Study of the structure of matter -”the science that seeks to understand the behavior of matter by studying the behavior of atoms and molecules”
From the smallest (atoms) to the largest (materials) Atomic or molecular properties affect material properties Study of the changes matter undergoes (reactions)
What is Chemistry?
Learning Outcomes • Determining Physical and Chemical Changes and Properties
• Converting between Temperature Scales; Fahrenheit, Celsius, and Kelvin
• Calculating the Density of a Substance
• Reporting Scientific Measurements to the Correct Digit of Uncertainty
• Working with Significant Figures
• Using Conversion Factors
• Solving Problems Involving Equations
Atoms vs. Molecules
http://emergentchaos.com/archives/2009/09/atoms-photographed.html
Free atoms are not common in nature. Can you name a few materials that may have free atoms? Can you name a material that is not made up of molecules?
Matter
Note: It may be useful to get a periodic table you like and always bring it to class and lab
Atoms and Molecules
• Elements – A substance that cannot be decomposed into simpler
substances. Contains one kind of atom (H, Na, O) • Basic building block of matter • Period table
• Atoms – The smallest part of an element that can exist
chemically • All matter can be described in terms of the interactions of
atoms with each other • Molecules
– One of the fundamental units forming a chemical compound
• Combination of two or more atoms.
ATOMS are the basic building blocks of everything
ELEMENTS are the kinds of atoms that we can have
Atoms can join together - they form bonds together - to make MOLECULES
H C
Structure of
diamond
Ethane – C2H6
Atoms and Molecules
Problem Solving
1. Observe a problem 2. Write down the facts related to problem 3. Analyze the data, identify the type of
problem to be solved, and outline a plan to answer the problem
4. Carry out your plan to obtain tentative result (experiement)
5. Evaluate answer as reasonable or not -(for science problems)
Problem Solving A large portion of solving problems in chemistry requires math. You MUST be good at math to succeed in chemistry To understand a problem, you have to be well acquainted with the material. Read the chapters BEFORE class. Lecture should be a review used for clarification Chemistry questions are often regarded as ”tricky” They usually involve more than one concept at once You cannot memorize chemistry questions. You have to apply yourselves.
Three terms often used interchangeably are “hypothesis," “law," and "theory” - but not in science!
Hypothesis: A tentative interpretation or explanation of the observations
Observed, but not proved (working assumption) Can be supported or refuted by experimentation or
continued observation Dalton's Hypothesis: All matter is composed of atoms
If a hypothesis successfully passes many tests, it can form the basis
for a theory
Hypothesis
• Law: A brief statement that summarizes past observations and predicts future ones.
• Scientific laws describe things. They do not explain them.
• Based on a series of similar experimental observations
Newton’s law of gravity predicts the behavior of falling objects, but it does not explain WHY it happens.
Law of conservation of mass
Easier – think of a scientific law as a principle instead
Law
Theory
• Theory: A well established, tested explanation of basic natural phenomena
– Verified multiple times – A theory can be modified – One scientist cannot create a theory; only a hypothesis – More complex and dynamic than a law - tries to explain
not only what nature does…but WHY
The atomic theory states (among other things) that all matter is composed of atoms
1. What is the best definition of the term "theory," as it is used in science? a) A theory is a guess or hunch about something that has occurred in nature. b) A theory is a comprehensive set of ideas explaining a phenomenon in nature. c) A theory is based on verifiable laws and can be proven true. d) A theory is a hypothesis that uses laws and observation to make an assumption. 2. While speaking to a colleague, a scientist makes the following statement: "I propose
that Bald Eagle eggs in northern Maine will have thinner shells than those from birds in southern Alaska due to increased levels of pesticides in the water." This statement is a
a) theory b) law c) conclusion d) hypothesis 3. Which statement below correctly identifies the difference between laws and theories? a) Laws describe phenomena, while theories explain why phenomena exist. b) Laws are a statement of fact, while theories are a statement of opinion. c) Laws explain why phenomena exist, while theories explain how. d) Laws are a prediction of phenomena, while theories are an explanation.
Matter: Anything that occupies space and has mass. Substance: The specific matter you are talking about.
Different Physical States of Matter: Solid – Rigid - Fixed shape and volume. Atoms/Molecules remain
in fixed place and only vibrate. Liquid – Relatively incompressible fluid - Fixed volume but no
fixed shape. Atoms/molecules tightly packed, but can move past one another allowing flow.
Gas – Easily compressible fluid - No fixed shape or volume.
Atoms/Molecules are widely spaced.
The Classification of Matter
Definitions for Vocabulary
• Pure Substances – A kind of matter that cannot be separated into other
kinds of matter by any physical process • NaCl and Water are substances, but salt water is not
• Compounds
– A substance formed by the combination of elements in fixed proportions by a chemical reaction
• Cannot be separated by physical means • Water is a compound
• Mixtures
– A material that can be separated by physical means into two or more substances
• Variable composition – example: salt water • Air is a mixture (O2 and N2)
Mixtures
Heterogeneous – A mixture that consists of physically distinct
parts, each with different properties
Homogeneous – A mixture that has uniform properties
throughout (solution)
Mixture :a substance consisting of two or more substances mixed together (not in fixed proportions and not with chemical bonding)
Mixtures can be separated by physical means Is this a homogeneous or heterogeneous mixture?
