Chapter 2 – Measurements and Calculations

transcript

Honors Chemistry, Chapter 2Page 1

Evidence of Chemical Change

• Evolution of a Gas (Bubbles, Odor)

• Formation of a Precipitate (Formation of Cloudiness in a Clear Solution, Solids Collecting at the Bottom or Top)

• Release of Energy (Heat, Light)

• Color Change

Scientific Method

• Observing and Collecting Data– Qualitative (Bubbles Formed)– Quantitative (1 gram/liter of catalyst speeded

the reaction by 25%)– Chemists Study Systems (Region Selected for

Study)

• Formulate Hypothesis – Generalization about Data – Testable Statement

Scientific Method

• Testing Hypothesis (Experimentation)– Supported, Retained– Not Supported, Discarded, Modified

• Theorizing – Create a Model– Model: An Explanation of How Phenomena

Occur and How Data or Events are Related.• Visual• Verbal• Mathematical

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Eric Severeid)

Units of Measure

• Measurements Are Quantitative Information

• Quantity: Something That Has Size or Amount

SI Measurement

• SI Units Are Defined in Terms of Standards of Measurement

• Seven Basic Units

• All Others Derived From Seven Basic Units

SI Base UnitsQuantity Symbol Unit Abbreviation

Length l meter m

Mass m Kilogram Kg

Time t second s

Thermodynamic Temperature T Kelvin K

Amount of a Substance n mole mol

Electric Current I ampere ALuminous Intensity Iv candela cd

SI Prefixes

Prefix AbbreviationExponent Multiplier Meaning Example Using Length

tera- T 10121000000000000 1 terameter (Tm)

giga- G 1091000000000 1 gigameter (Gm)

mega- M 1061000000 1 megameter (Mm)

kilo- k 1031000 1 kilometer (km) = 1000 m

hecto- h 102100 1 hectometer (hm) = 100 m

deka- da 10110 1 dekameter (dam) = 10 m

1001 1 meter (m)

SI Prefixes

Prefix AbbreviationExponent Multiplier Meaning Example Using Length

1001 1 meter (m)

deci- d 10-10.1 1 decimeter (dm)

centi- c 10-20.01 1 centimeter (cm)

milli- m 10-30.001 1 millimeter (km)

micro- 10-60.000001 1 micrometer (m)

nano- n 10-90.00000001 1 nanometer (nm)

pico- p 10-120.000000000001 1 picometer (pm)

Derived Units

• Area A m2

• Volume V m3

• Density D kg/m3 (=m/V)

• Molar Mass M kilograms/mol

• Concentration c mol/liter

• Molar Volume Vm m3/mol

• Energy E joule

Helpful Hint

• Relationship Between D, m, and V:

Factor Label Method

• Express 4.5 kg as grams• Begin by Expressing as a Fraction: 4.5 kg

1• Identify Conversion Factor: 1 kg = 1000 grams• Express as a Fraction:

1 kg 1000 g1 = --------------- or -------------- 1000 g 1 kg

Factor LabelContinued

• Write Equation Including Proper Factor

• Cancel Units

• Multiply Numbers to Get Final Result

4.5 kg 1000 g--------- x -------------- = 4500 g 1 1 kg

Scientific Measurements

• Accuracy – The Closeness of Measurements to the Correct or Accepted Value

• Precision – The Closeness of a Set of Measurements

Accuracy vs. Precision

High Precision

High Accuracy

High Precision

Low Accuracy

Accuracy vs. Precision

Low Precision

Low Accuracy

Low Precision

High Accuracy

(on average)

Percent Error

Valueaccepted - Valueexperimental

%Error = ---------------------------------------

Valueaccepted

Significant Figures

• All the Digits Known With Certainty Plus One Final Digit Which is Somewhat Uncertain

Rules for Significant Figures

1. Zeros Appearing Between Nonzero Digits are Significant

2. Zeros Appearing in Front of All Nonzero Digits are Not Significant

3. Zeros Appearing to the Right of the Decimal Point And at the End of the Number are Significant

Rules for Significant Figures

4. Zeros at the End of a Number but to the Left of the Decimal Point May or May Not be Significant. If a Zero Has Not Been Measured or Estimated but is Just a Placeholder, it is Not Significant. A Decimal Point Placed After Zeros Indicates They are Significant.

Rules for Rounding

If the Digit Following the Last Digit to be Retained is:

> 5 Then Round Up

< 5 Then Round Down

5 Followed by non Zero Digits

Then Round Up

Rules for Rounding

If the Digit Following the Last Digit to be Retained is:

5 Followed by Non-Zero Digit(s), and Preceeded by an Odd Digit

Round Up

5 Followed by Non-Zero Digit(s), and

Preceeded by an Even Digit

Leave Unchanged

Significant Figures With Addition/Subraction

• When Adding or Subtracting Decimals, the Answer Must Have the Same Number of Digits to the Right of the Decimal Point as There are in the Measurement Having the Fewest Digits to the Right of the Decimal Point.

Significant Figures With Multiplication/Division

• When Multiplying or Dividing, the Answer Can Have no More Significant Figures Than are in the Measurement with the Fewest Number of Significant Figures.

• (Conversion Factors Have Unlimited Digits of Accuracy.)

Scientific Notation

• Move the Decimal Point Left or Right Until the Mantissa is Greater Than or Equal to 1.0 and Less Than 10

• Express the Number as: M x 10n Where n Represents the Number of Places the Decimal Point was Moved, Positive if the Decimal is Moved Left and Negative if the Decimal is Moved Right

Direct Proportion

• Y = kX

• Example Mass vs. Volume Data for Aluminum

• Slope of the Line (k) is the Density

Measurement of a Series of Blocks

Block Number Mass (g) Volume (cm3)

1 1.20 0.442 3.69 1.393 5.72 2.104 12.80 4.685 15.30 5.716 18.80 6.907 22.70 8.458 26.50 9.649 34.00 12.80

10 36.40 13.50

Plot Mass vs. Volume

Mass (g) As a Function of Volume (V)

0 5 10 15

Volume - cubic centimeters

Mass (g)

Calculate Slope and Intercept

• Y = mX + b= slope x Volume + intercept

• Slope = 2.69 g/cm3

• Intercept = 0.09 grams (!) (Actually Zero)

• From Table of Densities: Sample is Aluminum (Al)

Inversely Proportional

• k = XY or Y = k/X

• As X Increases, Y Decreases

• Example: Pressure-Volume Data

Pressure Volume Data for Nitrogen

Pressure (k-Pa) Volume (cm3) P x V

100 500 50000150 333 49950200 250 50000250 200 50000300 166 49800350 143 50050400 125 50000450 110 49500

Volume vs. Pressure For Nitrogen

0 200 400 600

Pressure (kPa)

Volume (cm3)

Chapter 2 – Measurements and Calculations

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