Electrochemistry - Calculation of Ecell.

One of the uses in the calculation of E_cell is that it can used to predict the feasibility of a redox reaction. When E_cell^o >0, the redox reaction is feasible. The converse implies that the redox reaction is not feasible.

The following steps are used in the calculation:

(1) Predict which is the species that will be oxidised.

(2) Predict which is the species that will be reduced.

(3) Write the half-equation for the species that is oxidised; ensuring that it is found on the RHS.

(4) Write the half-equation for the species that is reduced; ensuring that it is found on the LHS.

(5) Apply anti-clockwise rule to calculate E_cell^o

The video below takes you through the above steps, demonstrate how the E_cell^o is calculated for a reaction between Zn and Cu²⁺ .



Lastly, when calculating the E_cell^o, please be mindful that we assume that the standard condition of 1 moldm^-3 was used. However, when a smaller (or different) concentration is used, the reduction potential of the species is affected because of Le Chatelier's Principle as the equilibrium is affected.

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Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}

Electrochemistry - Reduction Potentials in electrolysis

In an earlier entry on the topic of Electrochemistry -Reduction Potential, I spoke about my interpretation of reduction potential, whose main tenet is for us to decide which substance is more susceptible to reduction and which is less. Therefore, having the positive and negative sign is IMPORTANT.

In this entry I would be writing about the application (or uses) of the Standard Reduction Potential. There are three main uses, (a) to determine the Ecell of an electrochemical cell. (b) to determine whether a redox reaction will occur and (c) deciding which substance will be electrolysed. These will be discussed in three series of posts. The third and final one is:

(c) Using reduction potential for electrolysis.

In (a), we actually converted chemical energy into electrical energy. While in electrolysis, the converse actually occurs. We are supplying electrical energy such that a chemical reaction occurs.

Hence, we are looking out for reduction process whose reduction potential is less positive, therefore requiring energy aid for reaction to occur.

While in the case of oxidation processes, we are looking for their corresponding reduction potential to be more positive. This would me that the reduction step is favourable and the oxidation is not, hence, energy will have to be supplied such that the oxidation step can occur.

In addition, most of the times the electrolytes given are quite clear to whether it would be made to oxidised or reduce. Only water is ambiguous as it can be made to oxidised or reduced.

When there are two species which can be reduced (or oxidised), we will compare the reduction potential (or corresponding reduction potential) and the one with lower energy requirement will be reduced (or oxidised).

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Article written by Kwok YL 2007.

Disclaimer and remarks:

• If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.
• This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.
• Due to limitation of the blog and my limited code knowledge, I am unable to do superscripts and subscripts, hopefully in time to come, I would be able to do so.

Electrochemistry - Reduction Potential in Feasibility of Redox Reactions

In an earlier entry on the topic of Electrochemistry -Reduction Potential, I spoke about my interpretation of reduction potential, whose main tenet is for us to decide which substance is more susceptible to reduction and which is less. Therefore, having the positive and negative sign is IMPORTANT.

In this entry I would be writing about the application (or uses) of the Standard Reduction Potential. There are three main uses, (a) to determine the Ecell of an electrochemical cell. (b) to determine whether a redox reaction will occur and (c) deciding which substance will be electrolysed. These will be discussed in a series of three posts. The second one is:

(b) To determine feasibility of a redox reaction

Firstly, you need to identify which is the species that will be oxidised and which will be reduced. For example, in the question of asking whether I^- will cause the reduction of Br₂. In this situation I^- will be oxidised to I₂, while Br₂ will be reduced to Br^-.

Therefore, by using logical deduction, for the redox reaction between Br₂ and I^- to feasible, it must be that Br₂ tends to reduce more favourably than I₂. (If the converse is true, wouldn't the formation of I₂ and Br^- then cause I₂ to oxidise Br^-?)

Thus, if we substract the reduction potential of I₂ from the reduction potential of Br₂, we will get a positive number. This positive number is the Ecell of this redox reaction and it informs us that the redox reaction is spontaneous.

Therefore, we will have the equation Ecell = Ered - Eoxd . (Since reduction potential of Br₂ is greater than I₂ (+1.07V against +0.54V), thus Br₂ will oxidise I^-)

This equation can also be used to predict whether a reaction is non-spontaneous. For example, to predict whether a reaction between F^- and I₂ will occur, we are suggesting that F₂ and I^- are formed.

However, the reduction potential of I₂ is smaller than the reduction of F₂. Therefore the redox reaction between F^- and I₂ wouldn't occur. To prove it, we calculate Ecell of the reaction (Ecell = +0.54V - (+2.87)) which is smaller than zero.

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Article written by Kwok YL 2007.

