Overview of Chemistry 111. Section 2 MWF 10:10 © Julian Tyson 2004
This course addresses the questions, “What does chemistry have to do with (a) everyday life and (b) my future studies?”
The course will (a) give you an insight into the world of the chemist, (b) give you some chemical knowledge that will help in your studies in other disciplines, (c) give you some knowledge that will make you a better informed citizen, and (d) provide an insight into the way that science works (i.e. what scientists do and how they do it). Often what scientists do is concerned with solving problems and making a decision based on information provided by the application of science.
What is chemistry?
Chemistry is about understanding the properties of materials. What sort of properties? These might include mechanical strength, corrosion resistance, water solubility, effect of heat, electrical conductivity, magnetic properties, color (interaction with light), and effect on living organisms.
Chemistry is about understanding how to make new materials that have required properties.
Chemistry provides a means to measure the chemical composition of materials of interest. Many materials only have the desired properties if the chemical composition is appropriate.
Chemistry is about understanding processes that involve the breaking and making of bonds between atoms and molecules.
Chemistry is explaining what is observed on the macroscopic scale in terms of the behavior and properties of molecules and atoms (i.e. behavior on the microscopic scale). [1.5] (These reference numbers are to the relevant section of the textbook)
Chemistry involves constructing models of atoms and molecules and chemical processes to help with explanations of behavior at the microscopic level or some essential feature of a chemical system. [3.2] But note: chemists often don’t use vocabulary that indicates they are talking about a model
Behavior on the microscopic scale.
Energy and the phenomena associated with energy (such as vibration of atoms or rotations of molecules) are quantized. That is the quantity can only have certain discrete values. There are not many analogies of this on the macroscopic scale, but a useful one is the vibration of a string or a column of air (standing waves). There can only be integer numbers of nodes and antinodes in any vibrating string, for example. [7.1]
Small particles can also behave as waves. This is relevant for how electrons are distributed in atoms. [7.4]
Matter consists of atoms, which in turn are made up of protons neutrons and electrons. Atoms consist of protons and neutrons, held together tightly by really strong forces that overcome the mutual repulsion of the positive protons, forming the nucleus of the atom surrounded by the same number of electrons as there are protons. The electrons are held by the electrostatic forces of attraction between the positive nucleus and the negative electrons, which overcomes the mutual repulsion of the negative charges on the electrons. Certain combinations of neutrons, protons and electrons are stable for long period of time. Atoms are distinguished from each other by their behavior, which varies as the number of protons and electrons varies. Combinations containing from 1 to about 100 p and e are stable. Each stable combination of p and e is an element that has a name and a one- or two-letter chemical symbol. It is the p/e number that defines the element. Some p/e combinations can have several different numbers of neutrons (known as isotopes). Many elements occur in nature as a mixture of isotopes. In the atom, the protons and neutrons occupy a small volume; the bulk of the atom is made up of the electrons, which are distributed around the nucleus in a three-dimensional pattern, unique to each element. [2.1-2.3]
Atoms combine to form two and three-dimensional structures, most of which are known as molecules: some are macromolecules and some are crystals (in general, chemists talk about chemical species or entities). There appears to be an infinite number of ways in which atoms can be linked to form these structures. Some molecules are named by an internationally agreed systematic naming system, some are named by commonly accepted (but non-systematic) names based on their constituent elements, and some have trivial names. [3.1-3.4] Chemical species in which the number of electrons is not equal to the number of protons are called ions. If the electron number exceeds the proton number the species is an anion. If the proton number exceeds the electron number the species in a cation.
