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Structural formula

Many chemical compounds, especially hydrocarbons, can exist in different geometric configurations. A structural formula represents the arrangements of atoms in a way that a chemical formula cannot.

One of the advantages with structural formulae is the ability to tell the structure of the compound (see isomer). A simple example of this may be seen with the hydrocarbon butane, C₄H₁₀. The four carbons may be arranged in a linear pattern, or in a branched, “T” pattern. The first arrangement is known as orthobutane or n-butane, while the second is isobutane.

Molecular formula: C₄H₁₀.

Structural formulae:

n-butane

isobutane

Note that for organic compounds, line drawings of structural formula are assumed to have carbon atoms at the vertices and termini of all line segments not marked with the atomic symbol of an element (other than carbon). Each carbon atom is in turn assumed to bear enough hydrogen atoms to give the carbon atom four bonds. Equivalent full and abbreviated forms are shown in the adjacent figures.

A structural formula can be precisely described using IUPAC nomenclature. In the case of isobutane, the proper IUPAC name is methylpropane.

Multiple planes

Glycine

When substituents of a molecule exist in different planes, their position can be described using solid and dotted wedges, with the former showing a substituent coming out of the plane, and the latter going into it. This system is useful in describing differences between chiral molecules.

External links

• Structural formulae
• Structural formulae

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Stirling effect

The Stirling Effect is an effect where a vapour condenses straight into a solid through convection. It is an important factor to consider in the operation of steam engines and aeroplanes.

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Stepwise reaction

A stepwise reaction is a chemical reaction with at least one reaction intermediate or reactive intermediates and involving at least two consecutive elementary reactions.

The rate law of an elementary reaction is rather simple. On the other hand, when combining multiple elementary steps, the rate law can become rather complex. Moreover, when speaking about catalytic reactions, the diffusion may also limit the reaction. In general, however, there is one very slow step, which is the rate-determining step, i.e. the reaction doesn’t proceed any faster than the rate-determining step proceeds.

Organic reactions, especially when involving catalysis, are often stepwise. For example, a typical enol reaction consists of at least these elementary steps:

1. Deprotonation next to (α to) the carbonyl: HC–C=O → C=C–O^–
2. Attack of enolate: R^δ+ + C=C–O^– → R–C–C=O

R^δ+ is an electron acceptor, for example, the carbon of a carbonyl (C=O). A very strong base, usually an alkoxide, is needed for the first step.

Reactive intermediates may be trapped in a trapping reaction. This proves the stepwise nature of the reaction and the structure of the intermediate. For example, superacids were used to prove the existence of carbocations.

External links

• IUPAC Gold Book definition

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Standard conditions for temperature and pressure

It is absolutely necessary to define the standard reference conditions of temperature and pressure when expressing a gas volume or a volumetric flow rate because the volume of a gas varies with the temperature and pressure of the gas. The data in this article show quite clearly that there is no universally accepted single definition of the standard conditions of temperature and pressure. For that reason, simply stating that a gas flow rate is 10,000 m³/h (i.e. cubic meters per hour) at “standard conditions” has no meaning unless the actual conditions are clearly stated.

Definitions used in the past

For many years, most engineers, chemists, physicists and other scientists using the metric system of units defined the standard reference conditions of temperature and pressure for expressing gas volumes as being 0°C (273.15 K) and 101.325 kPa (i.e. 1 atmosphere of absolute pressure). During those same years, the most commonly used standard reference conditions for people using the Imperial or customary USA system of units was 60 °F (520 °R) and 14.696 psia (i.e. 1 atmosphere of absolute pressure) because it was almost universally used by the oil and gas industries worldwide.

The above two definitions are no longer the most commonly used definitions in either the metric, the Imperial or the customary USA system of units. Some of the many different definitions currently in use are presented in the following section.

It was also common in the past, when using the metric system of units, to refer to a Normal Cubic Meter (Nm³) and to define it as being at 0°C (273.15 K) and 101.325 kPa (i.e. 1 atmosphere of absolute pressure). As shown in the following section, that notation is no longer appropriate unless the specific reference conditions are explicitly stated, since there are currently many different metric system definitions of what constitutes standard reference conditions.

