INTRODUCTORY CHEMISTRY Concepts and Critical Thinking Seventh Edition by Charles H. Corwin

Transcription

1 Lecture INTRODUCTORY CHEMISTRY Concepts and Critical Thinking Seventh Edition by Charles H. Corwin Introduction to Chemistry by Christopher G. Hamaker Illinois State University

2 Evolution of Chemistry The Greeks believed in four basic elements: 1. Air 2. Fire 3. Water 4. Earth All substances were combinations of these four basic elements.

3 The Scientific Method Science is the methodical exploration of nature followed by a logical explanation of the observations. An experiment involves scientists who explore nature according to a planned strategy and make observations under controlled conditions.

4 The Scientific Method, Continued The scientific method is a systematic investigation of nature and requires proposing an explanation for the results of an experiment in the form of a general principle. The initial, tentative proposal of a scientific principle is called a hypothesis. After further investigation, the original hypothesis may be rejected, revised, or elevated to the status of a scientific principle.

5 Applying the Scientific Method Step 1: Perform a planned experiment, make observations, and record data. Step 2: Analyze the data and propose a tentative hypothesis to explain the experimental observations. Step 3: Conduct additional experiments to test the hypothesis. If the evidence supports the initial proposal, the hypothesis may become a scientific theory.

6 Applying the Scientific Method, Continued After sufficient evidence, a hypothesis becomes a scientific theory. A natural law states a measurable relationship.

7 Modern Chemistry Chemistry is a science that studies the composition of matter and its properties. Chemistry is divided into several branches: Organic chemistry is the study of substances containing carbon. Inorganic chemistry is the study of all other substances that don t contain carbon. Biochemistry is the study of substances derived from plants and animals. Green chemistry is the design of chemical processes that reduce waste and hazardous substances

8 Chemistry Connection: Worth Your Salt? Salt was once so valuable, it was used to pay Roman soldiers Table salt is obtained by three major processes: Salt Mining Solution Mining Solar Evaporation of Salt Water Table salt is necessary for the human body, but too much can cause high blood pressure

9 Different people learn chemistry differently. What do you see in the picture? Learning Chemistry Some people see a vase on a dark background; some people see two faces.

10 Problem Solving Connect the dots using only four straight lines. Experiment until you find a solution.

11 Problem Solving, Continued Connect the dots using only four straight lines. Experiment until you find a solution. Did you have to use five straight lines? No matter which dot we start with, we still need five lines.

12 Problem Solving, Continued Are we confining the problem? We need to go beyond the nine dots to answer the problem.

13 Chemistry: The Central Science Knowledge of chemistry is important to understanding the world around us.

14 Chemistry Connection: A Student Success In 1886, pure aluminum metal cost over $100,000 per pound. Charles Hall and Paul Héroult both independently discovered a method for obtaining pure aluminum from aluminum ore. The industrial process for obtaining aluminum metal is referred to as the Hall-Héroult process. Today, pure aluminum costs less than $1 per pound.

15 Chapter Summary Scientists use the scientific method to investigate the world around them. Experiments lead to a hypothesis, which may lead to a scientific theory or a natural law. Chemistry is a central science with many branches. The impact of chemistry is felt in many aspects of our daily lives.

16 Interlude Prerequisite Science Skills by Christopher G. Hamaker Illinois State University

17 Measurements A measurement is a number with a unit attached. Every measurement has a degree of inexactness, termed uncertainty. We will generally use metric system units. These include: The centimeter, cm, for length measurements The gram, g, for mass measurements The milliliter, ml, for volume measurements

18 Length Measurements Let s measure the length of a candy cane. Ruler A has 1-cm divisions, so we can estimate the length to ± 0.1 cm. The length is 4.2 ± 0.1 cm. Ruler B has 0.1-cm divisions, so we can estimate the length to ± 0.05 cm. The length is 4.25 ± 0.05 cm.

19 Uncertainty in Length Ruler A: 4.2 ± 0.1 cm; Ruler B: 4.25 ± 0.05 cm. Ruler A has more uncertainty than Ruler B. Ruler B gives a more precise measurement.

20 The mass of an object is a measure of the amount of matter it affected. Mass is measured with a balance and is not affected by gravity. Mass and weight are not interchangeable. Mass Measurements

21 Volume Measurements Volume is the amount of space occupied by a solid, a liquid, or a gas. There are several instruments for measuring volume, including: Graduated cylinder Syringe Buret Pipet Volumetric flask

22 Significant Digits Each number in a properly recorded measurement is a significant digit (or significant figure). Significant digits express the uncertainty in the measurement. When you count significant digits, start counting with the first nonzero number. Let s look at a reaction measured by three stopwatches.

23 Significant Digits, Continued Stopwatch A is calibrated to seconds (0 s); Stopwatch B to tenths of a second (0.0 s); and Stopwatch C to hundredths of a second (0.00 s). Stopwatch A reads 35 s; B reads 35.1 s; and C reads s. 35 s has two significant figures s has three significant figures has four significant figures.

24 Significant Digits and Placeholders If a number is less than 1, a placeholder zero is never significant. Therefore, 0.5 cm, 0.05 cm, and cm all have one significant digit. If a number is greater than 1, a placeholder zero is usually not significant. Therefore, 50 cm, 500 cm, and 5000 cm all have one significant digit.

