Energetics
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Question 1
Which statement about this reaction is correct?
2Fe(s) + 3CO2(g)  Fe2O3(s) + 3CO(g) ΔHƟ=+26.6kJ
13.3kJ of energy are
absorbed for every mole
of Fe reacted.
26.6kJ of energy are
absorbed for every mole
of Fe reacted.
26.6kJ of energy are
released for every mole
of Fe reacted.
53.2kJ of energy are
released for every mole
of Fe reacted.
Wrong Answer!
For 2 moles of Fe, 26.6kJ of energy is absorbed to break
the bonds. 1 mole of Fe will require 13.3kJ of energy.
Question 2
Which of the following is observed when the
change in enthalpy is positive for the dissoving
of a salt in water in an insulated copper beaker?
Heat is evolved to the
surroundings and the
beaker feels cold.
Heat is evolved to the
surroundings and the
beaker feels warm.
Heat is absorbed from
the surroundings and
the beaker feels warm.
Heat is absorbed from the
surroundings and the beaker
feels cold.
Wrong Answer!
A positve enthalpy change means that heat flows from
the surroundings into the system.
Question 3
All the following processes are exothermic except
2C2H5(g)  C4H10(g)
Cl(g) + e-  Cl-(g)
F2(g)  2F-(g)
4Fe(s) + 3O2(g) 
2Fe2O3 (s)
Wrong Answer!
Bond breaking is always endothermic.
Question 4
What can be deduced about the relative stability of the
reactants and products and the sign of ΔH, from the
enthalpy level diagram below?
Relative stability
reactants
ΔH
Sign of ΔH
products more stable
-
products more stable
+
reactants more stable
-
products
reactants more stable
+
Wrong Answer!
The vertical axis represents potential energy, energy or
enthalpy. The reactants are high in energy and hence
unstable; the products are lower in energy and hence
more stable. The difference is the released in the form of
heat energy.
Question 5
What is the specific heat capacity of an alcohol in
Jg-1K-1 if 560.0J of heat are required to raise the
temperature of a 64.0g sample of ethanol from
295.0K to 310.0K?
0.583
0.194
8.75
0.292
Wrong Answer!
Heat energy = mass x specific heat capacity x change in temp
560 = 64.0 x c x (310 – 295 )
c = 0.583 Jg-1K-1
Question 6
When 0.050 mol of nitric acid is reacted with 0.050 mol of
potassium hydroxide in water, the temperature of the
system increases by 13.70C. Calculate the enthalpy of
reaction in kJmol-1
HNO3(aq) + KOH(aq)  KNO3(aq) + H2O(l)
Assume that the heat capacity of the system was 209.2J 0C-1.
+57.3 kJmol-1
+2.87 kJmol-1
-2.87 kJmol-1
-57.3 kJmol-1
Wrong Answer!
Heat energy = 13.7 x 209.2 = 2866.04J
Enthalpy of reaction per mole of KCl = (1/0.05) x 2866.04
= 57328 Jmol-1
Sign is negative because energy is given out.
Question 7
The bond energies for H2 , I2 and HI are 432, 149
and 295 kJmol-1 respectively. From these data,
what is the enthalpy change(in kJ) for the
reaction below?
H2(g) + I2(g)  2HI(g)
+286
+9
-9
-286
Wrong Answer!
Enthalpy change = [(432 + 149)] – [(295 x 2)] = -9 kJ
Question 8
Consider the following equation:
6CO2(g) + 6H2O(l)  C6H12O6(s) + 6H2O(g)
ΔH=2824kJmol-1
What is the enthalpy change associated with the
production of 100.0g of C6H12O6?
157 kJ
282 kJ
508 kJ
1570 kJ
Wrong Answer!
Amount of C6H12O6= (100.0g/180gmol-1) = 0.55 mol-1
Enthalpy change = 0.55 x 2824 = +1568 kJ
Question 9
The specific heat capacities of some metals are given below.
Metal
Specific heat capacity (Jg-1K-1)
copper
0.385
magnesium
1.020
mercury
0.138
platinium
0.130
If 100kJ of heat is added to 10.0g samplesof each of the metals above,
which are all at 250C, which metal will have the lowest temperature?
copper
mercury
magnesium
platinium
Wrong Answer!
Q = mcΔT, q/mc = ΔT, hence if c is increased, then ΔT will
decrease. A high heat capacity means more energy is required
to raise the temperature.
Question 10
The bond energy for the H-F bond is equal to the
enthalpy change for which process?
HF(g)  ½F2(g) + ½H2(g)
HF(g)  H(g) + F(g)
½F2(g) + ½H2(g)  HF(g)
H+(g) + F-(g)  HF(g)
Wrong Answer!
The H-F bond enthalpy is the amount of energy (in kJ)
required to break 1 mole of HF covalent bond into
gaseous hydrogen and fluorine atoms (under standard
thermodynamic conditions)
Question 11
When a sample of a pure hydrocarbon (melting
point 850C) cools, the temperature is observed
to remain constant as it solidifies. Which
statement accounts for this observation?
The heat released in the
change of state equals the
heat loss to the surroundings.
The solid which forms
insulates the system,
preventing heat loss.
The temperature of the system
has fallen to room
temperature.
Heat is gained from the
surroundings as the solid
forms, maintaining a constant
temperature.
Wrong Answer!
During freezing intermolecular forces (van der Waals’
forces) are formed and heat energy is released to the
surroundings.
Question 12
Consider the following reactions:
N2(g) + 3H2(g)  2NH3(g)
Bond enthalpies (in kJmol-1) involved in the reaction are:
NΞN
a
H-H
b
N-H
c
Which expression could be used to calculate the enthalpy
of reaction?
a + 3b - 2c
a - 3b + 6c
6c – a + 3b
a + 3b – 6c
Wrong Answer!
Enthalpy change = ∑(bond broken) - ∑(bond made)
= (NΞN + 3 x H-H) – (6 x N-H)
Question 13
The enthalpy changes for two different hydrogenation
reactions of C2H2 are:
C2H2 + H2  C2H4 ΔHƟ1
C2H2 + 2H2  C2H6 ΔHƟ2
Which expression represents the enthalpy change for the
reaction below?
C2H4 + H2  C2H6 ΔHƟ = ?
ΔHƟ1 + ΔHƟ2
ΔHƟ1 - ΔHƟ2
ΔHƟ2 – ΔHƟ1
- ΔHƟ1 - ΔHƟ2
Wrong Answer!
Using the energy cycle by Hess law, the enthalpy change
is obtained by reverting ΔHƟ1 and adding to ΔHƟ2
C2H4 + 2H2  C2H6 ΔHƟ
ΔHƟ1
ΔHƟ2
C2H2 + 2H2
Question 14
2KHCO3(s)
+ 2HCl(aq)
K2CO3(s) + CO2(g) + H2O(l)
+ 2HCl(aq)
Ɵ
ΔH
2
ΔHƟ1
2KCl(aq) + 2CO2(g) + 2H2O(l)
This cycle may be used to determine ΔHƟ for the
decomposition of potassium hydrogen carbonate.
Which expression can be used to calculate the ΔHƟ ?
ΔHƟ = ΔHƟ1 + ΔHƟ2
ΔHƟ = ΔHƟ1 - ΔHƟ2
ΔHƟ = ½ ΔHƟ1 - ΔHƟ2
ΔHƟ = ΔHƟ2 – ΔHƟ1
Wrong Answer!
Congratulations!