[candiishop]

1) A first-order reaction has a rate constant of 7.5 ? 10-3/s. The time required for the reaction to be 60% complete is:

a) 3.8 × 10-3 s
 
b) 6.9 x 10-3 s
 
c) 68 s
 
d) 120 s
 
e) 130 s

I got C from this equation ln [A]t/ [A0] = -kt


2) Carbon-14 is a radioactive isotope which decays with a half-life of 5730 years. What is the first-order rate constant for its decay, in units of years-1?

a) 5.25 x 10-5
 
b) 1.21 x 10-4
 
c) 1.75 x 10-4
 
d) 3.49 x 10-4

I got b from this working out:

0.693/K = t1/2
0.693/K = 5730
K= 1.21x10^-4

3) What is the slope of an Arrhenius plot for the following reaction?
              2NOCl ----> 2NO + Cl2

Temperature (K)        k (L/mol•s)             
              400                            6.6 x 10–4
              500                            2.9 x 10–1
              600                            16.3
 
a) 8.18 x?10–2 K
 
b) 5.06 x 10–2 K
 
c) -1.21 x 104 K
 
d) -1.96 x 104 K
 
e) not enough information to calculate the slope

I tried looking for similar example questions in the text book but found none. Are we supposed to use the K= Ae^ -Ea/RT? Sorry.. im so lost.

Thanks alot

[green-goblin]

An Arrhenius plot uses the logarithmic form of the arrhenius equation. If you plot (ln k) versus 1/T you will  get a linear plot with a negative slope. The slope will have a value of -Ea/R

[candiishop]

Oh i see. So that leaves out C, D are the answers.. since it's a negative slope. Btw.. how do we work out the slope (-Ea/r)? Do we use rise/run? Also, are my answers for the other 2 questions are correct?

Thanks