Boron Trichloride

13 05 2012

Molecules based around a triangle shape:

A molecule with no lone pairs and 3 electron pair bonds (3 pairs of bonded electrons), the shape is planar instead of pyramidal. Plane means a flat or level surface, so planar means a flat two-dimensional shape.

Planar shape allows 120° angle between bonds, so the separation between electrons is greater than in the example we used in a previous post, ammonia (NH3), which had a trigonal pyramidal shape.

Boron trichloride, BCl3 is an example of a molecule with a trigonal planar shape. If you build a model like the one I built below you notice it forms an equilateral triangle, this is because it is made up of 3 bonded pairs of electrons, one for each chlorine atom, and there is no lone pair adding extra repulsion.

Write out the electronic configuration and dot cross structure of phosphorous trichloride (PCl3), based on this you will see how many pairs of electrons are bonded or lone. Using this information you can make a model like the one I made below:

How would you describe the shape? It has a pyramidal structure like ammonia. You can predict this from drawing out its electron configuration:

We have established pyramidal shape in previous posts, and explored trigonal in this current post, but some molecules have a trigonal bipyramid shape.

Phosphorous pentachloride (PCl5) has 5 repelling bonds. Look again at the model we made before of boron trichloride, if you add another piece through the middle with two chlorine atoms attached you see that this constructs the model of PCl5. This helps you see that it has a trigonal planar section but with an added dimension creating a trigonal bipyramidal shape.

The trigonal bipyramid shape looks like 2 pyramids joined base to base.

Sulfur tetrafluoride (SF4) and chlorine trifluoride (ClF3) are further examples of trigonal bipyramid shaped molecules. ClF3 is T-shaped with 2 ‘equatorial’ lone pairs. Try making models of these yourself.

Molecules with an octahedron shape:

Sulfur hexafluoride (SF6) has 6 repelling bonds connecting to the fluorine corners of an octahedron. There is a 90° angle between its adjacent bonds.

Draw the electronic structure of BrF5, from this we can guess that its shape would be a square pyramid.

 

credit: chemeddl.org

To summarise:

3 bonds – triangular molecule

4 bonds – tetrahedral molecule

3 bonds + 1 lone pair – pyramidal molecule

2 bonds + 2 lone pairs – bent molecule

5 bonds – trigonal bipyramid

6 bonds – octahedral molecule





Water H2O

10 05 2012

Water is something we can’t live without; roughly 70% of an adult human body is composed of water. I will be writing a followup post about the properties, but for now lets just focus on the molecular structure.

I made the model below to show its shape. The red centre is oxygen, the two whites are hydrogen, and the two empty red prongs represent two lone pairs.

The presence of two lone pairs means lots of repulsion between them, which pushes the two sets of bonded pairs closer together, so the two atoms of hydrogen are forced closer together to form a V shape.

Based on this we can predict the shape for hydrogen sulfide (H2S), bent for the same reasons H2O is bent, see below:

and also the oxonium ion H3O+

This oxonium ion is pyramidal as it is isoelectronic with ammonia, see previous post.

I highly recommend visiting Wolfram Alpha, simply type in a formula, name or equation and a treasure of information comes up.





Ammonia

10 05 2012

Ammonia NH3 contains 3 electron pair bonds (N-H) and one lone pair, represented on the model I made below as the empty tip of the pyramid shape. The blue centre represents Nitrogen, and the 3 white bits are Hydrogen.

The pyramidal shape is a result of the electron pairs (bonded and lone) repelling each other. Remember where the most repulsion occurs:

Most electron repulsion between:       lone pairlone pair

Strong repulsion between:                      lone pairbonded pair

Least repulsion between:                         bonded pairbonded pair

These repulsion rules explain why the bonded pairs are pushed downwards to form the base of the pyramid, they don’t want to be anywhere near the lone pair at the tip of the pyramid. But they also don’t want to be near each other, so they go as far as they can away without breaking their bonds to the nitrogen centre.

credit: wikimedia.org

What shape do you think the Ammonium ion NH4+ would have?

Ammonium is isoelectronic CH4 (Methane) so has the same shape, tetrahedral. Remember ‘iso’ in Greek means ‘equal’, so in chemistry it means ‘the same’ or ‘no change’, remember we studied Isotopes of elements in Lesson 2.

Methane (credit: wikimedia.org)

Here’s an ammonium ion I drew below:

NH3 + H+  = NH4+

Dative bond: Both electrons of a shared pair are donated by one of the bonded atoms.

In the example of ammonium this symmetrical ion has a dative bond that is hard to distinguish from its other covalent (electron sharing) bonds because of its symmetry. But as you can see there are two dots instead of dot cross, and the + sign atop the brackets surrounding the ion represents the electron the hydrogen atoms gives away. Electrons are negative so giving them away means you become more positive.

