Bai 4 xúc tác, hóa lí 2

CATALYSIS


• Definition
• Classification
• Homogeneous catalysts
• Enzymes
• Heterogeneous catalysts

• General features
• Catalysts interact with reactants and lead the reaction following a different mechanism
with lower activation energy.
• Catalytic action is specific.
• Catalysts do not change thermodynamic properties of the reaction.


HOMOGENEOUS CATALYSIS

• Phương trình động học tùy theo cơ chế cụ thể.
Đa số trường hợp xúc tác đồng thể trải qua giai đoạn cân bằng nhanh giữa xúc
tác và tác chất.
• Example 1: iodide-catalyzed decomposition of hydrogen peroxide
2 H2O2(aq) → 2 H2O(l) + O2(g)
(1) H+ + H2O2  H3O2+ (k1 and k-1)
(2) H3O2+ + I- → HOI + H2O(k2)
(3) HOI + H2O2 →I- + O2 + H2O

(k3)

a) Derive the rate law for the overall reaction.
b) It is known that the second step is rate-determining (very slow compared to

other steps). Show that equilibrium (1) is established and remains during the
reaction.


• In acid catalysis the crucial step is the transfer of a proton to the
substrate:
X + HA → HX+ + A− (fast)
HX+ (+B)→ products (slow)
Acid catalysis is the primary process in the solvolysis of esters and
keto–enol tautomerism:


• In base catalysis, a proton is transferred from the substrate to a base:
XH + B → X− + BH+
X− (+N)→ products
Base catalysis is the primary step in the isomerization and halogenation of organic
compounds, and of the Claisen and aldol condensation reactions. The basecatalyzed version of keto–enol tautomerism, for instance, is


Self-test questions


ENZYME CATALYSIS

Enzymes are homogeneous, biological catalysts.
Principal features:
-For a given initial concentration of substrate, [S]O, the initial rate of product
formation is proportional to the total concentration of enzyme, [E]O.
-For a given [E] O and low values of [S]O, the rate of product formation is
proportional to [S]O.

-For a given [E] O and high values of [S] O, the rate of product formation becomes
independent of [S] O, reaching a maximum value known as the maximum velocity, vmax.


Michaelis–Menten mechanism

E + S  ES k1,k-1
ES → P + E k2

k2 [ E ]o k2 [ E ]o [ S ]
V

K M [S ]  K M
1
[S ]

where is the Michaelis constant, characteristic of a given enzyme acting on a given substrate
and having the dimensions of a molar concentration.
• When [S]o << KM the rate is proportional to [S]o:
• When [S]o >> KM the rate reaches its maximum value and is independent of [S] o:


k2 [ E ]o
Vmax
V

KM
KM
1
1

[S ]
[S ]

Turnover frequency, or catalytic constant, of an enzyme, kcat,
is the number of catalytic cycles (turnovers) performed by the
active site in a unit of time kcat = k2

To identify KM and k2


Example

The enzyme carbonic anhydrase catalyses the hydration of CO2 in red blood cells to
give bicarbonate (hydrogen carbonate) ion:
CO2(g) + H2O(l) → HCO3-(aq) + H+(aq)
The following data were obtain for the reaction at pH = 7.1, 273.5 K and an enzyme
concentration of 2.3 nM..
Calculate Michaelis constant and maximum rate of this reaction at given conditions.



XÚC TÁC DỊ THỂ
(HETEROGENEOUS CATALYSIS)
• 5 GIAI ĐOẠN CỦA PHẢN ỨNG DỊ THỂ
Khuếch tán (diffusion) tác chất đến bề mặt phân chia pha (interface)
Hấp phụ (adsorption) tác chất lên bề mặt phân chia pha
Phản ứng hóa học
Giải hấp phụ (desorption) sản phẩm khỏi bề mặt phân chia pha
Khuếch tán sản phẩm khỏi bề mặt phân chia pha

Example : Using the Langmuir isotherm (đường đẳng nhiệt)

 

The data given below are for the adsorption of CO on charcoal at 273 K.
Confirm that they fit the Langmuir isotherm, and find the constant K
and the volume corresponding to complete coverage. In each case V
has been corrected to 1.00 atm (101.325 kPa).
P, kPa 13.3
26.7
40.0
53.3
66.7
80.0
93.3
V, cm3 10.2
18.6
25.5
31.5
36.9
41.6
46.1



Measuring the enthalpy of
adsorption
• The data below show the pressures of CO needed for the volume of
adsorption (corrected to 1.00 atm and 273 K) to be 10.0 cm3 using

the same sample as in Example 23.4. Calculate the adsorption
enthalpy at this surface coverage.
T, K
200
210
220
230
240
250
P, kPa
4.00
4.95
6.03
7.20
8.47
9.85