Reaction rates:
Interpreting energy profiles for reactions and imp pts
Transition state-peak of energy profile
Ea for reactants- energy from reactants starting pt to peak=t.s
Delta H- change in energy of reactants and products
Ea for products-energy from product starting pt to peak=t.s.
Ea= activation energy- energy needed for rxn to occur
Delta H- exothermic rxn- negative delta H, endothermic rxn-positive delta H
Delta H-product H- reactant H
• Low Ea, High T → large k → faster reaction. (T-temperature)
We determine an instantaneous rate at time t:
• by calculating the negative of the slope of the curve of concentration of a reactant
versus time at time t.
•
• by calculating the slope of the curve of concentration of a product versus time at
time t
Expressions for rate of reaction- decomposition/formation
Rate of rxn= rate of disappearance of reactant
For example A product
Rate= d[A]/dt change in the concentration of A over the time of the reaction
Rate= -k[A] (negative because its disappearing, the conc is decreasing)
* the rate law is determined experimentally*
• Rate = -ΔReactant/ΔTime = how fast a reactant disappears.
Rate = ΔProduct/ΔTime = how fast a product forms.
Balanced chemical eqns from the rates
using the equation
aA+bB -> cC +dD
where the lower case letters ( a) represent the coefficient of the balanced equation
and the upper case letters (A) represent the molecule.
It is seen that the Rate of Disappearance are: -∆[A]/∆t*1/a=-∆[B]/∆t*1/b
and the Rate of Formation are: ∆[C]/∆t*1/c=∆[D]/∆t*1/d
Since Rate of Disappearance and Rate of Formation are equal to one another
-∆[A]/∆t*1/a=-∆[B]/∆t*1/b=∆[C]/∆t*1/c=∆[D]/∆t*1/d
Calculate avg rxn rates
Determine order of reaction
Zero order reaction- rate=k-constant, independent of concentration
in a first-order reaction the rate is proportional to one concentration; in a secondorder reaction it is proportional to the product of two concentrations or to the
square of one concentration; and so on.
Reaction order of each species if identical with the stoichiometric coefficient of that
species
Reaction order = sum of all exponents of the concentration variables in the rate law.
Reaction
Reaction Type
Rate Law(s)
Order
Unimolecular
1
r = k[A]
Bimolecular
2
r = k[A]2, r = k[A][B]
Termolecular
3
r = k[A]3, r = k[A]2[B], r = k[A][B][C]
Zero order
0
r=k
reaction
Determination of rate law- calculate k and its units
They should give you experimental data to fill this out
1st look at chart and see what happens when a certain reactant’s concentration is
doubled etc.
2-find rate law from that ex r=k[A][B]^2
3-plug in one trial in the data given to above eqn to solve for k
4-determine units by also plugging in units and canceling