Question

I want to ask in the half oxidation and reduction reaction .Like Cu+2 + 2e- = Cu,
it is the reduction reaction as it gain of 2 electrons but why (+ )sign is used in this ....does it means lose of electrons but in the reaction of Zn = Zn+2 +2e-, it is oxidation reaction which also shows the same sign (+ )
and showing loss of electron.

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Answer 1

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bcoz those who gain electrons always have positive charge and those who loses elections have negative charge

Answer 2

Answer:

Standard electrode reduction potential for different metals are tabulated for:

Reduction half reactions, indicating the tendencies of the electrodes to behave as cathodes.

Tendency to behave as cathode is considered towards Standard Hydrogen Electrodes (SHE).

Now the negative sign of standard reduction potential for any metal indicates that an electrode when joined with standard hydrogen electrode (SHE) will not act as cathode and reduction reaction half reaction will not be spontaneous. Instead the said metal will act as anode and oxidation (loss of electrons) will occur spontaneously on this electrode.

For example, standard reduction potential of zinc is -0.76 volts. So, when zinc electrode is joined with SHE, it will not act as cathode as reduction half reaction will not occur spontaneously, instead Zinc electrode will acts as anode (-ve electrode) i.e., oxidation occurs on this electrode spontaneously. The +ve sign of standard reduction potential indicates that the electrode when joined with SHE acts as cathode and reduction occurs on this electrode spontaneously.

Positive sign of reduction potential physically signifies non-spontaneous reduction reaction and negative sign indicates spontaneous reduction reaction. From electrochemical point of view this is justified as below:

ΔG=−nFE∘ Gibbs free energy of an electrochemical cell.

E is electrode potential

ΔG is Gibbs free energy, The difference between the enthalpy of a system and the product of its entropy and absolute temperature; a measure of the useful work obtainable from a thermodynamic system at constant temperature and pressure.

F = Faraday Constant

n is number of electrons transferred in the balanced equation

Negative values of E (Reduction potential) will lead to positive values of ΔG and vice versa. ΔG is positive, the reaction is non-spontaneous, and when ΔG is negative, the reaction is spontaneous. So, we can summarize as:

If E°cell > 0, then ΔG is negative and the reaction is spontaneous. This is the principle of galvanic cell where chemical energy is converted to electrical energy. Electrons supplied by electrodes move in external circuit.

If E°cell < 0, then ΔG is positive and the reaction is non-spontaneous. This is the principle of electrolytic cell. Due to non-spontaneous reaction, electrical energy is to be supplied to initiate the reaction.

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