In molecules, atoms bind to small numbers (1, 2, 3, 4, 5 and 6) of other atoms. [1.2]
In molecules (and ions and crystals), the electron distribution in space holds nuclei together. Only the outer electrons are involved in bonding; inner electrons retain their atomic character and hang around the nucleus to which they belong. [9.1]
Atoms and molecule are in constant motion, with a distribution of speeds that results from the collision of atoms/molecules with the walls of vessel and with each other. In any given molecule or crystal structure, the atoms are vibrating (i.e. are moving relative to each other in a regular fashion within the molecule). [1.5]
If the molecules move independently with large distances between them compared with the particle size, the material is a gas. The behavior of gases can be described by a simple theory based on the kinetic energy of particles that undergo elastic collisions with the container walls and with each other. Many proprieties can be summarized in the equation PV = nRT where P is pressure, V is volume, n is number of moles (see later), T is temperature and R is a constant. The units of the various quantities have to be appropriate. A gas whose properties vary in accordance with this equation is known as an ideal gas. [12.1-12.3]
If the molecules have some short-range order but can still move throughout the entire volume of the container, on the macroscopic scale the material is a liquid. If the molecules (or ions) are constrained into long-term order both spatially and temporally, the material is a solid. [1.5]
The total energy of a material is made up of the total kinetic energy of the constituent atoms. This total energy is a function only of temperature. The temperature at which all motion ceases is absolute zero, -273.13 oC (or 0 kelvin, K). [12.2]
Chemical processes involve breaking bonds (either within or between molecules) and then (re)forming the bonds. If the process involves the bonds between atoms, new chemical compounds are formed, and the process is known as a chemical reaction. It takes energy to break bonds and this energy comes from collisions between reactants that have sufficient energy to break the bonds. When bonds are formed energy is given out. If the energy given out when bonds form is greater than the energy absorbed when bonds are broken, the process will continue and the reaction goes until one of the reactants is consumed. The fraction of collisions between molecules that possess sufficient energy depends on temperature, and may be very small because the probability of such a collision may very low. Such a reaction is slow.
Many of the reactions we observe around us (or are used to keep students interested in chemistry) are not in closed systems and proceed until one of the reactants is completely used up. Many chemical processes that proceed spontaneously at room temperature and atmospheric pressure give out energy. However, some reactions that proceed spontaneously take in heat energy. The reaction is able to proceed, if the forward reaction increases the number of chemical entities considerably, as the rate of the reverse reaction becomes very slow (it is extremely difficult to reassemble the initial species from the entities formed, especially if they can escape from the reaction vessel). A more detailed explanation for this phenomenon is beyond the scope of Chem 111, but will be encountered in Chem 112 around section 19.5.
Chemical processes in closed systems are reversible. That is reactions proceed in the forward and reverse direction. When the rates of these processes are equal, no change at the macroscopic level is observed. The system has reached equilibrium. [16.1] Many reactions in a solvent (such as water) behave as though they were closed systems.
Chemists like to classify things. They like to find patterns in chemical behavior; they look for patterns and explanations that can be generalized. This search for underlying patterns and explanations, which are formulated as laws or rules or hypotheses, is mirrored in every other discipline. And, just as with any other discipline, there is a vocabulary to be learnt and some basic facts to be committed to memory. Elements have chemical properties that are periodic i.e. regular patterns of behavior occur. Elements can be grouped in a two-dimensional representation known as the periodic table. Students need to know the names of the various regions of the table and how (relative) chemical properties relate to position in the table. [2.6 – 2.7]
Patterns of chemical behavior are often summarized as being due to some “driving force”. The existence of some driving force(s) is often offered as an explanation of “why do chemical processes occur”. For example, chemical processes occur because chemical systems try to achieve a configuration at which their energy is a minimum.
Processes at room temperature and atmospheric pressure dominate chemical classifications. It is important to know the physical form of elements, especially the common elements, at room temperature and atmospheric pressure. [2.7]
The forces holding the molecules together in atoms and liquids are electrostatic because most molecules have dipoles, or dipoles can be induced. For an ideal gas, it is assumed there are no forces between the molecules. [9.10, 12.2, 12.8] A molecular species has a dipole if the “center of gravity” of the positive charge does not coincide with the center of gravity of the negative charge. Such molecules are called polar. Non-polar molecules will have transient dipoles as the atoms vibrate.
Chemical reactions are process in which one set of combinations of atoms is converted into another. In the world of chemistry, matter can neither be created nor destroyed (not the case when dealing with reactions involving only the nucleus of atoms). It is necessary to be able to give names to the compounds disappearing (the reactants) and the compounds formed (the products). [3.1–3.4]
A large part of chemistry is about understanding these reactions: understanding why they occur, how they occur, being able to predict whether they will occur, being able to design reactions to produce desired products, and understanding and predicting the energy that is involved [6.8], being able to predict the amount of product or the amount of reactant needed to give a known amount of product [4.3].
Chemists count atoms and molecules by weighing them. As there are such large numbers of atoms and molecules in macroscopic amounts, it is convenient to use a large unit to specify the numbers. The unit is called the mole and is the number of carbon atoms in 0.001 kg of the material consisting of entirely of atoms that have 6 protons, 6 neutrons and 6 electrons (about 6.022 x 1023). [2.5]
Reactions can occur between pure compounds or elements, or can be mediated by a solvent. Water is commonly used. It is useful to know what compounds are likely to be soluble in water and whether they dissociate to form ions. [5.1-5.3] Solutions in water are called aqueous solutions.