Ammonium is a positive ion so is called a cation. Here’s a model of amonium I made:

 

Methane:





Tetrachloromethane

9 05 2012

Molecular shape: the basic principles

Positioning of the bonds, and the bond angles, that form an atom are determined by electron repulsion between pairs of electrons (lone or bonded pairs).

Tetrahedron shaped molecules

Tetrachloromethane (CCl4), otherwise known as carbon tetrachloride, is an example of this, and it has four electron pair bonds. The 4 chlorine atoms are at the corners of a tetrahedron with the carbon atom at its centre.

 

The angle between adjacent bonds in this molecule is 109° 28’, and there is no other shape in which it could exist with bond angles greater than this.

Why?

The maximum angle has already been reached due to repulsion between the 4 pairs of bonded electrons. They want to be as far apart as possible without breaking the structure, think of it like magnets, electrons are negative and don’t want to be together or near each other particularly, they are attracted to positive things like protons in the nucleus of an atom.

Here’s a rough dot cross diagram I did on a post-it, the dots represent Carbon’s 4 electrons in its outer shell/energy level, therefore valence of 4, the crosses obviously represent Chlorine’s outer electrons (we know this because of its electronic configuration, see Lesson 2: Atomic Structure, Isotopes and Electronic Configuration)





Why I’m Breaking Bad

9 05 2012

Bryan Cranston said in an interview that the show tests morality to the extreme across a spectrum, where Walter starts as “good”, achieves the audience’s sympathy, then veers into bad, still maintaining sympathy and audience support though, and this will be pushed to breaking point in season 5. That’s something I’ve always loved about television series that films don’t always have the time or ways of achieving, in series your perceptions of a character can change throughout, characters you dislike can be redeemed, characters you empathise with or relate to can change and cause you to question your feelings, maybe even your life.

I like that Breaking Bad doesn’t tell you what to think, there’s no right or wrong way to feel when you watch it. Isn’t that the whole point? Walter discovers that right and wrong are arbitrary, he tells his DEA Agent brother in law that the alcohol they  are drinking at that moment would have gotten them thrown in jail during prohibition. Walter’s morality does not orbit around the law anymore, instead adjusting to what he feels is right for his family. When he discovers he has terminal cancer, he starts to “cook” meth and save money for his family.

But then he realises it is not just about his family anymore. “Get out of my territory” Walter growls to some would-be meth dealers in a supermarket parking lot. You can see the satisfaction and acceptance on his face, that this is a part of him now. Being the “cook”, being Heisenberg, doesn’t just grant him money for his family, it gives him the control which he previously lacked in his life. He now has control over his actions, perhaps even over other people (he has been known to manipulate Jesse), being the provider for his family, control over his life, the danger of the job giving him a way out that even the cancer can’t beat, releasing frustration and feelings that are buried in the constricting polite facade of his everyday, mundane, underachieving life as a high school chemistry teacher.

Don’t we all secretly want to Break Bad?





Lesson 4: Formulae and Equations

8 05 2012

Click to download Lesson 4: Formulae and Equations (Word document) Formatting looks much better when you download it! When pasting from Word to this post some of my tables and colours were altered.

Empirical Formula is the simplest whole number ratio of each type of atom in a compound.

Example:

Chemical Molecular Formula Empirical Formula
Glucose C6H12O16 CH2O
Ethanoic Acid CH3CO2H CH2O

Looking at the molecular formula for Ethanoic Acid you can see there are

2 Carbon, 2 Oxygen and 4 Hydrogen atoms, this whole molecular formula has then been divided by a common multiple, in this case 2, so in the Empirical Formula you have:

1 Carbon, 1 Oxygen and 2 Hydrogen atoms.

In some questions you will be asked to calculate the Empirical Formula of a compound.

As an example, let’s work out H2SO4 by breaking it into its component parts (ratio H : S : O). If the question says we have 2.45g of H2SO4 containing 0.05g Hydrogen, 0.8g Sulfur and 1.6g Oxygen. Create yourself a little table, and work out the details from top to bottom:

In basic form, your working table could look like this:

H S O
Mass (g) 0.05 0.8 1.6
Molar mass (gmol-1) 1 32 16
Moles 0.05 0.025 0.1

Then divide all moles values by the smallest; you’re left with the result of the simplest ratio of how many of each atom makes up the compound.

Let’s try another Question…

Analysis shows that 0.6075g of Magnesium combines with 3.995g of Bromine to form a compound. Find the Empirical Formula of this compound.

