Final answer:
To balance the electrochemical cell reaction and determine spontaneity, one must write the half-reactions, combine them, and use standard reduction potentials to calculate E°cell, indicating spontaneity with a positive value.
Step-by-step explanation:
The question pertains to electrochemistry, specifically writing a balanced equation for a galvanic cell and calculating the standard potential to determine the spontaneity of the reaction. To write the balanced equation for the provided cell schematic (Pt(s) | H₂(g) | H+ (aq)||Br₂(aq), Br⁻(aq) | Pt(s)), we need to consider the oxidation and reduction half-reactions occurring at each electrode. Hydrogen gas is oxidized to protons (oxidation), while molecular bromine is reduced to bromide ions (reduction).
Oxidation at the anode: H₂(g) → 2H+(aq) + 2e-
Reduction at the cathode: Br₂(aq) + 2e- → 2Br⁻(aq)
Combining both half-reactions gives us the full balanced equation:
H₂(g) + Br₂(aq) → 2H+(aq) + 2Br⁻(aq)
To calculate the standard cell potential (E°cell), you would look up the standard reduction potentials for each half-reaction in a table of standard potentials. Assuming E°cell is positive after calculation, the reaction is spontaneous under standard state conditions as positive E°cell indicates spontaneity.
Achieving balanced chemical equations requires the application of coefficients and understanding that no coefficients are written for a one, subscripts in a chemical formula should never be changed, and that all coefficients should be in the lowest possible ratio.
An instantaneous reaction is different from a spontaneous reaction as it refers to the speed of the reaction, not its spontaneity. A reaction that releases free energy and moves to a more stable state is spontaneous, which may or may not be instantaneous.