THINK LIKE A SCIENTIST
Science begins by making observations. A project is defined and experiments are conducted. At the end of the project a discussion is presented to explain what is happening. This discussion is known as the thesis.
This is followed up with projects by varying the conditions or the material for the system until a fairly clear picture of how the system works. At this point science will define the system (known as the model) and says that for this particular model the system is expected to work in a particular manner. This is known as a hypothesis.
Using the hypothesis more experiments are conducted, within the scope defined by the model, to test whether the results agree with the prediction. For example if you conducted the experiment with sodium you may then try it out with potassium. Every disagreement presents a good opportunity to understand the system better. This may then be repeated with cesium and so on. When all parameters are being thoroughly studied and a very clear understanding is obtained, and the prediction is in very good agreement with all experimental results obtained the concept becomes a theory.
When the theory becomes very fundamental to science it is accorded the status of a law.
So science has become a global endeavor. Someone will continue where someone else has left off. Of course the group that started the study will have a first-mover advantage; what we now known as "cutting" edge research.
IDEAL versus REAL
Systems are normally complex with many parameters to consider. So science normally will limit the study to a few parameters, a simplified version of the real thing, a hypothetical situation. Meaning that under these conditions this is what happens?
For example, to know how gases behave under different temperature and pressure we will have to consider the electrostatic interactions of every molecule when it passes each other. There are millions of molecules in the system and the molecules have attractive and repulsive forces between them. Then there are so many different types of molecules of different sizes. There are hydrogen, oxygen, carbon dioxide, etc. Also there are systems that are a mixture of gases, like air.
So for a start it will be very helpful to ignore all forms of intermolecular interactions and study how the temperature and pressure will effect the random movement of molecules in a gas. The model proposed is a container of gases at low pressure and high temperature. Under these conditions the molecules should be very far apart and moving very fast, making the interaction between them unimportant. This will be ideal, and so the model is known as an "ideal gas" model. The law derived, naturally, is known as the ideal gas law.
It has nothing to do with the gas except that it is in an ideal condition. However science always focused on application. The ideal condition is actually close enough to gases in normal everyday situation. 1 atmospheric pressure is a low enough pressure and at room temperature gas are moving at a very slow pace (Just look at the cigarette smoke in an isolated room).
In the industry, gases are normally at high pressure. Because of this the equation derived for the ideal model failed. Since it is too complex, we tinker here and there and came up with equations that can predict to a good degree the behaviour of gases under high pressure. To distinguish between the two, we referred to this model as the real gas model. So we end up confusing students by referring to gases under abnormal condition as "real" gases, and normal gases as "ideal" gases". At all time we are referring to real gases under two different conditions
MICRO versus MACRO
In science we like to discuss theories and concepts at the atomic and molecular level. But in experiments we can only measure them if they are in sufficient quantity. We found an ingenious way in the mole concept, defining it as a fixed amount of atoms and molecules. This allows us to "imagine" a mole as a very large atom or molecule that we can see and measure. When we talk of one mole, our mind can actually think of one atom or one molecule. If one mole of sodium interacts with one mole of chlorine in our experiment we can "see" each atom of sodium reacting with an atom of chlorine.
CATCH 22 TERMS
Along the way science has to come up with rather general terms. The first of such terms a student will most likely come across is matter. What we want to say is everything in this world that is made of atoms and molecules. But since students have not been taught atoms and molecules we have to use a definition without using the words atom and molecule. So we end up by saying "anything that has mass".
Later on we have to discuss subatomic matters like protons, neutrons, electrons, etc, to students who do not known about these matters we come up with the term particles to classify them.
When we have to introduce element and compound we conveniently use the term substance. When we wish to refer to a component of a molecule or an unstable substance we use the term radical.
So when you come across terms that are very genral just tolerate it for the moment and continue with the lesson. Once you are slightly ahead the term becomes redundant.
LESSON 1 : ATOMS & MOLECULES
Chemists think in terms of atoms and molecules. An atom is made up of protons, neutrons and electrons. The protons and neutrons form the nucleus at the centre and the electrons orbit around it. We refer to them as particles. Neutrons are neutral particles. Protons are positively charged particles and electrons are negatively charged. One proton has one unit of positive charge and one electron has one unit of negative charge. Although a neutron has no charge it has the same mass (or weight) as a proton, whereas an electron is so small its mass is negligible.
We start with an atom with one proton and ended up with an atom with as much as 103 protons. The number of protons in an atom is known as the atomic number (Z). As more and more protons squeeze themselves into the nucleus of the atom, the nucleus becomes unstable. The larger atom would break up by itself, in most cases emitting energy and harmful particles. We say that these atoms are radioactive.
Molecules are formed from atoms. All matters in their natural states are neutral in charge. So all atoms and molecules are neutral in charge.
Natural state (day-to-day or ambient state) means normal temperature, pressure, and in a normal atmosphere of nitrogen, oxygen and carbon dioxide and moisture. Only five atoms - helium, neon, argon, krypton, and xenon - exist as stable (or non reactive) atoms in their natural states. They are commonly referred to as noble gases (since such atoms are gases).
LESSON 2 : CONFIGURING THE ELECTRONS OF AN ATOM
For particles smaller than atom (subatomic particles) we found that Newtonian (normal) physics does not work. For example electron has both particle and wave properties. A special physics (and mathematics) - Quantum Mechanics - was developed. Using Quantum Mechanics we found that the orbital of the electron in an atom is defined by a set of Quantum Numbers (QN); (n, l, m and s). Their values are all integers except for s (the Spin QN) can only be ��. The lowest value for n (the Principal QN) is one. The lowest value for l (the Azimuthal QN) is zero. m (the Magnetic QN) can take any value between -l to +l including zero.
