chem121 chapter 1 beekman sh

8/9/2019 Chem121 Chapter 1 Beekman SH

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Chapter 1

Matter and Measurement

R. Spinney 2010

What is Chemistry? The Study of MATTER science that describes

the physical properties, composition, structureand reactions of matter.

Chemistry studies the products, amounts, energyand rates of change of reactions.

Matter Anything that has mass and occupiesspace.

Atom The smallest building block of matter.*

Molecule Groups of atoms held together in aspecific connectivity and shape.

* In reality atoms are made up of smaller building blocks calledprotons, neutrons and electrons, (we will learn about these particlesin Chapter 2).


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What is Chemistry? (contd)

Composition what matter is made ofand in what proportions chemicalformulas.

Structure the (3D) arrangement ofcomponents of matter in a sample.

Chemical Reactions involve changes in

composition and structure but NOT thedestruction of matter

What is Chemistry? (contd)

Memorization is a part of any science, you needto know the definitions and common variables:you need to know: Table 1.2 and the first 36, groups 1, 2, 6, 7, 8 element

names & symbols

- Others will be added as we continue.


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Classification of Matter

Macroscopic (our world) scale:

Gas: No definite volume or shape and is

compressible.

Liquid: Constant volume, no definite shape,

slightly compressible.

Solid: Rigid with a definite shape, difficult to

compress.

Classification of Matter

Microscopic (molecular) scale:

Gas: Particles are far apart, move at high speed,

collide often with itself and other particles.

Liquid: Molecules are packed closely together but stillmoving. The arrangement of molecules is rapidlychanging

Solid: particles are packed close together, held tightlyby strong forces of attraction often in orderedarrangements. Arrangement changes very slowly ornot at all.


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Types of Pure Substances

Elements : only one kindof atom

Examples: Oxygen (O2), Gold (Au),Silicon (Si), H2, N2 and the Halogens (F2,Cl2, Br2, I2)

Compounds: Two or more elements

(different atoms)Examples: NaCl, H2O

Compounds Combination of elements in specific ratios.

Compounds have different properties than their

elements: NaCl vs. Na and Cl.

Law of Constant Composition

Different samples of any pure compound containthe same elements in the same proportion by

mass.

Salt is always NaCl, water is always H2O


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Mixtures

Combination of two or more pure substances.

Variable composition.

Two types of mixtures:

Homogeneous : composition and propertiesuniform throughout

Also called solutions (solid, liquid, or gas)

Heterogeneous : composition and propertiesnon-uniform throughout

Properties of Matter

Physical properties: the identifyingcharacteristics of matter. Some propertiescan be readily measured with our senses,such as odor and color. Instruments areused to measure others, such as hardness,

melting point, etc. Chemical properties: describe the

reactivity of a substance. Examples include:"ethanol burns in air", "sodium reactsvigorously with water", etc.


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Terminology

Intensive property: does not depend on the

amount of material. Density, temperature, etc.

Extensive property: depends on the amount.

Mass, volume, etc.

Chemical change: substances transform into

chemically different substances. Reactions.

Physical change: no change in chemicalcomposition. Melting, boiling, break a solid.

Units of Measurement

Many properties of matter are quantitative.

A measured quantity must have BOTH anumber and a unit !

The units most often used for scientificmeasurement are based on those of themetric system. Called SI units.


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Measurements

Volume is based on a cubic length = m3

Chemistry uses non SI units, i.e. L (= 10-3 m3), mL and cm3

MeasurementsFractions & multiples of SI units are designated by prefixes.


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Converting Units, SI System

2.3 kg is equal to how many ng?

2.3 kg

1 kg

1000 g 1x109 ng

1 g=

2.3x1012 ng

1= 2.3x10

12 ng

Accuracy and Precision

Accuracy how close are you to the correct value. Accuracy depends on the quality of the measuring device, i.e. a

ruler calibrated to meters vs. centimeters vs. millimeters.

Precision deals with the reproducibility of ameasurement: how close are repeated measured valuesto one another? Precision depends on the users ability to make a measurement

with a specific device, i.e. your experimental technique.

Error: Random vs. Systematic. Random: unavoidable

Systematic: you made a mistake


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Accuracy and Precision

i) good precision, poor accuracy (systematic error)

ii) good precision & accuracyiii) poor precision & accuracy (random error)

Significant Figures

Reflect the quality of our measurement.

Limited by the measuring device, i.e. thecalibration of the device (plus one moredecimal place estimated when possible).

When rounding calculated numbers, wepay attention to significant figures so wedo not overstate the accuracy of ouranswers.


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Significant Figures (contd)

We know the measurement is 21.6 cm.

