Enthalpy 1 due 9-12-16
1.
/25
%
NAME:
Some mean bond enthalpy values are given in the table below.
Bond
Mean bond enthalpy / kJ mol–1
H―H
+436
I―I
+151
H―I
+299
What is the enthalpy change for the reaction shown below in kJ mol–1?
H2(g) + I2(g) → 2HI(g)
A
+436 + 151 – 299 = +288
B
–436 – 151 + 299 = –288
C
+436 +151 – (2 × 299) = –11
D
–436 – 151 + (2 × 299) = +11
(Total 1 mark)
2.
Which of the equations shown below represents the reaction for which H is the standard
enthalpy change of formation, Hοf 298, for ethanol, C2H5OH. Ethanol melts at 156 K and boils
at 352 K.
A
2C(g) + 6H(g) + O(g)  C2H5OH(g)
B
2C(s) + 3H2(g) + O2(g)  C2H5OH(l)
C
2C(s) + 3H2(g) + O(g)  C2H5OH(g)
D
2C(s) + 3H2(g) + ½O2(g)  C2H5OH(l)
(Total 1 mark)
The Westlands School
1
Enthalpy 1 due 9-12-16
3.
(a)
/25
%
NAME:
Enthalpy changes can be calculated using average bond enthalpy data.
(i)
The enthalpy change to convert methane into gaseous atoms is shown below.
CH4(g) → C(g) + 4H(g)
∆H = +1664 kJ mol–1
Calculate the average bond enthalpy of a C—H bond in methane.
(1)
(ii)
Use the data in the table below and your answer to (a)(i) to calculate the enthalpy
change for
2C(g) + 2H2(g) + Br2(g) → CH2BrCH2Br(g)
Bond
Average bond
enthalpy / kJ mol–1
Bond
Average bond
enthalpy / kJ mol–1
C—C
+348
H—H
+436
Br—Br
+193
C—Br
+276
(3)
The Westlands School
2
Enthalpy 1 due 9-12-16
(b)
/25
%
NAME:
The standard enthalpy of formation of 1,2-dibromoethane, CH2BrCH2Br, is
–37.8 kJ mol–1.
Suggest the main reason for the difference between this value and your calculated value
in (a)(ii).
................................................................................................................................
................................................................................................................................
................................................................................................................................
(1)
(Total 5 marks)
4.
An experiment was carried out to find the enthalpy change for the reaction of zinc powder with
copper(II) sulphate solution.
Zn(s) + CuSO4(aq)  ZnSO4(aq) + Cu(s)
50cm3 of copper(II) sulphate solution, of concentration 1.0 mol dm–3, was put into a polystyrene
cup and the temperature of the solution measured. After one minute, 5.0 g of zinc powder was
added, the mixture stirred with a thermometer and the temperature measured every 30 s.
(a)
(i)
What is meant by a spectator ion?
...........................................................................................................................
...........................................................................................................................
(1)
(ii)
Give the formula of the spectator ion in this reaction.
...........................................................................................................................
(1)
(iii)
Write the equation for this reaction, omitting the spectator ion.
(1)
The Westlands School
3
Enthalpy 1 due 9-12-16
(b)
/25
%
NAME:
How would you measure the 50 cm3 of copper(II) sulphate solution?
.....................................................................................................................................
.....................................................................................................................................
(1)
(c)
Give TWO reasons why it is better to use a polystyrene cup, rather than a metal container,
to obtain more accurate results.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
(2)
(d)
Calculate the number of moles of each of the reactants and hence deduce which reactant
is completely used up.
Use the Periodic Table as a source of data.
Moles of zinc powder
Moles of copper(II) sulphate
Reactant used up .................................................................
(3)
(e)
The following results were obtained.
Time /s
0
60
90
120
150
180
210
Temperature /°C
22
22
60
65
63
61
59
The Westlands School
4
Enthalpy 1 due 9-12-16
(i)
/25
%
NAME:
On the graph paper below, plot the results of this experiment.
70
60
50
Temperature
/ºC
40
30
20
10
0
0
40
80
120
Time / s
160
200
240
(2)
(ii)
Explain the shape of your graph
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
(2)
The Westlands School
5
Enthalpy 1 due 9-12-16
(iii)
/25
%
NAME:
The maximum recorded temperature in this experiment was 65°C. Use your graph
to estimate a more accurate maximum temperature.
...........................................................................................................................
(1)
(f)
(i)
Calculate the energy change in this experiment using your answer to (e)(iii) and the
relationship
energy change = mass of × specific heat capacity × temperature rise
solution
of solution
/J
/g
/J °C–1 g–1
/°C
You may assume that
• 1.0 cm3 of solution has a mass of 1.0 g
• The specific heat capacity of the solution is 4.2 J °C–1 g–1
(1)
(ii)
Use your answers to (d) and (f)(i) to calculate ∆H for this reaction. Include a sign
and units in your answer.
(3)
(Total 18 marks)
The Westlands School
6