Chemistry Reference
In-Depth Information
H
OH
O
N
O
CH
3
Figure 11.12
The structure of adrenochrome.
(d)
Angiotensin II is an octapeptide composed of eight amino
acids joined to each other by peptide bonds. Peptidase
enzymes present in the body can hydrolyse these bonds to
liberate free amino acids. It is often the case that potent
biological molecules (e.g. hormones such as adrenaline, or
neurotransmitters such as acetylcholine) are quickly broken
down either chemically or by enzymes. This rapidly
terminates the biological activity.
A9.1
For a first-order process, ln(
a
x
)
ln
a
kt
and a plot of
ln(
a
x
) vs
t
should give a straight line graph with slope equal to
k
. This graph was plotted and a slope of
0.351 was obtained.
0.351 day
-1
.
Thus, negative slope
k
and hence
k
(a)
For a first-order process, ln(
a
x
)
ln
a
kt
and a plot of
A9.2
ln(
a
x
) vs
t
should give a straight line graph with slope
equal to
k
. This graph was plotted and a slope of
0.00412 was obtained. Thus, negative slope equals
k
and
hence
k
0
.
00412 s
-
1
. The fact that this equation yields a
straight line confirms the rate is first order with respect to
peroxide.
(b)
Using the linear form of the Arrhenius equation,
(
-
-
)
E
1
ln
k
ln
A
-
RT
E
/
R
, from
which
E
, the activation energy, may be calculated (units are
J mol
-1
or kJ mol
-1
).
To determine the frequency factor, a pair of values of ln
k
and 1/
T
are chosen, the above graph is plotted, and the intercept
with the vertical axis is determined. This is ln
A
, from which
A
,
the frequency factor, is found. The units of
A
are the same as for
k
, i.e. s
-1
.
A plot of ln
k
vs 1/
T
will yield a graph of slope
