CHEM Chapter 1

Transcription

1 CHEM 1110 Chapter 1

2 Chapter 1 OVERVIEW What s science? What s chemistry? Science and numbers Measurements Unit conversion States of matter Density & specific gravity Describing energy Heat and its transfer First things first

3 What is Science? The whole of science is nothing more than a refinement of everyday thinking. -Albert Einstein

4 What s chemistry? The Central Science Physics Chemistry Biology

5 What is Chemistry? The study of the matter, its composition, properties, and the changes it undergoes.

6 Scientific Method A way of solving problems or answering questions. Starts with observation- noting an recording facts Hypothesis- a possible explanation as to the cause of the observation, based on research and previous knowledge

7 Scientific Method Experiment- designed to test the hypothesis only two possible answers hypothesis is right hypothesis is wrong Generates data -observations from experiments. Modify hypothesis - repeat the cycle

8 Observations Hypothesis Experiment Cycle repeats many times. By you and by others The hypothesis gets more and more certain. Becomes a theory A thoroughly tested model that explains why things behave a certain way.

9 Observations Hypothesis Experiment Another outcome is that certain behavior is repeated many times Scientific Law is developed Description of how things behave Usually an equation Law - how Theory- why

10 Science and numbers Why does science have to use # s?

11 Types of observations Qualitative- descriptive, but not true measurements Hot Large Quantitative- describe with numbers and units 100 C 15 meters Scientists Prefer Quantitative data; More precise, testable, no bias

12 How good are the measurements? Scientists use two words to describe how good the measurements are Accuracy- how close the measurement is to the actual value. Precision- how well can the measurement be repeated.

13 Differences Accuracy can be true of an individual measurement or the average of several. Precision requires several measurements before anything can be said about it.

14 Let s use a golf analogy

15 Accurate? No Precise? Yes

16 Accurate? Yes Precise? Yes

17 Precise? No Accurate? Maybe?

18 Accurate? Yes Precise? We cant say!

19 In terms of measurement Three students measure the room to be 10.2 m, 10.3 m and 10.4 m across. Were they precise? Were they accurate?

20 Why? Chemistry often deals with very large and very small numbers. There are 602,000,000,000,000,000,000,000 molecules of water in 18 ml one electron has a mass of g We need a shorter way of writing these numbers

21 Standard Exponential Form another name for scientific notation. consists of two parts a number between 1 and 10 multiplied by 10, raised to some power 602,000,000,000,000,000,000,000 = 6.02 x g = 9.11 x 10-28

22 Putting a number into scientific notation determine how many times you have to move the decimal place to make it into a number between 1 and use that as the power of x 10 6 Put this number into scientific notation

23 What if the number is smaller? if you make the number bigger by moving the decimal point, make the exponent smaller and visa-versa x 10-4 Put this number into scientific notation

24 Getting back to original number Move the decimal point the same number of times as the exponent If the exponent gets bigger the number gets smaller Change 2.99 x 10 8 Change 4.87 x 10-3

25 Using your calculator EE and EXP button stand for x 10 to the 4.5 x 10-4 push 4.5 push either EXP or EE push 4 +/- or -4 see what your display says.

26 Practice these problems (4.8 x 10 5 ) x (6.7 x 10-6 ) (6.8 x 10-6 ) (3.2 x 10 4 ) Remember when you multiply you add exponents 10 6 x 10-4 When you divide you subtract exponents.

27 Adding and Subtracting You can t add or subtract numbers until they are to the same power of ten. Your calculator does this automatically. (4.8 x 10 5 ) + (6.7 x 10 6 ) (6.8 x 10-6 ) - (3.2 x 10-5 ) Remember- standard form starts with a number between 1 and 10 to start.

28 Really large, Scientific Notation and really small numbers

29 PEMDAS

30 Significant figures (sig figs) How many numbers mean anything. When we measure something, we can (and do) always estimate between the smallest marks. Measure in cm

31 Significant figures (sig figs) The better marks the better we can estimate. Measure in cm. Scientist always understand that the last number measured is actually an estimate

32 Significant figures (sig figs) The measurements we write down tell us about the ruler we measure with The last digit is between the lines What is the smallest mark on the ruler that measures cm?

33 Rules for Determining Significant Digits All non zero numbers are significant 1,2,3,4,5,6,7,8,9 All zeros sandwiched in between non zero numbers are significant 101, Placeholder zeros are not significant Use the Atlantic-Pacific Rule

34 Pacific Atlantic Present Absent If the decimal point is absent, start at the Atlantic (right), find the first non zero, and count all the rest of the digits

35 Pacific Atlantic Present Absent If the decimal point is PRESENT, start at the Pacific (left), find the first non zero, and count all the rest of the digits

36 Sig figs. How many sig figs in the following measurements? 458 g 4085 g 4850 g g g g 4050 g g g g

37 Rounding rules Look at the number behind the one you re rounding. If it is 0 to 4 don t change it. If it is 5 to 9 make it one bigger. Round to four sig figs. to three sig figs. to two sig figs. to one sig figs

38 Numbers without sig figs Counted numbers 12 eggs in a dozen 32 students in a class Definitions 1 m = 100 cm 16 ounces is 1 pound No estimated numbers

39 Watch the Sig Figs When rounding, you don t change the size of the number. You should end up with a number about the same size. Use place holders- they re not significant. Round to 3 sig figs Round to 3 sig figs

40 Adding and subtracting with sig figs The last sig fig in a measurement is an estimate. Your answer when you add or subtract can not be better than your worst estimate. have to round it to the least place of the measurement in the problem.

41 For example First line up the decimal places Then do the adding Find the estimated numbers in the problem This answer must be rounded to the tenths place.

42 RULE: When adding or subtracting your answer can only show as many decimal places as the measurement having the fewest number of decimal places. Example: When we add 3.76 g g g = g We look to the original problem to see the number of decimal places shown in each of the original measurements. 2.1 shows the least number of decimal places. We must round our answer, 20.69, to one decimal place (the tenth place). Our final answer is 20.7 g

43 Practice

44 RULE: When multiplying or dividing, your answer may only show as many significant digits as the multiplied or divided measurement showing the least number of significant digits. Example: When multiplying cm x 3.10 cm x cm = cm 3 Check the number of significant digits in each of the original measurements: significant digits significant digits significant digits. Our answer can only show 3 significant digits shows 9 significant digits, we must round to show only 3 significant digits. Our final answer becomes 5950 cm 3.

45 Multiplication and Division 3.6 x has 2 s.f. 653 has 3 s.f. answer can only have 2 s.f. 2400

46 Multiplication and Division Same rules for division. practice 4.5 / x.043