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Chem 59-250 Introductory Inorganic Chemistry What is Inorganic Chemistry?

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PowerPoint PresentationWhat is Inorganic Chemistry?
Inorganic chemistry is the chemistry of all the elements – organic refers to a few at the top right-hand part of the periodic table.
To understand inorganic chemistry properly we need to be aware of aspects of physical chemistry, analytical and even organic chemistry.
Because Inorganic comprises all the elements, we need some way to understand the underlying connections and to figure out why things work the way they do.
Fortunately, the nature of the elements themselves provides us with such a guide and we can understand a lot from the arrangement of the periodic table itself.
Chem 59-250
Mendele’ev 1869-1871 made first modern periodic table
Tables before based on alchemy, or “ideal” numbers (triads or octaves)
Mendele’ev arranged elements by increasing mass and similar properties (density and reactivity) but had no underlying theoretical framework.
One of the biggest successes of this table was the prediction of unknown elements (Sc, Ga, Ge)
Amazingly, it wasn’t until more than 40yrs later (with the quantum interpretation of atomic structure) that people would understand the structure of the periodic table.
In any event, the rationale he used (elements connected by similar chemistry) ended up being useful.
Chem 59-250
The group structure of the periodic table is still useful.
If one wants to differentiate between the groups of the periodic table. Much reactivity can be understood in this manner, which is the reason for the typical arrangement of the table (although the colours are a nice touch)
Group names alkaline metals, alkaline earth metals, earth metals, nothing, pnictogens, chalcogens, halogens, noble gases, transition metals, lanthanides and actinides
Oddities in older tables are left over from Mendele’ev derived tables e.g. A and B designations, order of B groups
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Another common scheme differentiates by the physical properties of the elements. In this case, metals are colourless, semi-metals are green, non-metals are yellow and the noble gases are blue.
This explains the diagonal line that we often see on the periodic table.
More importantly, this view of the periodic talble shows us that there seems to be an obvious organization of the properties of non-metals (top right), semi-metals, and metals (everything else).
Why?
Such views of the table seem to indicate that there is an underlying reason why something is metallic or not and we will see that this is truly the case later.
Chem 59-250
Other properties that are much more important for an understanding of the chemistry of seemingly unrelated elements are sometimes best depicted in other ways.
We will see later that electronegativity is one of the most important concepts that allows us to understand inorganic chemistry.
You may also notice that, in regard to the last slide that we saw, the trends of electronegativities seem to match the trends in metallic and non-metallic character.
We will see later that there is a reason for this correspondence, although electronegativity is a theoretical construct (not a fundamental measurable property).
WebElements is a good site to take a look at.
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This alternative version of a periodic table emphasizes the electronic configuration of the elements and the symmetrical nature of the periodic table.
E.g. Arsenic, one of the elements that I worked with a lot during my PhD.
It is not as effective in showing the chemical relationship group nature, which is one reason why it’s not generally used.
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Chem 59-250
In this course we will focus on the chemistry of the Main Group elements, which are those of the s- and p-blocks. These are the elements of groups 1 and 2 and groups 13-18, respectively and contain most of the elements that we encounter on an everyday basis.
Furthermore, an understanding of the structure, bonding and reactivity of these elements provides the necessary foundation for the understanding of how things work at the molecular level.
One of the unfortunate things about this table is the green band calling periods 2 and 3 “typical elements” – the opposite is actually true, but these include the elements of organic chemistry and have been studied the most.
Because the periodic table will be our guide throughout this course, I want you to be able to identify all the elements up to Kr (#36). What I mean by this is that I will give you a blank periodic table at the start of each midterm and you should be able to fill in those elements. I don’t usually want people to memorize anything, but this is important.
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Complex (polyatomic ions) Na2(SO4)
Network Solids diamond, graphite (C) “red” phosphorus (P)
Network ions Mg3(Si2O5)(OH)2 (talc)
Network Solids SiO2, polymers
Solid/Liquid Metals Hg, Ga, Na, Fe, Mg
Because Inorganic chemistry includes everything in the periodic table, there are an incredible number of different types and forms of compound that we want to understand.