Mixtures
Homogeneous vs. Heterogeneous
“homogenous means that it's all mixed together exactly the same; heterogeneous means that it's all jumbled together with different clumps and different amounts of things in different places.”
For each box determine whether you are looking at : 1- a solid, liquid, or gas 2 - elements, compounds, or mixtures 3 - a heterogeneous or homogeneous mixture.
Physical and Chemical Changes
What is the difference between physical and chemical changes?
Physical Change: Change that only alters state or appearance
• Reversible • No new compounds are formed • Can separate mixtures by physical means
Boiling of a liquid Melting of a solid Dissolving a solid in a liquid
Chemical Change: A change in which one or more kinds of matter are transformed into a new kind of matter or several new kinds of matter
• Irreversible • New compounds are formed • To recover reactants requires chemical
reactions
Physical and Chemical Changes
Chemical reactions like - combustion - oxidation
We characterize or identify materials by their various properties
• Physical Property – A characteristic that can be observed for a
material without changing its chemical identity • Melting point, boiling point, density, smell, color
• Chemical Property
– A characteristic of a material involving its changing chemical composition
• Flammability, acidity, toxicity, corrosiveness • Can usually be identified when there is an
observable change: such as in color, or if heat is given off
Potassium is a soft, silver-colored metal that melts at 64°C. It reacts vigorously with water, with oxygen, and with chlorine. Identify all of the physical and the chemical properties given in this description
• Physical – Soft, silver-colored metal, melts at 64°C
• Chemical
– Reacts vigorously with water, reacts with oxygen, reacts with chlorine
Units of Measure
English System – inches, yards, pounds, miles, pints Metric System (called the International System of Units –S.I) - centimeters, meters, grams, liters
To record a measurement you need both the measured number and the unit
SI Prefixes Multiple of Base Unit
Prefix Symbol
106 mega M 103 kilo k 10-1 deci d 10-2 centi c 10-3 milli m 10-6 micro µ 10-9 nano n 10-12 pico p
The advantage of the metric system is that it is a decimal system
Know your prefixes, and their relative size to the base unit!
1m = 1 x 109 nm 1cm = 1 x 101 mm 1 km = 1 x 103 m
Notice how I only wrote positive exponents? If you put the ‘1’ by the larger prefix, this will always be the case. (there are more little things in a big thing)
Conversions will be easier if you always follow this convention.
SI Units
Write the following without scientific notation • 5.89 x 10-12 s
• 0.2010 m
• 2.56 x 10-9 g
• 6.05 x 103 m
Temperature
The Celsius scale is the temperature scale in general scientific use*.
– However, the SI base unit of temperature is the kelvin (K), a unit based on the absolute temperature scale.
– The conversion from Celsius to Kelvin is simple since the two scales are simply offset by 273.15°.
K = °C + 273.15
*When doing calculations for gases we always convert to kelvin!
Temperature
• The Fahrenheit scale is at present the common temperature scale in the United States. – The conversion of Fahrenheit to Celsius, and
vice versa, can be accomplished with the following formulas
°C = °F - 32 1.8 °F = 1.8 (°C) + 32
SI Derived Units
• Units created by combination of SI base units – The SI unit for speed is meters per second, or m/s.
– Volume is defined as length cubed and has an SI unit of
cubic meters (m3).
– Traditionally, chemists have used the liter (L), which is a unit of volume equal to one cubic decimeter.
• 1L = 1 dm3 • 1cc = 1 cubic centimeter = 1mL
Derived Units • The density of an object is its mass per unit volume,
Mass is given in g
Volume is given in: – mL for liquids : g/mL – cm3 for solids : g/cm3 – L for gases : g/L
VolumeforUnitsSImassforUnitsSI
Vmd ==
The Reliability of a Measurement The number of digits in a reported measurement indicated the certainty associated with that measurement. More digits = more certainty Fewer digits = less certainty The number of digits you record for a measurement is not something each person chooses depending on how they feel…it is tied directly to the equipment used to measure and everyone using that specific equipment will always report to the same certainty.
The Reliability of a Measurement
• Precision – How close a series of measurements are to one
another – Reproducibility
• You did 5 measurements and obtained the same value
• Accuracy – Refers to the closeness of a single measurement
to its true value • You measured a 5cm long piece of metal and
determined it had a length of 4.99cm
Poor accuracy Good precision
Poor accuracy Poor precision
Good accuracy Good precision
What about Good accuracy and poor precision?
There are 10 divisions between 9 and 10 The rod is slightly larger than 9.1cm, but much less than 9.2cm Mentally divide the smallest space (0.1cm) into 10 equal parts We know the length is 9.1x cm, we just have to estimate the x 9.12 cm ± 0.01cm
What is the length of the nail to the correct number of significant figures?
