Chapter 1Chemical Foundations

Nottoway High SchoolDual Enrollment Chemistry

2014-2015Dr. Gur

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1.1 Chemistry: An Overview

Science: A process for Understanding Nature and Its Changes. 

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We can explain both of these things in convincing ways using the models of chemistry and the related physical and life sciences. 

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What is Chemistry?

Chemistry can be defined as the science that deals with the materials of the universe and the changes that these materials undergo.

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1.2 The Scientific Method

1. Making observations.

Observations may be qualitative or quantitative.

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A Qualitative observation does not involve a number..

A Quantitative observation (a measurement) involves both a number and a unit.

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2. Formulating hypotheses.

A Hypothesis is a possible explanation for an observation.

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3. Performing experiments. 

An experiment is carried out to test a hypothesis.

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Variables: the factors that influence the

outcome of an experiment.  Note: change only one

variable at a time.

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Independent Variable: the variable that is intentionally changed or manipulated by the experiment. (x-axis). Dependent Variable: the variable being measured or watched, sometimes called the outcome. (y-axis)

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Constants all other factors which remain the same throughout an experiment. Controlusually distilled water. 

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Repeated trials: repeat experiments until the results are consistent, usually two or three times. Conclusion: write a conclusion based on the data gathered from the experiment. The data will either support or contradict the hypothesis.

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1.3 Units of Measurement

A quantitative observation, or measurement, always consists of two parts: a number and a unit.

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The International System of units, or the SI system, is based on the metric system.

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SI units of mass, length, time, and temperature are the kilogram, meter, second, and Kelvin.

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Volume (V) is the amount of three- dimensional space occupied by a substance.

1 dm3 ≡ 1 liter (L)

1 liter = 1000 cm3 = 1000 mL

1.4 Uncertainty in Measurement

All measurements have a degree of uncertainty.

When one is making a measurement, the custom is to record all of the certain numbers plus the first uncertain number.

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The numbers recorded in a measurement are called significant figures.

Sample Exercise 1.1 Uncertainty in Measurement

In analyzing a sample of polluted water, a chemist measured out a 25.00-mL water sample with a pipet (see Fig. 1.7). At another point in the analysis, the chemist used a graduated cylinder (see Fig. 1.7) to measure 25 mL of a solution. What is the difference between the measurements 25.00 mL and 25 mL?

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Solution

25 mL = 24 mL – 26 mL

25.00 mL = 24.99 mL – 25.01 mL

Precision and Accuracy

Accuracy refers to the agreement of a particular value with the true value.

Precision refers to the degree of agreement among several measurements of the same quantity. It reflects the reproducibility of a given type of measurement.

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Random error means that a measurement has an equal probability of being high or low.

Systematic error occurs in the same direction each time.

Figure 1.10

(a) Neither accurate nor precise (large random errors).

(b) Precise but not accurate (small random errors, large systematic error).

(c) Bull’s eye! Both precise and accurate (small random errors, no systematic error).

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Sample Exercise 1.2 Precision and AccuracyTrial Volume Shown Volume Shown

by by theGraduated BuretCylinder

1 25 mL 26.54 mL2 25 mL 26.51 mL3 25 mL 26.60 mL4 25 mL 26.57 mL

Average 25 mL 26.54 mL

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Solution

If the volume is 26.50 mL, the Graduated Cylinder is precise but not accurate; however, the Buret is both precise and accurate.

1.5 Significant Figures and Calculations

Rules For Counting Significant Figures

1. Nonzero integers. Nonzero integers always count as

significant figures. 1457 4 significant figures4.723 significant figures

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2. Zeros. a. Leading zeros are zeros that precede all the nonzero digits. They do not count as significant figures.

0.0025 2 significant figures

0.000341 3 significant figures

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b. Captive zeros are zeros between nonzero digits. They always count as significant figures.

1.008 4 significant figures

0.083908 5 significant figures

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c. Trailing zeros are zeros at the right end of the number.

They are significant only if the number contains a decimal point.

100 1 significant figures100. 3 significant figures

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3. Exact numbers have an unlimited number of significant

figures.a. Numbers that are

determined by counting

10 studentsunlimited number

of significant figures

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b. By Definition

1 inch ≡ 2.54 cm

unlimited number of significant figures

Sample Exercise 1.3 Significant Figures

Give the number of significant figures for each of the following results.

a. A student’s extraction procedure on tea yields 0.0105 g of caffeine.

b. A chemist records a mass of 0.050080 g in an analysis.

c. In an experiment a span of time is determined to be 8.050 x 10-3 s.

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Rules for Significant Figures in Mathematical Operations

1. For multiplication or division, the number of significant figures in the result is the same as the number of significant figures in the least precise measurement used in the calculation.

