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Chemical Properties of Material

Chemical and physical properties are often tabulated together in most handbooks. In general, the data associated with a compound contain name, empirical and structural formula, molecular weight, Chemical Abstract (CA) registry number, melting point, boiling point, density, color, solubility, oxidation or reduction potential, and various spectroscopic peaks. However, other literature must be consulted on chemical reactivity.

Chemical reactions usually involve the breakage and formation of some chemical bonds. All chemical reactions involve the redistribution of electrons among species involved. Chemical properties show the nature of its reactivity, the type of compounds and the category of reactions. Think of a compound, and classify it according to the following criteria.

Acids and bases

Definitions of acids and bases
Arrhenius Substance produce
H+ in solution
Substance produce
OH- in solution
Bronsted Proton donor proton acceptor
Lewis Electrophile Nucleophile
Chemicals can be classified according to their acidity or basicity. A base and an acid readily react. There are three definitions of acid and base first defined by Arrhenius, Bronsted, and Lewis. G.N. Lewis, wanted to consider all chemical reactions as acid-base reactions, and he redefined acidity and basicity as electrophile (electron loving sites) and nucleophile (positive or nuclei loving sites) in order to cover most reactions. Thus, chemicals can be classified as acids and bases according to their reactivity. However, such classification works best for only some compounds. The concept works well for chemists, because they use an acid to react with a base. For example, water deposit (usually CaCO3) is slightly basic, and it can be dissolved in an acid such as vinegar. The design of household cleaning agents often makes use of the acid and base properties.

Oxidants and reductants

Many reactions involve the transfer of electrons from one species to another. Substances such as sodium, zinc, and oxalic acid that give up electrons are reducing agents, or reductants. They are oxidized in reactions. Electron-accepting compounds such as ferric ion (Fe3+), permanganate (MnO4-), oxygen, and hydrogen peroxide (H2O2) are oxidants, they don't always contain oxygen. Oxidation and reduction happen together, and they are referred to as redox reactions. The strength of a compound to accept one or more electrons is indicated by its electromotive force (emf) or reduction potential. Values of emf at standard conditions for common compounds or ions are listed in handbooks. The list indicates the relative redox strength of substances, but their strengths vary with conditions such as concentration, acidity, and temperature.

Redox reactions occur in corrosion and combustion. For example, oxidation of iron in wet condition produces loose rust in the reaction,

4Fe + 3O2 + 2H2O ® 2Fe2O3.H2O.

Exchange reactions

In exchange reactions, two compounds exchange ions or group of atoms. Usually, these reactions involve more than one phase and they are called heterogeneous reactions. These reactions stop only when one of the reactants is exhausted, and they are ideal for manufacturing process. A solid or precipitate is formed when two ions having great affinity for each other are poured together from their solutions. For example, Ca2+ ions form a solid CaCO3 when they meet the carbonate ions, CO32-, Ca2+ + CO32- ® CaCO3 (solid). Due to limited solubility, a solid precipitated out. Similarly, gas-generating reactions from none-gas reactants tend to exhaust one of the reactants.

Decomposition reactions

Substances synthesized at stringent conditions such as low temperature and high pressure may be unstable and undergo reactions by themselves to become another one or more substances, at ordinary condition. However, there are two factors governing the stability of a substance: kinetics and thermodynamics. The former refers to the speed of reaction whereas the latter the energy content of the system. If the speed is slow, the compounds may stay unchanged for an indefinite period.

Composition

The presence of various elemental and chemical components in a mixture is called composition. For a pure compound, composition means percentages of elements present. Thus, the term composition must be used with specification. Physical and chemical properties of mixtures depend on the composition. For example, water and dibutylamine boil at 373 and 433 K respectively, but a 50 - 50 % mixture boils at 370 K, lower than either one. Properties of metallic solid solutions (alloys) are drastically different from those of their components. Often, the composition is expressed in units of concentrations: percentage (by weight or by mole), molarity, etc.