25 Exact Numbers When we count something, it is an exact number. Significant digit rules do not apply to exact numbers. An example of an exact number: There are 7 quarters on this slide.

26 Rounding Off Nonsignificant Digits All numbers from a measurement are significant. However, we often generate nonsignificant digits when performing calculations. We get rid of nonsignificant digits by rounding off numbers. There are three rules for rounding off numbers.

27 Rules for Rounding Numbers 1. If the first nonsignificant digit is less than 5, drop all nonsignificant digits. 2. If the first nonsignificant digit is greater than or equal to 5, increase the last significant digit by 1 and drop all nonsignificant digits. 3. If a calculation has several multiplication or division operations, retain nonsignificant digits in your calculator until the last operation.

28 Rounding Examples A calculator displays and 3 significant digits are justified. The first nonsignificant digit is a 3, so we drop all nonsignificant digits and get 15.7 as the answer. A calculator displays and 3 significant digits are justified. The first nonsignificant digit is a 5, so the last significant digit is increased by one to 8. All the nonsignificant digits are dropped, and we get 18.8 as the answer.

29 Rounding Off and Placeholder Zeros Round the measurement 183 ml to two significant digits. If we keep two digits, we have 18 ml, which is only about 10% of the original measurement. Therefore, we must use a placeholder zero: 180 ml. Recall that placeholder zeros are not significant. Round the measurement 48,457 g to two significant digits. We get 48,000 g. Remember, the placeholder zeros are not significant, and 48 grams is significantly less than 48,000 grams.

30 Adding and Subtracting Measurements When adding or subtracting measurements, the answer is limited by the value with the most uncertainty. Let s add three mass measurements. The measurement g has the greatest uncertainty (±0.1 g). The correct answer is g g 0.75 g g g

31 Multiplying and Dividing Measurements When multiplying or dividing measurements, the answer is limited by the value with the fewest significant figures. Let s multiply two length measurements: (7.28 cm)(4.6 cm) = cm 2 The measurement 4.6 cm has the fewest significant digits two. The correct answer is 33 cm 2.

32 Exponential Numbers Exponents are used to indicate that a number has been multiplied by itself. Exponents are written using a superscript; thus, (4)(4)(4) = 4 3. The number 3 is an exponent and indicates that the number 4 is multiplied by itself three times. It is read 4 to the third power or 4 cubed. (4)(4)(4) = 4 3 = 64

33 Powers of 10 A power of 10 is a number that results when 10 is raised to an exponential power. The power can be positive (number greater than 1) or negative (number less than 1).

34 Scientific Notation Numbers in science are often very large or very small. To avoid confusion, we use scientific notation. Scientific notation utilizes the significant digits in a measurement followed by a power of 10. The significant digits are expressed as a number between 1 and 10. D.DD x 10 n power of 10 significant digits

35 Applying Scientific Notation Step 1: Place a decimal after the first nonzero digit in the number, followed by the remaining significant digits. Step 2: Indicate how many places the decimal is moved by the power of 10. A positive power of 10 indicates that the decimal moves to the left. A negative power of 10 indicates that the decimal moves to the right.

36 Scientific Notation, Continued There are 26,800,000,000,000,000,000,000 helium atoms in 1.00 L of helium gas. Express the number in scientific notation. Place the decimal after the 2, followed by the other significant digits. Count the number of places the decimal has moved to the left (22). Add the power of 10 to complete the scientific notation x atoms

37 Another Example The typical length between a carbon and oxygen atom in a molecule of carbon dioxide is m. What is the length expressed in scientific notation? Place the decimal after the 1, followed by the other significant digits. Count the number of places the decimal has moved to the right (7). Add the power of 10 to complete the scientific notation x 10-7 m

38 Scientific Calculators A scientific calculator has an exponent key (often EXP) for expressing powers of 10. If your calculator reads 7.45 E-17, the proper way to write the answer in scientific notation is 7.45 x To enter the number in your calculator, type 7.45, press the exponent button (EXP) and type in the exponent followed by the +/ key.

39 Chapter Summary A measurement is a number with an attached unit. All measurements have uncertainty. The uncertainty in a measurement is dictated by the calibration of the instrument used to make the measurement. Every number in a recorded measurement is a significant digit.

40 Chapter Summary, Continued Placeholding zeros are not significant digits. If a number does not have a decimal point, all nonzero numbers and all zeros between nonzero numbers are significant. If a number has a decimal place, significant digits start with the first nonzero number and all digits to the right are also significant.

41 Chapter Summary, Continued When adding and subtracting numbers, the answer is limited by the value with the most uncertainty. When multiplying and dividing numbers, the answer is limited by the number with the fewest significant figures. When rounding numbers, if the first nonsignificant digit is less than 5, drop the nonsignificant figures. If the number is 5 or more, raise the first significant number by 1, and drop all of the nonsignificant digits.

42 Chapter Summary, Continued Exponents are used to indicate that a number is multiplied by itself n times. Scientific notation is used to express very large or very small numbers in a more convenient fashion. Scientific notation has the form D.DD x 10 n, where D.DD are the significant figures (and is between 1 and 10) and n is the power of 10.