Chemistry 202/212
Worksheet 9
Winter 2005
January 20
Oregon State University
1. Express the rate of the reaction in terms of each reactant and product.
2 N2H4(l) + N2O4(l) → 3 N2(g) + 4 H2O(g)
rate of reaction 
- N 2 H 4   N 2 O 4  N 2  H 2 O



2t
t
3t
4t
2. For the reaction
5 Br-(aq) + BrO3-(aq) +6 H+(aq) → 3 Br2(aq) +3 H2O
It was found that at a particular instant bromine was being formed at the rate of 0.039
mol·L-1·s-1. At that instant, at what rate is
(a) water being formed?
rate of formation Br2 = rate of formation H2O = 0.039 mol·L-1·s-1
(b) bromide ion being oxidized?
rate of consumption Br- = 5/3·rate of formation Br2 = 0.065 mol·L-1·s-1
(c) H+ being consumed?
rate of consumption H+ = 6/3·rate of formation Br2 = 0.078 mol·L-1·s-1
3. Ammonia is produced by the reaction between nitrogen and hydrogen gases.
(a) Write a balanced equation using smallest whole-number coefficients for the
reaction.
N2(g) + 3 H2(g) → 2 NH3(g)
(b) Write an expression for the rate of reaction in terms of [NH3].
rate of reaction 
NH 3 
2t
(c) The concentration of ammonia increases from 0.257 M to 0.815 M in 15.0 min.
Calculate the average rate of reaction over this time interval.
rate of reaction 
0.815M  0.257 M
 0.186mol·L - 1·s - 1
2  (15 min)
4. Experimental data for the following
hypothetical reaction are plotted in the
figure.
(a) Find the instantaneous rate at 40s.
0.004
0.0035
0.003
[X] mol/L
Draw a line tangent to the curve at 40s.

0.0008  0.0026mol  L1
rate 
50  0s
5
rate  3.6  10 mol  L1  s 1
Reaction Rate for X -> Y + Z
0.0025
0.002
0.0015
0.001
(b) Find the average rate over the 10
to 50s interval

0.0008  0.0024mol  L1
rate 
50  10s
5
rate  4  10 mol  L1  s 1
0.0005
0
0
10
20
30
40
50
60
tim e (s)
(c) Compare the two rates
The magnitude of the average rate is greater than the instantaneous rate.
(d) Does the rate remain constant over time?
No, the magnitude of the rate decreases over time.
(e) What does the rate depend upon?
The rate is probably dependent upon the concentration of reactants or products.
5. A reaction has two reactants X an Y. What is the order with respect to each reactant
and the overall order of the reaction described by the following rate expressions? What
are the units for the rate constants (k) if the rate is expressed in mol·L-1·s-1.
X
Y
Overall
(a) rate = k1[X]2·[Y]
second order
first order
third order
(b) rate = k2[X]
first order
zero order
first order
(c) rate = k3[X]2·[Y] 2
second order
second order fourth order
(d) rate = k4
zero order
zero order
zero order
6. The equation for the reaction between iodide and bromate ions in acidic solution is
6 I- (aq) + BrO3- (aq) + 6 H+ → 3 I2 (aq) +Br- (aq) +3 H2O
The rate of the reaction is followed by measuring the appearance of I2. The following
data are obtained:
[I-]
[BrO3-]
[H+]
Initial Rate (mol/L·s)
0.0020
0.0080
0.0020
8.89 · 10-5
0.0040
0.0020
0.0020
0.0015
0.0080
0.0160
0.0080
0.0040
1.78 · 10-4
1.78 · 10-4
3.56 · 10-4
7.51 · 10-5
0.0020
0.0020
0.0040
0.0030
(a) What is the order of the reaction with respect to each reactant?
[I-]
first order
[BrO3-]
first order
[H+]
second order
(b) Write the rate expression for the reaction.
reaction rate = k[I-][BrO3-][H+]2
(c) Calculate k.
rate
k
1
I  BrO 31  H 
 

 
2
3.56  10 4 mol/L·s
0.0020mol/ L0.0080mol / L 0.0040mol / L 2
k = 1.39·106 L3·mol-3·s-1
(d) What is the hydrogen ion concentration when the rate is 5.00·10-4 mol/L·s and
[I-] = 0.5 [BrO3-] = 0.0075 M?
H 

rate


1
k  I  BrO 31
 

5.00  10 4 mol / L  s
1.39  10 6 L3  mol 3  s 1 0.05mol / L 0.0075mol / L 


[H+] = 0.00031 M
7. In the first order decomposition of acetone at 500ºC,
CH3COCH3 (g) → products
It is found that the concentration is 0.0300 M after 200 min and 0.0200 M after 400
min. Calculate the following:
(a) the rate constant
 0.0300M 
ln 
  k  (400 min  200 min)
 0.0200M 
k = 2.03·10-3 min-1
(b) the half-life
 A0
ln 
 1 / 2A0

  k  t1 / 2

t1/2 = ln2/k =342 min
(c) the initial concentration
 A0 
ln 
  k  200 min
 0.0300M 
[A]0 = 0.0300M·e(k·200 min) = 0.0450 M
8. For the zero-order decomposition of ammonia on tungsten
NH3(g) → 1/2 N2(g) + 3/2 H2(g)
The rate constant is 2.08·10-4 mol·L-1·s-1.
(a) What is the half-life of a 0.250 M solution of ammonia?
[A] = [A]0 – kt
1/2[A]0 = [A]0 – kt1/2
1/2[A]0 = kt1/2
t1/2 = ½(0.250 mol/L)/( 2.08·10-4 mol·L-1·s-1) = 601 s = 10 min
(b) How long will it take for the concentration of ammonia to drop from 1.25 M
to 0.388 M?
0.388 M = 1.25 M - ( 2.08·10-4 mol·L-1·s-1)·t
t = 4140 s = 1.15 hrs
9. Write the rate expression for each of the following elementary steps.
(a) NO + O3 → NO2 + O2
rate = k·[NO]·[O3]
(b) 2 NO2 → 2 NO + O2
rate = k·[NO2]2
(c) K + HCl → KCl + H
rate = k·[K]·[HCl]