Si-Units

In the system of units that Lavoisier used in the eighteenth century, there were 9216 grains to the pound (the / Vre). English chemists of the same period used a system in which there were 7000 grains to the pound—unless they were trained as apothecaries, in which case there were 5760 grains to the pound  

The amount of substance is discussed in Chapter 3, and the electric current (ampere) is introduced in Chapter 20. Luminous intensity wiii not be used in this book. 

The first measurements were probably based on the human body (the length of the foot, for example). In time, fixed standards developed, but these varied from place to place. Each country or government (and often each trade) adopted its own units. As science became more quantitative in the seventeenth and eighteenth centuries, scientists found that the lack of standard units was a problem. They began to seek a simple, international system of measurement. In 1791 a study committee of the French Academy of Sciences devised such a system. Called the metric system, it became the official system of measurement for France and was soon used by scientists throughout the world. Most nations have since adopted the metric system or, at least, have set a schedule for changing to it. 

In 1960 the General Conference of Weights and Measures adopted the International System of units (or SI, after the French le Systeme International d Unites), which is a particular choice of metric units. This system has seven SI base units, the SI units from which all others can be derived. Table 1.2 lists these base units and the symbols used to represent them. In this chapter, we will discuss four base quantities length, mass, time, and temperature.  

One advantage of any metric system is that it is a decimal system. In SI, a larger or smaller unit for a physical quantity is indicated by an SI prefix, which is a prefix used in the International System to indicate a power of ten. For example, the base unit of length in SI is the meter (somewhat longer than a yard), and 10 meter is called a centi-meter. Thus 2.54 centimeters equals 2.54 X 10 meters. The SI prefixes are fisted in Table 1.3. Only the highlighted ones will be used in this book mega- (10 ), kilo- (10 ), deci- (10 ), centi- (10 ), milli- (10 ), micro- (10 ), nano- (10 ), andpico- (10 ). 

In the following treatment, the SI system is discussed first, and the other systems are then derived from it. The procedure reverses the historical order, as the SI units evolved from the cgs system. Because of the growing importance of the SI system, it should logically be given a preference. If, in time, the other systems are phased out, they can be ignored and the SI system then used exclusively. 

The SI system covers the entire field of science and engineering, including electromagnetics and illumination. For the purposes of this book, a subset of the SI units covering chemistry, gravity, mechanics, and thermodynamics is sufficient. The units are derivable from (1) four proportionalities of chemistry and physics  

BASIC EQUATIONS. The basic proportionalities, each written as an equation with its own proportionality factor, are  

Equation (1.1) is Newton s second law of motion, showing the proportionality between the resultant of all the forces acting on a particle of mass m and the time rate of increase in momentum of the particle in the direction of the resultant force. 

Equation (1.2) is Newton s law of gravitation, giving the force of attraction between two particles of masses m and mi, a distance r apart. 

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Show what Eq. V-7 (for k ) becomes when written in the SI system. Calculate k for 0.05M, 2 2 electrolyte at 25°C using SI units repeat the calculation in the cgs/esu system and show that the result is equivalent. 

Make a calculation to confirm the numerical illustration following Eq. V-42. Show what form the equation takes in the SI system, and repeat the calculation in SI units. Show that the result corresponds to that obtained using cgs/esu units. 

To first order, the dispersion (a-a) interaction is independent of the structure in a condensed medium and should be approximately pairwise additive. Qualitatively, this is because the dispersion interaction results from a small perturbation of electronic motions so that many such perturbations can add without serious mutual interaction. Because of this simplification and its ubiquity in colloid and surface science, dispersion forces have received the most significant attention in the past half-century. The way dispersion forces lead to long-range interactions is discussed in Section VI-3 below. Before we present this discussion, it is useful to recast the key equations in cgs/esu units and SI units in Tables VI-2 and VI-3. 

We will quote a numerical constant in some of these equations to help with actual calculations. The units can be very confusing because it is conventional to use non-SI units for several quantities. The wavenumber value, i>, is usually taken to be in cm The extinction coefficient is conveniently taken in units of 1 moH ... 

FIGURE 2 12 Boiling points of unbranched alkanes and their 2 methyl branched iso mers (Temperatures in this text are expressed in de grees Celsius C The SI unit of temperature is the kelvin K To convert degrees Celsius to kelvins add 273 15 )... 

The SI units of frequency are reciprocal seconds (s ) given the name hertz and the symbol Hz m honor of the nineteenth century physicist Heinrich R Hertz The constant of proportionality h is called Planck s constant and has the value... 

The SI unit for magnetic field strength is the tesla (T) named after Nikola Tesla a contemporary of Thomas Edison and who like Edison was an inventor of electrical devices... 

Frequency (Section 13 1) Number of waves per unit time Although often expressed in hertz (Hz) or cycles per sec ond the SI unit for frequency is s ... 

Tesla (Section 13 3) SI unit for magnetic field strength Tetrahedral intermediate (Section 19 14 and Chapter 20) The key intermediate in nucleophilic acyl substitution Formed by nucleophilic addition to the carbonyl group of a car boxyhc acid derivative... 

Physical quantity Name of SI unit Symbol for SI unit Definition... 

Physical quantity SI unit SI unit of SI base units... 

Physical quantity Name of unit Symbol for unit Value in SI units ... 

The angstrom and bar are approved for temporary use with SI units however, they should not be introduced if not used at present. 

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