Chapter 1: Introduction

Chemistry 1020: Interpretive chemistry

Andy Aspaas, Instructor

What is chemistry?

• “The science that deals with the materials of the universe and the changes that these materials undergo.”

• Chemistry in relation to other sciences

Chemistry around us

• Advances from chemistry

– Medicine– Agriculture– Energy– Plastics

• Problems from chemistry?

Scientific problem solving

• The scientific method: process behind all scientific inquiry

• Flexible, changes when new information is learned

• Start with a question, problem or observation

• Hypothesis: possible explanation

• Experimentation: controlled process of gathering new information

• Observations, do they support the hypothesis?

• Theory: a tested hypothesis, can still be revised

Law vs. theory

• Natural law: generally observed behavior, result of measurements

• Theory: our attempt to explain why certain behaviors happen

• Scientific method is still limited by human imperfection

How to learn chemistry

• Reading, vocabulary, memorization are only a start

– Should be considered a minor part of your learning process in chemistry

• Problem solving skills are even more important!

– Why practice homework problems are assigned– Struggle with them, use answers carefully– Mistakes can be valuable

Chapter 2: Scientific Notation

Chemistry 1020: Interpretive chemistry

Andy Aspaas, Instructor

Types of observations

• Observations are a key part of any type of scientific research

• Qualitative: a description (a white solid was formed)

• Quantitative: a specific measurement (the product weighs 1.43 grams)

Measurements and numbers

• Measurements must contain both a number and a unit - without both, the measurement is meaningless

• Many numbers in measurements are very large or very small

– Distance from earth to sun: 93,000,000 miles– Width of an oxygen atom: 0.00000000013 meters

• Is there an easier way to deal with such ungainly numbers?

Scientific notation

• Used to make very large or very small numbers more manageable

• Multiply a number between 1 and 10 by any power of 10

• 200 in scientific notation?

• For even larger numbers, count the number of places the decimal point must move, and make that the power of 10

• 230,000,000,000 in scientific notation?

Scientific notation

• Works with small numbers too

• For small numbers, move the decimal point to the right, and use that as the negative power of 10

• Left is positive, “LIP”

• Using a calculator

– The E or EE button on your scientific calculator

Units of measurement

• Unit: which scale or standard is used for a particular measurement

• English system: US residents are most familiar with

• Metric system: used in most of the rest of the world

• SI, or International System, used in scientific work

– Based on metric system– Agreed upon by scientists worldwide

Some fundamental SI units

Quantity Name of unit Abbreviation

mass kilogram kg

length meter m

time second s

temperature kelvin K

• Most other SI units can be derived from these

Prefixes to SI units

Prefix Symbol Meaning Power of 10

mega M 1,000,000 106

kilo k 1000 103

deci d 0.1 10-1

milli m 0.001 10-3

micro µ 0.000001 10-6

nano n 0.000000001 10-9

Length

• Fundamental SI unit for length: meter

– A little longer than a yard

• Using prefixes as the power of 10

– 1 mm = 10-3 m = 0.001 m

• 1 inch = 2.54 cm

• Measured with a ruler or caliper

Volume

• Amount of 3-dimensional space occupied by an object

• Unit: liter (L)• 1 L = 1 dm3 (cubic decimeter)• 1 millileter (mL) = 1 cm3

– Commonly used volume unit in chemistry• Volume measurements:

– Graduated cylinder– Syringe– Buret

Mass

• The specific amount of matter present in an object– Measured on a balance

• Not to be confused with weight– (Force of gravity acting on the mass of an object)– Dependent on the strength of gravity– Earth vs. moon?– Measured on a scale

• Mass used much more commonly in chemistry• SI fundamental unit: kilogram

Uncertainty in measurement

• Analog measurements - measured mechanically against some type of physical scale– Estimate required for last digit of measurement– Last digit = the uncertain digit– Can be expressed as ± amount of the uncertain

digit (4.542 ± 0.001) • Digital measurements - read from a display

– Last digit still uncertain even though you don’t do an estimation

Accuracy vs. Precision

• Accuracy: how close a single measurement or set of measurements are to their true value

• Precision: how similar a number of measurements are

• Dartboard example

• Beaker of water example

Significant figures

• Sum of all certain numbers in a measurement plus the first uncertain number

• Indicates the amount of precision with which a measurement can be made

• Since each measurement contains uncertainty, that uncertainty must be tracked when manipulating the measurements

How many sig figs does a measurement have?

• Nonzero integers are always significant (1 thru 9)

• Leading zeroes (on the left) are never significant

• Captive zeroes are always significant

• Trailing zeroes (at the end) are only significant if there’s a decimal point

• Exact numbers (obtained by counting) have an infinite number of sig figs

Rounding off

• Calculators don’t understand sig figs

• Will return as many digits to you as possible

• You must round the answer to the correct number of sig figs

• Look at the digit to the right of the last sig fig

– 0-4, just drop it and everything to the right– 5-9, increase last sig fig by one, drop rest– Look only at the one digit to the right of the last

sig fig, ignore all others!

Determining sig figs in calculations

• When multiplying or dividing, find the measurement with the smallest number of sig figs

– Answer must be rounded to that many sig figs

• When adding or subtracting, find the measurement with the smallest number of decimal places

– Answer must be rounded to that many decimal places

• Practice!

Dimensional analysis introduction

• We do this all the time without even thinking about it

• Example: planning a party

– 15 guests– 3 drinks per guest– How many drinks should you buy?

• Conversion factor: a ratio of two measurements with different units that are equal to each other

• Expressed as a fraction, two possible orders!

Dimensional analysis calculations

• Set up an equation like this

Known quantity x conversion factor = unknown quantity

• Orient conversion factor so units of known quantity are cancelled

• Multiply the known by the conversion factor

• The only remaining unit should be the one you’re solving for

• Correct for sig figs

• Does the answer make sense?

• Practice, practice, practice, practice, practice!

Temperature scales

• Fahrenheit scale: used in the US

• Celsius scale: used in most rest of world, and by most scientists

• Kelvin scale: SI base unit of temperature

– 0 K is lowest possible theoretical temperature

Temperature scales

Fahrenheit Celsius Kelvin

Absolute zero -460 °F -273 °C 0 K

Water freezes 32 °F 0 °C 273 K

Body temp 98.6 °F 37 °C 310 K

Water boils 212 °F 100 °C 373 K

Temperature conversions

• Celsius to Kelvin

– Temperature units are the same size– Zero points are different

TK = T°C + 273

• Kelvin to Celsius

– Solve above for T°C

T°C = TK - 273

Fahrenheit and Celsius

• Different degree units and zero points

T°F = 1.80(T°C) + 32

T°C = (T°F - 32) / 1.80

Density

• Density: amount of matter present in a given volume of substance

Density = mass / volume

– Units could be kg/L, g/cm3, g/mL, etc.