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Published on: 28/05/2021
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Questions + Answers key
Take MCQ Chemistry Test1.
The standard reduction potential for \({ Cu }^{ 2+ }|Cu\) \(+0.34V\) . Calculate the reduction potential at \( pH=14\) for the above couple.\({ K }_{ sp }\) of \(Cu{ (OH) }_{ 2 }\) is \(1.0\times { 10 }^{ -19 }\).
2.
What pressure of H2 would be required to make e.m.f. of the hydrogen electrode zero in pure water at 25°C?
3.
If change on the electron is \(1.60\times { 10 }^{ -19 }C\) and 96500 C deposit of 107.9 g of silver from its solution, calculate the value of Avogadro's number. (At. mass of Ag = 107.9 u)
4.
Consider the following half-cell reactions:
\({ Br }_{ 2 }+{ 2e }^{ - }\longrightarrow { 2Br }^{ - } \ \ E°=1.09V\\ { I }_{ 2 }+{ 2e }^{ - }\longrightarrow { 2I }^{ - } \ \ E°=0.54V\)
If \({ I }_{ 2 }\) and \({ Br }_{ 2 }\) are added to solution containing I M concentration of \({ I }^{ - }\) and \({ Br }^{ - }\) respectively.
(i) Write an equation for the overall cell reaction and \(E°\) of the cell.
(ii) Write the cell in which the reaction takes place, using Pt as the inert electrolyte for eacg electrode.
(iii) How will the increase in the concentration of \({ Br }^{ - }\) affect \({ E }_{ cell }\)?
5.
What are the signs of \(\Delta G,K\) and \({ E° }_{ cell }\) for a spontaneous cell reaction?
1.
For pH = 14, [H+]= 10-14
\([OH^-]={K_w\over [H^+]}={10^{-14}\over 10^{-14}}=1M\)
Cu(OH)2 ⇌ Cu2++ 2OH-
Ksp= [Cu2+] [OH-]2
1.0 x 10-19 = [Cu2+](1)2
[Cu2+] = 1.0 X 10-19
For the cell reaction,
Cu2++ 2e- ⟶ Cu
\(E=E^0{0.059\over 2}log{1\over [Cu^{2+}]}\)
\(=0.34-{0.059\over 2}log{1\over 1.0\times 10^{-19}}\)
\(=0.34-{0.059\over 2}\times19=-0.22V\)
2.
\(For \ water \ at \ 298K,[{ H }^{ + }]={ 10 }^{ -7 }M\)
\(Reduction \ reaction: \ { H }^{ + }+{ e }^{ - }\longrightarrow \frac { 1 }{ 2 } { H }_{ 2 }\)
\( \therefore E=-\frac { RT }{ F } In\frac { { p }_{ { H }_{ 2 } }^{ 1/2 } }{ [{ H }^{ + }] } =-0.0591log\frac { { p }_{ { H }_{ 2 } }^{ 1/2 } }{ [{ H }^{ + }] } \)
\(=-.0.0591log\frac { 1 }{ { 10 }^{ -7 } } =-0.4137=-0.414V\)
3.
1 mol = \(107.9g\)
1 mol of Ag is deposit by \(96500C\)
If charge of electron is \(1.60\times { 10 }^{ -19 }C\)
No. of electrons in 1 mol of silver = \(\frac { 96500C }{ 1.60\times { 10 }^{ -19 }C } \)
= \(6.03\times { 10 }^{ 23 }{ e }^{ - }\)
Avagadro number = \(6.03\times { 10 }^{ 23 }\)
4.
(i) \({ Br }_{ 2 }+2{ I }^{ - }\longrightarrow 2{ Br }^{ - }+{ I }_{ 2 }\)
\({ E }^{ \theta }={ E }^{ \theta }\left( \frac { 1 }{ 2 } { Br }_{ 2 }|{ Br }^{ - } \right) -E°\left( \frac { 1 }{ 2 } { I }_{ 2 }|{ I }^{ - } \right)\)
\(=1.09-0.54=0.55V\)
(ii) \(Pt|{ I }_{ 2 },{ I }^{ - }(aq)||{ Br }^{ - }(aq){ Br }_{ 2 }|Pt\)
(iii) The value of \({ E }^{ \theta }\) will be reduced.
5.
\(\Delta G=\_ ve,K=+ve,{ E° }_{ cell }=+ve\)
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