AP Chemistry

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1. Chemical foundations

Overview

to see atoms, use microscope called STM (scanning tunneling microscope)

uses electron current from tiny needle to probe surface

chemistry helps understand the connection between macroscopic (visible) and microscopic worlds

two fundamental concepts of chemistry

matter is composed of various types of atoms

one substance changes to another by reorganizing the way that the atoms are attached to each other

science is a process to understand nature and its changes

making observations (collecting data)

suggesting a possible explanation (formulating a hypothesis)

doing experiments to test the possible explanation (testing the hypothesis)

Scientific method

science is not only a set of facts but also a plan of action or procedure to process and understand certain types of information

scientific method

making observations

qualitative: does not involve a number

quantitative (measurement): involves a number and unit

formulating hypotheses

hypothesis: possible explanation for an observation

performing experiments

to test a hypothesis

gathering new information to decide whether the hypothesis is valid

always produce new observations

restart process

theory/model: set of tested hypotheses that give overall explanation of some natural phenomenon

interpretations of observations

change as more information becomes available

human inventions; educated guesses

natural law: statement that expresses generally-observed behavior

law of conservation of mass: total mass of materials is not affected by a chemical change in those materials

natural law (what) vs. theory (why)

natural law: summary of observed (measurable) behavior

theory: explanation of behavior

scientific methods are only as effective as the humans using them

hypotheses and observations are not totally independent; focusing on one theory may limit ability to see other possible explanations

scientists are human

prejudices

misinterpretation of data

politics

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Units of measurement

a quantitative observation always has two parts that must be present for the measurement to be meaningful

number

scale (unit)

SI system (la Système international d’unités or the International System of Units): standardized system based on metric system

volume

derived from length

not a fundamental SI unit

e.g. volume of cube calculated by cubing the length of its sides

liter (L): cubic decimeter

1000 L = cube with volume 1 m³

1 L = (1 dm)³ = (10 cm)³ = 1000 cm³ = 1000 mL

devices for liquid volume

graduated cylinder

pipet/pipette (with calibration mark)

buret (with valve/stopcock to control liquid flow)

volumetric flask (with calibration mark)

mass and weight are not the same

mass: measure of resistance of object to change in state of motion; force necessary to give certain acceleration

weight: response of mass to gravity; varies with strength of gravitational field

Fundamental SI units

Fundamental SI units

Physical quantity

Name of unit

Abbreviation

mass

kilogram

length

meter

time

second

temperature

kelvin

electric current

ampere

amount of substance

mole

mol

luminous intensity

candela

There are no rows in this table

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Prefixes used in SI system

Prefixes used in SI system

Prefix

Symbol

Meaning

Exponential notation

exa

1,000,000,000,000,000,000

10^18

peta

1,000,000,000,000,000

10^15

tera

1,000,000,000,000

10^12

giga

1,000,000,000

10^9

mega

1,000,000

10^6

kilo

1,000

10^3

hecto

100

10^2

deka

10^1

—

10^0

deci

0.1

10^-1

centi

0.01

10^-2

milli

0.001

10^-3

micro

0.000001

10^-6

nano

0.000000001

10^-9

pico

0.000000000001

10^-12

femto

0.000000000000001

10^-15

atto

0.000000000000000001

10^-18

There are no rows in this table

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Uncertainty in measurement

measurement always has some degree of uncertainty; depends on precision of device

significant figures: all certain digits plus first uncertain digit (estimated number)

uncertainty in last number usually assumed to be ±1 unless otherwise indicated

shows how precise a measurement is

25 mL (±1) means between 24 mL and 25 mL

25.00 mL (±0.01) means between 24.99 mL and 25.01 mL

reliability of measurements

accuracy: agreement of particular value with true value

precision: degree of agreement among several measurements of same quantity (reproducibility)

errors

random/indeterminate error: measurement has equal probability of being high or low

systemic/determinate error: occurs in same direction each time

precision often used as indication of accuracy

average of series of precise measurements often assumed to be accurate

only valid if systemic errors are absent

Calculating significant figures

counting significant figures

nonzero integers: always count as significant figures

zeros:

leading zeros: precede all nonzero digits; do not count as significant figures (e.g. 0.0025 has 2 significant figures)

captive zeros: between nonzero digits; always count as significant figures (e.g. 1.008 has 4 significant figures)

trailing zeros: at right end of number; only significant if number contains decimal point (100 has 1 significant figure; 100. has 3 significant figures)

