CHEMISTRY 12 – ACTIVATION ENERGIES WORKSHEET
Use the following diagram to answer question 1
1) The above diagram shows reactant molecules approaching one another.
a) What is happening to the kinetic energy and the potential energy? (2 marks)
KE increasing and PE decreasing
b) Draw the structure of the activated complex? (1 mark)
0
Cl
\
/
N
'
'
0
a
2) Describe the relationship between activation energy and the rate of a chemical reaction. (2 marks)
If the activation energy is high, the rate of a chemical reaction is slow and if the activation energy is
low, the rate of a chemical reaction is fast
3) If ΔH = - 40 kJ and Ea(f) = 20 kJ, what is the value of Ea(r)? (2 mark)
Eats
20
Each
=
Each
=
Each
60
=
DH
+
40
-
KJ
4) Consider the following reaction:
formation
H2 (g) + I2 (g) → 2 HI (g)
ΔH = +28 kJ
decomposition
The activation energy for the formation of HI is 167 kJ. What is the activation energy for the
decomposition of HI? Is the decomposition reaction endothermic or exothermic? (2 marks)
Each
167
Each
Each
=
+
Each
=
=
139 KJ
SH
+28
since
is
the formation
endothermic
,
the
decomposition
is
exothermic
5) A reaction has Ea(f) = 40 kJ and Ea(r) = 10 kJ. Is the reaction exothermic or endothermic? Explain.
(2 marks)
Each
40
DH
Eacrs
=
10
=
=
endothermic
+
+30
,
DH
+
SH
KJ
since
AH
positive
is
6) Sketch a PE diagram for the reacting system where: (3 marks)
ΔH = - 30 kJ / mol
Ea = 50 kJ / mol
the
DH
=
-30
KJ
7) Consider the following potential energy diagram for a reversible reaction: (3 marks)
a) Calculate the activation energy for the forward reaction
15 kJ
b) Calculate ΔH for the forward reaction
- 10 kJ
c) Calculate the activation energy for the reverse reaction
25 kJ
8) Consider the following reaction:
H2 (g) + I2 (g) → 2 HI (g)
What is true of the activated complex, in terms of KE and stability, relative to the reactants?
(1 mark)
The activated complex has lower KE and has less stability
9) Describe what happens to the KE and PE as an activated complex forms products? (1 mark)
KE increases and PE decreases
10) Consider the following reaction:
CO + NO2 → CO2 + NO
ΔH = -234 kJ
The activation energy of the forward reaction is 134 kJ. Is the reverse reaction endothermic or
exothermic? What is the activation energy for the reverse reaction? (2 marks)
Each
134
Eacr )
Eacr )
=
Each
=
368
=
DH
+
-234
endothermic
KJ
11) In a certain reaction ΔH = -136 kJ and Ea = 96 kJ. Is the reverse reaction endothermic or
exothermic? What is the activation energy for the reverse reaction? (2 marks)
EACH
96
Each
=
Each
=
Eacr )
=
DH
+
-
232 KJ
136
endothermic
12) Define “activated complex”. (2 marks)
A chemical species that is short-lived and unstable due to its high PE
13) Consider the following potential energy diagram for a reversible reaction: (5 marks)
a) Calculate the activation energy for the reverse
reaction 50 kJ
b) Calculate ΔH for the forward reaction + 50 kJ
c) Calculate the activation energy for the forward
reaction 100 kJ
d) Calculate the energy of the activated complex
200 kJ
e) Is the reaction endothermic or exothermic in the
reverse direction?
Exothermic