Welcome to AP Chemistry

Scientific Method l A method of solving problems/answering questions l Observation- what is seen or measured l Hypothesis- educated guess of why things behave the way they do. (possible explanation) l Experiment- designed to test hypothesis l leads to new observations, l and the cycle goes on

Scientific Method l Theory (Model) –A set of tested hypotheses that give an overall explanation of some natural phenomenon why things behave the way they do –explains why something happens l Law –The same observation applied to many different systems –Example - Law of Conservation of Mass l Laws are summaries of observations

Scientific Method l Theories have predictive value. l The true test of a theory is if it can predict new behaviors. l If the prediction is wrong, the theory must be changed. l Theory- why l Law - how

Significant Figures l Meaningful digits in a MEASUREMENT l Exact numbers are have unlimited significant figures l If it is measured or estimated, it has sig. figs. l All numbers except zero are significant. l Some zeros are, some aren’t

Which zeroes count? Zeros  leading zeros  captive zeros  trailing zeros sig figs sig figs

Which zeroes count? Zeros  Captive zeros always count as significant figures.  Captive zeros always count as significant figures has 4 sig figs.

Which zeroes count? Zeros  Trailing zeros are significant  Trailing zeros are significant only if the number contains only if the number contains a decimal point has 4 sig figs.

Which zeroes count? Exact numbers have an infinite number of significant figures. 1 inch = 2.54 cm, exactly

Doing the math Multiplication and division # sig figs in the result equals the number in the least precise measurement used in the calculation 6.38  2.0 =  13 (2 sig figs)

Doing the math Addition and subtraction # sig figs in the result equals the number of decimal places in the least precise measurement =  22.5 (3 sig figs)

SI Measurement System l Every measurement has two parts –Number –Scale (unit) l SI system (le Systeme International) based on the metric system –Prefix + base unit –Prefix tells you the power of 10 to multiply by - decimal system -easy conversions

The Fundamental SI Units

Prefixes l giga- G 1,000,000, l mega - M 1,000, l kilo - k 1, l deci-d l centi-c l milli-m l micro-  l nano-n

Mass and Weight l Mass is measure of resistance to change in motion l Weight is force of gravity. l Sometimes used interchangeably l Mass can’t change, weight can

Uncertainty l A digit that must be estimated is called uncertain. l A measurement always has some degree of uncertainty l Basis for significant figures l Precision- how repeatable l Accuracy- how correct - closeness to true value.

Uncertainty l Random error - equal chance of being high or low- addressed by averaging measurements - –expected in all measurements l Systematic error- always the same direction each time –you want to avoid this type of error l you can have precision without accuracy l You can’t have accuracy without precision

Dimensional Analysis Using the units to solve problems

Dimensional Analysis l Use conversion factors to change the units l Conversion factors = 1 l 1 foot = 12 inches (equivalence statement) l 12 in = 1 = 1 ft. 1 ft. 12 in l 2 conversion factors l multiply by the one that will give you the correct units in your answer.

Examples l Science fiction often uses nautical analogies to describe space travel. If the starship U.S.S. Enterprise is traveling at warp factor 1.71, what is its speed in knots? l Warp 1.00 = 5.00 times the speed of light l speed of light = 3.00 x 10 8 m/s l 1 knot = 2000 yd/h exactly

Temperature l A measure of the average kinetic energy l Different temperature scales, all are talking about the same height of mercury.

100ºC212ºF 100ºC =212ºF 0ºC 32ºF 0ºC = 32ºF

Temperature

Density l Ratio of mass to volume l D = m/V l Useful for identifying a compound l Useful for predicting weight l An intensive property- does not depend on how much material there is