Cinétique de réaction Flashcards

1
Q

Define rate of reaction

A

Generally defined as change in conc. of rxts or pdts per unit time

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2
Q

Define rate equation

A

relates rate of rxn to conc of rxts raised to appropriate powers

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3
Q

Define order of reaction

A

order of reaction w.r.t. rxt is power to which conc of that rxt is raised in rate eq n

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4
Q

Define overall order of reaction

A

sum of powers to which conc. of rxts r raised in rate eqn

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5
Q

Define rate constant

A

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)

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6
Q

Define zero-order reaction. What are its characteristics

A

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

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7
Q

Define first order reaction. What are characteristics of such rxn?

A

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

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8
Q

Define half-life of reaction, t1/2

A

time taken for conc of rxt to decrease to half its og value
- for 1st order rxn, t1/2=(ln2)/k

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9
Q

What’s a formula to find number of half-lives in a first order rxn after time t?

A

[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

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10
Q

Define second order reaction. What are some of its characteristics?

A

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

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11
Q

Elaborate on pseudo 1st order rxn

A

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

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12
Q

What are the types of experiments performed to find order of rxn?

A
  • 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

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13
Q

What are the methods to deduce order of rxn?

A
  1. Initial rates method (discontinuous measurement) by:
    a. inspection/calculation based on table of data
    b. determination & comparison of gradient of graphs
  2. Half-life method (continuous measurement)
    by determine half-lives of conc-time graphs
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14
Q

What are 2 methods to find order or rxn? Briefly describe the steps

A
  • 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)

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15
Q

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?

A
  • 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.

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16
Q

What is rate-determining step?

A

Slowest step in a rxn mechanism

17
Q

Rate eqn is based on what in an elementary rxn and non-elementary rxn?

A

Elementary rxn:
based on stoichiometric eqn (rxn in one step)

Non-elementary rxn:
based on slow/rate-determining step

18
Q

Order of rxn w.r.t. rxts is reflected by what in an elementary rxn and non-elementary rxn?

A

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]

19
Q

Explain collision theory

A

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

20
Q

Define activation energy (Ea)

A

min. amt. energy molecular collis n must possess for a chemical rxn to occur

21
Q

What is transition state?

A

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

22
Q

Slow step has … Ea

A

largest

23
Q

What are the factors affecting rate of rxn?

A
  • Concentration
  • Temperature
  • Presence of catalyst
  • Pa, surface area and presence of light
24
Q

How does conc. affect rate of rxn?

A

When conc increase,
- no of rxt particles per unit volume increase
- frequency of effective collis n increase
Thus, rate of rxn increase

25
Q

How does Maxwell-Boltzman Distribution curve look like when:
- temperature is changed?
- catalyst is used

A

*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

26
Q

How does temp affect rate of rxn?

A

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

27
Q

How does addition of catalyst affect rate of rxn?

A

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

28
Q

Define catalyst

A

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

29
Q

What are the four types of catalysts?

A
  • Autocatalyst
  • Enzymatic catalyst
  • Homogeneous catalyst
  • Heterogeneous catalyst
30
Q

Define autocatalyst

A

one of the pdts of chemical rxn formed that carries out its catalytic action as soon as it is formed in rxn

31
Q

What are enzymes? What are its properties?

A

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

32
Q

How does substrate conc affect rate of enzyme-catalysed rxn?

A
  1. At low [substrate]
    - rate of rxn ∝ [substrate]
    - active sites in enzymes not fully occupied => rxn approximately 1st order w.r.t. substrate
  2. When [substrate] increase,
    - rate no longer ∝ [substrate]
    - more active sites occupied => rxn mixed order w.r.t. substrate
  3. 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
33
Q

Elaborate on homogenous catalysts

A
  • 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

34
Q

Elaborate on heterogeneous catalyst

A
  • 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

  1. NOx reduced to N2 by excess CO present (that is oxidised to CO2) w rhodium as catalyst
  2. 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
35
Q

How does pa, surface area & light affect rate of rxn?

A

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