Question

Based on the reduction potentials listed in the textbook appendix, which of the following redox reactions do you expect to occur spontaneously?  
W. 2Al(s)+3Pb2+ (aq) → 2Al3+ (aq)+3Pb(s)  
X. Fe(s)+Cr3+ (aq) → Fe3+ (aq)+Cr(s)  
Y. Ca2+ (aq)+Zn(s) → Ca(s)+Zn2+(aq)  
Z. 2Cu+(aq)+Co(s) → 2Cu(s)+Co2+ (s)

a. W only
b. X, Y and Z
c. Y only
d. X and Z
e. Z only
f. X and Y
g. W, X and Z
h. X and Y
i.W and Z

Answer 1

Answer: The redox reactions that occur spontaneously are Reaction W and Reaction Z.

Explanation:

For the reaction to be spontaneous, the Gibbs free energy of the reaction must come out to be negative.

Relationship between standard Gibbs free energy and standard electrode potential follows:

$\Delta G^o=-nFE^o_{cell}$

For a reaction to be spontaneous, the standard electrode potential must be positive.

To calculate the $E^o_{cell}$ of the reaction, we use the equation:

$E^o_{cell}=E^o_{cathode}-E^o_{anode}$       .......(1)

Substance getting oxidized always act as anode and the one getting reduced always act as cathode.

• For reaction W:

The chemical reaction follows:

$2Al(s)+3Pb^{2+}(aq.)\rightarrow 2Al^{3+}(aq.)+3Pb(s)$

We know that:

$E^o_{Al^{3+}/Al}=-1.66V\\E^o_{Pb^{2+}/Pb}=-0.13V$

Calculating the $E^o_{cell}$ using equation 1, we get:

$E^o_{cell}=-0.13-(-1.66)=1.53V$

As, the standard electrode potential is coming out to be positive. So, the reaction is spontaneous.

• For reaction X:

The chemical reaction follows:

$Fe(s)+Cr^{3+}(aq.)\rightarrow Fe^{3+}(aq.)+Cr(s)$

We know that:

$E^o_{Fe^{3+}/Fe}=0.77V\\E^o_{Cr^{3+}/Cr}=-0.74V$

Calculating the $E^o_{cell}$ using equation 1, we get:

$E^o_{cell}=-0.74-(0.77)=-1.51V$

As, the standard electrode potential is coming out to be negative. So, the reaction is not spontaneous.

• For reaction Y:

The chemical reaction follows:

$Zn(s)+Ca^{2+}(aq.)\rightarrow Zn^{2+}(aq.)+Ca(s)$

We know that:

$E^o_{Ca^{2+}/Ca}=-2.87V\\E^o_{Zn^{2+}/Zn}=-0.76V$

Calculating the $E^o_{cell}$ using equation 1, we get:

$E^o_{cell}=-2.87-(-0.76)=-2.11V$

As, the standard electrode potential is coming out to be negative. So, the reaction is not spontaneous.

• For reaction Z:

The chemical reaction follows:

$Co(s)+2Cu^{+}(aq.)\rightarrow Co^{2+}(aq.)+2Cu(s)$

We know that:

$E^o_{Cu^{+}/Cu}=0.34V\\E^o_{Co^{2+}/Co}=-0.28V$

Calculating the $E^o_{cell}$ using equation 1, we get:

$E^o_{cell}=0.34-(-0.28)=0.62V$

As, the standard electrode potential is coming out to be positive. So, the reaction is spontaneous.

Hence, the redox reactions that occur spontaneously are Reaction W and Reaction Z.