400 Chemistry
Intro to Chemistry Notes
Chemistry = the study of matter, its composition and structure, and the changes
(reactions) it undergoes
The language of chemistry…
Chemical symbols = letters representing the elements
Note: Start memorizing the names and symbols for elements #1 through 92
Periodic Table= and organized arrangement of elements according to their properties
Periods = horizontal rows
Groups or families = vertical columns
Metals- left and center (mostly solids, ductile, shiny, conductive)
Nonmetals- right (mostly liquids and gases, dull/brittle solids, nonconductive)
Metalloids- in between along zig-zag (properties of both, semiconductors)
Alkali metals- group 1 (IA)
Alkaline earth metals- group 2 (IIA)
Transition metals- middle section
Halogens- group 17 (VIIA)
Noble or inert gases- group 18 (VIIIA)
Chemical equation= symbolic “sentence” showing how chemicals react and what
products result
Chemical equations are like sentences….
A+B→C+D
A, B are reactants (starting materials)
C, D are products (what results)
Arrow is read as “yields” or “produces”
May see “heat” (ex: 54 kJ) as a reactant (left) or product (right)
Heat on the left = Endothermic, heat is needed for the reaction to take place
Heat on the right = Exothermic, heat is released as part of the reaction
Mathematics
Measurement and mathematics are extremely important in chemistry!
2 types of data: qualitative and quantitative
Qualitative= non-numerical, descriptive observations
Quantitative= numerical measurements or data
To communicate effectively, all quantitative measurements or data MUST include units
SI units and prefixes are used for consistency:
Quantity
SI base
unit
Mass
kilogram
Length
meter
Time
second
Temperature Kelvin
Amount
Mole
Electric
Ampere
current
Luminous
Candela
intensity
Abbreviation
kg
m
sec or s
K
mol
A or amp
cd
Note: Temperature, for our purposes, is often measured in Kelvin
Celsius temp + 273 = Temp in K
Kelvin temp -273 = Temp in Celsius
Celsius temp = 5/9 (deg F – 32)
Fahrenheit temp = 9/5 (deg C) + 32
Common SI prefixes:
Giga (G) = a billion
Mega (M)= a million
Kilo (k)= a thousand
Deci (d)= one tenth
Centi (c)= one hundredth
Milli (m)= one thousandth
Micro (μ)= one millionth
Nano (n)= one billionth
Pico (p)= one trillionth
Scientific Notation:
Positive exponent means “times ten to that power” (move the decimal point to the right
that number of places)
Negative exponent means “divided by ten to that power” (move the decimal point to the
left that number of places)
***Note: Make sure you are using scientific notation correctly on your calculator!
[And that means using the EE or EXP button!]***
45,000 = 4.5 x 104
0.00005 = 5 x 10-5
Significant Figures
Because there is a certain amount of uncertainty associated with every measurement, we
must carefully express our measurements to show how uncertain they are.
Significant figures = the meaningful digits in a measured or calculated quantity
Note: All measurements must be reported with the last digit as the uncertain or estimated
value. (Demo)
Rules for Determining Significance:
1.
2.
3.
4.
All nonzero digits are significant (ex: 543 has 3 sig figs)
Zeroes between other numbers are significant (ex: 505 has 3 sig figs)
Leading zeroes are placeholders and are not significant (ex: 0.004 has 1 sig fig)
Trailing zeroes to left of implied decimal point are not significant (ex: 5000 has 1
sig fig)
5. Trailing zeroes to right of decimal point are significant (ex: 45.000 has 5 sig figs)
6. Counting and exact measurements (conversion factors) have unlimited or infinite
significance (ex: 30 apples has infinite sig figs, 12 inches = 1 foot has infinite sig
figs)
Examples:
24.7 m
0.73 in
1.503 L
42.00 mL
20 mL
60 minutes = 1 hour
23 students
1.50030 psi
0.000420500 mol
Note: When you do calculations in chemistry, you must always use significant
figures.
Main point = An answer cannot be more accurate than the least accurate measurement
from which it was calculated!!
For example, a block of 41 g of aluminum has a volume of 15 cm3. When you divide to
get its density, you might be tempted to record a value of 2.7333333 g/cm3. But that
answer is more accurate than our measuring equipment! You must round your answer by
knowing how many significant figures are allowed in it. In this case: 2.7 g/cm3 is correct.
Mathematics with Significant Figures
Adding and subtracting: Add or subtract normally and round the answer to the number of
decimal places that is the lowest of the ones you started with.
Ex: 1.5 m + 2.00 m + 1.702 m = 5.202 m which must be rounded to match the decimal
places in 1.5 m, Final answer = 5.2 m
Multiplying and dividing: Multiply or divide normally and round the answer to the
number of significant figures that is the lowest of the ones you started with.
Ex: 7.55 m x 1.1 m = 8.305 m2 which must be rounded to match the sig figs in 1.1 m,
Final answer = 8.3 m2
Multiple operations in sequence (or chain calculations): Carry out the first calculation
and round to the appropriate number of sig figs, then using that rounded answer, carry out
the next calculation and round to the appropriate number of sig figs, and continue
accordingly.
Density: Incorporating measurement, units, and sig figs
Density is an intensive, physical property of matter
Density = mass distributed over a volume
Density(D) = mass (m) / volume (V) [can rearrange as needed to solve for m or V]
Units of density are derived from units of mass and volume
Mass- units are usually kg, g, or mg
Volume- units may be m3, cm3, mL, or L
Note: 1 cm3 = 1 mL
Specific gravity= comparison of the density of a substance to a reference density
Note: Water is usually the reference density since its density is 1.0 g/mL
Specific gravity of a substance= its density / density of water
Specific gravity has no units…. Why?? (density units / density units = 1)
Other Points on Measurement and Mathematics
Accuracy vs. Precision
Accuracy= how close a single measurement is to a target or accepted value
(depends on the instrument used)
Precision= how close several measurements are to each other, has to do with
repeatability (depends on skill of person using the instrument)
Evaluate Measurements by Calculating Error
Error = Theoretical or accepted value – Experimental or actual value
Percent error = |Error|/ Theoretical or accepted value x 100%
[Note: In any numerical lab, you should calculate and analyze the percent error.]
Independent and Dependent Variables
Independent variable = what you vary in an experiment
Dependent variable = what you observe changes in as a result of changing the
independent variable
Often, it is useful to graph a dependent variable (y axis) as a function of
independent variables (x axis)