1.1 What Chemistry Is About

Transcription

1 1.1 What Chemistry Is About Where does the mass of a tree come from? Most of the solid matter in a tree comes from air Fire: a chemical reaction that changes matter from one form to another The answer is mostly from the air! A tree takes in carbon dioxide from the air and water from the ground. Carbon dioxide is a gas, but it is still matter. You don t feel the weight of a gas like air because the matter is spread very thinly. However, there is a lot of air more than enough to create majestic trees! An incredible chemical reaction called photosynthesis plucks carbon atoms from carbon dioxide in the air and compacts them into molecules of a sugar called glucose. The glucose along with water and a tiny amount of minerals from the soil is used to build the tree. In Chapter 10 we will learn a lot more about photosynthesis and other chemical reactions that are important to life on Earth. When a tree grows, where does the matter come from? How does the tree get bigger? We know that matter cannot come from nothing. A big redwood tree might weigh 500,000 pounds. If all this weight came from the soil, we would see big holes in the ground around large trees. But, we do not! Experiments reveal that only a tiny fraction of mass is lost from the soil around a tree. If the matter in a tree does not come from the soil, then where does all this matter come from? What is fire? Is it a chemical? Is it matter? Is it energy? In early times it was believed that air, earth, water, and fire were the major elements of the universe. You can feel fire, see it, and smell it, so our senses tell us it is present. Water, air, and earth are all made up of many atoms connected together, so they do represent types of matter! Fire is a form of heat and light energy, not matter. Chemical reactions transform one kind of matter, such as wood, into other kinds of matter, such as ashes, smoke, and water. Fire is produced when matter changes its form. For example, when wood burns, the carbon reacts with oxygen, producing energy and carbon dioxide. This energy is the heat and the light that we feel from a fire. 4 A NATURAL APPROACH TO CHEMISTRY

2 Chemistry in your body Why should you eat foods high in antioxidants? What do antioxidants do? What is the purpose of vitamins? Different kinds of vitamins We are told to eat foods that are high in antioxidants, such as blueberries, cranberries, artichokes and beans. How does this help us? The answer has to do with the chemistry of life. When energy in food is used by our cells, chemicals called free radicals are produced as a byproduct. Free radicals are very reactive. Think of a pinball game where the ball is the free radical. Anything the pinball hits may become damaged. The ball continues to bounce around and damage useful chemicals until it is absorbed by something. In the pinball machine, an absorbed ball falls to the bottom and is no longer in play where it can cause more damage. In your body, antioxidants absorb free radicals before they can do much damage. Once neutralized by an antioxidant, free radicals are harmless. That is why it is important to have high levels of antioxidants in your body. Antioxidants become even more important with age because your metabolism becomes less efficient. This causes the number of free radicals to increase. Antioxidants have been shown to help prevent age-related diseases such as cancer. Vitamins are chemicals that the body needs but does not produce itself from the raw materials in food. For example, vitamin C is used by the body to make collagen. Collagen is found in joints and tendons that connect muscles and bones. If you don t get enough vitamin C from fresh fruits and vegetables, you may develop scurvy. Scurvy is a disease that used to sicken or even kill sailors in Columbus s day. Some vitamins dissolve in fat (vitamins A, D, E, and K) and others dissolve in water (vitamin C and B complexes). Water-soluble vitamins need to be consumed each day since they are excreted in urine. Fatsoluble vitamins are stored in your liver, so they do not need to be replaced in your body every day. Humans require vitamins in very small amounts; you can get sick from either too much (overdose) or too little (deficiency). That s why scientists came up with nutritional guidelines that give approximate daily recommended amounts of each one. Read the label on a vitamin bottle. Do you find terms that you recognize? Discuss with your classmates and your teacher. A NATURAL APPROACH TO CHEMISTRY 5

