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Chapter 1 Notes The Scientific Method: General process of advancing scientific knowledge through observations, the framing of hypothesis and laws or theories and the conducting of more experiments to test these laws or hypotheses. Process:
- Experiment: an observation of natural phenomena carried out in a controlled manner so that the results can be duplicated and rational conclusions obtained.
The key words here are controlled and duplicated. What does it mean for the experiment to be carried out in a controlled manner? ____________________________________________________ Hopefully you predicted this means to know the variables, then ideally change one variable at a time to see how that variable affects the results of the experiment. Variables must be controlled so that results can be duplicated.
And to be duplicated? If an experiment can’t be performed again under the exact same conditions to obtain the same result (i.e. it can’t be duplicated, then the initial results are meaningless).
Let’s take an example: Say you work in the pharmaceutical industry. The generic reaction below is used to produce C, which is an important antibiotic.
A + B → C
Why does the company want to make C? ____( to sell it for profit )____________________
So how efficiently would the company like to be able to make C? (i.e. what yield would they like reaction to proceed in?). ( 100% yield would be preferred!)
What are the variables that one could vary to produce C in a higher yield. List as many as you can think of. ( temperature, amount of compounds, time of reaction, concentrations, etc.)
- A hypothesis can then be made: a tentative explanation of some regularity of nature. Thought process to organize knowledge and predict future events.
After a series of experiments to test hypothesis, perhaps the scientists sees a relationship or regularity in the results.
- If the regularity or relationship is fundamental, we call it a law : a concise statement or mathematical equation about a fundamental relationship or regularity of nature. Also known as a theory.
Cannot prove a theory absolutely, can only disprove it. It may be possible to devise a further set of experiments that will show the theory to be limited or that someone will develop a better theory.
Example: The physics of motion obeys Isaac Newton’s laws. All things known obeyed these laws for two centuries, then a new set of experiments showed that particles moving near the speed of light, or that were very very small) do not obey Newton’s laws. First observation led to the theory of relativity Second observation led to the development of quantum mechanics
All things are composed of something, that something is what scientists call matter. Matter: general term for material things around us, whatever occupies space and can be perceived by our senses.
Clasify by Physical State (solid, liquid, gas) Matter Chemical Constitution
mixing
Substances^ Physical Process Mixtures separation (such as distillation, extraction...)
Elements 118 known (^) Compounds Homogeneous (^) Heterogeneous
Chemical Process
Matter can be classified by its physical state (solid, liquid, gas). Lets define solid, liquid and gas as a scientist would…we all can conceive of what a solid, liquid and gas is, but how would you describe it to a child? And what are the phase changes call when converting from one phase to another. sublimation
= could be a water (H 2 O) molecule for example
gas (^) liquid solid
condense freeze
evaporate (^) melt
deposition
easily (^) compressed not easily^ compressed not easily^ compressed
- assumes the shape of its container - assumes the shape of its container - does not assumes the shape of its container fixed volume and shape - rigid
So what is a vapor? Gaseous state of any kind of matter that normally exists as a solid or liquid…measure of humidity is a measure of how much water vapor is in the air.
Matter can have its physical state changed by a physical change – a change in the physical change of matter does not alter its chemical composition. For example, the change from ice to water to steam does not change the composition of the matter (they are all H 2 O).
How do we measure matter? 1960 – The International Conference of Weights and Measures adopted the International System of Units (SI units). There are 7 base SI units from which all other units can be derived: Base Units: Derived Units (some examples)
- _length = ___meter__________ area = m*m = m^2
- mass = kilogram ____________ volume = mmm = m^3
- time = second____________ density = mass / volume (kg / m^3 )
- temperature = Kelvin____________ pressure (Pascall, Pa) = kg / (m * s^2 )
- amount of substance = mole________ force (Newton, N) = (kg * m ) / s^2
- electric current = amp ____________ speed = m / sec
- luminous intensity = candela__________ Energy (joule, J) = (kg * m^2 ) / s^2 1 meter defined as the distance traveled by light in a vacuum in 1 / 299,792,458 seconds.
The SI base unit of density = mass / volume (kg / m^3 ) is not a very convenient unit in chemistry because 1 kg of material is a lot of sample and the meter is large (3.28 ft) and a m^3 is thus a very large volume (it would be a box 3.28 ft wide by 3.28 ft deep by 3.28 ft tall – this box would hold a very large volume of a solid or liquid!). A more convenient unit of density in chemistry is g / cm^3. One gram is a reasonable (easily managed) amount of material. One inch = 2.54 centimeters…..so consider a box that is 1 in wide by 1 in deep by 1 in tall – this is not a very large box and thus is a fairly small volume of a solid or liquid. 1 cm^3 = 1 milliliter. Thus, density in chemistry is often also reported as g / ml….you should memorize this! When you read or hear the word density, you should immediately think grams/milliliter!
SI decimal system (base units of 10) (fill in the rest of the missing information below) Multiple 1018
prefix exa
symbol E
Multiple 10 -
prefix deci
symbol d 10 15 1012
peta tera
P
T
centi milli
c m 10 9 106 103 102
giga mega kilo hecto
G
M
k h
micro nano pico femto
μ n p f 10 1 deka da 10 -18^ atto a
1 angstrom ( Å ) = 10-10^ m (not an SI unit)…commonly used in chemistry since atoms have diameters in the angstrom range. (example: an oxygen atom has a diameter of 1.3 Å)
The SI system is very convenient to use since to convert between units all you have to do is know how many powers of 10 there are between the units – the number of powers of ten between the units is the same number of positions you will move the decimal point in converting between the units.