Molecular structure

As a discipline, chemistry studies molecules, correlating properties to their structures, and their changes. In the study of molecular structure, the ultimate goal is to learn their 3-dimensional atomic bonding and arrangements, from which the bond distances and bond angles are calculated. In cases three-dimensional information are not possible, the interconnectiivity and type of bonding between atoms are sought. Often structural formulas representing the bonding are given to reflect molecular structures. For example, SO42- indicates that the sulfate ion has four equivalent oxygen atoms bonded the central sulfur. Thus, a familiarity with structural notations is important.

Crystal structure

Crystals of most organic compounds were studied in order to determine their molecular structures. Often the arrangement of molecules in the solid state is interesting. One such structure is shown here. Molecules of cyanoacetamide NºCCH2C(=O)NH2 are planar, and a network of hydrogen bonds hold the molecules together forming layers. The negative site NºC acts as an acceptor in the hydrogen bonding.

Properties of the crystals are overshadowed by their chemical properties. For inorganic compounds and minerals, their properties as crystals are often used in their applications. Thus, crystal structural information is particularly valuable. However, we should realize that most bulk materials are composed of many crystals, some large and some very small. A chunk of metal may be an aggregate of millions of crystals called a grain. The range of grain size, the average grain size, and their orientations plus the shape of the grain boundaries play important roles in determining the properties of the bulk material. Sizes, arrangements and orientations of grains in the bulk material is often called texture structure, and the texture structures of metals and polymers are particularly important in their mechanical, appearance, and dimensional properties. Surface treatments such as heat quenching alter their property.

Secondary and tertiary structures

When it comes to biological materials such as proteins and enzymes, the nature of folding and spiral of the chains are considered as secondary structure. The spatial relationship of various segments due to secondary structures is called tertiary structure. The secondary and tertiary structures provide cavities suitable for promoting certain chemical reactions, which are the biological in nature.

Thermodynamic properties

Heats of Combustion /kcal mole-1
methane,CH4 212.8
ethane, C2H6 372.8
propane,C3H8 530.6
maltose,C12H22O11 1349.3
lactose,C12H22O22 1350.0
sucrose,C12H22O22 1350.0
glucose,C6H12O6 669.9
fructose,C6H12O6 672.0
methanol,CH3OH 173.6
ethanol, C2H5OH 326.7
Energy drives all physical and chemical changes. The study of these phenomena is called thermodynamics. Thus, many types of data are associated with materials. They range from heat of fusion (the energy required to melt a mole of solid), energy of formation (energy released when a substance is formed from the respective elements), energy of solution (energy released when one mole of the substance dissolve in water), to heat of combustion (energy released when one mole substance is burned).

Heats of combustion for some substances are given in a separate box. Thermodynamics is a very broad field taught in chemistry, physics, engineering, and biological sciences. We are only able to mention some properties here. Internal energy, free energy, enthalpy, and entropy are concepts of thermodynamics, and they are given in most year-one chemistry texts., although not in details due to time constrain. An interested reader may pursue further study in thermodynamics by reading.

Chemical bond properties

Not only the shape and bonding in a molecule are important, quantitative information such as bondlength, bond angle, bond energy, and intermolecular distances are chemical bond properties. We shall discuss these in later chapters.

Spectroscopic data

When a beam of light passes a medium, photons with appropriate wavelengths are absorbed. Plots of the transmitted intensity against frequency are called spectra. Depending on the region of the electromagnetic radiation used, these are called infrared (IR), visible, or ultraviolet (UV) spectra. Excited atoms or ions in the plasma also emit radiation, and plots of intensity against frequency are called emission spectra. If the wavelengths are in the order of , the patterns are called X-ray spectra. Some molecules contain nuclei that are magnetically active, their absorption of radiation is affected by the strength of the magnetic field, in which they are placed. The technique is called nuclear magnetic resonance (NMR) spectroscopy. Using laser as light sources and analyzing scattered light pattern are techniques used in Raman spectroscopy. The technique based on the weight and charge of a particle is call mass spectroscopy. Spectroscopic techniques are useful for compound identification and chemical analysis.

Activities

How do the definitions of acid and base evolve?

Name three oxidants and three reductants.

Do an artistic draw of a molecular structure of a compound of your choice.

If mathane, ethane and propane are sold at the same price per unit weight, which one is the best buy in terms of energy?

What properties are associated with the chemical bond?

What is a spectrum? What can spectra be used for?

Chemical data

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