Elements (and their various allotropes)
Although the meaning of elements is clear, the division between ionic and covalent is more arbitrary – as we shall see in the next couple weeks
Covalent = sharing
Ionic = stealing
All of the subdivisions I have listed here are sort of arbitrary as well and are different than those that you will find in different textbooks.
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Molecular Solids P4, S8, C60
Network Solids diamond, graphite (C) “red” phosphorus (P)
Solid/Liquid Metals Hg, Ga, Fe, Na, Mg
No drawing for atomic and molecular gases
Molecular species all at the top right of periodic table
Allotropes of C, and Phosphorus include both molecular and network structures (network just implies infinite arrangement of covalently bonded atoms)
Although metals may look similar to network solids, in the “body center cubic” structure of iron depicted at bottom right, those are not the same kind of bond as is the covalent network structure.
Properties of metals can vary a lot from liquid mercury and almost gallium, to soft sodium, to hard tungsten – we will get into the reasons for this later.
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Ionic Compounds
Network ions Mg3(Si4O10)(OH)2 (talc)
Ionic compounds range from the familiar binary salts (which are usually very hard) that are arranged in various types of essentially infinite lattices. E.g. the sodium chloride structure that we are all familiar with.
The ones I have termed complex, contain complex ions such as nitrate, sulfate, phosphate, tetrachloroaluminate etc. And properties depend on the nature of the cations and anions and how they pack together.
Ionic compounds containing network ions are also quite common, especially for minerals. As in the case of the elements, the arrangement of the network has a large effect on the properties of the material. Talc is constructed from linked units of (Si4O10) fragments – the polyhedra, which makes layers. Some layers are held together by the cations but others are not – this allows layers to slide, so talc is not a hard material like salt.
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Covalent Compounds
Complex Molecular As(C6H5)3, organometallic compounds
Network Solids SiO2, polymers
The covalent compounds of inorganic chemistry are much more interesting than those of organic chemistry – one of the reasons that I decided to study inorganic chemistry in the first place.
For the most part, covalent organic structures are all based on tetrahedral, planar, or linear arrangements of atoms.
Water, ammonia and carbon dioxide are typical of organic–type bonding, but even simple inorganic structure can have many more interesting structures like the trigonal bipyramid of PF5.
The polyatomic and organometallic type compounds can have incredible structures. E.g. boranes and carboranes have cluster structures like the icosahedral one shown here – I solved the two structures shown in the middle using X-ray crystallography.
There are also many network-type compounds such as quartz and many polymers.
One thing you may notice is that the structures can vary drastically depending on the element (CO2 vs SiO2) – we will find out why this is as the course progresses.
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Entropy Change, DS°
Enthalpy is “heat content” of a substance
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Standard Enthalpy of Formation, DH°f
DH° for the formation of a substance from its constituent elements
Standard Enthalpy of Fusion, DH°fus Na(s) Na(l)
Standard Enthalpy of Vapourization, DH°vap Br2(l) Br2(g)
Standard Enthalpy of Sublimation, DH°sub P4(s) P4(g)
Standard Enthalpy of Dissociation, DH°d ½ Cl2(g) Cl(g)
Standard Enthalpy of Solvation, DH°sol Na+(g) Na+(aq)
Enthalpies of formation can be found in books such as text books and CRC handbook.
These ones include phase changes and dissociation of molecules into atoms, which will be important for calculations we will do later.
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Na(g) Na+(g) + e- DH°ie = 502 kJ/mol
Al(g) Al+(g) + e- DH°ie = 578 kJ/mol
Al+(g) Al2+(g) + e- DH°ie = 1817 kJ/mol
Al2+(g) Al3+(g) + e- DH°ie = 2745 kJ/mol
Thus:
Al(g) Al3+(g) + e- DH°ie = 5140 kJ/mol
Ionization enthalpies are always positive – it requires energy to rip an electron away from a nucleus.
