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## Introduction to Chemistry

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**“The world is full of obvious things. Which nobody by any**chance ever observe.”-- Sherlock Holmes • Careful observation is the foundation of chemistry as an experimental science, leading us to question what we have observed – how, what why? • The answers to these questions are sought in experiments, which may be described as observations made under controlled conditions • Observation and experimentation – the twin pillars of the scientific method**Scientific Method…**• The scientific way of knowing – often called the scientific method – is sometimes presented as a rigid sequence of events • It is not however a rigid path – it is a process of discovery! • Discovery begins when we make observations and then try to understand what we have observed by asking key questions and proposing possible answers • This process of discovery begins as we design and conduct experiments to test whether our answers to these questions are valid!!**What are the steps of the Scientific Method?**• Make an observation(s) • Propose a hypothesis • Design and conduct a controlled experiment • Analyze the results • Form conclusions**Controlled Experiment…**• Experiments should be designed so that the effects of different variables on the behavior of a substance can be studied independently • A controlled experiment is when only one variable at a time is changed • There are two types of variables in an experiment: • Independent variable – the one that is deliberately changed • Dependent variable – the thing that changes because of the independent variable**An Example…**• Students were trying to determine if the amount of a Sodium chloride added to calcium carbonate effected the amount of heat given off • Give 3 possible variables for this experiment • Give the independent and dependent variables**What is chemistry?**• Chemistry is the study and investigation of the structure, composition and property of matter and the changes it undergoes • The properties of materials are always related to their structure • Hence, structure determines properties**Measuring and Calculating in Science…**• Chemistry is a quantitative science because it involves measuring and calculating • A measurement must have a number and a scale (called a unit) to be meaningful • It can also be a qualitative science because it can involve describing what is happening in a reaction**What makes a measurement?**• In order to make measurements, we must meet three requirements… • Know what we are trying to measure • Have a standard with which to compare whatever we are measuring • Have a method for making comparisons**Exact Numbers…**• A number that is the result from a definition or an exact count • For example – there are 12 apples or π = 3.14 • All are significant • Do not limit the number of sig figs**Uncertainty in Measurement…**• A measurement always has some degree of uncertainty • The amount of uncertainty depends on the precision of the measuring device • In science it is customary to report a measurement by recording all certain digits plus the first uncertain (estimated) digit – these numbers are called the significant figures of a measurement**Estimating Uncertainty in a measurement…**• Remember all measurements are a result of known values and one estimated number • When finding the uncertainty in a measurement, we look at the estimated number • For example: 0.023 • The 3 is the estimated number • It is in the 1000th place • The estimated uncertainty is written +0.001 • This measurement has a very small uncertainty**Rules for Counting Significant Figures…**• Nonzero integers are always significant figures • Zeros – there are three classes of zeros • Leading zeros precede all the nonzero digits and are not significant figures • Captive zeros are between nonzero digits and always count as significant figures • Trailing zeros are at the right end of the number and are only significant if the number contains a decimal point**Try a few…**• Tell how many sig figs are in each measurement and tell the uncertainty in each measurement • 1508 cm • 300.0 ft • 20.003 g • 0.00705 L**Bell Ringer…**• Tell the number of sig fig • 94300 • 0.000400670 • 100000. • 56.00 • Tell the uncertainty of each measurement • 3.45 • 6.0 • 12 • 4.725**Bell Ringer…**• Each of the following are statements from different labs – tell if they are quantitative or qualitative • Bubbling • Heat given off • 23.6 cm wide • A strong odor • pH of 5.4 • 273 K**Sig figs in Mathematical Operations…**• To this point we have learned to count the significant figures in a given number, but we must also consider how uncertainty accumulates as calculations are carried out**Rules for Sig figs in Mathematical Operations…**• For multiplication or division the number of sig figs in the answer is the same as the number in the least precise measurement used in the calculation • 4.56 x 1.4 = 6.38 → 6.4 (correct answer) • For addition or subtraction the number of sig figs in the answer has the same number of decimal places as the least precise measurement used in the calculation • 12.11 + 18.0 + 1.013 = 31.123 → 31.1 (correct answer)**Bell Ringer…**• Carry out the following mathematical operations and give each result with the correct number of significant figures • 1.05 x 10-3 / 6.135 • 21 – 13.8 • 20 X 23.00 • 14.75 + 34.25**Bell Ringer…**• The actual length of a certain plank is 26.782 cm. Which of the following measurements is the most accurate? Are the measurements precise? • 26.5 cm • 26.8 cm • 26.202 cm • 26.98 cm**Rules for Rounding…**• In most calculations you will need to round numbers to obtain the correct number of sig figs • When rounding, use only the first number to the right of the last significant figure • In a series of calculations, carry the extra digits through to the final result, then round • If the digit to be removed • Is less than 5, then the preceding digit stays the same 1.33 → 1.3 • Is equal to or greater than 5, the preceding digit is increased by 1 1.36 → 1.4**Precision and Accuracy…**• Two terms often used to describe the reliability of measurements are precision and accuracy • Precision – the degree of agreement among several measurements of the same quantity. It also is known as the degree of reproducibility of the measurement • Accuracy – the agreement of a particular value with the true value**Bell Ringer…**• Decide if the following lab data is accurate or precise or both • 13.2 mL, 13.3 mL, 13.1 mL, 13.2 mL • The actual value is 13.0 mL • Make each of the following have 3 sig figs • 34098 • 0.0003219 • 7154 • 76.78**Types of errors…**• There are two types of errors in measurements • Random error (indeterminate error) – a measurement that has an equal probability of being high or low. This type of error occurs in estimating the value of the last digit of a measurement • Systemic error (determinate error) – occurs in the same directions each time. The measurement is either always too high or too low**In groups…**• There are 365 days/year, 24 hours/day, 12 months/year and 60 minutes/hr. Use this data to determine how many minutes are in a month. • Now use the following data to calculate the number of minutes in a month: 24 hours/day, 60 minutes/hour, 7 days/week, and 4 weeks/month. • Why are these answers different? Which, if any, is more correct and why?