Reactions can be classified into a number of types: acid-base, precipitation, gas-forming, and redox. Redox reactions are ones in which electrons are transferred from one reactant to another. [5.4-5.7]
Chemists use a code (chemical equation notation) to describe certain features of chemical reactions: the identity of the reactants, of the products and the relative numbers of molecules of each. These equations can be used to calculate information about the masses of the reactants and products. [4.1-4.6]
The formulae of compounds can be calculated from measurements of the elemental percent compositions. [3.6]
Electrons in atoms behave as though they had wave characteristics and have spatial distributions (called orbitals) and energies (meaning how tightly they are bound) that depend on the element in question (i.e. is governed by the number of protons and the number of electrons). Some electrons occupy regions of space closer to the nucleus and shield the outer electrons from the nuclear charge. [7.5]
Electrons in atoms are assigned energy based on how hard it is to remove them from the atoms (i.e. overcome the attraction of the nucleus). The energy required to remove an electron is the ionization energy. The larger this value is the “lower” the energy of the electron (this is because the energies are assigned negative values, being the energy released when an electron is put into this particular region in the atom). The energies of electrons in atoms are quantized, i.e. they can only have certain discrete values. [7.5]
The numbers and energies of these orbitals are governed by a hierarchy of quantum numbers related by simple rules. At the top of the hierarchy is the principal quantum number, n, which takes values 1, 2, 3, 4, and so on. For each value of n, there is/are certain values of l (ell), the orbital angular momentum quantum number which can take values 0, 1, 2 and so on up to n-1 (one). The orbitals are named for the value of l (ell): l (ell) = 1 (one) is an s orbital, l (ell) = 2 is a p orbital, l = 3 is a d-orbital and l = 4 is an f orbital. For each value of l, there is a third quantum number, ml, which takes values 0, ±1, ±2, and so on up to ±l (ell). As n increases the electrons are located for the most part further away from the nucleus. Each set of orbitals corresponding to a value of n is called a shell. Each set of orbitals corresponding to a value of l (ell) is called a subshell (except for n=1 (one)).
The electronic structure of the elements can be predicted on the basis of some simple rules: no two electrons can occupy the same region of space (i.e. orbital) unless they are “spinning” in opposite directions (there are only two directions of electron spin); electrons occupy the lowest energy orbitals first. Orbital energy increases with principal quantum number n, but shielding effects cause the 3d orbitals to have lower energy than the 4p, the 4d to have lower energy than the 5p, the 5d to have lower energy than the 6p, and the 6d to have lower energy than the 7p. In addition, the 4f orbitals have lower energy than the 5d, and the 5f orbitals have lower energy than the 6d. [8.1-8.3]
Elements are classified in a table in which the elements, starting at the top left, are listed in order of increasing proton number (known as the atomic number) so that elements with similar chemical properties are grouped together. This table is known as the periodic table. The major regions of the table relate to which orbitals are being filled. Thus the table is divided into s, p, d and f blocks. Each new row in the table corresponds to a new value of n. Columns in the table, which contain elements with similar chemical properties, are known as groups. Rows in the table are known as periods [8.4]. There is debate over the locations of H and group 12 in the table in terms of their chemical properties.
The electron configurations of the elements in the last column on the right of the table are particularly stable configurations. When elements are mixed together, they try to achieve these group 8A electron configurations. It provides a “driving force” for chemical behavior. For elements in the 2nd and 3rd periods, the configuration involves an octet of electrons (as the 3p orbitals are filled before the 3d orbitals). [9.4]
Bonding between atoms can be classified as ionic (complete transfer of electrons from one atom to another) or covalent (electrons are shared in some way between atoms). Usually a covalent bond consists of a pair of electrons [9.1-9.4]. Unless the participating atoms are identical, the electrons are not shared equally between the two atoms and the bond is polar [9.7]. The extent to which an atom attracts the electrons in a bond is governed by a characteristic called electronegativity [9.7]. It takes energy to break chemical bonds and energy is given out when bonds form. The energy involved at constant pressure (reaction vessels open to the atmosphere) is known as the enthalpy, H. It is usually in the form of heat, but sometimes in the form of light. The flow of heat is detected by temperature changes. If a chemical reaction gives out heat, the energy change for the reaction, DH, is negative. If a reaction takes in heat, its DH value is positive. These processes are known as exothermic and endothermic processes, respectively [6.5].
