Chemistry 1020--Lecture 3--Notes
Measurement in science: numbers have units.
"How old are you?" Ans: "20" would be incorrect. Mean 20 yrs.
Units are part of calculations. How many breaths have you taken in your lifetime? (Make an assumption--30 breaths/minute). Then do the unit conversion:
20 yrs x (365 days/yr) x (24 hr/day) x (60 min/hr) x 30 breaths/min =315,360,000 breaths
But not justified in writing this many figures even if that is what the calculator gives. Both the 20 yrs and 30 breaths have uncertainty in the second digit, so are good to only two significant figures. Therefore the answer should be written to two significant figures, and as a large number is better written in exponential notation: 3.2 x 108 breaths.
(Note: at about 3 x 1022 particles of air/breath, this is 9 x 1030 particles of air breathed over the 20 year period).
How many significant figures in the following?
In multiplication or division, the answer should have the same number of significant figures as the least significant number in the calculation. In addition and subtraction, the least significant decimal position is determined from the least significant decimal position in the numbers being added or subtracted. For example 2.15 + 0.1239888, the answer would be significant only to the hundredths position. (Image adding 2.15?????? to 0.123988, you see there is uncertainty in the thousandths position).
For some practice in counting significant figures, go to this link.
Elements: Recall two kinds of pure substances. Elements are substances that cannot be broken down into something simpler. Elements are organized in the periodic chart. They are composed of identical particles called atoms, except that some elements are made of molecules containing more than one atom, for example N2, O2, etc.
Compounds composed of two or more elements in definite proportion by weight. (Law of definite composition).
Describe a compound by a formula, using the symbols of the elements to stand for atoms. Several kinds of formulas:
Empirical formula: gives relative numbers of atoms. Ionic compounds, such as NaCl, are made up of a lattice of positively charged sodium ions and negatively charged Cl atoms, in a 1:1 ratio. There is no identifiable molecule. So formulas for ionic compounds are always empirical formulas.
Molecular formula gives the actual number and kind of atoms bound together in a molecule--applies only to compounds composed of molecules rather than ions: H2O, CH4, CO2, etc. Empirical formula of a molecular compound could be different from the molecular formula: For example: hydrogen peroxide: EF: HO; MF: H2O2; glucose: EF: CH2O; MF: C6H12O6.
Metals with Non-metals (usually ionic)
name metallic element first, then non-metallic one ending in ide· (see examples, page 15, fig. 1.5 and 1.6) For compounds between group IA and IIA metals with group VIA and VIIA non-metals, only one combination usually possible, don't need information as to number of each. For transition metals, often more than one compound can be formed, so must include that information in the name (CuCl is copper (I) chloride, CuCl2 is copper (II) chloride, for example). Will discuss why later.
Non-metal with non-metal (usually covalent)
need to describe relative numbers, using the prefixes mono, di, tri, etc. (table 1.5). For example: CO is carbon monoxide, CO2 is carbon dioxide, P2O5 is diphosphorus pentoxide. Often skip mono for first element. Usually start with the element that is further left on the periodic chart, but there are exceptions (ex. NH3 for ammonia, CH4 for methane).
Pay attention to names and formulas as we go along. Learn the ones we talk about most.
The language of chemistry. The way chemical change is expressed.
Reactants ------> Products
Examples: (Illustrate need for balancing equations)
carbon + oxygen -----> carbon dioxide
hydrogen + oxygen ------> water
methane + oxygen -----> carbon dioxide + water
hexane + oxygen --------> carbon dioxide + water
sulfur + oxygen -------> sulfur dioxide
sulfur dioxide + oxygen -------> sulfur trioxide
carbon dioxide + water -------> carbonic acid
sulfur trioxide + water --------> sulfuric acid
Incomplete reactions: limited oxygen forms carbon monoxide
Common Air Pollutants Look at table 1.2 and see where these exceed permissible limits)
Carbon monoxide (from incomplete combustion reactions, particulary automobile engines)
Ozone (an allotropic form of oxygen. More reactive and toxic to living tissues. Formed as by product of sunlight reacting with nitrogen oxides and volatile organic compounds produced in automobile engines)
Nitrogen oxides (formed in high temperature of automobile engine)
Sulfur oxides (formed primarily in power plants that burn coal with a high sulfur content).
Catalytic converters in automobiles have helped decrease the levels, at least from 1975 to 1991 (table 1.7). What about today? Browse the EPA web site to see if you can find current figures.
Next: The two faces of ozone. Why is it good in the stratosphere?