Chemistry Chapter 2

Scientific Method  serendipity has played a role in science  most of what we know has come by careful research and experimentation  scientific method- logical approach to solving problems by observing, collecting data, formulating hypotheses, testing hypotheses, & formulating theories supported by data

 quantitative data -involves numbers  measurements using rulers, thermometers, graduated cylinders, etc.  for ex- temp 25 o C  qualitative data - is descriptive  for example- sulfur is a yellow chemical

 experiments are controlled to test one variable and collect data  system- a specific portion of matter in a given region of space is studied in an experiment or observation

 when scientists have a question they want answered, they usually state it in an “if- then” statement  hypothesis- testable statement (if-then)

 control- part of experiment that remains the same  variable- part of experiment that is changed  during the experiment, any change observed is usually due to the effects of the variable

Units of Measurement  What is wrong with this recipe? Banana Nut Bread 3 flour1 vanilla 2 eggs2 mashed bananas 2 sugar½ nutmeg

 measurements represent quantities  quantity- something that has magnitude, size, or amount (UNIT)  most ALL m’ments are a NUMBER and a UNIT

SI System  a standard system of m’ment  7 base units  system is monitored by International organizations  commas are NOT used in numbers = for example: not 75,000  (many other countries use commas as decimal points)

 few differences between SI system and metric  base units specific for certain quantities (table 1)  prefixes are used to indicate quantities larger or smaller than the base unit  prefixes are based on 10 (table 2)

Most common prefixes  kilo – means 1000  deci – means tenth (0.1)  centi - means hundredth (0.01)  milli - means thousandth (0.001)  commit these to memory

 the prefixes are used with the base units to measure larger or smaller quantities  for ex: length of room- meter distance to Sylacauga-kilometer length of book- centimeter width of fingernail- millimeter

MASS  measure of the quantity of matter  base unit: SI- kilogram metric- gram  triple-beam balance

Weight  measure of the force of gravity between 2 objects  can change, mass DOESN’T  SI unit - Newton  scale

Time  interval between 2 occurrences  SI unit- seconds  stopwatch/clock

Length  distance between 2 points  SI unit- meter  ruler

Temperature  matter is composed of molecules, ions, and atoms which are in constant motion (i.e. have kinetic energy)  temp measure of the average kinetic energy of all these particles  increase heat, increase movement of particles, increase KE

 SI unit- Kelvin (K) measures extreme temps  metric- Celsius ( o C) based on the freezing and boiling point of water  thermometer

Derived Units  combinations of SI units  produced by multiplying or dividing std units

Volume  amount of space an object takes up  SI unit- 1m 3  metric- liter (L) 1cm 3 and 1mL are smaller and usually used in the lab 1cm 3 = 1mL  graduated cylinder

Volume  can be calculated using a ruler and this formula:v = l x w x h  volume relationships: 1dm 3 = 1L = 1 000cm 3 = 1 000mL 1 000mL = 1 000cm 3

Density  mass per unit volume  density = mass volume D = m v  units can be g/mL, g/cm 3 (whatever units are used to measure mass and volume will be the units of density

 can be used to identify substances  can use the formula to find mass or volume also  density of H 2 O = 1g/mL

How reliable are the measurements you make?  2 important terms indicate reliability: 1. accuracy- how close the m’ment is to the true value 2. precision- how close a set of m’ments for a quantity are to each other (regardless of accuracy)

% error  used to evaluate results obtained in lab  always positive number  % error =

 An automobile is traveling at 88 km/h. What is its speed in cm/s.

Density pop quiz 1. A 30.0 cm 3 sample of quartz has a density of 2.65g/cm 3. What is the mass? 2. The density of a sample of cork is 0.24g/cm 3. What is the volume of a 35.0g sample? 3. What is the density of a piece of marble with the following dimensions: 552g and 212 cm 3 ?

Significant Digits  In science, significance means measured, not importance.  the # of sig digs in a m’ment depends on the scale of instrument used  m’ment includes 1 uncertain, or estimated, digit

To find sig digs: 1. find decimal point 2. find 1 st non-zero digit in the sequence 3. that digit and everything to the right is significant 4. if no decimal point, count from the 1 st non-zero digit to the last non-zero digit

 when doing calculations on calculator, the answer cannot have any more sig digs than the value in the problem

 answers in addition & subtraction must contain no more digits to the right than the # with the least digit to the right in the prob = 40.2

answer in multiplication or division must contain no more sig digs than the # with the fewest digits in the prob 18.3 x = x = 4.25

Rounding Rules 1. # 1-4 round down = #6-9 round up 36.7 = # 5 -round down if # preceding 5 is even 32.5 = = 688 round up if # preceding 5 is odd 43.5= = 760.

4. if there are #s after the 5, round up no matter what the preceding # is = = 79

Scientific Notation  very small and very large numbers are written in this shorthand method  #s are written in this format: M x 10 n M = 1 to n = whole number exponent

convert into sci not: x x 10 -7

 convert into std numbers: 3.8 x x

adding/subtracting in sci not  exponents must be same  moving decimal to LEFT increases exp  moving decimal to RIGHT decreases exp 4.5 x x 10 7

multiplying/dividing in sci not  multiply – ADD exponents  divide- SUBTRACT exponents 2.74 x 10 3 x 3.1 x 10 8 = 9.58 x x 10 6

Proportions: 2 types 1. direct proportions- if 2 quantities can be divided and you get a constant value y=kx

results in a straight line as x increases, y increases

2. two quantities are inversely proportional if their product is constant xy = k

 forms a hyperbola  if x increases, y must decrease