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
How does Maxwell-Boltzman Distribution curve look like when:
- temperature is changed?
- catalyst is used
*Note, curve always start fr origin
Temperature changed:
- two curves, one is lower, shift to right
Catalyst used:
- Same curve
- Catalyst portion bigger than no catalyst portion
How does temp affect rate of rxn?
When temp increase
- no of rxt particles w energy >= Ea increase
- frequency of effective collis n increase
- result in larger rate constant k
Thus, rate of rxn increase
How does addition of catalyst affect rate of rxn?
A catalyst provides an alternative rxn pathway of LOWER Ea. When it is used,
- no of rxt particles w energy >= Ea increases
- frequency of effective collis n increase
- result in larger rate constant k
Hence, rate of rxn increase
Define catalyst
A substance increasing rate of rxn by provide alternative rxn pathway of lower activation energy (Ea) while it remains chemically unchanged at end of rxn
What are the four types of catalysts?
- Autocatalyst
- Enzymatic catalyst
- Homogeneous catalyst
- Heterogeneous catalyst
Define autocatalyst
one of the pdts of chemical rxn formed that carries out its catalytic action as soon as it is formed in rxn
What are enzymes? What are its properties?
specialised globular proteins increasing rate of biological rxn by provide alternative rxn pathway of lower Ea
Properties:
- highly specific (can act on certain substrates [biological rxt])
- enormous catalytic power at biological pH & moderate temp
How does substrate conc affect rate of enzyme-catalysed rxn?
- At low [substrate]
- rate of rxn ∝ [substrate]
- active sites in enzymes not fully occupied => rxn approximately 1st order w.r.t. substrate - When [substrate] increase,
- rate no longer ∝ [substrate]
- more active sites occupied => rxn mixed order w.r.t. substrate - At very high [substrate]
- rate of rxn constant as rate is independent of [substrate]
- ALL active sites are occupied (ie. enzyme is saturated w substrate)
- Rxn is zero-order w.r.t. substrate
Elaborate on homogenous catalysts
- same physical stat as rxt
- provide alternative rxn pathway w lower Ea by first form intermediate that is subsequently consumed to form pdts
- regenerated at end of rxn
eg1
Rxn btw S2O8 2- (aq) and I- (aq) is kinetically unfavourable due to high Ea since both ions negatively charged and like charges repel
Rxn is accelerated in presence of homogenous catalyst (in same physical state) like Fe 2+ (aq) or Fe 3+(aq)
eg2
SO2 in atmosphere cause acid rain. It is oxidised to give SO3. Oxidat n of atmospheric SO2 catalysed by NO2 (another atmospheric pollutant)
SO3 then dissolves in rainwater to form H2SO4, which kills aquatic life in lakes, destroys limestone & marble of buildings
Elaborate on heterogeneous catalyst
- in diff physical state as rxt
- provide alternative rxn pathway of lower Ea by increasing local conc of rxt particles on catalyst surface & oso weakening chemical bonds in them for rxn
eg1 Haber Process
Iron (s) used [diff state to rxt N2(g), H2(g)]
eg2 catalytic converter in vehicles
A catalytic converter in exhaust sytem of motor vehicles speed up convers n of pollutants eg CO, NOx & unburnt hydrocarbons (CxHy) into harmless pdts eg H2O, CO2, N2
- NOx reduced to N2 by excess CO present (that is oxidised to CO2) w rhodium as catalyst
- Unburnt hydrocarbons and CO oxidised to CO2 & H2O (and O2 reduced to H2O) w Pt and Pd as catalysts
eg 3 Hydrogenati n of alkenes eg ethene on nickel surface
- rxt molecules r ADSORBED (not ‘absorb’) onto catalyst surface thru form n of temporary bonds
- adsorpt n weakens covalent bonds within rxt molecules, so lower Ea
- Rxt molecules r brought closer tgt, increasing surface conc. of rxt & rxn can occur btw rxt molecules more easily
- Products formed diffuse away fr catalyst surface
How does pa, surface area & light affect rate of rxn?
pa:
when pa of gaseous rxn system is increased
- gaseous rxt particles r brought closer tgt
- no of gaseous rxt particles per unit volume increase
- frequency of effective collis n increase
therefore rate of rxn increase
surface area:
when surface area of rxt is increased (eg use more fine powder form),
- larger accessible area for collis n (since larger total exposed surface area)
- frequency of effective collis n increase
hence rate of rxn increase
light (applicable to photochemical rxn)
when light intensity in photochemical rxn is increased
- amt of light energy absorbed by rxt particles increase
- no of rxt particles w energy >= Ea increase
- frequency of effective collis n increase
thus rate of rxn increase