**Dimensional Analysis…**• It is often necessary to convert a given result from one system of units to another • The best way to do this is by a method called unit factor method OR dimensional analysis**Converting from One Unit to Another…**• To convert from one unit to another, use the equivalence statement that relates the two units • Derive the appropriate unit factor by looking at the direction of the required change (to cancel unwanted units) • Multiply the quantity to be converted by the unit factor to give the quantity with desired units**Bell Ringer…**• A marathon race is 26 miles and 385 yards. • What is the distance in rods • What is the distance in meters • What is the distance in furlongs? 5.5 yards = 1 rod 40 rods = 1 furlong 8 furlongs = 1 mile 1 meter = 39.37 inches 1 yard = 36 inches**What if there is more than one unit present?**• When more than one unit is present, decide which unit you want to convert first • Convert it first • Then convert the second unit ***do not get confused!!! EX: How fast is a car going 35 miles/hour going in yards/second? 1 mile = 1760 yards;1 hour = 60 minutes; 1 minute = 60 seconds**A few problems…**• How many doughnuts can one purchase for $123 if doughnuts cost $3.25/doz? • Convert 9.85 L to gal. 1.06 qt = 1.00 L and 4 qt = 1 gal • A certain size of nail cost $1.25/lb. What is the cost of 3.25 kg of these nails? 1kg = 2.2 lb**Metric System Review…**• Scientists recognized that long ago a standard system of units had to be adopted if measurements were to be useful • The system agreed upon in 1960 was the International System or le Systeme International (SI system) • The SI system is based on the metric system and units derived from the metric system • Because fundamental units are not always convenient, the SI system employs prefixes to change the size of the unit**Derived units…**• Many SI units are combinations of quantities • These units are produced by multiplying or dividing standard units**Dimensional Analysis with metric units…**When converting with metric, always use that value of the unit as compared to the base unit • Convert 35.4 mm to m • Convert 2327.9 cg to kg • How many grams are in 53.24 dg?**Bell Ringer…**• Why do we use the metric system? • Convert 35.4 mm to m • Convert 2327.9 cg to kg • How many grams are in 53.24 dg? • Convert the following: • How many inches are in 3.0 meters? • A baby weighs 8.5 lbs. How many grams is that? • How many gallons of Coke would you drink if you drank entire 2 liter?**Science fiction often uses nautical analogies to describe**space travel. If the starship U.S.S. Enterprise is traveling at warp factor 1.71, what is its speed in knots? Warp 1.71 = 5.00 times the speed of light The speed of light = 3.00 x 108 m/s 1 knot = 2000 yd/hr**Mass**• The measure of the resistance of an object to a change in its state of motion OR the amount of “stuff” in an object • A scale is used to mass an object**Mass vs. weight…**• An important point concerning measurements is the relationship between mass and weight • Weight is the force gravity exerts on mass, therefore weight varies with the strength of the gravitational field • Therefore if you went to the moon your weight would change but not your mass • Many times the terms mass and weight are sometimes used interchangeably, although this is incorrect!**Volume…**• The derived SI unit of volume is cubic meters (m3) • Many times this unit is way too large to be a practical way of expressing volume in a chemistry lab • Instead, a smaller unit cubic centimeters (cm3) is used • When dealing with the volumes of liquids and gases, the non-SI unit liter(L) is often used • Again the liter is often too large so the unit milliliter (mL) is used • This means 1 cm3 = 1mL**Review…**• Round the following to 3 sig figs • 96747210 • 91 • 0.0006589 • How many sig figs are in each in #1? • What is the uncertainty of each measurement? • 34.09 • 6.0222 • 12 • What is the difference between precision and accuracy?**Review…**• Convert the following: • How many grams are in 548.9 mg? • How many feet are in 34.2 m? • How many liters are in 2 gallon and 3.4 quarts?**It can be tricky with volume conversions…**• How many mL are in 14.65 kL? • How many L are in 48.6 cm3? • How many dm3 are in 29100 mL?**Bell Ringer…**• A piece of metal has the mass of 3.45 kg. What is its mass in g? • A container has 2.3 L of gas in it? What is its volume in mL? • A container has 750.00 mL of liquid in it. What is its volume in m3?**What is Temperature?**• A measure of the AVERAGE kinetic energy • When looking at the different temperature scales, all are talking about the same height of mercury**Temperature Conversions…**• There are three systems used to measure temperature • Degrees Fahrenheit (°F) • Degrees Celsius (°C) • Kelvin (K) • Each has a different way of converting between the values**Notice -- 0°C = 32°F and 100°C = 212°F**If we subtract these values then… 100°C = 180°F * Find the value of 1°C 1°C = (180/100) °F 1°C = 9/5 °F How the equation for °F to °C was derived…**Converting…**1. Converting from °C to Kelvin TC = TK – 273.15 TK = TC + 273.15 • Converting from °C to °F TF = TC x 9°F + 32°F 5°C**More Converting…**• Converting °F to °C TC = (TF - 32°F)5°C 9°F**Try these…**1. Normal body temperature is 98.6°F. Convert this to the Celsius and Kelvin scales. 2. Liquid nitrogen, which is often used as a coolant for low-temperature experiments has a boiling point of 77 K. What is this temperature on the Fahrenheit scale?