# Enthelpy of Reaction and Efficiency of Galvanic Cells

• Dario56
In summary, the efficiency of electrochemical energy devices such as batteries and fuel cells is defined as the ratio of Gibbs energy change of the system to enthalpy change of the system for a reversible cell. The enthalpy of reaction is used as the energy input, but it is not clear why this is the case since the physical interpretation of enthalpy change in electrochemical cells is not straightforward. This is because when there is no non-PV work, enthalpy change is equal to heat exchanged with surroundings at constant pressure and temperature. However, in electrochemical cells, there is also electrical work, making the interpretation of enthalpy change more complex. More research is needed to fully understand the reasoning behind using enthalpy of
Dario56
TL;DR Summary
Why is Enthalpy of Reaction Used as Energy Input in Galvanic Cells When Defining its Efficiency?
For electrochemical energy devices such as batteries or fuel cells working reversibly, efficiency is defined as:

η=ΔrG⦵/ΔrH⦵

Since cell operates reversibly Gibbs energy change of the system is equal to electrical work done by the cell, so we can instead just use standard Gibbs energy of reaction as energy output. What I am not clear about is why is enthalpy of reaction used as energy input? Since electrochemical energy devices transform chemical energy of electroactive materials to electrical energy, energy input should be chemical energy, but I am not sure why enthalpy of reaction represents chemical energy? Enthalpy of reaction is given by this equation:

ΔrH⦵=ΔrG⦵+TΔrS⦵

Since cell operates reversibly, ΔrG⦵ represents reversible electrical work (maximum non-PV work done by the cell) and TΔrS⦵represents heat exchanged with surroundings to keep cell at constant T in reversible operation since change in Gibbs energy as measure of non-PV work is only applicable for closed systems at constant p and T.

Dario56 said:
Summary:: Why is Enthalpy of Reaction Used as Energy Input in Galvanic Cells When Defining its Efficiency?

For electrochemical energy devices such as batteries or fuel cells working reversibly, efficiency is defined as:

η=ΔrG⦵/ΔrH⦵

Since cell operates reversibly Gibbs energy change of the system is equal to electrical work done by the cell, so we can instead just use standard Gibbs energy of reaction as energy output. What I am not clear about is why is enthalpy of reaction used as energy input? Since electrochemical energy devices transform chemical energy of electroactive materials to electrical energy, energy input should be chemical energy, but I am not sure why enthalpy of reaction represents chemical energy? Enthalpy of reaction is given by this equation:

ΔrH⦵=ΔrG⦵+TΔrS⦵

Since cell operates reversibly, ΔrG⦵ represents reversible electrical work (maximum non-PV work done by the cell) and TΔrS⦵represents heat exchanged with surroundings to keep cell at constant T in reversible operation since change in Gibbs energy as measure of non-PV work is only applicable for closed systems at constant p and T.
Can you please provide a reference for that efficiency equation?

Chestermiller said:
Can you please provide a reference for that efficiency equation?
This equation isn't correct, my mistake. It should be ratio of Gibbs energy change of system divided by enthalpy change of system for reversible cell. There should be no standard reaction Gibbs energy in equation.
Chestermiller said:
Can you please provide a reference for that efficiency equation?
Basically, I am not sure why is enthalpy change of system taken as energy input since I don't know what is physical interpretation of enthalpy change in electrochemical cell. When there is no non-PV work, it is equal to heat exchanged with surroundings at constant p and T. But, in electrochemicall cells we have electrical work, so interpretation of enthalpy change of system isn't as straightforward, so I am not sure why is it taken as energy input?

## 1. What is the definition of Enthalpy of Reaction?

The Enthalpy of Reaction, also known as the Heat of Reaction, is the amount of heat energy released or absorbed during a chemical reaction at constant pressure. It is represented by the symbol ΔH and is measured in units of joules (J) or kilojoules (kJ).

## 2. How is the Enthalpy of Reaction calculated?

The Enthalpy of Reaction is calculated by taking the difference between the sum of the products' enthalpies and the sum of the reactants' enthalpies. This can be expressed as: ΔH = ΣH(products) - ΣH(reactants).

## 3. What is the Efficiency of a Galvanic Cell?

The Efficiency of a Galvanic Cell is a measure of how much of the energy produced by the cell is actually converted into electrical energy. It is represented as a percentage and is calculated by dividing the electrical energy output by the total energy input.

## 4. How can the Efficiency of a Galvanic Cell be improved?

There are several ways to improve the Efficiency of a Galvanic Cell. These include using electrodes with higher surface area, optimizing the concentration of electrolyte solutions, and reducing the resistance in the external circuit by using thicker wires or shorter distances.

## 5. What factors affect the Enthalpy of Reaction and Efficiency of Galvanic Cells?

The Enthalpy of Reaction and Efficiency of Galvanic Cells can be affected by several factors, including temperature, concentration of reactants and products, surface area and type of electrodes, and the type of electrolyte solution used. Additionally, the presence of impurities or side reactions can also impact these values.

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