Mg Br
Mass (g)÷Molar mass (gmol-1) 0.6075g ÷ 24gmol-1 3.995 ÷ 80gmol-1
Moles 0.025 ≈ 0.3mol 0.5mol
Divide mol values by the smallest 0.3 ÷ 0.3 0.5 ÷ 0.3
Simplest Ratio 1 1.6666… ≈ 2
Empirical Formula Mg Br2

The simplest ratio of Magnesium is 1 so just write Mg, which is one atom. For Bromine the ratio was 2, so you have to write Br2 two atoms being the simplest ratio. Simply it is a 1:2 ratio.

Molecular Formula shows the exact number of atoms in a molecule.

It is used for compounds that form covalent molecules (covalence is where elements share electrons in a chemical bond.) Remember that Noble gases are monoatomic (mono means one, single) and other elemental gases are diatomic (di means two or double)

Diatomic example: in molecular form Hydrogen gas is H2, Chlorine gas is Cl2, and Nitrogen gas is N2.

CH4 Methane is made up of 1 Carbon atom and 4 Hydrogen atoms.

We can use the ‘octet’ rule with ionic formula (which is explained in the next heading) to predict molecular formula, but only for s and p block elements not d block. (In Lesson 1 we discussed that the noble gases all have a ‘stable octet’ of 8 electrons in their outer shell making them very unreactive, because they don’t want to give away or accept electrons).

So if we wanted to predict the molecular formula of chlorine oxide…

Cl: 8 – 7 = 1 so Cl1-

O:  8 – 6 = 2 so O2+

The charges of each element must always balance, but currently we have 1- and 2+

You cannot subtract from the 2+ or add to the 1-, the charge does not change, but the amount of an element can change. In this case we need 2 atoms of Chlorine to balance out the 2+ charge of Oxygen. So we can predict the molecular formula to be:

Cl2O

So we have Cl 1- (x2) with O 2+, balanced together so that the overall charge always equals zero. This compound is called dichlorine monoxide.

Ionic Formula

This is the formula of an ionic compound whereby the charges of the elements are made to balance.

 Notice how in each equation Chlorine has a different charge, the ionic charge of the element itself has not changed (it is still 1- in each), but each time it has needed more atoms of itself to balance with the positive charge of the element it is bonding with. So sometimes it is just Cl, other times 2 atoms of Cl are needed, Cl2, and sometimes 3, Cl3.

We will go into more details on ionic compounds, and charges on the periodic table soon.

Structural Formula

The structural formula shows the arrangement of atoms and groups in a molecule.

Essential for organic compounds.

Example

Ethanol

oxidised to

Ethanal (Acetaldehyde)

CH3CH2OH

=>

CH3CHO

For organic reactions we often write unbalanced equations showing only the structural formula of the principle organic reactant and products. (reactant Þ products).

For inorganic reaction we usually write balanced equations showing all the reactants and products.

Examples of an inorganic equation:

1) Ordinary equation

Mg + H2SO4  => MgSO4 + H2

2) Ionic equation

Mg + 2H+ => Mg2+ + H2

Hydrogen gains electrons so is reduced. (Oxidation is loss of electrons, Reduction is gain of electrons. Remember OIL RIG”: Oxidation Is Loss, Reduction Is Gain.)

Half Equations

Half equations are used to show the loss and gain of electrons in a reaction, in other words to demonstrate the oxidation and reduction.

Oxidation Is Loss

Mg => Mg2+ + 2e-

The 2e- are the two electrons lost by Magnesium, electrons are negative so a loss of them makes the atom more positive, and hence Mg becomes Mg2+

Reduction is Gain

2H+ + 2e- => H2

In this half equation Hydrogen is initially positive, until 2 electrons (2e-) are added and it becomes negative. Remember, it must balance.





Lesson 3: Mass and Moles

7 05 2012

 Download lesson as Word document, click: Lesson 3 

Mass and Moles

Objective: To explore different types of mass of elements, and be introduced to the measurement of moles.

Relative Isotopic Mass

This is the mass of an atom of an Isotope compared with 1/12 of the mass of an atom of Carbon-12.

Example: Oxygen-16 has a relative Isotopic mass of 16.0, Sodium-23 has a relative Isotopic mass of 23.0.

 

Relative Atomic Mass (Ar)

The relative atomic mass is the weighted mean mass of an atom of an element, this is calculated using the different masses and relative abundances of all the Isotopes of a particular element.

For example:

35                        37

     Cl                         Cl

17                        17

If I am told that 75% of Chlorine atoms have an atomic mass of 35, and 25% have an atomic mass of 37, I can calculate the relative atomic mass.

(35×75) + (37×25) ÷ 100 = 35.5

35.5 is therefore the relative atomic mass of Chlorine, an average taken using the types and amounts of other Isotopes of the element.

