rate equation with differentials,, yk this girl!!
d(conc products)/d(time)
change in product conc/ change in time
what is a rate equation eg
k [a] [b]
[a] to the power of 1 means
first order
sn1
change of this is proportional to change in rate
[a] to the power of 2 means
second order
sn2
change of this is proportional to double the change in rate
what relates the rate constant and the activation energy
arrhenius equation
k = Ae ^ -Ea/Rt
what is at the top of an energy curve
local maxima peaks
transition states
higher in enery
less stable
what is a transition state
occurs during bond making / breaking
dotted arrow diagram
at local maxima peaks
higher in energy
cant be observed or isolated
transient species (not around for a long time)
what is on the slightly lower parts of an energy curve diagram
intermediates
occur at local energy minima
what is an intermediate
carbocation intermediate can be observed and isolated
lower in enery, more stable
bonds arent in the process of being formed or broken (no dotted lines but a + or -)
sn1
what does it mean to be a transient species
to not be around for a long time
how can a transition state be drawn
dotted line
square brackets
= sign with | in the middle in top right hand corner of the [ ]
what is the hammond postulate
shows if the transition state is closer to the starting materials or the products
hammond postulate for an exergonic reaction
Ts resembles starting materials as that line is closer to the local energy maxima peak.
hammond postulate for endergonic reactions
Ts resembles products as the line for products is closer to the local energy maxima
