Graphite vs Diamond Properties - Their structure

This is by far my favourite topic for chemistry. So many comparison questions have been set regarding these 2 carbon allotropes.

Let's start of with the structure of graphite. Graphite has a unique hexangonal structure which is a result of the carbon atoms forming 3 bonds with each other (as opposed to 4 bonds normally).

As you can see from the diagram, each carbon is only bonded to 3 other atoms and they are arranged in hexangonal layers and the layers are held together by weak intermolecular force of attraction (or van de waal's forces).

Lets look at diamond now. Carbon atoms in diamond bonded to 4 other carbon atoms in a tetrahedral structure. This rigid and tough structure gives diamond its ultra tough properties, like hardness, high melting boiling points.

For comparison of properties regarding graphite and diamond stay tune!

Enjoy!

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Json Lim

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Titration Salt Making Procedures

Salt making is by far my favourite topic in chemistry. For any salt making procedures, we need to know the following information.
a)The reactants you are using to produce them,
b)The chemical equation,
c)Write up steps to produce the salt.

This blog entry is for titration, hence the first thing to decide is the reactants needed reaction.
A) Reactants
i)An Acid - Well you know the usual.
ii)An Alkali - To identify an alkalis, they are any group 1 hydroxides, ammonium hydroxides and calcium hydroxide.

That's all.
Let's take this salt as an example, sodium sulfate.
As the name of the salt suggest, the sulfate part comes from the acid used and the sodium part comes from the alkali.
Logically these are the reactants,
Acid - sulfuric acid (since the salt is a sulfate)
Alkali - sodium hydroxide (since the salt is a sodium substance)

So there is it, the reactants.
Next the chemical equation.
Since Alkali + Acid -------> Salt + Water
We will have this word equation
Sodium hydroxide + sulfuric acid --------> Sodium sulfate + water
2NaOH(aq) + H2SO4(aq) ------> Na2SO4(aq) + 2H2O(l)

Last part is the write up. This is the template we can use.
1. Pipette 25 cm3 of acid into a conical flask. Add 2-3 drops of a suitable indicator.
2. Add alkali from a burette gradually until the indicator changes colour. Note the volume of the alkali needed to neutralise the acid.
3. Repeat the titration by adding this volume of alkali to another 25 cm3 of acid.
4. Evaporate the salt solution until it becomes saturated.
5. Filter the crystals and dry between filter paper.

So in our answer we change the keywords to fit our reagents, so this is how our eassay would look like. Of course the eassay will not be in point form, i put it this way to aid understanding. The keywords that i changed are in red.
1. Pipette 25 cm3 of sulfuric acid into a conical flask. Add 2-3 drops of a suitable indicator.
2. Add sodium hydroxide from a burette gradually until the indicator changes colour. Note the volume of the sodium hydroxide needed to neutralise the sulfuric acid.
3. Repeat the titration by adding this volume of sodium hydroxide to another 25 cm3 of sulfuric acid.
4. Evaporate the sodium sulfate solution until it becomes saturated.
5. Filter the sodium sulfate crystals formed and dry between filter paper.

There you go, this method is salt making for titration.

Enjoy!
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Json Lim
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Salt Making - Selection of Method

There are a total of 3 salt making techniques that we need to master for O levels. The 3 methods are,
a)Titration
b)Precipitation
c)Acid Reactions (In my tuition class, we call this excess reagent method)

The selection technique is very simple. Simply ask yourself if the salt is soluble. This should ALWAYS be the first thing you ask yourself.

If the salt is soluble, use either titration or acid reactions.
For titration, it is for salts that contain the following ions: Group 1 ions and Ammonium ion.
E.g. sodium chloride, potassium nitrate, ammonium sulfate, lithium ethanoate.

For acid reactions, it is for all other salts that are soluble.
E.g. copper (II) sulfate, magnesium chloride, barium nitrate, silver nitrate.
That's all for the selection technique for soluble salts.