In Wave Mechanics (wave function mathematics) the absolute value of the amplitude is taken as the probability of finding the wave. So the absolute value of the wave function of the electron was taken as the probability of finding the electron in the vicinity of the nucleus. This means the orbital of the electron.
The energy of the orbital is given by the value of the Principal QN, an slightly influenced by the Azimuthal QN. The shape of the orbital is determined by the Azimuthal QN. The orientation of the orbital under a magnetic field is depends on the Magnetic QN. The Spin QN defines the direction of rotation of the electron particle about its axis.
When the mathematicians were working on the Quantum Numbers they named the energy shell for n=1 as K-shell. The next (n=2) became L, the M, N, until Q (i.e. when n=5). There was no chemistry input into the naming.
LESSON 3 : WHY DO ATOMS REACT?
It is a universal law that all systems proceed towards a state of lowest energy (stable state) possible. This is the driving force in a chemical reaction. The lowest possible energy state for an atom is that of a complete valence shell, like in noble gases.
When this is achieved by the giving and taking of electrons the product formed will be two ions - cation (+ve) and anion (-ve) - held to each other by electrostatic attraction. The bond between them is known as an ionic bond and the product is known as a salt or ionic molecule, by some chemists.
For some atoms the lower energy state can only be achieved by sharing the covalence electrons. In most atoms this meant an outer shell of eight electrons. So we often refer to this as the "octet rule". The bond formed is known as a covalent bond. In the early development of chemistry this was graphically represented by a 2-dimensional Lewis pictures. Today we need to see molecules in 3-dimension and this has to be worked out using Wave Mechanics.
LESSON 4 : CHEMICAL EQUATION
For a chemical reaction to take place two or more chemicals must collide with each other with an impact intensity (or energy) that will cause the valence electrons to go into action. In chemistry we are interested in how much energy of impact is required (that is the temperature required) and how fast the reactants can form the products (that is the rate of reaction).
After a sufficient period of time we will find the amount of products remain constant even though there are still reactants present. We say the reaction has reached an equilibrium. In general it is best to assume that all reactions show an equilibrium; that means not 100% complete.
Consider a reaction 2A + B = C + 3D, the rate of reaction can take any of these forms.
| - | 1 --- 2 |
d[A] ----- dt |
= | - | d[B] ----- dt |
= | d[C] ----- dt |
= | 1 --- 3 |
d[D] ----- dt |
When it reaches equilibrium, the equilibrium constant (K) will be given by:
| K | = | [C][D]� ---------- [A]�[B] |
When we write a chemical reaction equation we are actually writing the equation for the reaction at chemical equilibrium. For all chemical equations it is important to observe the Law of the Conservation of Matter. That is all atoms and charges must be accounted for; no atoms or charge is created or lost.
LESSON 5 : ELEMENTS AND COMPOUNDS
So far we were in the microscopic world of atoms and molecules. To move into the macroscopic world we "multiply" the number of atoms and molecules by a conversion factor (the Avogadro's number). We called this fixed amount a mole.
All is fine until we come to the weight of a mole of atoms or molecules. Atoms of the same type may have different number of neutrons. This we said are isotopes of the atom. For example, majority of the carbon atom in nature has six neutrons but there are small fractions with 7 or 8 neutrons. So a mole of carbon in nature will weight more than 12.0000 grams. So it is important for us to specifically say that one mole of carbon-12 weighs 12.0000 grams.
LESSON 6 : IONISATION POTENTIAL (IP)
Science always take a systematic approach. If bonding between atoms is by giving, taking or sharing of the valence electrons, it will be useful to find out the Ionisation Potential; the energy required to remove a valence electron from an atom, and the Electron Affinity; the energy required to remove the electron from a negatively charged anion. If IP is extremely low and EA is extremely high than ionic bonding is favoured. If IP is extremely high and EA low then covalent bonding is favoured.
For covalent bonds, the electron sharing will be based on the IP and EA values for an atom. The sum effect of IP and EA is known as the electronegativity of atom.
Studying the trends of the IP and EA, it is easy to relate the chemical properties to the electron configuration of the atoms, and organise the atoms into a Periodic Table.
LESSON 7 : SOLUBILITY
The chemicals can only collide with each other if they are in motion, so we can assume that reaction cannot react in solid forms. Gaseous reactions are of limited application it requires high pressure. At low pressure it is too "dilute". So most reactions are conducted in a solution.
In industry we would very much like to have high percent conversion when the reaction reach equilibrium. One technique used is to engineer the precipitation of the products. Since solid do not react, the products cannot revert back to the reactants. This can easily be done if one of the products is not soluble in the solvent used. (The other way is to selectively remove one of the products continuously.)
Ionic and polar molecules are more likely to dissolve in polar solvents. They will not dissolve non-polar molecules. The number of solvent molecules surrounding an ion in a solution is known as the coordination number. In most cases the value is six. Two from the opposing ends of the x, y, z axis. Non-polar molecules are more likely to dissolve in non-polar solvents. They will not dissolve polar molecules.
LESSON 8 : PROTONIC ACID
One of the most elementary classes of chemicals are the protonic acids. A protonic acid is any compound that can react with water to release a proton, H+, which is responsible for the acidity. However being such a small ion (radius about 10 ‾�� cm) its charge per unit area is extremely high and so it is easily attracted to the lone electron pair of the oxygen in the water molecule to form the hydronium ion (or hydroxonium ion).
Another name for protonic acid is Bronsted acid, as the overall reaction is generally represented by the Bronsted equilibrium equation. The strength of the acid depends on the concentration of the hydronium ion in the aqueous solution. The pH scale was defined for this purpose.
| pH | = | - | log10a({H3O}+) | ; log1010x = x |
