Electrochemical reactions are essential in chemistry, providing insights into the spontaneity of reactions and the associated entropy changes. This set of notes aims to elucidate how to predict the spontaneity of reactions using electrochemical data and to understand the indirect changes in the surroundings' entropy due to the transfer of heat energy.
Predicting the Spontaneity of a Reaction
Standard Electrode Potentials (E°)
- The standard electrode potential (E°) is a measure of the tendency of a species to gain electrons (be reduced), measured under standard conditions.
- It is provided for half-reactions, with the standard hydrogen electrode (SHE) set at 0 V.
- A positive E° indicates a favourable reduction half-reaction, while a negative E° suggests a tendency to lose electrons (be oxidised).
A standard hydrogen electrode (SHE)- Used by scientists as a reference electrode with a potential zero.
Image courtesy of Sandip
Calculating Cell Potentials (E°cell)
- To determine the full electrochemical cell's potential, use: E°cell = E°cathode - E°anode.
- A positive E°cell indicates a spontaneous reaction under standard conditions.
- Conversely, a negative E°cell implies a non-spontaneous reaction under standard conditions.
Nernst Equation
- The Nernst Equation helps calculate cell potentials under non-standard conditions: E = E° - (RT/nF) * ln(Q).
- Here, R is the gas constant, T is the temperature in Kelvin, n is the number of moles of electrons transferred, F is the Faraday constant, and Q is the reaction quotient.
Image courtesy of Science Query
Spontaneity and Electrode Potentials
- A positive E°cell suggests a spontaneous reaction, with a larger positive value indicating a stronger driving force for spontaneity.
- A negative E°cell suggests a non-spontaneous reaction, requiring external energy input to proceed.
Indirect Entropy Change of the Surroundings
Transfer of Heat Energy
- During an electrochemical reaction, energy transfer to the surroundings often occurs as heat.
- This transfer results in a change in the entropy of the surroundings.
- The direction and magnitude of this entropy change depend on the exothermic or endothermic nature of the reaction.
Exothermic Reactions
- In exothermic reactions, heat is released to the surroundings.
- This results in an increase in the entropy of the surroundings.
- The release of energy is associated with a decrease in the system's entropy, but the overall entropy of the universe increases.
Endothermic Reactions
- In endothermic reactions, heat is absorbed from the surroundings.
- This results in a decrease in the entropy of the surroundings.
- Although the system's entropy increases due to energy absorption, the overall change in the universe's entropy may be positive or negative, depending on the specific reaction.
Image courtesy of udaix
Relationship with Gibbs Energy
- The change in Gibbs Energy (ΔG) is also a critical factor in determining spontaneity and is related to entropy changes.
- A negative ΔG indicates a spontaneous reaction, while a positive ΔG indicates a non-spontaneous reaction.
- The relationship between ΔG, entropy change (ΔS), and enthalpy change (ΔH) is given by ΔG = ΔH - TΔS.
By understanding and applying these principles, students can predict the spontaneity of electrochemical reactions and comprehend the associated entropy changes in the system and its surroundings.
FAQ
The Faraday constant (F) in the Nernst equation represents the total charge of one mole of electrons. It is calculated as the product of the elementary charge (the charge of one electron, approximately 1.602 x 10-19 C) and Avogadro’s number (approximately 6.022 x 1023 mol-1). Thus, F is approximately 96485 C mol-1. In electrochemical reactions, the number of moles of electrons transferred is a crucial factor, and the Faraday constant allows for the incorporation of this aspect into calculations of cell potential under non-standard conditions using the Nernst equation.
During a spontaneous electrochemical reaction (positive E°cell), the system usually releases energy, often in the form of heat, to the surroundings. This release of energy results in an increase in the entropy of the surroundings because the molecules in the surroundings gain energy and their movement becomes more disordered. The second law of thermodynamics states that for a process to be spontaneous, the total entropy change of the universe (system + surroundings) must increase. In a spontaneous electrochemical reaction, even though the system might lose entropy as reactants form products, the increase in entropy of the surroundings is greater, leading to an overall increase in the entropy of the universe, driving the reaction forward.
The sign of the cell potential (E°cell) is crucial in determining the direction of electron flow in an electrochemical cell. A positive E°cell indicates a spontaneous reaction, meaning electrons will flow naturally from the anode (oxidation) to the cathode (reduction). In this scenario, the cell can do work on its surroundings. However, if E°cell is negative, the reaction is non-spontaneous, and an external voltage (greater than the magnitude of E°cell) needs to be applied to drive the reaction in the desired direction. This scenario is common in electrolytic cells, where electrical energy is used to drive a non-spontaneous reaction.
A reaction with a negative ΔG value is generally spontaneous. However, the spontaneity of a reaction can be influenced by temperature. The relationship ΔG = ΔH - TΔS shows that both enthalpy (ΔH) and entropy (ΔS) changes, as well as temperature (T), play a role in determining ΔG. For reactions with a positive ΔS (increase in disorder), an increase in temperature will make the reaction more spontaneous. Conversely, for reactions with a negative ΔS (decrease in disorder), increasing the temperature could make the reaction non-spontaneous (ΔG positive), even if ΔG is negative at lower temperatures.
A spontaneous reaction (positive E°cell) in an electrochemical cell may not proceed at a noticeable rate due to kinetic barriers. The reaction’s spontaneity only indicates the thermodynamic feasibility, not the speed at which the reaction will occur. A reaction might have a high activation energy, requiring a significant input of energy to initiate the process. Additionally, the presence of a catalyst, the concentration of reactants, and the surface area of electrodes can also influence the rate of reaction. Even a spontaneous reaction can be practically useless if it proceeds at an imperceptibly slow rate.
Practice Questions
The overall cell potential (E°cell) can be determined by subtracting the anode's standard electrode potential from the cathode's standard electrode potential: E°cell = E°cathode - E°anode. Using the given values, E°cell = +0.50 V - (-0.30 V) = +0.80 V. A positive value for E°cell indicates that the reaction is spontaneous under standard conditions. This is because a positive cell potential suggests that the reaction has a tendency to move towards the formation of products, leading to a favourable reduction half-reaction.
If an electrochemical reaction absorbs heat from its surroundings, it is an endothermic process. An endothermic reaction results in a decrease in the entropy of the surroundings because heat energy is taken up from the surroundings and used by the system. As the entropy of the surroundings decreases, the overall entropy change of the universe might be negative or positive, depending on the specific reaction. For the reaction to be spontaneous, the Gibbs Energy change (ΔG) should be negative. Considering the relationship ΔG = ΔH - TΔS, the absorption of heat suggests a positive ΔH. The spontaneity will then depend on the magnitude of TΔS. If TΔS is larger than ΔH, the reaction will be spontaneous; otherwise, it will be non-spontaneous.