If the salt is insoluble, use precipitation. That's the only method.
E.g. barium sulfate, lead (II) sulfate, lead(II) chloride, silver chloride, calcium sulfate. Notice no nitrate salts here since all nitrates are soluble.



Enjoy!
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Json Lim
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Comparing the MP/BP of Ionic and Covalent Compouds

This question is by far my favourite and it seems like most students have difficulty giving a decent answer for this straight forward question. Lets look at how a sample question would look like.

Question normally ask you to state which substance has a higher MP or BP and explain why.

For our answer, it will be definitely be the ionic compound of magnesium oxide. For those who are lost, this is because ionic compounds have high MP and BP while simple covalent compounds have low MP and BP. If you have no clue how to differentiate between ionic and covalent compounds refer to this post.

Now the next step is to explain why does magnesium oxide has high MP and BP.

This is the suggested template.

a)Start by identify the type of compounds for the substances.

For our question magnesium oxide is the ionic compound and the oxide of phophorous is a simple covalent molecule.

b) Next, tell the marker the kinds of bonds broken when the substances undergo change of state.

This is the general rule of selection when it comes to identifying the kinds of bond broken.

Ionic Compounds --- break strong ionic bonds during change of state (aka melting and boiling)
Giant Covalent Compounds --- break strong covalent bonds during change of state (aka melting and boiling) Simple Covalent Molecules --- break weak intermolecular force of attraction or van da waal's forces of attraction during change of state (aka melting and boiling)

c)Last part sum up your answers by telling the marker the amount of energy needed to carry out the change of state.

That's all. I am gonna end this post by writing a sample answer for this type of question.

Qn: Which of the 2 substances, magnesium oxide and phophorous oxide has a higher melting point. Explain.

Ans: Magnesium oxide is an ionic compound and phosphorous oxide is a simple covalent compound. During change of state, magenesium oxide breaks strong ionic bonds while phosphorous oxide breaks weak intermolecular force of attraction between the molecules. Hence a lot of energy is needed to break the strong ionic bonds therefore MP is high, while lesser energy is needed to break the weak intermolecular force of attraction hence MP of simple covalent molescules is low.

Enjoy!

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Json Lim

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Acid Reactions Part 1

Acids are basically substances that produce hydrogen ions in the presence of water. For this post, the acids that we are going discuss are,
a)nitric acids
b)sulfuric acids
c)nitric acids
d)ethanoic acids

Acids react with a 4 main substances for our syllabus, namely,
a)metals
b)metal oxide
c)metal hydroxide
d)metal carbonate


Below is the flow chart you are expected to observed when an acid reacts with the above substances.

Enjoy!

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Json Lim
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Atomic Structure - The basics

Atomic structure is the study of how particles are arranged in an atom. To understand this, we need to know that atoms consist of 2 parts,
a)nucleus
b)electron shells.

The Nucleus
Simply put it, it is the part of the atom which you will find 2 kinds of particles, namely protons and neutrons.

Protons
Positively charged particle and it has a relative mass unit of 1.

Neutrons
Neutral and uncharged particle and it has a relative mass unit of 1.
Ok that basically sums up the particles found in the nucleus, lets go on to the electron shells.

Electron shells
Well as the name suggests, electrons are found here. Basically the electron shells are like holla hoops around a person and the electrons found in the electron shells spinning around the person at great speed. In an atom, the electron shells are found outside the nucleus and the electrons are spinning around the nucleus

Electrons
Negatively charged particles and have negligible mass compared to the other 2 particles.

A quick recap,

Subatomtic Particles Relative charge Relative mass Location
Proton +1 1 In the nucleus
Neutron 0 1 In the nucleus
Electrons -1 0 or 1/1840 Electron shells outside
nucleus

Cheers

Enjoy!


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Json Lim
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Types of Oxide

First of all, what do we consider to be oxides? Generally when a substance is made up of an element and oxygen only, its considered to be an oxide.

Meaning
The compound CaO, is considered an oxide since it is made up of calcium element and oxygen only. While NaOH, sodium hydroxide is not an oxide as it contains 3 elements. Hence in order to be an oxide it has to follow the sequence below.