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Corrected

4.56 x 1.4 = 6.384 6.4

3 2 4 2Significant Significant Significant SignificantFigures Figures Figures Figures

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Corrected

8.315/298 = 0.0279027 0.0279

4 3 6 3Significant Significant Significant SignificantFigures Figures Figures Figures

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2. For addition or subtraction, the result has the same number of decimal places as the least precise measurement used in the calculation.

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12.11 2 decimal places

18.0 1 decimal place

1.013 3 decimal places

31.123 31.1 1 decimal place

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0.6875 4 decimal places

-) 0.1 1 decimal place

0.5875 0.6 1 decimal place

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Rules for Rounding

1. In a series of calculations, carry the extra digits through to the final result, then round.

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2. If the digit to be removed a. is less than 5, the

preceding digit stays the same.1.33 1.3

b. is equal to or greater than 5, the preceding digit is increased by 1. 1.36 1.4

Sample Exercise 1.4 Significant Figures in Mathematical Operations

Carry out the following mathematical operations, and give each result with the correct number of significant figures.

a. (1.05 x 10-3)/6.135b. 21 – 13.8

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c. As part of a lab assignment to determine the value of the gas constant (R), a student measured the pressure (P), volume (V), and temperature (T) for a sample of gas, where

R = PVT

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The following values were obtained:

P = 2.560T = 275.15V = 8.8

Calculate R to the correct number of significant figures.

1.6 Dimensional Analysis

We can convert from one system of units to another by a method called the unit factor method, or more commonly dimensional analysis.

Unit 1 x Unit factor = Unit 2

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Consider a pin measuring 2.85 centimeters in length. What is its length in inches?

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The equivalence statement is

2.54 cm ≡ 1.00 in

The unit factors are

2.54 cm or 1.00 inch

1.00 inch 2.54 cm

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2.85 cm x 1.00 inch 2.54 cm

= 2.85 in 2.54

= 1.12 in

Sample Exercise 1.5 Unit Conversions I

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

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7.00 in x 2.54 cm 1 in

= (7.00)(2.54) cm

= 17.78 cm = 17.8 cm

1.7 Temperature

Temperature can be measured on three different scales: Fahrenheit, Celsius, and Kelvin.Boiling point of water: 2120 F, 1000 C, 373 KFreezing point of water: 320 F, 00 C, 273 K

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Temperature Conversion

Celsius to Kelvin Scales

T K = T 0C + 273

Kelvin to Celsius Scales

T 0C = T K - 273

1.8 Density

Density is the amount of matter present in a given volume (mass per unit volume). That is,

Density = Mass Volume

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Sample Exercise 1.13 Determing Density

A chemist, trying to identify the main component of a compact disc cleaning fluid, finds that 25.00 cm3 of the substance has a mass of 19.625 g at 20oC.

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The following are the names and densities of the compounds that might be the main component:

Compound Density in g/cm3 at 200CChloroform 1.492Diethyl ether 0.714Ethanol 0.789Isopropyl Alcohol 0.785Toluene 0.867

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Which of these compounds is the most likely to be the main component of the compact disc cleaner?

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Density = Mass Volume

= 19.625 g25.00 cm3

= 0.7850 g/cm3

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This density corresponds exactly to that of isopropyl alcohol.

CH3

CH3- C – OH.

H

1.9 Classification of Matter

Matter, best defined as anything occupying space and having mass, is the material of the universe.

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Matter exists in three states: solid, liquid, and gas.

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A solid is rigid.

A liquid has a definite volume but no specific shape.

A gas has no fixed volume or shape.

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Most of the matter around us consists of mixtures of pure substances.

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Mixtures have variable composition.

Wood, gasoline, wine, soil, tap water, and air are all mixtures.

Mixtures and Pure Substances

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Mixtures can be classified as homogeneous mixtures or heterogeneous mixtures.

A homogeneous mixture is called a solution.

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A pure substance is one with constant composition.

Compounds and elements are pure substances.

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Water (H2O), ammonia (NH3), and(Carbon dioxide (CO2) are compounds.

Gold (Au), silver (Ag), oxygen (O2), and hydrogen (H2) are elements.

Physical Change

A physical change is a change in the form of a substance, not in its chemical composition.

Boiling and freezing are physical changes.

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Mixtures can be separated by methods involving only physical changes:

DistillationFiltrationChromatography

Separation of Mixtures

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Distillation is a method for separating the components of a liquid mixture (water and ethanol).

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Filtration is a method for separating the components of a mixture containing a solid and a liquid (sand and water).

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Chromatography employs a system with two phases of matter: a mobile phase an a stationary phase.

Chemical Change

A Chemical reaction is the change of substances into other substances through a reorganization of the atoms.

CH4 + 2O2 2H2O + CO2

Natural gas Oxygen Water Carbon dioxide

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