Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 3 September

© R. Paselk 2005
 
     
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Dimensional (Unit) Analysis and Problem Solving

Memorize: 1 mL = 1 cm3; 1 inch = 2.54 cm; density of water = 1 g/mL; 1 liter is about 1 quart; 0° C = 32 °F, 100°C = 212 °F

Dimensional analysis can be convenient check on your work, or even a way to determine the best approach to a problem. This simply means to include all of the units for each factor in an equation, and then to check to see that the units on both sides of the equation are equal.

For example, how long is a one foot ruler? Know conversion for cm to inches: 2.54 cm = 1 inch (no sig figs, defined ) [ans. = 30.48 cm]

Let's try some density problems. First recall that the units of density are g/cm3 or g.cm-3.

Let's look at a pressure problem. What I want to emphasis here is that you don't need to know what the units refer to, you only need to know the conversions.

Heat and Specific Heat: Earlier we spoke of heat as a measure of energy transferred between objects of different temperatures. We are already familiar with the units of temperature, what are the units of heat?

Let's look at a specific heat problem. Specific heat is the amount of heat it takes to raise 1 g of a specific substance 1 °C. Specific heats for other substances are relative to water, so no units (comparing results in canceling out units).

The heat transferred in a process (q) is summarized in the equation:

Heat = q = mCspgreek symbol deltaT

where m is the mass of substance and Csp is the specific heat of the substance.

Example: 750 calories of heat is transferred to 100.0 g of water at 20.00 °C. What will the new temperature of the water be assuming no heat is lost to the container of the surroundings?

Atoms and Atomic Structure

Atoms

Atoms are the smallest constituents of elements. The first successful atomic theory was that of John Dalton (1803).

Dalton's atomic theory states:

  1. All matter is composed of ultimately small particles, called atoms.
  2. Atoms are permanent and indivisible - they can neither be created nor destroyed.
  3. Elements are characterized by their atoms. All atoms of a given element are identical in all respects. Atoms of different elements have different properties.
  4. Chemical change consists of a combination, separation, or rearrangement of atoms.
  5. Chemical compounds are composed of atoms of two or more elements in fixed ratios.

All of these statements are close to reality, and nearly describe chemical behavior. But here are exceptions. Thus atoms can be created and destroyed via nuclear processes. They consist of different forms called isotopes. Atoms are not the smallest particles, etc.

Atoms are now known to consist of three different types of particles: electrons, protons and neutrons (the common form of one very important atom, hydrogen, has only two kinds: a proton and an electron). The protons and neutrons reside in a small inner portion called the nucleus while the electrons reside in a relatively large cloud centered on the nucleus. Important properties of these particles are listed in the table below:

 Particle Charge Relative Mass Mass
Electron (e-) -1 1/1840 9.11 x 10-28g
Proton (p or H+) +1 ª1 1.67 x 10-24g
Neutron (n) 0  ª1  1.67 x 10-24g

Some important terms which you must know are:


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Last modified 3 September 2009