Cinétique de réaction Flashcards
Define rate of reaction
Generally defined as change in conc. of rxts or pdts per unit time
Define rate equation
relates rate of rxn to conc of rxts raised to appropriate powers
Define order of reaction
order of reaction w.r.t. rxt is power to which conc of that rxt is raised in rate eq n
Define overall order of reaction
sum of powers to which conc. of rxts r raised in rate eqn
Define rate constant
rate constant, k, is proportionality constant in rate eqn
- its units depend on overall order of rxn
- its value depends on temp, Ea
(when temp increase, k increase; when Ea decrease, k increase -> when k increase, rate of rxn increase)
Define zero-order reaction. What are its characteristics
a rxn is said to b zero-order w.r.t. reactant A if rate of rxn is independent of [A]
- shows horizontal straight line in rate vs [A] graph
- shows -ve gradient straight line in [A] vs time graph
Define first order reaction. What are characteristics of such rxn?
a rxn is said to b first-order w.r.t. reactant A if rate of rxn is directly proportional to [A]
- shows +ve gradient in rate vs [A] graph
- shows half smile parabola in [A] vs time graph
-> constant half-life, t1/2
- shows sad face curve in [pdt] vs time graph
-> if x is final conc, [pdt] increase from 0 to x/2, x/2 to 3x/4, 3x/4 to 7x/8, & so on, taking same time each step => can deduce half life
Define half-life of reaction, t1/2
time taken for conc of rxt to decrease to half its og value
- for 1st order rxn, t1/2=(ln2)/k
What’s a formula to find number of half-lives in a first order rxn after time t?
[A]t = [A]0 x (0.5)^n
where
n is no of half lives,
[A]0 is initial conc of A,
[A]t is conc of A at time t
Define second order reaction. What are some of its characteristics?
a rxn is said to b second order w.r.t. reactant A if rate of rxn is directly proportional to [A]²
- quadratic graph in rate vs [A] graph
- straight line in rate vs [A]² graph
- half smile curve in [A] vs time graph
-> half life not constant
NOTE: when half-life not constant, oni proves not 1st order rxn; 2nd order rxn not oni order w no uniform half-life
Elaborate on pseudo 1st order rxn
Consider rxn in which rate = k[A][B]
if [A] esentially constant, then rate = k’[B], where k’=k[A].
this occurs:
- when A present in large excess/high conc such that its conc hardly changes during rxn
- if A is catalyst
-> this is pseudo first-order rxn w.r.t. B
Graph of [B] vs time will indicate constant half-life t1/2
=> thus can use t1/2 = (ln2)/k’ and oso find true k if [A] is known
What are the types of experiments performed to find order of rxn?
- Discontinuous measurement
where rate-conc r/s is established
OR - Continuous measurement
where conc-time r/s is established
-> Rate of rxn can b found by monitoring conc changes using either chemical or physical properties
What are the methods to deduce order of rxn?
- Initial rates method (discontinuous measurement) by:
a. inspection/calculation based on table of data
b. determination & comparison of gradient of graphs - Half-life method (continuous measurement)
by determine half-lives of conc-time graphs
What are 2 methods to find order or rxn? Briefly describe the steps
- Calculation
rate 1 k[A]^x[B]^y…
______ = ________________________
rate 2 k[A]^z[B]^w…
->And solve for value of order using log
- Inspection
eg Compare expt 1 and 2
when [A] x 2 while keeping [C] and [D] constant,
rate of reaction x 4
Thus, order of rxn w.r.t. A = 2
*NOTE: If time is given, know that rate is inversely proportional to time (ie when time x 2, rate x 0.5)
If qn asks u to use an equation (eg ideal gas eqn) to explain how two variables (eg partial pa of gas and concentration) are related, what do u do?
- group constants and separate
- variables to each side of eqn
pV=nRT
p=(n/V)RT
p = c (RT), where c is concentration in mol dm^-3
p=kc, where k is a constant
OR
p ∝c
-> Thus, at constant temperature T, partial pa of gas, p, is directly proportional to its conc.
What is rate-determining step?
Slowest step in a rxn mechanism
Rate eqn is based on what in an elementary rxn and non-elementary rxn?
Elementary rxn:
based on stoichiometric eqn (rxn in one step)
Non-elementary rxn:
based on slow/rate-determining step
Order of rxn w.r.t. rxts is reflected by what in an elementary rxn and non-elementary rxn?
elementary:
stoichiometric ratio in stoichiometric eqn
eg
mA+nB->…
rate eqn: Rate=k[A]^m[B]^n
non-elementary:
stoichiometric ratio in slow/rate-determining step
eg
slow step: xA+yB ->…
rate=k[A]^x[B]^y
*note, intermediates (rxt not in overall eqn) shd not appear in rate eqn
eg Rate = k[A][B], but A is intermediate. Based on step 1, [A] ∝[C]²
Thus, rate = k[C]²[B]
Explain collision theory
States that chemical rxn oni occur if there are effective collis n, which occur when rxt particles:
- collide w energy>= Ea
- collide w correct orientat n
Rate of rxn increases w increase in frequency of effective collis n
Define activation energy (Ea)
min. amt. energy molecular collis n must possess for a chemical rxn to occur
What is transition state?
The pt where rxt r abt to change to products. It has highest energy. (ie peak of energy vs rxn pathway graph) Species present at this pt known as activated complex
Slow step has … Ea
largest
What are the factors affecting rate of rxn?
- Concentration
- Temperature
- Presence of catalyst
- Pa, surface area and presence of light
How does conc. affect rate of rxn?
When conc increase,
- no of rxt particles per unit volume increase
- frequency of effective collis n increase
Thus, rate of rxn increase