Element A + Oxygen -----> A Oxide.

There are 2 major categories of oxide, namely metallic and non metallic oxide.

Metallic Oxide
Basic Oxide
a)The first type of metallic oxide is known as basic oxide. This group of oxide reacts with acid in a neutralization reaction and if it dissolves in water, it will form alkali (the soluble basic oxides are all the group 1 oxides and calcium hydroxide based on O levels syllabus).

Amphoteric Oxides
b)The second type of metal oxide is made up of zinc oxide, aluminium oxide and lead (II) oxide.
This group of oxides react with both acid and alkalis.


Non Metallic oxides
c)Acidic oxides
Acidic oxides are non metal oxides that produces hydrogen ions when dissolved in water. Otherwise, any non-metal oxides can be classified as acidic oxides. However there are a few exceptions and these are the neutral oxides

d) Neutral oxides
This last group of oxide are made up of all monoxides (carbon monoxide or nitrogen monoxide etc) and water. They do not have an effect on any indicators. Cheers.


Enjoy!

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Json Lim
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Formulae of Ions Part 1 - First Twenty Elements

How do we find decide the ionic charges of the various ions? For O level examinations, you are expected to know how to derive the ionic charges for certain elements.

Technique.
Lets select an element, e.g. Magnesium.
Magnesium has the following information based on the periodic table.
Number of protons: 12
Number of neutrons: 12
Number of electrons: 12

The electronic configuration of magnesium is 2,8,2.
The next thing is to ask yourself, in order to obtain a stable electronic configuration (full outer electron shell), the atom needs to do something.
a) Lose 2 electrons so that the configuration becomes 2,8 (full outer electron shell)
b) Gain 6 electrons so that the configuration becomes 2,8,8 (full outer electron shell again)

From all the atoms' point of view, it is easier to lose 2 electrons than to gain 6. So in order to form a stable electronic configuration, magnesium will choose to lose 2 electrons and the updated sub atomic particles looks like this.
Number of protons: 12
Number of neutrons: 12
Number of electrons: 10
Since electrons are negatively charged and protons are positively charged, there will be a total of 12 positive charges and 10 negative charges and hence the overall charge is +2.
Therefore the charge on magnesium is +2

Lets take another example, oxygen.
Subatomic particles of oxygen atom looks like this
Number of protons: 8
Number of neutrons: 8
Number of electrons: 8
Electronic configuration: 2,6
In order to be stable oxygen needs to gain 2 electrons (instead of losing 6 electrons) to form a stable electronic configuration.

After gaining 2 electrons, the new numbner of subatomic particles looks like this.
Number of protons: 8
Number of neutrons: 8
Number of electrons: 10
In this ion, there are 8 positive charges and 10 negative charges, hence it has an overall charge of -2. Therefore the ion of oxygen has a charge of -2.

This is how you find the ionic charge of the first 20 elements. Cheers.

Enjoy!


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Json Lim
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Differentiating Ionic and Covalent Compounds

A common question asked by my students all the time, how do you know if a substance is an ionic or covalent compound?

The general rule is basically very simple and i am sure your teachers have mentioned many times.

Ionic compounds - formed by substances that is made up with positive and negative ions.
The EASIEST method to decide if a substance is ionic basically is to look at the formula of the compound. For example, magnesium chloride, basically made up of magnesium ion (metallic) and chloride ion (non-metallic). So to recognize an ionic compound, look out for a metal and a non-metal in formula. This however is just a general guide as there are exception cases.

For example, a compound like ammonium chloride, NH4Cl. Notice there are no metals, in ammonium chloride but it is still considered an ionic compound. Reason is because it is made up of the positive ammonium ion and negative chloride ion. However, it is still quite safe to to identify ionic by the metal - non metal rule.

Finally for covalent substances, its the easiest to identify. As long as the substance does not contain any metals it can be taken to be a covalent substance. That of course taking into account the annoying ammonium based substances which defy this rule.

Enjoy!

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Json Lim
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