Geology Reference
In-Depth Information
Exercises
increased by 10°C. Calculate the activation energy
for a reaction that exactly conforms to this relation-
ship. (Gas constant R = 8.3143 J K 1 mol 1 .)
3.3 Use the viscosity measurements given below to
verify that the Arrhenius equation is applicable to
the flow of a silicate melt, and determine the acti-
vation energy. ( Hint : viscosity is the resistance to
flow, to which flow rate is inversely proportional.)
3.1 The table below gives the Geiger counter count-rate
at various times during an experiment on radioac-
tive iodine-131 ( 131 I). Treating the decay of 131 I as a
chemical reaction in which the count rate may be
taken as being a measure of the concentration of 131 I,
confirm that the reaction is first-order and determ-
ine the decay constant. From the linear graph you
have drawn, calculate the half-life of 131 I (Box 3.2).
Temperature
(°C)
Viscosity of rhyolite
melt (N s m 2 )
1325
2042
Time (hours)
Counts (sec −1 )
1345
1585
0
18,032
1374
1097
25
16,410
1405
741
50
15,061
100
12,590
(Gas constant as given in Exercise 3.2.)
3.4 Calculate the half-life of 87 Rb (λ 87
200
8789
Rb =×. year 1 ).
What percentage of the 87 Rb incorporated in the
Earth 4.6 × 10 9 years ago has now decayed to 87 Sr?
3.5 Demonstrate that the units of diffusion coefficient
D i in Equation 3.11 are m 2 s 1 , when concentration c i
is expressed in mol m −3 and the net flux f i is expressed
in mol m 2 s 1 .
14210 11
300
6144
400
4281
500
3002
3.2 There is a rule of thumb that says that the rate
of  many room-temperature chemical reactions
approximately doubles when the temperature is
 
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