More later on the magnitudes
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Chem 59-250
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The enthalpy change for the gain of an electron
Cl(g) + e- Cl-(g) DH°ea = -349 kJ/mol
O(g) + e- O-(g) DH°ea = -142 kJ/mol
O-(g) + e- O2-(g) DH°ea = 844 kJ/mol
Electron Affinity, EA = -DH°ea + 5/2 RT
EA = -DH°ea
Electron Affinity is the negative of the electron attachment enthalpy
Chem 59-250
Chem 59-250
They will provide us information about the strength of
bonding in solids.
Polyatomic:
Thus:
Average O-H bond energy = 918 / 2
EO-H = 459 kJ/mol
DH = 1724 kJ/mol
EN-H = 391 kJ/mol
We can estimate N-N bond energy to be:
1724 – 4(391) = 160 kJ/mol
DHrxn = (436 + 745) – (414 + 351+ 464) kJ/mol
DHrxn = -48 kJ/mol
For H2N-NH2(g): EN-N = 160 kJ/mol
For F2N-NF2(g): EN-N = 88 kJ/mol
For O2N-NO2(g): EN-N = 57 kJ/mol
They are only a rough approximation and predictions must be
made cautiously.
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At STP: DG° = DH° - (298.15 K) DS°
The two factors that determine
if a reaction is favourable:
If it gives off energy (exothermic)
DH = SHproducts - SHreactants
DS = SSproducts - SSreactants
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DG lets us predict where an equilibrium will lie through
the relationship:
DG = -RT ln K
So if DG < 0, then K > 1 and equilibrium lies to the right.
There are three possible ways that this can happen with
respect to DH and DS.
aA + bB + cC + … hH + iI + jJ + …
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i.e. DH < 0 and DS > 0 then DG < 0.
S(s) + O2(g) SO2(g) DH° = -292.9 kJ/mol
TDS° = 7.5 kJ/mol
DG° = -300.4 kJ/mol
If enthalpy drives the reaction:
i.e. DH < 0 and DS < 0, but |DH| > |TDS|, then DG < 0.
N2(g) + 3 H2(g) 2 NH3(g) DH° = -46.2 kJ/mol
TDS° = -29.5 kJ/mol
DG° = -16.7 kJ/mol
If entropy drives the reaction:
i.e. DH > 0 and DS > 0, but |DH| < |TDS|, then DG < 0.
NaCl(s) Na+(aq) + Cl-(aq) DH° = 1.9 kJ/mol
TDS° = 4.6 kJ/mol
DG° = -2.7 kJ/mol
Measure change in equilibrium constants with temperature
to get DH° using the relationship:
Measure the equilibrium constant for the equilibrium,
then determine DG° using the relationship ? :
DG° = -RT ln K
Often not that easy…
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gives access to DG° through the following relationship:
DG° = - nF DE°
F = Faraday’s constant = 96.4867 kJ mol-1 V-1 (e-)-1
Note: if DG° < 0, then must be DE° > 0
So favourable reactions must have DE° > 0
This is more important for Analytical chemistry, but we might talk more about redox reactions later.
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Sn4+(aq) + 2 e- Sn2+(aq) DE° = 0.15 V
thus for:
2 Al(s) + 3 Sn4+(aq) 2 Al3+(aq) + 3 Sn2+(aq)
DE° = -(-1.67 V) + (0.15 V) = 1.82 V for 6 electrons
So: DG° = - nF DE° = - (6 e-)F (1.82 V) = -1054 kJ/mol
Standard is H+ e- -> ½ H2
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Relative Energy vs. Oxidation State (under certain conditions)
Provides:
What is the energy of electron gain
What is the energy of electron loss
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Some important information provided by Frost diagrams:
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Oxidation state diagrams (Frost Diagrams)
The most useful aspect of Frost diagrams is that they allow us to predict whether a RedOx reaction will occur for a given pair of reagents and what the outcome of the reaction will be. This is described in the handout.
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Chem 59-250 Introductory Inorganic Chemistry What is Inorganic Chemistry?
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