There are three ways in which DH for a reaction can be estimated from tabulated data: (1) from the bond enthalpies of the bonds broken and the bonds formed [9.8], (2) by Hess’s law, which involves DH values for reactions of each of the reactants and products with some common reactant [6.7], and (3) from standard enthalpies of formation of each of the reactants and products (i.e. via a hypothetical route in which the reactants are dissociated into the constituent elements and then reconstituted into the products [6.8].
Bonding controls the shapes of molecules. The shapes of molecules can be predicted by examining the numbers of (valence) electron pairs around each atom and assuming they repel each other about equally in three-dimensional space [9.9]. From the shape of the molecule and knowledge of the polarity of the bonds, the molecular polarity can be deduced [9.10]. In turn, the extent to which molecules of a given substance will interact with each other can be predicted and the relative boiling and melting points of compounds can be accounted for [13.2-13.4].
Atomic orbitals “overlap” with each other to produce hybrid orbitals, atomic orbitals (hybridized and unhybridized) overlap to produce molecular orbitals [10.1-10.2]. This overlap is a wave phenomenon and can be in a constructive manner (the result is a bonding orbital) or a destructive manner (the result is an antibonding orbital) [10.3]. The end-on overlap of orbitals produces a sigma bond; the sideways overlap of orbitals produces a pi-bond. In many compounds of p-block elements, sigma bonds are formed by the overlap of hybrid orbitals while pi bonds are formed by the overlap of unhybridized p-orbitals.
Elements in the 3rd and higher periods can use d orbitals for bonding. [9.6]
Hydrogen is a unique element in the sense that there are no other elements that have the properties of hydrogen recurring on a periodic basis. Hydrogen is the only element that uses all of its electrons for bonding. It is the only element that cannot shield the nuclear charge from the electron. No matter which shell the electron is in, it experiences the full nuclear charge. [13.3]
In compounds containing C, H, O, and N that are uncharged and have four or more atoms, C always has 4 bonds and no lone pairs, N has 3 bonds and one lone pair, O has 2 bonds and two lone pairs, and H is always a terminal atom (i.e. is bonded to only one other atom). [Tyson’s rules]
A fundamental feature of molecules and atoms is their interaction with electromagnetic radiation. Much information about the nature of the structure of atoms and molecules can be and has been deduced from this interaction (the science of the interaction of light with matter is known as spectroscopy). Light behaves as both an oscillating electromagnetic field and as a stream of particles (known as photons), whose energy is given by E = hc/l where h is Planck’s constant, c is the speed of light, and l is the wavelength [7.1]. If the energy of the light beam matches the energy difference between two energy levels in the atom or molecule, light can be absorbed and an electron moved from a lower energy orbital to a higher energy orbital. If an electron in a higher energy orbital moves to a lower energy orbital, light may be emitted. If the frequency of the light beam matches the frequency of some molecular vibration or rotation, energy may be transferred by a process of resonance. [7.3]
Chemists make extensive use of spectroscopy to probe the molecular and atomic structure, and composition of materials. The extent of interaction of electromagnetic radiation with a chemical substance can be used to determine the identities and amounts of the substances present. [4.6]
An important application of chemistry is to the measurement of chemical composition: both the identification of the chemical species (qualitative analysis) and the determination of how much (quantitative species). Many areas of science, technology and engineering are supported by the provision of reliable information about chemical composition. This is not just related to research and development in academic or industrial laboratories, but is also relevant to our everyday lives. Often answering the question “Is it safe?” requires information about the chemical composition of relevant materials, regardless of what “it” is.
Chemical reactions in aqueous solution are a major component of analytical chemistry, as many of the diagnostic tests and/or measurements that are applied require the material being examined to be dissolved. Many quantitative analyses are performed by measuring the amount of product formed after adding a reagent that is selective for the target species (analyte). The amount of product can be related to the amount of reactant by (a) the stoichiometry of the reaction or (b) by measuring the product formed for a series of solutions containing known amounts of the analyte and constructing a calibration graph (a plot of magnitude of what is measured as a function of concentration of analyte).
Often, other components of the sample interfere in the measurement process. A common way of dealing with this is to separate the analyte from the rest of the sample (the matrix). An understanding of the differences in chemical properties between analyte and matrix species is needed to devise suitable separation schemes.
The interaction of analyte species, or species derived from the analyte by appropriate reactions, with light is widely used the basis of chemical analysis. The range of energies of light involved is often a unique feature of the compound and can be used for qualitative analysis, whereas the magnitude of the interaction can be used for quantitative analysis.