Periodicity - Electrical Conducitiy Trend

Across the period, there is a change in the electrical conductivity of the elements. Clearly, Na, Mg and Al are metals and hence are good conductors of electricity. While, P₄, S₈, Cl₂ and Ar are simple discrete molecules; the absence of mobile charge particles prevent them from conducting electricity. Lastly, Si as we commonly know is a semi-conductor.

The diagram below depicts the electricial conductivity across Period 3.

Electricial conductivity trend.

In order for us to explain the trend, we need to know the structure the substance exists as. If the substance is a metal, it will have a sea of delocalised electrons available which acts as mobile charge carriers for electricity to be conducted. Thus, simplistically more available the sea of electrons, the better the conductivity. Therefore, structure provides important information to whether there are any charge particles which can move easily.

Flowchart to explaining the conductivity trend.

Interestingly, the explanation to why Si acts as a semi-conductor can be found here. But it is not part of the "A" level syllabus. If you can appreciate the article, you may wish to think about what is the more accurate representation of metallic bonds.

Lastly, there is a slight anonamlly. In this post, I accounted that Al has a higher electrical conductivity as it has a greater pool of delocalised electrons as Al has more valence electrons - Ironically, this contradicts the reasoning to why Al's melting point is not much higher than Mg. However, the details of this reasoning is beyond the scope of this discussion.
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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.}

Periodicity - Ionisation Energy Trend

Across the period, another trend which we need to explain is how the first ionisation energy varies as atomic number increases. The diagram below shows the first ionisation energy trend.

1st IE trend across the period.

Generally, as we move across the period, the 1st ionisation energy increases. This is because despite an electron is added, the number of inner shells remains the same hence the shielding effect is relatively constant. While the nuclear charge increases. Therefore, the combination of these two reasons result in an overall increase in the 1st ionisation energy.

There are a couple of dips seen and these are exceptions to the trend. Generally, the dips are present because the causes for the dips are more significant than the effect due to increase in effective nuclear charge. The mind map below provides you with a structure which will explain the trend, including the exceptions.

Mindmap to explain trend across Period 3.

In addition, there is another explanation to why the ionisation energy differs between period. We move from Period 2 to Period 3, the size of the atoms get larger. The added inner shell of electrons results in the valence shell of elements in Period 3 to be further away from the nucleus. Hence, electrostatic attraction between nucleus and valence shell becomes weaker, therefore 1st ionisation energy is smaller.

Trends to notice across the period

Across the period, there are 4 main trends to take note. These trends usually appear even in the general trend for 2^nd and 3^rd I.E and their explanation used to explain is similar. However, do take note that sometimes we do not see the dip (e.g. when you compare the 2^nd I.E of S and Cl we don't see the expected drop) and that is because of the bold phrase of the article.

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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.}

Periodicity - Melting Point Trend.

In the understanding of periodicity, we will need to explain for the melting point trend across the period. Below is a picture of a graph which shows the melting point trend of the elements across the period.

Diagram to show melting point trend.

Across the period from Na to Ar: Na, Mg and Al exist as giant metallic structures. While Si exists as giant covalent molecule (macromolecular structure). While P, S, Cl and Ar exists as simple discrete molecules. These elements' standard states formula of P₄, S₈, Cl₂ and Ar respectively.

Hence, to explain the melting point trend across the period, it is recommended to define the structures the substance exists as first before proceeding further. The following diagram constructs the explanation needed.

Mindmap which constructs the explanation.

Lastly, Al contains 3 valence electrons. Although these 3 should contribute to the sea of electrons, it does not happen because it will be very difficult for Al to lose so many electrons to contribute to the sea of electrons. Therefore, the increase in melting point from Al to Mg is much smaller than the increase form Mg to Na.

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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 - 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.

^{-- -- -- -- --
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.}

Chemical Energetics - Applying Gibbs Free Energy

From the chart below, we can observe that there are certain chemical reactions whose change in Gibbs Free Energy change with temperature. Hence, we are able to perform mathematical calculations to predict at which temperatures the reaction will happen, become spontaneous and cease to occur.

Table to show change in Gibbs Free Energy and T
(1) Predict the temperature for reactions to happen or become spontaneous.

The difference between the two terms is quite simple. Spontaneous chemical reaction implies that the change in Gibbs Free Energy is smaller than 0. Reaction whose change in Gibbs Free Energy is equals to 0 can happen; they are just reversible. Hence, for reactions to happen, the change in Gibbs Free Energy has to be smaller or equals to 0.

Steps to predict Temperature
(2) Predict the temperature for reactions to become non-spontaneous.

In non-spontaneous reactions, we are implying that we need to determine temperatures where the reaction's change in Gibbs Free Energy becomes greater than 0.

Steps to predict Temperature
(3) Units.

The temperature used in the formula to calculate change in Gibbs Free Energy is in Kelvin (K). While the unit for change in Enthalpy is in KJ mol^-1 and the unit for change in Entropy is J mol^-1K^-1. Hence, you need to do the necessary conversion and exercise care when apply the formula.

^{-- -- -- -- --
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.}

Homework - Change in venue.

Dear 1CH304,

There is a new online assignment, please read the following:

Usage of Google Site - Ground Rules

Regards
Mr Kwok