Calculating the potential of a battery

O calculating the potential of a battery is performed when you want to know in advance (before assembly) what the voltage will be, emf (electromotive force) or ddp (potential difference) that a given cell will generate from two metals with different characteristics.

The term was used in advance because, if we want to measure the voltage of any device, either a cell or a battery, we just need to use a multimeter.

Each of the metals that participate in the composition of the cell's anode and cathode has different abilities to undergo oxidation (ability to lose electrons) and reduction (ability to gain electrons). This ability to oxidize or reduce is called a potential. Therefore, the metals that make up the electrode of a cell may have:

• Standard reduction potential (E_red): is the ability of the electrode to suffer the phenomenon of reduction. This potential is measured in volts (V) and its value is the same as the standard oxidation potential, but with the opposite sign.

• Standard oxidation potential (E

  _oxy): it is the capacity of the electrode to suffer the phenomenon of oxidation. This potential is measured in volts (V) and its value is the same as the standard reduction potential, but with the opposite sign.

NOTE: A standard potential can be influenced by the temperature and concentration of the solutions used in assembling the pile.

To perform calculating the potential of a battery, we must know the values ​​of the standard potentials, whether oxidation or reduction, and apply in the following Mathematical Equation:

ΔE = E_red (larger) - And_red (smaller)

or

ΔE = E_oxy (larger) - And_oxy (smaller)

• ΔE = variation of the potential of a battery (emf or ddp).

This equation uses the potential variation because, as in the device, oxidation and reduction occur, the measure of the potential will be given simply by subtracting the potential of each of the electrodes (metals).

NOTE: We can only say that we are working with a stack when the çcalculating the potential results in a positive value.

Now follow some examples of questions that address the çcalculating the potential of a battery:

Example 1: (UFSC-SP) Given the semi-reactions:

2 Fe⁺² → 2 Fe⁺³ + 2 and E_oxy = - 0.77 V

2 Cl^-1 → 1 Cl₂ + 2 and E_oxy = - 1.36V

Calculate the potential of the reaction below (stack) and say if it is spontaneous or not, checking the correct option:

2 Fe⁺² + Cl₂ → 2 Fe⁺³ + 2 Cl^-1

a) – 0.59V, it is not spontaneous

b) 0.59 V, it is not spontaneous

c) 0.59 V, is spontaneous

d) – 2.13 V, it is not spontaneous

e) 2.13 V, is spontaneous

Resolution:

As the exercise asks you to determine the value of the battery's potential, it is enough to analyze the oxidation potentials provided to judge which is the highest and which is the lowest:

2 Fe⁺² → 2 Fe⁺³ + 2 and E_oxy = - 0.77 V (This is the biggest)

2 Cl^-1 → 1 Cl₂ + 2 and E_oxy = - 1.36V (This is the smallest)

Then, just apply it in the formula for calculating the potential of a battery:

ΔE = E_oxy (larger) - And_oxy (smaller)

ΔE = - 0.77 - (-1.36)

ΔE = - 0.77 + 1.36

ΔE = + 0.59V

As the potential found has a positive value, we have a spontaneous process. Letter ç).

Example 2: (UFMS) Consider the semi-reactions, indicated below, with their respective standard reduction potentials, in volts (V):

Ag⁺ + and → Ag E_red = 0.80V

Ass⁺² + 2 and → Cu E_red = 0.34V

Calculate the potential of the reaction below (stack) and say if it is spontaneous or not, checking the correct option:

Cu + 2 Ag⁺→ Cu⁺² + 2 Ag

a) – 4.6 V

b) - 0.46 V

c) + 0.46 V

d) + 1.14V

e) - 1.14 V

Resolution:

As the exercise asks you to determine the value of the stack's potential, you simply need to look at the provided reduction potentials to judge which is the largest and which is the smallest:

Ag⁺ + and → Ag E_red = 0.80V (This is the biggest)

Ass⁺² + 2 and → Cu E_red = 0.34V (This is the smallest)

Then, just apply it in the formula for calculating the potential of a battery:

ΔE = E_red (larger) - And_red (smaller)

ΔE = 0.80 - (0.34)

ΔE = 0.80 - 0.34

ΔE = + 0.46V

As the potential found has a positive value, we have a spontaneous process. Letter ç).

By Me. Diogo Lopes Dias

Source: Brazil School - https://brasilescola.uol.com.br/quimica/calculo-potencial-uma-pilha.htm