Disclaimer and remarks:

• If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.
• This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.
• Due to limitation of the blog and my limited code knowledge, I am unable to do superscripts and subscripts, hopefully in time to come, I would be able to do so.

electrochemistry - Reduction Potential in electrochemical cell

In an earlier entry on the topic of Electrochemistry -Reduction Potential, I spoke about my interpretation of reduction potential, whose main tenet is for us to decide which substance is more susceptible to reduction and which is less. Therefore, having the positive and negative sign is IMPORTANT.

In this entry I would be writing about the application (or uses) of the Standard Reduction Potential. There are three main uses, (a) to determine the Ecell of an electrochemical cell. (b) to determine whether a redox reaction will occur and (c) deciding which substance will be electrolysed. These will be discussed in a series of three posts. The first one is:

(a) To Determine Ecell of an electrochemical cell.

An electrochemical cell consists of an anode and a cathode, each in their respective half-cell. The cells are linked together by a salt bridge. Using Cu²⁺ + e <-> Cu, generally a Cu half-cell will contain Cu and Cu²⁺.

When this half-cell is connected to another half-cell (e.g the H₂ half-cell), a potential difference is caused because of the different ease of reduction of the two species.

Since, the reduction potential of Cu²⁺ is more positive than the H⁺ (+0.34V against 0V), electrons flows from the H₂ half-cell (which is less willing to be reduced thus oxidation occurs) to the Cu²⁺ half-cell (where reduction is more favoured). Thus, cell voltage will be +0.34 - 0 = +0.34V.

From this, it results in the equation Ecell = Ereduction half-cell - Eoxidation half-cell. when Ecell > 0, the reduction potential of the reduction half-cell must be greater than the oxidation half-cell. And in simple terms, where one particular cell prefers reduction more than the other cell.

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Article written by Kwok YL 2007.

Disclaimer and remarks:

• If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.
• This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.
• Due to limitation of the blog and my limited code knowledge, I am unable to do superscripts and subscripts, hopefully in time to come, I would be able to do so.

Electrochemistry - Reduction Potentials.

In this entry, I shall begin talking about standard reduction potentials. Hopefully, it will clear some doubts and instill some understanding.

In essence, this term actually is about a REDUCTION process that is occuring at STANDARD condition and we are trying to find out how FAVOURABLE the process will be.

But how did this come about? The following is a simple description to how standard reduction potentials are obtained.

Reduction Potential

Are you curious to why is the standard reduction potential in your data are all written with the reversible arrow? In simple terms (as compared to the actual explanation), the explanation is as follows:

Let's assume we observed that A is reduced to A^- in the atmosphere. But how do we determine that A indeed likes to be reduced when we compare it to another substance B?

The easiest way would be to put A into a solution of A^- and when an equilibrium between A and A^- is reached, and if there is more A^-, we would see that the remaining A being increasing positive since some of it has to lose electrons so that the other A particles can be reduced.

This steps is repeated to see if substance B likes to reduce and its the same for all the other substances.

But, how are we going to compare the ease to reduce between A and B? This can be answered by connecting an A/A^- half-cell to B/B^- half-cell and attached a voltmeter between them and allow both to reach equilibrium.

At equilibrium, the electrode of the half-cell which has a more positive charge will pull electrons from the other electrode, thus at the other half-cell would be made to prefer oxidation.

Therefore, the direction of electron flow will indicate to us which substance would favour reduction more and rendering (or forcing) the other to favour oxidation.

However, we can't measure the electron flow, we actually measure the potential difference. Which implies the cell that prefers reduction more than the other (at equilibrium) would have it electrode gain a positive charge.

While, the half-cell that prefers reduction less (than the other cell thus is made to allow oxidation to occur such that it can supply electrons to the other cell) will gain a negative charge hence, we have a measurable potential difference.

However, we are many different elements in the periodic table and obviously there will be an overwhelming number of comparisons to make and it will make such comparison tedious. Thus, all the elements are compared to the H⁺/H2 system. Essentially, their reduction is compared with respect to the reduction of H⁺. What we have done, is to arbituary set the reduction potential of H⁺ as zero.

When a substance ease of reduction is greater than H₊, its reduction potential will be positive. Converse is true for substances who prefers to remained in the oxidised form, thus metal/cation half-cell usually have a negative reduction potential (taken with respect to the H⁺/H₂).

Concentration and reduction potentials

In addition, if you recall Le Chatelier's principle, concentration changes causes equilibrium position to change and that will result in your reduction potential to differ from one concentration to another (How concentration will affect the value of reduction potential is beyond the scoop of your syllabus, you may refer to the Nernst equation to find out more). Hence, some further standardisation is required, therefore standard conditions is set.

Standard Condition

In electrochemistry, like in chemical energetics, refers to standard conditions as the following. Temperature of 298 K, pressure of 1 atm, concentration of any aqueous solution to be 1 M.

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Article written by Kwok YL 2007.

Disclaimer and remarks:

• If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.
• This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.
• Due to limitation of the blog and my limited code knowledge, I am unable to do superscripts and subscripts, hopefully in time to come, I would be able to do so.