Chem1612 2014-N-12 Worksheet With Answers - The University Of Sydney - 2014 Page 12
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28CHEM1612
2009-N-15
November 2009
Marks
• The standard reduction potential of phosphorous acid to hypophosphorous acid is
3
−0.499 V, with the following half-reaction:
+
−
H
PO
(aq) + 2H
(aq) + 2e
→ H
PO
(aq) + H
O(l)
3
3
3
2
2
What would the reduction potential be for this half reaction at a temperature of 25 °C
in an aqueous solution with pH of 2.3 and concentrations of [H
PO
(aq)] = 0.37 M
3
3
and [H
PO
(aq)] = 0.00025 M?
3
2
+
+
-2.3
As pH = -log
[H
(aq)], [H
(aq)] = 10
M. With [H
PO
(aq)] = 0.37 M and
10
3
3
[H
PO
(aq)] = 0.00025 M, the reaction quotient, Q, is given by:
3
2
��
����
����
(��.����������)
��
��
=
Q =
= 27
!
��
!��.��
��
��
����
���� [��
���� ]
��.���� (����
)
��
��
The reduction potential for this 2 electron reduction is given by the Nernst
equation:
����
(��.������ ×(����!������)
E = E° -
lnQ = (-0.499 –
× ln(27)) V = -0.541 V
����
�� ����������
Answer: -0.541 V
2
• A number of bacteria can reduce the nitrate ion in the presence of sulfur. A
simplified unbalanced redox reaction can be written as:
–
S(s) + NO
(aq) → SO
(g) + NO(g)
3
2
Balance this redox equation for acidic conditions.
+
–
3S(s) + 4H
(aq) + 4NO
(aq) → 3SO
(g) + 4NO(g) + 2H
O(l)
3
2
2
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