|Lecture Notes: 1 September||
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 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.
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:
Another form of energy we need to be familiar with is:
Note that these forms of energy are readily interconverted.
The metric system originated in the French Revolution as a rational system of measurements to rescue France from the chaos of pre-revolutionary measurements and thus prevent tax collectors from cheating.
Wanted to base system on "natural" universal standards. Thus for length they chose the size of the Earth: specifically the meter was defined as one ten-millionth (10-7) of one quarter of the Earth's meridian* passing through Paris (the line along the "surface" of the Earth from the north pole though Paris to the equator). For mass the Kilogram was defined as the mass of a cube of water 0.1 meter on a side. Of course these are not convenient, so other standards were quickly created: the meter became the distance between two lines on a platinum-iridium bar stored in a vault in Paris, while the kilogram became a cylindrical mass of platinum-iridium alloy stored in the same vault.
Today the various units are defined by international agreement to give the SI (Systéme International) units:
Note Table 3.2 in your text (p 38). You should know and be able to interconvert the prefixes in the table below:
Other common prefixes which you should be familiar with but do not need to memorize include: tera- (T, 1012), giga- (G, 109), mega- (M, 106), pico- (p, 10-12), and fempto- (f, 10-15).
Memorize: 1 mL = 1 cm3; 1 inch = 2.54 cm; 1 liter is about 1 quart; density of water = 1 g/mL; 0° C = 32 °F, 100°C = 212 °F, -40 °C = -40 °F
A convenient check on your work, or even a way to determine the best approach to a problem, is to use dimensional analysis. 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]
© R A Paselk
Last modified 1 September 2009