Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107	Fundamentals of Chemistry	Fall 2008
Lecture Notes: 2 September	© R. Paselk 2005
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Matter, cont.

A fourth state of matter commonly occurs under special conditions: a plasma. A plasma is an ionized fluid - can be contained by magnetic fields.

• Chemical properties of substances describe behaviors which lead to changes in composition. Chemical properties describe reactivity under various circumstances (does it burn in air, react with acids or bases, corrode in sea water etc.) Note that chemical changes result in different compositions and different physical properties.

Conservation Laws

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:

C₈H₁₈ + 12 1/2 O₂ 8 CO₂ + 9 H₂O

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 are equal in mass). 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.

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.

Measurements

Accuracy and Precision

First we need to define and distinguish between two terms: accuracy and precision. Consider the two targets below:

Which target is the work of the better marksman? I would say B, because she always hits nearly the same place - all we have to do to get all of her shots in the center is to adjust her sights. On the other hand, A is scattered all over. Sure he hit the center once, but, on average he needs lots of shots to do it, adjsuting his sights will do us no good!

• Looking at the average of the dots on each target (represented by an x) we see that A has very good accuracy, but lousy precision.
  • Accuracy refers to how closely we approach the "real" or actual value (center of the target). Note that ALL measurements have some scatter, so accuracy generally refers to an average value.
  • Precision refers to how closely values are repeated (are the hits "clustered"). Precision is often described as repeatability.
• B on the other hand, has poor accuracy, but very good precision.

For the most part, it is more important to be precise, than it is to be accurate, since we can always adjust our instrument, or our data, to bring the results to the proper value.

Exponential or scientific notation

It is often convenient to express numbers in exponential or scientific notation to indicate significant figures, and to just avoid writing the huge numbers of zeros we often run into in the natural world. [examples] See lab book exercises.

Significant Figures

For measurements we want to be sure we convey the precision (repeatability) of our measurements using significant figures. [examples] See lab book exercises.

• Look at example of making measurements with a ruler.
  • Which digits should we keep? Which did we measure?

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Last modified 2 September 2008