For the following cell,
Zn(s) | ZnSO4 (a) | | CuSO4 (a) | Cu (s)
when the concentration of Zn2+ is 10 times the concentration of Cu2+, the expression for
G (in J mol–1) is
[F is Faraday constant; R is gas constant; T is temperature; E° (cell) = 1.1 V]
Zn - 2e → Zn2+
Cu2+ + 2e → Cu
-----------------------
Zn + Cu2+ → Zn2+ + Cu
= 2.303RT - 2.2F
This question involves calculating the Gibbs free energy change (ΔG) for an electrochemical cell under non-standard conditions. The cell is a Daniell cell with zinc and copper electrodes. The key concept is the relationship between ΔG, the standard cell potential (E°), and the reaction quotient (Q) via the Nernst equation.
Step 1: Write the cell reaction and identify the standard cell potential.
The cell notation is: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
The cell reaction is: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
The standard cell potential, E°(cell), is given as 1.1 V.
Step 2: Recall the formula relating ΔG and the cell potential.
The Gibbs free energy change is given by:
where:
n = number of electrons transferred in the reaction
F = Faraday constant
Ecell = cell potential under the given conditions
Step 3: Apply the Nernst equation to find Ecell.
The Nernst equation for this cell at temperature T is:
For the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), the reaction quotient Q is:
It is given that [Zn²⁺] = 10 [Cu²⁺]. Substituting this into the expression for Q:
The number of electrons transferred, n, is 2. Substituting E°(cell) = 1.1 V, Q=10, n=2 into the Nernst equation:
Since ln(10) = 2.303 log₁₀(10) = 2.303, the equation becomes:
Step 4: Substitute Ecell into the ΔG formula.
Distribute the -2F term:
Therefore, the expression for ΔG is J mol⁻¹.
Final Answer: The correct expression is .
Nernst Equation: This equation is used to calculate the cell potential under non-standard conditions. The general form is: . At 298 K, it is often written using log base 10: .
Gibbs Free Energy and Cell Potential: The relationship shows that a positive cell potential (spontaneous reaction) corresponds to a negative ΔG. Under standard conditions, this becomes .
Reaction Quotient (Q): For a general reaction aA + bB → cC + dD, the reaction quotient is . Pure solids and liquids are not included in the expression for Q.