Another example:

 

24                       25                        26

    Mg                               Mg                        Mg

12                       12                          12

 

78.6%                10.1%                   11.3%       < abundance of each Isotope of the element magnesium.

(24 x 78.6) + (25 x 10.1) + (26 x 11.3)

________________________________    = 24.3

100

As you can see Mg 24 is the most common / abundant Isotope, so the final average is nearest to 24. Relative atomic mass allows for the most accurate average mass of a particular element.

What is the difference between Relative Isotopic Mass and Relative Atomic Mass?

This is simple; Relative Atomic Mass takes into account ALL the Isotopes of a particular element (as demonstrated above), whereas Relative Isotopic Mass means the mass of just ONE Isotope of a particular element.

Example: Chlorine-35 has relative ISOTOPIC mass of 35. Chlorine-37 has a relative isotopic mass of 37. But if we want the relative ATOMIC mass, we must add the isotopes, multiply by abundance, and divide by one hundred to find an average mass (this exact question was covered above).

Relative Formula Mass

Also known as relative molecular mass (Mr). Relative Formula Mass is the weighted mean mass of a molecule (compared with 1/12 of the mass of an atom of Carbon-12).

So, where Relative Atomic Mass dealt with an atom of a whole element, Relative Formula Mass deals with the mass of a molecule (a molecule is made up of two or more chemically bonded atoms).

Many elements and compounds are made up of simple molecules like N2, O2 or CO2

Compounds with giant structures do not exist as molecules, for example: Ionic compound NaCl, or covalent compound SiO2, so ‘relative formula mass’ is seen as more accurate than saying molecular mass.

Remember: Atoms, Molecules, and Compounds;

A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds.

Molecular hydrogen (H2), molecular oxygen (O2) and molecular nitrogen (N2) are not compounds because each is composed of a single element. Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element. The smallest bit of each of these substances would be referred to as a molecule. For example, a single molecule of molecular hydrogen is made from two atoms of hydrogen while a single molecule of water is made from two atoms of hydrogen and one atom of oxygen.” http://education.jlab.org/qa/compound.html

To calculate the relative formula mass (Mr) of a substance all you have to do is add up the relative atomic masses of all the elements.

H2O     Water there fore has a relative formula mass of 18.

Hydrogen has an atomic mass of 1, there are 2 atoms of Hydrogen in water so multiply by 2.

Oxygen has an atomic mass of 16, so 16 + 2 = 18.

What is the relative formula mass of K2CO3 ?

 

K2 – 1 atom of Potassium has atomic mass of 39.1, but there are 2 atoms here so x2 is 78.2

C  – see on the periodic table, atomic mass 12

O – We already established has atomic mass 16, but there are 3 atoms of it here so x3 is 48

78.2 + 12 + 48 = 138.2

 

Introduction to the mole

The mole is a measure of the amount of a substance.

1 mole of an element is equal to that particular element’s relative formula mass or relative atomic mass. (1mol = RFM and RAM)

Example: 1 mole of oxygen has a molar mass/relative formula mass of 16gmol-1 (the unit gmol-1 literally means grams per mole, so there is 16g per mole of oxygen, 1 mole of oxygen has a mass of 16g)

As we know 16 is oxygen’s relative formula mass, and its relative atomic mass listed on the periodic table.

If we have 2 moles of oxygen, then its ‘molar mass’ (same as relative formula mass) would be 32gmol-1

But what if we are given the weight of an element in grams and told to calculate the amount of moles?

If I have 50g of Oxygen, all I need to do is divide that by Oxygen’s molar mass/relative formula mass (16gmol-1) and I get 3.125 moles. (Test this by multiplying 3.125mol by 16gmol-1).

This is all illustrated in the calculation formula triangle below:

1 mole of a substance contains 6.02×1023 particles/atoms, this number is known as Avogadro’s number and you can’t escape it in chemistry.

A book I highly recommend is Calculations in AS / A Level Chemistry by Jim Clark, goes into detail with moles but keeps it simple and fun, with plenty of practice questions. The book is not just about moles, it’s everything, and has been essential. You can get a second hand version pretty cheap here: http://www.amazon.co.uk/Calculations-AS-A-Level-Chemistry/dp/0582411270/ref=sr_1_cc_1?s=aps&ie=UTF8&qid=1335368574&sr=1-1-catcorr

The bottom row multiplies, the top row divides. So…

moles x molar mass = mass

molar mass x moles = mass (same thing)

mass ÷ moles = molar mass

mass ÷ molar mass = moles

It’s very straightforward, so the above may seem patronising but it’s just in case anyone finds triangles tricky. The other formula triangles are below.








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