5.75 cm ± 0.05cm or 5.70 cm ± 0.05cm
Significant figures are those digits in a measured number that include all certain (definite) digits plus a final one having some uncertainty
Significant Figures
In measuring with the ruler, the first two digits (9.1) are certain
The next digit was estimated, therefore it had some uncertainty
Number of significant figures refers to the number of digits that indicate the precision of the value
9.12cm - 3 sig. figs 9.123cm - 4 sig. figs
Significant Figures
Significant Figures To count the number of significant figures in a measurement, observe the following rules:
1. All nonzero digits are significant. 3.333
2. Zeros between significant figures are significant. 3.003
3. Zeros preceding the first nonzero digit are not significant. 0.03333
4. Zeros to the right of the decimal after a nonzero digit are significant. 3.300
Significant Figures
5. Zeros at the end of a nondecimal number may or may not be significant.
Use scientific notation to avoid ambiguity
• 300 cm – can not tell # of sig figs • 3.00x102 - Three sig figs
Sometimes a line or decimal is used: 1200 = 2 sig figs 1200. = 4 sig figs 1200 = 3 sig figs
If the number has a decimal in it: Like reading; go to the right until you hit a non-zero number. Everything after that is significant.
Exact Numbers
Have NO uncertainty ‘infinite sig figs’ 1 – accurate counting of objects [eg. five chairs] 2 – defined quantities [10 mm in 1 cm or 2.54cm in an inch] 3 – integral numbers [to halve something, divide by 2]
Significant Figures in Calculations
When multiplying and dividing measured quantities, give as many significant figures as the least found in the measurements used.
8.71 x 0.0301 / 0.031 =
Significant Figures in Calculations
When adding or subtracting measured quantities, give the same number of decimals as the least found in the measurements used. 0.71 + 92.2 =
Keeping track of sig figs when adding and/or subtracting is difficult. Many students frequently lose points because they do not understand how to keep track of sig figs when adding or subtracting. You cannot simply look at the calculation and know how many sig figs the answer has!
Significant Figures in Calculations
When you have both kinds of operations in a calculation follow the rules as you go step by step Calculate the following : (847.89 - 847.73) x 14673 847.89 – 847.73 = 0.16 0.16 x 14673 = 2347 BUT ONLY TWO SIG FIGS!! Best to change to scientific notation 0.16 x 14673 = 2.3x103 (= 2300)
Rounding
• Dropping of nonsignificant digits in a calculation result and adjusting the last digit
Round 1.2151 to 3 Sig. Figs.
• Look at the leftmost digit to be dropped • 5 or greater = add one • Less than 5 = do not change
Rounding gives 1.22 to 3 sig. figs.
Rounding example: 1.2151 rounded to three significant
figures is: 1.22 1.2143 rounded to three significant
figures is: 1.21
Examples
How many significant figures should be reported for the difference between 235.2497 and 235.22?
What is the best answer to the following expression: (4.9800-4.9680) x 0.2500 ?
Perform the following calculation and round your answer to the correct number of significant figures:
28.7289189146
×..
Perform the following calculation and round your answer to the correct number of significant figures: 59.1453.0 −
Perform the following calculation and round your answer to the correct number of significant figures:
421.0456.0 −
Perform the following calculation and round your answer to the correct number of significant figures:
)421.0456.0(35.92 −
Units: Dimensional Analysis
• Dimensional analysis (or the factor-label method) is the method of calculation in which one carries along the units for quantities. – Suppose you simply wish to convert 20 yards to feet.
– Note that the units have cancelled properly to give the
final unit of feet. Always use this method for conversion calculations in this class.
feet 60 yard 1 feet 3 yards 20 = × feet 60 yard 1 feet 3 yards 20 = ×
Units: Dimensional Analysis
• The ratio (3 feet/1 yard) is called a conversion factor. – The conversion-factor method may be used to
convert any unit to another, provided a conversion equation exists.
– Relationships between U.S. units and metric units will be given on tests.
Relationships of Some U.S. and Metric Units
Length Mass Volume
1 in = 2.54 cm 1 lb = 0.4536 kg 1 qt = 0.9464 L 1 yd = 0.9144 m 1 lb = 16 oz 4 qt = 1 gal
1 mi = 1.609 km 1 oz = 28.35 g
1 mi = 5280 ft
Unit Conversion
• Sodium hydrogen carbonate (baking soda) reacts with acidic materials such as vinegar to release carbon dioxide gas. Given an experiment calling for 0.348 kg of sodium hydrogen carbonate, express this mass in milligrams.
x 0.348 kg x 10 3 g
1 kg
10 3 mg
1 g = 3.48 x 10 5 mg x x 0.348 kg x
10 3 g
1 kg
10 3 mg
1 g = 3.48 x 10 5 mg 0.348 kg x
10 3 g
1 kg
10 3 mg
1 g = 3.48 x 10 5 0.348 kg x mg
10 3 g
1 kg
10 3 mg
1 g = 3.48 x 10 5 mg