In the same manner, it is also no longer appropriate to refer to a Standard Cubic Foot (scf) unless the specific reference conditions are explicitly stated, again because there are currently many different definitions of the standard reference condition in both the Imperial and the customary U.S. systems of units. In particular, OPEC and a majority of the natural gas industry in North America have adopted 60°F and 14.73 psia as their standard reference conditions for expressing natural gas volumes and flow rates (rather than the 60°F and 14.696 psia commonly used previously).

Definitions in current use

There are a great many different definitions of the standard reference conditions currently being used. Table 1 presents twelve such variations of standard condition definitions - and there are quite a few others as well.

As shown in the table, the IUPAC (International Union of Pure and Applied Chemistry) currently defines standard reference conditions as being 0°C and 1 bar (i.e. 100 kPa) of absolute pressure rather than the 1 atmosphere (i.e. 101.325 kPa) of absolute pressure used in the past. In fact, the IUPAC’s current definition has been in existence since 1997.

As further shown in the table, the oil and gas industries have to a large extent changed from their past usage of 60°F and 14.696 psia to their current usage of 60°F and 14.73 psia. This is especially true of the natural gas industry in North America as well as elsewhere.

It should also be noted that the International Organization for Standardization (ISO), the U.S. Environmental Protection Agency (EPA) and National Institute of Standards and Technology (NIST) each have more than one definition of standard reference conditions in their various standards and regulations.

The table makes it quite obvious that it is absolutely necessary to clearly state the temperature and pressure reference conditions whenever expressing a gas volume or gas volumetric flowrate. It is equally important to state whether the gas volume is expressed on a dry basis or a wet basis. As noted in the table, some of the current definitions of the reference conditions include a specification of the percent relative humidity (% RH).

Notes:

• 101.325 kPa = 1 atmosphere = 1.01325 bar ≈ 14.696 psi
• 100.000 kPa = 1 bar ≈ 14.504 psi
• 14.503 psi = 750 mm Hg = 99.992 kPa ≈ 1 bar
• All pressures are absolute pressures (not gauge pressures)
• 59°F = 15°C
• 60°F ≈ 15.6°C
• dry = 0 percent relative humidity = 0 % RH

The full names of the entities listed in Table 1:

• IUPAC: International Union of Pure and Applied Chemistry
• NIST: National Institute of Standards and Technology
• ISA: ICAO’s International Standard Atmosphere
• ISO: International Organization for Standardization
• EEA: European Environment Agency
• EGIA: Electricity and Gas Inspection Act (of Canada)
• EPA: U.S. Environmental Protection Agency
• SATP: Standard Ambient Pressure and Temperature
• CAGI: Compressed Air and Gas Institute
• SPE: Society of Petroleum Engineers
• OSHA: U.S. Occupational Safety and Health Administration
• SCAQMD: California’s South Coast Air Quality Control District
• OPEC: Organization of Petroleum Exporting Countries
• EIA: U.S. Energy Information Administration
• Std. Metro: U.S. Army’s Standard Metro (used in ballistics)

Molar volume of a gas

It is equally as important to indicate the applicable reference conditions of temperature and pressure when stating the molar volume of a gas as it is when expressing a gas volume or volumetric flow rate. Stating the molar volume of a gas without indicating the reference conditions of temperature and pressure has no meaning and it can cause much confusion.

The molar gas volumes can be calculated with an accuracy that is usually sufficient by using the Universal Gas Law for ideal gases:

P · V = n · R · T

… is the usual expression of the Universal Gas Law and it can be rearranged thus:

V ÷ n = R · T ÷ P

The molar volume of any ideal gas may be calculated at various standard reference conditions as shown below:

• V ÷ n = 8.3145 × 273.15 ÷ 101.325 = 22.414 m³/kgmol at 0°C and 101.325 kPa absolute pressure
• V ÷ n = 8.3145 × 273.15 ÷ 100.000 = 22.711 m³/kgmol at 0°C and 100 kPa absolute pressure
• V ÷ n = 10.7316 × 519.67 ÷ 14.696 = 379.48 ft³/lbmol at 60°F and 14.696 psia absolute pressure
• V ÷ n = 10.7316 × 519.67 ÷ 14.730 = 378.61 ft³/lbmol at 60°F and 14.73 psia absolute pressure

The technical literature can be very confusing because many authors fail to explain whether they are using the universal gas law constant R which applies to any ideal gas or whether they are using the gas law constant R_s which only applies to a specific individual gas. The relationship between the two constants is R_s = R ÷ M, where M is the molecular weight of the gas.

