Chem 109 - General Chemistry - Spring 2015
Lecture Notes 2: 23 January
What is Chemistry?
Chemistry is the study of matter and its transformations.
- "Classical" chemistry involves mostly electron transfers and/or interactions of charges (electron and nuclear). As we'll see only some electrons in atoms are involved - the outer or valence electrons of atoms.
- Nuclear chemistry is an extension of chemistry where nuclei are transformed changing one kind of atom (element or isotope) to another. This is a completely separate realm of phenomena, largely unimportant in everyday life (unless you work at a nuclear power plant!). We will NOT cover nuclear chemistry.
More specifically, chemistry is the scientific study of matter. So what do we mean by science? Two common "definitions" for Science:
- The body of knowledge and rules/laws/theories we have discovered regarding the natural world.
- The method of discovery and confirmation used by scientists. Classically we describe this process as the "Scientific Method" summarized in the steps below:
- Identify a problem based on initial observations
- Make a hypothesis (a tentative, testable, explanation of the observations)
- Collect data via planned Observations and/or Experiments ("asking nature a question")
- "Clean" simple experiments vs. statistical inference
- Controls - everything the same except the variable of interest.
- Analyze and Evaluate results - modify hypothesis if necessary, and repeat data collections etc.
- Create a theory (an explanation of observations consistant with results of experiments etc.)
- The theory is a "model of reality"
- Note we also use models which are not intended to represent reality, but rather are used to solve particular problems within a defined "universe" which may mimic the behavior of a restricted subset of "reality."
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.
© R A Paselk
Last modified 23 January 2015
