Uddingston Grammar School
CfE Higher Chemistry
Unit 1: Chemical Changes and Structure
Sub-Topic A: Controlling the Rate
Homework 2: Enthalpy
1. A reaction was carried out with and without a catalyst as shown in the
energy diagram.
What is the enthalpy
change, in kJ mol–1, for the
catalysed reaction?
A –100
B –50
C +50
D +100
2. The following potential diagram is for a reaction carried out with and
without a catalyst.
The activation energy for the
catalyzed reaction is
A 30 kJ mol–1
B 80 kJ mol–1
C 100 kJ mol–1
D 130 kJ mol–1
3.
The enthalpy change for the
forward reaction can be
represented by
A
x
B
y
C
x+y
D
x–y
Uddingston Grammar School
4. A potential energy diagram can be used to show the activation energy (EA)
and the enthalpy change (ΔH) for a reaction. Which of the following
combinations of EA and ΔH could never be obtained for a reaction?
A
B
C
D
EA
EA
EA
EA
= 50 kJ mol–1 and ΔH = –100 kJ mol–1
= 50 kJ mol–1 and ΔH = +100 kJ mol–1
= 100 kJ mol–1 and ΔH = +50 kJ mol–1
= 100 kJ mol–1 and ΔH = –50 kJ mol–1
5.
Which set of data applies to the above reaction?
Enthalpy change
Activation energy/ kJmol –1
A
Exothermic
60
B
Exothermic
80
C
Endothermic
60
D
Endothermic
80
6. The following potential energy diagram represents the energy changes in a
chemical reaction.
The activation energy for the
reaction, in kJ mol -1, is
A
20
B
30
C
50
D
70
Uddingston Grammar School
7. The potential energy diagram below refers to the reversible reaction
involving reactants R and products P.
What is the enthalpy change, in kJmol-1, for the reverse reaction P
A.
B.
C.
D.
R?
+30
+10
–10
–40
8.
When a catalyst is used, the activation energy of the forward reaction is reduced to
35kJmol-1. What is the activation energy of the catalysed reverse reaction?
A.
B.
C.
D.
30kJmol-1
35kJmol-1
65kJmol-1
190 kJmol-1
9. Which of the following diagrams represents an exothermic reaction which
is most likely to take place at room temperature?
Uddingston Grammar School
Uddingston Grammar School
10. The potential energy diagram for the reaction is shown.
CO(g) + NO2(g)
→
CO2(g) + NO(g)
ΔH, in kJ mol–1, for the forward reaction is
A
+361
B
–93
C
–227
D
–361
11. Which of the following is not a correct statement about the effect of a
catalyst?
The catalyst
A
B
C
D
12.
provides energy so that more molecules have successful collisions
lowers the energy that molecules need for successful collisions
provides an alternative route to the products
forms bonds with reacting molecules.
Uddingston Grammar School
In area X
A
B
C
D
molecules always form an activated complex
no molecules have the energy to form an activated complex
collisions between molecules are always successful in forming
products
all molecules have the energy to form an activated complex
13. Which of the following describes the effect of a catalyst?
Activation energy
Enthalpy of reaction
A
decreased
decreased
B
decreased
no change
C
no change
decreased
D
decreased
increased
14. Soil bacteria are mainly responsible for releasing nitrogen in urea into
the soil so that it can be taken up by plants. The first stage in the process
is the hydrolysis of urea using the enzyme urease.
(a) Determine the enthalpy change, in kJ mol−1, for the reaction.
1
(b) Acid is a less effective catalyst than urease for this reaction. Copy the graph and
add a curve to the potential energy diagram to show the change in potential energy
when acid is used as the catalyst.
1
Uddingston Grammar School
15. Enzymes are biological catalysts.
(a) Name the four elements present in all enzymes.
1
(b) The enzyme catalase, found in potatoes, can catalyse the Decomposition of
hydrogen peroxide.
2H2O2(aq) → 2H2O(l) + O2(g)
A student carried out an experiment to determine the effect of pH on enzyme
activity. Describe how the activity of the enzyme could be measured in this
experiment.
1
(c) A student wrote the following incorrect statement.
“When the temperature is increased, enzyme-catalysed reactions will always
speed up because more molecules have kinetic energy greater than
the activation
energy.” Explain the mistake in the student’s reasoning.
1
16. Methanoic acid, HCOOH, can break down to carbon monoxide and water by
two different reactions, A and B.
Reaction A (catalysed)
HCOOH(aq) + H+(aq)
CO(g) + H2O(l) + H+(aq)
Reaction B (uncatalysed)
HCOOH(aq)
heat
CO(g) + H2O(l)
(a) What is the evidence in the equation for Reaction A that the H+(aq) In acts as a
catalyst ?
1
(b) Explain whether Reaction A is an example of heterogeneous or homogeneous
catalysis.
1
Uddingston Grammar School
17. Nitrogen dioxide gas can be prepared in different ways.
It is manufactured industrially as part of the Ostwald process. In the first stage
of the process, nitrogen monoxide is produced by passing ammonia and oxygen
over a platinum catalyst.
NH3(g) + O2(g)
→
NO(g) + H2O(g)
(a)
Balance the above equation.
(b)
Platinum metal is a heterogeneous catalyst for this reaction.
What is meant by a heterogeneous catalyst?
(c)
Explain how a heterogeneous catalyst works.
TOTAL: 25 MARKS
1
1
3