References

1. ^a b ”Compendium of Terminology”, 2nd Edition, 1997, IUPAC Secretariat, Research Triangle Park, P.O. Box 13757, NC, USA (pre-1997 and post-1997 definitions)  IUPAC Compendium
2. ^{^} ”NIST Standard Reference Data Base 7 Users Guide”, December 1969, NIST, Gaithersburg, MD, USA  NIST Data Base 7
3. ^{^} ”Stationary Emissions-Measurement of Velocity and Volume Flow Rate of Gas in Ducts”, ISO 10780, International Standards Organization, Geneva, Switzerland  ISO
4. ^{^} ”Handbook of Physics and Chemistry”, 56th Edition, pp.F201-F206, CRC Press, Boca Raton, FL, USA
5. ^{^} ”Natural Gas-Standard Reference Conditions”, ISO 13443, International Standards Organization, Geneva, Switzerland  ISO
6. ^{^} ”Extraction, First Treatment and Loading of Liquid & Gaseous Fossil Fuels”, Emission Inventory Guidebook B521, Activities 050201 - 050303, September 1999, European Environmental Agency, Copenhagen, Denmark  Emission Inventory Guidebook
7. ^a b ”Electricity and Gas Inspection Act”, SOR/86-131 (defines a set of standard conditions for Imperial units and a different set for metric units)  Canadian Laws
8. ^{^} ”Standards of Performance for New Sources”, 40 CFR–Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990  New Source Performance Standards
9. ^{^} ”Design and Uncertainty for a PVTt Gas Flow Standard”, Journal of Research of the National Institute of Standards and Technology, Vol.108, Number 1, 2003  NIST Journal
10. ^{^} ”National Primary and Secondary Ambient Air Quality Standards”, 40 CFR–Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998  National Ambient Air Standards
11. ^{^} ”Table of Chemical Thermodynamic Properties”, National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2.
12. ^{^} ”Glossary”, 2002, Compressed Air and Gas Institute, Cleveland, OH, USA  Glossary
13. ^a b ”The SI Metric System of Units and SPE Metric Standard (Notes for Table 2.3 on page 25)”, June 1982, Richardson, TX, USA (defines standard cubic foot and standard cubic meter)  SPE
14. ^{^} ”Storage and Handling of Liquefied Petroleum Gases” and “Storage and Handling of Anhydrous Ammonia”, 29 CFR–Labor, Chapter XVII–Occupational Safety and Health Administration, Part 1910, Sect. 1910.110 and 1910.111, 1993  Storage/Handling of LPG
15. ^{^} ”Rule 102, Definition of Terms (Standard Conditions)”, Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA  SCAQMD Rule 102
16. ^{^} ”Annual Statistical Bulletin”, 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, Vienna, Austria  OPEC Statistical Bulletin
17. ^{^} ”Natural Gas Annual 2004″, DOE/EIA-0131(04), December 2005, U.S. Department of Energy, Energy Information Administration, Washington, D.C., USA  Natural Gas Annual 2004
18. ^{^} ”Effects of Altitude and Atmospheric Conditions”, Exterior Ballistics Section, Sierra’s “Rifle and Handgun Reloading Manual, 5th Edition”, Sedalia, MO, USA  Exterior Ballistics
19. ^{^} ”Gas turbines - Procurement - Part 2: Standard reference conditions and ratings”, ISO 3977-2:1997 and “Gas turbines - Acceptance tests”, ISO 2314:1989, Edition 2, International Standards Organization, Geneva, Switzerland ISO

External links

• “Standard conditions for gases” from the IUPAC Gold Book.
• Related information in the feature articles at air-dispersion.com.

