Page 206 - Instant notes

F6
THE KINETICS OF REAL SYSTEMS

Key Notes
A chain reaction occurs when a reaction intermediate generated
in one step reacts with another species to generate another
reaction intermediate. A chain reaction mechanism typically
contains several types of elementary reaction steps including
initiation, propagation, branching and termination. Mechanisms
containing many branching reactions may lead to explosions.
Chain branching explosions can arise when an elementary
reaction produces more reaction intermediates than are
consumed, each of which then instigate further chain branching
reactions resulting in a catastrophic accelerated increase in
reaction rate. Whether explosion or smooth reaction occurs
depends on exact conditions of temperature and pressure.
In the lock and key hypothesis of enzyme action, enzyme and
substrate are in equilibrium with an enzyme-substrate complex
(ES) which can proceed through to products. The Michaelis-
Menten equation for the rate of formation of products is
v=k 2 [E] 0 [S]/(K M +[S]), where [E] 0 is total enzyme present and K M
is the Michaelis constant. The enzyme released from the ES
complex is available for further reaction and is therefore a
catalyst.
A Lineweaver-Burke plot is a linear relationship used to analyze
kinetic data on enzyme catalyzed reactions. The reciprocal of the
rate of reaction (1/v) is plotted against the reciprocal of substrate
concentration (1/[S]) for experiments with the same initial
enzyme concentration. The y-axis intercept of the plot is 1/v max
and the gradient is K M /v max .
Related topics Rate law determination (F2) Rate laws in action (F5)
Formulation of rate laws (F4)

Chain reactions

In many complex reaction systems, the product of one elementary reaction step is the
reactant in the next elementary reaction step, and so on. Such systems are called chain
reactions and the reactive intermediates responsible for the propagation of the reaction
are called chain carriers. Important examples of such processes include combustion