For example to convert 1289 millliters to kiloliters note that there is a difference of 6 powers of 10 (10-3^ versus 10^3 )….if converting from a smaller unit to a larger unit, move the decimal point to the left. If converting from a large unit to a smaller one, move the decimal point to the right.
1289 milliters = 0.001289 kiloliters (converted from a small unit to a larger unit so decimal moved to the left 6 places since there are 6 powers of 10 between 10-3^ and 10^3 ).
1.25 megagrams = 1,250,000,000,000,000. nanograms (10^6 versus 10-9^ = 15 powers of 10…converting a big unit (mega) to a small one (nano) so move decimal 15 places to the right).
*I think in the SI system at work (liters, grams, etc) but I think in the US system at home (gallons, cups, pounds, etc).
Precision: the closeness of the set of values obtained from identical measurements of a quantity (may or may not be accurate).
Accuracy: the closeness of a single measurement to its true value.
precise but not accurate not accurate or precise accurate and precise
Which is best? To be both accurate and precise in your measurements! How can you be sure if the results of a measurement are accurate? Compare the result to a known standard. For example, if you are weighing a compound on a balance, how can you know that the weight that the balance reports is accurate? You could weigh a known sample (say a platinum bar) provided by the balance manufacturer that is of known weight to determine if the balance reports the correct known weight of the standard sample – if it does, you can be confident that the weight of your sample is also accurate. It is easy to determine if your data is precise (since this is just a measure of how close all of the measurements are to each other).
Significant Figures: the number of digits reported for the value of a measured or calculated quantity, indicating the precision of the value. 1.77 cm has 3 significant figures for example.
Rules for determining the number of significant figures:
- All non-zero digits in a number with a decimal point are always significant. 9.12 cm (three significant figures), 11.387 (five significant figures), 0.912 cm (3 significant figures)
- Terminal zeros at the right of a decimal point are significant (all zeros to the right of a non- zero number are always significant in a number with a decimal point). 9.00 cm, 9.10 cm, and 90.0 cm all have three significant figures.
- Zeros between non-zero numbers in a number with a decimal point are always significant. 9.102 cm (four significant figures), 10.387 (five significant figures), 0.9102 cm (4 significant figures)
- Leading zeros in a number with a decimal point are not significant. 0.00313 (three significant figures), 0.00050 (two significant figures), 0.051090 (5 significant figures)
Example: 184.22 g - 2.324 g = 181.896 g (calculator). But since adding, 184.2s has two digits after the decimal place and 2.324 has three digits after the decimal place. Report answer to two digits after the decimal place…..so answer is 181.90 g (does not matter how many SF numbers a number has when adding and subtracting….the # of decimal places is all that matters.
Rounding
- Look at the left most digit to be dropped; if the digit is 5 or greater, round up (add to the last significant digit and drop all others digits to the right of this digit). Example: 1.2151 to 3 SF is 1.
- If left most digit to be dropped is less than 5 simply drop all digits right of the left most significant digit. Example: 1.2143 to three SF would be 1.
***** If doing a calculation consisting of 2 or more steps, DO NOT ROUND UNTIL THE LAST STEP!!!!!
Try these examples in your group:
a. (8.11 x 2.212) / 3.6467 = …..4.91932981…….4.91 = answer (3 significant figures)
b. 5.332 – 0.2122 = …..5.1198…….5.120 = answer (3 decimal places)
c. 19.225 + 17.0302 = …..36.2552…. 36.255 = answer (3 decimal places)
d. 9.2225 – 9.129 = …..0.0935…….0.094 = answer (3 decimal places)
e. 4.18 – 58.16 x (3.38 – 3.01) = …….4.18 – 58.16 x 0.37 (2 decimals) = 4.18 – 21.5192 (2 significant figures) = - 17.3392 (0 decimal places)…….17. = final answer
What volume is in this graduated cylinder to the correct # of sig figs? It would be fine to say 2.1 ml or 2.2 ml but it would not be fine to say 2.1258 ml or 2.25 ml. You should be 100% confident in the 2 ml but the next number is a guess. Report all numbers you are 100% confident in plus one digit that you guess at.
This cylinder has more graduations, so you can report the volume to more decimal places. You could report the volume as 2.68 ml or 2.67 ml or 2.69 ml for example. You are 100% confident in the 2 and the 6 but the last digit (second decimal place) is your first guess)
1 ml
2 ml
3 ml
1 ml
2 ml
3 ml
If you mix these two volumes, what would be an appropriate way to report the total volume (to the correct number of significant figures?). You could say 2.2 ml = 2.68 ml = 4.88 ml….4.9 ml (you are limited to reporting it to one decimal place because you are limited by the graduated cylinder on the left above which cannot measure volume as accurately (of course, you are subtracting numbers in this example so your answer will be reported to the same number of decimal places as the number with the least).
Temperature Know how to convert between Fahrenheit, Centigrade and Kelvin scales
Tk = (Tc x 1K/1ºC) + 273.15 K
TF = (T (^) c x 9ºF/5ºC) + 32 ºF
TC = (5ºC/9ºF) x (T (^) F - 32 ºF)
Dimensional Analysis The factor label method: a method for converting between units….some conversions require one step, some require a sequence of steps.
NASA ground flight control engineers, working in units of lbs-force/seconds, sent thrust commands to the orbiter in the units which did not match the flight controller units on the orbiter (Newtons/second) and which were not converted at any point. This resulted in incorrect thrust from the orbiter…..which then entered the Mars atmosphere at too low of an altitude (37 miles) causing the orbiter to burn up in the Mars atmosphere.
The cost of the mistake to not convert units:
$327.6 million (orbiter) $193.1 million (orbiter research and development) $91.7 million (launch) $42.8 million (mission operations)
Total cost = $655.2 million dollars (2013 dollars)…..a VERY costly simple mistake!!!
