Humboldt State University ® Department of Chemistry

Richard A. Paselk

	Science 331
Fall 2004	Lecture/Activity	Office: SA560a
Notes: 25 October		Phone: x 5719 Home: 822-1116
		e-mail: rap1

Conservation of Mass

Law of Conservation of Mass: Mass is neither created nor destroyed during a chemical change. (Strictly speaking there is no measurable change.) For example, if we burn gasoline (octane) in air we will get carbon dioxide and water:

If we were to weigh (determine the mass of) the carbon and oxygen vs. the carbon dioxide and water we would find them to be identical - the masses are the same on both sides of the equation (that's why its called a chemical equation, the two sides indicate are equal).

Looked at another way, if you count the atoms, the numbers of each kind of atom on each side are identical - so we can also say that atoms are conserved in chemical processes.

Energy

Law of Conservation of Energy: Energy is neither created nor destroyed in chemical processes. The problem here of course is - What is energy? Energy is the capacity to do work. So what's work? Work occurs when an object (mass) is moved against a force. Some common forms of energy important to our study include:

• Kinetic Energy (KE) - energy due to motion. KE = 1/2 mv².
• Potential Energy (PE) - energy due to position.

Another form of energy we need to be familiar with is:

• Heat - energy transferred between objects because of a difference in temperature.
  • Heat vs. Temperature: heat tells us how much energy is held (can be transferred from) and object or given mass of stuff. Temperature on the other hand is a measure of the energy per particle in a sample of matter. Thus for a gas temperature is a measure of the average Kinetic Energy (KE) of the gas particles, while the amount of energy we must add to the gas to achieve this average KE is the heat.
  • Note that most energy eventually ends up as heat (ex.: burning gasoline to move a car - heat in exhaust, friction in tire deformation, braking, etc.) or work (car is moved against its inertia).

Note that these forms of energy are readily interconverted.

Atoms and Atomic Structure

Elements are substances which cannot be broken down further into simpler substances.

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:

• Atomic number (Z) - the number of protons in the nucleus. This number is characteristic of a given element.
• Atomic mass number (A) - the sum of the protons and neutrons in a given atom (p + n).
• Atomic mass - the actual mass of an average atom in a sample. The characteristic atomic masses for Earth are shown on periodic tables.
• Atomic Mass Unit: the atomic mass unit = amu is a unit of mass for atoms. It is defined as 1/12 the mass of one atom of ¹²C, where the mass of ¹²C is defined as 12 exactly.

Isotopes

Isotopes are forms of elements which differ only in the number of neutrons. This means different isotopes of the same element have essentially the same chemical properties but slightly different physical properties. They can also different substantially in terms of their nuclear stability. Let's look at some examples of isotopes:

Symbol Z A p n e- 14C  6 14 6 8 6 238U6+ ? ? ? ? ? 35Cl- ? ? ? ? ? ?O? ? 18 ? ? 10

You should be able to fill in the blanks in a table like this with, the aid of a periodic table, on a quiz.

Chemical Periodicity

Look at the Periodic Chart on the wall. The pattern arises due to a repetition or periodicity of chemical properties. The vertical columns of the charts are called groups, while the rows are referred to a periods.

Note the numbering of the groups. The numbers from 1 - 18 are the internationally accepted numbers. We will also use the I - VIII "American" numbering system. Note that the "tallest" columns comprise what are referred to as the "representative elements" (IA - VIIIA).

Terms:

• Period: the rows of elements showing a repeating pattern of properties (e.g. Na - Ar).
• Group: a vertical column of elements on the table sharing a family resemblance of properties (e.g. Li - Fr).
• Representative elements: the elements of the s-block and p-block (blue and green on the table below).

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© R A Paselk

Last modified 27 October 2004