exact numbers: numbers determined by counting or definitions; infinite number of significant figures

e.g. 10 in 10 experiments, 2 in 2πr, 1 and 2.54 in 1 in = 2.54 cm (because 1 inch is exactly 2.54 centimeters)

exponential notation: expresses number as

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represent very large or very small number

easily indicate number of significant figures

significant figures in mathematical operations

multiplication or division: same number of significant figures as least precise measurement used in calculation

addition or subtraction: same number of decimal places as least precise measurement used in calculation

rounding

in series of calculations, carry extra digits through to final result, then round

if the digit to be removed:

is less than 5: preceding digit stays the same (e.g. 1.33 → 1.3)

is equal to or greater than 5: preceding digit increases by 1 (e.g. 1.33 → 1.4)

use only the first number to the right of the last significant figure

Solving problems systemically

approach a problem by asking questions

what is my goal?

where am I starting?

how do I proceed from where I start to where I want to go?

alternatively:

where am I going?

what do I know?

how do I get there?

Dimensional analysis

unit factor method/dimensional analysis: used to convert from one system of units to another

use equivalence statement that relates the two units

derive appropriate unit factor by looking at the direction of the change (to cancel the unwanted units)

multiply the quantity to be converted by the unit factor to give quantity with desired units

always include units for quantities used

examples

A pencil is 7.00 in long. What is its length in centimeters?

1 in = 2.54 cm

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A student has entered a 10.0-km run. How long is the run in miles?

1 km = 1000 m; 1 m = 1.094 yd; 1760 yd = 1 mi

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The latest model Corvette has an engine with a displacement of 6.20 L. What is the displacement in units of cubic inches?

1 ft³ = 28.32 L

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converting from one unit to another

use the equivalence statement that relates the two units

derive the appropriate unit factor by looking at the direction of the required change (to cancel the unwanted units)

multiply the quantity to be converted by the unit factor to give the quantity with the desired units

Temperature

three widely-used systems for measuring temperature: Celsius, Kelvin, Fahrenheit

Kelvin and Celsius

degrees in Kelvin and Celsius are the same size

conversion requires adjustment for the zero points

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Fahrenheit and Celsius

degree sizes and zero points are different

how to find formulas

subtract boiling and freezing points of water

Fahrenheit: 212 - 32 = 180

Celsius: 100 - 0 = 100

180℉ = 100℃

unit factor: 9℉/5℃ or 5℃/9℉

zero points: subtract 32 from Fahrenheit to get Celsius zero point

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-40℉ = -40℃

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Density

density: mass of substance per unit volume

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units: “mass/volume” (e.g. “g/cm³”) or “mass volume” with negative exponent (e.g. “g cm⁻³”)

used for:

identifying of substances (by comparing density of substance to known densities)

determining amount of antifreeze in a car (because sulfuric acid in the battery is consumed, changing the solution’s density)

Classification of matter

matter: anything occupying space and having mass

material of the universe

complex; many levels of organization

matter exists in three states

solid: fixed volume and shape; rigid

liquid: definite volume, no specific shape; shape of container

gas: no fixed volume or shape; shape and volume of container

mixture: variable composition

homogenous (solution): visibly indistinguishable parts

air (gases)

wine (liquids)

brass (copper and zinc)

heterogenous: visibly distinguishable parts

sand in water

iced tea with ice cubes

pure substance: constant composition

pure water is only H₂O molecules (most water in nature are mixtures)

can be compounds (combinations of elements) or free elements

physical change: change in form of substance, not in chemical composition

boiling/freezing water (state changes but remains water)

separating components of a mixture

distillation (solution)

process depends on differences in volatility (how readily substances become gases) of components

simple: mixture heated, most volatile component vaporizes then condenses back to liquid

filtration (solid and liquid)

simple: mesh (e.g. filter paper) passes the liquid and leaves the solid behind

chromatography (solid and liquid or gas)

two phases of matter

stationary phase (solid)

mobile phase (liquid or gas)

components have different affinities for the two phases; move through at different rates

paper chromatography

stationary phase: strip of porous paper (e.g. filter paper)

drop of mixture placed on paper, then dipped in liquid (mobile phase) that travels up the paper like a wick

compounds have constant composition and can be broken down into elements by chemical processes

chemical change: given substance becomes new substance(s) with different properties and composition

element: substance that cannot be decomposed into simpler substances through physical or chemical processes

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