Sonochemistry

In chemistry, the study of sonochemistry is concerned with understanding the effect of sonic waves and wave properties on chemical systems. Since acoustic waves have unique physical properties, the corresponding atomic and molecular chemistry is unique as well. Often these effects are most apparent in ultrasonic systems. This is demonstrated in phenomenon such as ultrasound, sonication, sonoluminescence and sonic cavitation.

For example, in chemical kinetics, it has been observed that ultrasound can greatly enhance chemical reactivity in a number of systems; effectively acting as a catalyst by exciting the atomic and molecular modes of the system (such as the vibrational, rotational, and translational modes).

Solventless reactions

It is possible to do a Solventless reaction where no solvent is used.

One type of reaction is one where a liquid reactant is used neat, for instance the reaction of 1-bromonaphthalene with P₄S₁₀ is done with no added liquid solvent. But the 1-bromonaphthalene acts as a solvent.

A reaction which is closer to a true solventless reaction is Knoevenagel condensation of ketones with dicyanomethane (malononitrile) where a 1:1 mixture of the two reactants (and ammonium acetate) is irradated in a microwave oven [1].

Collin Raston’s research group have been responsible for a number of new solvent free reactions [2] and ‘Towards benign synthesis of pyridines involving sequential solvent free aldol and Michael addition reactions’, Gareth W. V. Cave and Colin L. Raston, Chem. Commun., 2000, 2119 - 2120 and Gareth W. V. Cave and Colin L. Raston and in Journal of the Chemical Society, Perkin Transactions 1, 2001, (24), 3258 - 3264.

In some of these reactions all the starting materials are solids, they are ground together with some sodium hydroxide to form a liquid, which turns into a paste which then hardens to a solid.

Solvatochromic

The Solvatochromic effect or solvatochromic shift refers to a strong dependence of absorption and emission spectra with the solvent polarity. Since polarities of the ground and excited state of a chromophore are different, a change in the solvent polarity will lead to differential stablization of the ground and excited states, and thus, a change in the energy gap between these electronic states. Consequently, variations in the position, intensity, and shape of the absorption spectra can be direct measures of the specific interactions between the solute and solvent molecules. Due to the Franck-Condon principle (atoms do not change position during light absorption), the excited state solvent shell is not in equilibrium with the excited state molecule (”solute”). In fact, charge-transfer transitions of ground state ion-pairs give the largest changes in absorption spectra, and are thus, the most useful for measuring solvent polarity.

Solid-state chemistry

Solid-state chemistry is the study of solid materials, which may be molecular. Solid-state chemistry studies both the synthesis, the structure and the physical properties of solids. It therefore has a strong overlap with solid-state physics, mineralogy, crystallography, ceramics, metallurgy, thermodynamics, materials science and electronics with a focus on the synthesis of novel materials.

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Skeletal formula

A skeletal formula is a three-dimensional model of the molecule that demonstrates the molecular shape, including bond angles. It does not necessarily display the chemical elements within a molecule. These are drawn by using a line for each bond. Carbon atoms are understood to exist at each bend, conjunction, or end of a line. Hydrogen atoms are understood to be anywhere that they are required to give any atom the appropriate number of covalent bonds. All other elements are specifically noted in the appropriate place. Some common groups, such as benzene rings, have their own shorthand forms.

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Semipermeable membrane

Scheme of semipermeable membrane during hemodialysis, where red is blood, blue is the dialysing fluid, and yellow is the membrane.

A semipermeable membrane, also termed a selectively permeable membrane or a differentially permeable membrane, is a membrane which will allow certain molecules or ions to pass through it by diffusion and occasionally specialised “facilitated diffusion”. The rate of passage depends on the pressure, concentration and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute.

Depending on the membrane and the solute, permeability may depend on solute size, solubility properties, or chemistry. An example of a semi-permeable membrane is a lipid bilayer, on which is based the plasma membrane that surrounds all biological cells. Many natural and synthetic materials thicker than a membrane are also semipermeable. An example of this is the thin film on the inside of an egg.

See also

• Osmosis
• Reverse osmosis

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