However we can interpolate another decimal place as21.65 cm.

Reporting 21.655 cm would be implying a great degree

of accuracy than we can observe. Watch for this in the lab, only estimate ONE decimalplace.


1. All nonzero digits are significant.

2. Zeroes between two significant figures are themselvessignificant, i.e. 2.06 - 3 sig figs, 20.06 - 4 sig figs.

3. Zeroes at the beginning of a number are neversignificant, i.e. 0.004 1 sig fig, 0.435 3 sig figs.

4. Zeroes at the end of a number to the right of adecimal point are significant, i.e. 0.040 2 sig figs,

400.0 4 sig figs.

5. When a number ends in a zero to the left of a decimalpoint, the zeros are NOT significant, i.e. 400 - 1 sig fig41000000 2 sig figs.


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The last rule causes problems: is 400 meant to

be 1, 2, or 3 significant figures?

Use scientific notation to remove any ambiguity,

i.e.400 100 4 x 102

400 10 4.0 x 102

400 1 4.00 x 102

Note: correct scientific notation has only 1

nonzero digit to the left of the decimal place.

Exact Numbers

The previous rules apply to measuredquantities in which there is some degree ofuncertainty due to the measuring device.

Counts of things, for example, the numberof people in this room, are exact.


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Sig. Figs. in Calculations

We use measured quantities incalculations, but they have a error, and theanswer must reflect the same level of erroras the LEAST accurate value wemeasured.

Two rules, one for addition / subtractionthe second for multiplication / division.

Sig. Fig. Calculations, Addition

Addition / Subtraction the last place in the answer isthe last place common to all numbers. Round off to getcorrect number of sig. figs., i.e. 4 + 1.45 + 12.4 = ?

4 1

1.45 0.01

12.4 0.1

17.85 last place common to allunits placetherefore round the .85 (round up if

left most place is a 5 or greater, down

if < 5)

Therefore the correct answer is 18


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Sig. Fig. Calculations, Subtraction

Subtraction same, i.e. 12.4 4 = ?

12.4 0.1

-4 1

8.4

Since the left most number to be discarded is < 5 (.4) rounddown

The correct answer is 8

Sig. Fig. Calculations (contd)

12.4 1.45 =?

12.4

-1.45

10.95Round a 5 upalthough your book has a different

rule. I am not concerned with this rule.


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Multiplication / division rule is the answershould contain the same number of sig. figs. asquantity with the LEAST number of sig. figs. to

start with, i.e.

vol of a box 3.6 cm x 2.45 cm x 10.0 cm =

88.2 cm3 round to 2 sig figs,

therefore the correct answer is 88 cm3

Note: carry exact sig. figs. until the final step and then round so you dontintroduce rounding errors.


Division same as multiplication:

907.2 453.6 = 2

However the answer must reflect theaccuracy of the given quantities (4 sig.figs.) therefore the correct answer is 2.000


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It is common in science that a calculationinvolves several steps, where the answerfor any single step is based on thepreceding step. In these sequentialoperations the correct rules must beapplied at each step, i.e.

(12.4 9.45) 0.2212 = ?

(95 x 2.1) + 875.4 = ?


Answers:

(12.4 9.45) 0.2212 = 2.95 0.2212 = 13.336.. = 13

(95 x 2.1) + 875.4 = 199.5 + 875.4 = 1074.9 = 1.1 x 103


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Dimensional Analysis

Method of doing calculation where youcarry UNITS through the calculation.

Common in Sciences but useful in ANYcalculation.

Units will cancel out until you arrive at the

correct units for the quantity calculated.

Key is conversion factors , i.e.given unit x (desired unit / given unit) = desired unit

Dimensional Analysis (contd)

Density is the conversion factor between mass &

volume

What volume will 45 g of ether occupy?

The density of ether is 0.71 g/mL.

45 g 1 mL

0.71 g= 63.38 mL 63 mL


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A pitcher has a fast ball at 105 mi/hr. Whatis the speed in feet per second?

We know the conversion factors:

1 mi = 5280 ft

1 hr = (60 min/hr x 60 sec/min) = 3600 s,\

105 mi / hr x 5280 ft / mi x 1 hr / 3600 s = 154 ft / s

105 mi 5280 ft

1 mi=

1 hr

1 hr 3600 s154 ft/s


How long for the ball to reach the plate (adistance of 60.5 ft)?

60.5 ft 1 s

154 ft =0.392857s 0.393 s


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Dimensional Analysis

You MUST practice dimensional analysisto pass this class.

Rigorous, systematic, forces you to getunits correct.

Often, the work you are required to showis the dimensional analysis. It is how youget your answer.

chem121 chapter 1 beekman sh

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