Standard cell potential intensive property

To calculate standard cell potentials from the half-cell potentials in Table 9-1, there are four principles that we must know (1) When we reverse the direction of the chemical reaction, we change the sign of the potential. (2) If we multiply the coefficients in the equation by some number, we do NOT change the potential. Potential is an intensive property, and does not depend on the quantity of reagents. (3) When we add chemical equations for half-cells, we add the corresponding potentials. (4) A positive potential for a complete cell reaction means that the reaction proceeds spontaneously in the direction of the equation, and a negative potential means that the reaction goes spontaneously in the opposite direction. 

Since the standard cell potential is an intensive property. 

Since the number of electrons lost must equal the number gained, the half-reactions must be multiplied by integers as necessary to achieve the balanced equation. However, the value of%° is not changed when a half-reaction is multiplied by an integer. Since a standard reduction potential is an intensive property (it does not depend on how many times the reaction occurs), the potential is not multiplied by the integer required to balance the cell reaction. 

It is important to understand that the standard reduction potential is an intensive property (like temperature and density), not an extensive property (like mass and volume) [ M4 Section 1.4]. This means that the value of the standard reduction potential does not depend on the amount of a substance involved. Therefore, when it is necessary to multiply one of the half-reactions by a coefficient in order to balance the overall equation, the value of for the half-reaction remains the same. Consider a galvanic cell made up of a Zn half-cell and an Ag half-cell ... 

This reaction is represented by the same cell diagram given above, but the electron number is one consequently, the Gibbs energy is one-half of that previously calculated, but the value of Eceii is the same. This result supports the fact that the standard reduction potential is an intensive property but the Gibbs energy is an extensive property. Finally, the reaction tells us that when 0.5 mole of Cu is reduced, 32.8 kJ of energy is released and one mole of electrons passes from the anode to the cathode. 

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