3 Water: a very important chemical Most of your body is water Water is critical to life Sugar is a chemical that dissolves in water A physically fit human body is about 60% water by weight. Take away the water and the rest is mostly different proteins and fats, with about 6% other chemicals and minerals such as calcium, phosphorus and iron. During one hour of exercise, you may lose as much as two liters of water by sweating and breathing! You also lose small amounts of dissolved salts. If the lost water and dissolved salts are not replaced, your body stops working. Hydration is very important for athletes. Look at the back of hydration drink. What elements do you find listed on it? The process of living requires your body to change the chemicals in food into other chemicals needed by your blood, muscles, nerves, and the rest of your body. Solids like sugar cannot easily move around in your body. This is why most of the chemistry of life happens only in water. There are two main reasons for this: 1. Many substances dissolve in water, including sugars, salts, and proteins. 2. Once dissolved, chemicals can circulate and interact with other chemicals. A good example of a chemical that dissolves in water is sugar. Sugar is made of tiny particles (molecules) that are so small that more than a million would fit on the head of a pin. Water is also made of tiny particles! A tablespoon of sugar is solid. The sugar molecules are stuck to each other. When sugar is mixed with water, the tiny molecules separate from each other. Each sugar molecule disperses among many water molecules. In fact, the sugar seems to visibly disappear once it has been dissolved. One taste will tell you the sugar is still there, even though it has been dissolved. 6 A NATURAL APPROACH TO CHEMISTRY

4 Measurements and units Why units are necessary Units In science, it is often not enough to say it s hot. Scientists want to communicate precisely how hot. To describe how hot it is you might say 82 degrees Fahrenheit (82 F). The value 82 F has two parts: a number (82) and a unit ( F). The number tells you how much and the unit tells you what the number means. One without the other can lead to big mistakes. If you say 82 degrees without specifying the unit, a person from Canada might think you mean 82 Celsius (82 C), which is a fatally hot 180 F, while 82 F is comfortable for the beach! To understand chemistry, you need to speak the language of units. Just about everything that can be measured has its own unit. Below are some of the ones that are most useful. TABLE 1.1. Some units in chemistry Kinds of units you use in chemistry Quantities have more than one unit SI units Definition of measurement Unit Used for Unit Used for Celsius degree ( C) temperature second (s) time Fahrenheit degree ( F) temperature meter (m) length kelvin (K) temperature centimeter (cm) length kilogram (kg) mass joule (J) energy gram (g) mass watt (W) power liter (L) volume pascal (P) pressure milliliter (ml) volume ampere (A) electric current mole (mole) counting atoms candela (cd) luminous (light) intensity Notice that there can be more than one unit for the same quantity. Your laboratory balance will measure mass in grams. A graduated cylinder measures volume in milliliters. The seven units highlighted in green on Table 1.1 is the international system (SI) of base units and it is the system of units used in this book. When you put a substance on a balance, you are making a measurement of its mass. A measurement is a specific kind of information that describes a physical quantity with both a number and a unit. The value grams is a measurement because it has a number (104.5) and a unit (grams) that describe a physical quantity. measurement: information that describes a physical quantity with both a number and a unit. A NATURAL APPROACH TO CHEMISTRY 7

5 Mass and weight Mass Weight The kilogram Equal masses means equal amounts of matter Mass is a measure of how much matter there is. A grain of salt has very little mass and a planet has a great amount of mass. Note that size (or volume) does not by itself tell you how much matter there is. A block of foam and a brick can be the same size, but one contains much more matter than the other. Don t be fooled by size. The definitive, reliable way to communicate a quantity of matter is by giving its mass. A digital balance uses weight to measure mass. Weight is the force of gravity. The balance senses the weight of whatever you place on it. Then it calculates the mass by dividing by the number 9.8, which is the acceleration due to gravity. Pounds or ounces are units of weight, not mass, in the English system of units. Mass is measured in kilograms (kg) and grams (g). One kilogram is about the mass of a one liter bottle of water you buy at the grocery store. One gram is about the mass of a single peanut. There are 1,000 grams in 1 kilogram and kilograms in 1 gram. When we say an object has a mass of one kilogram, we are saying that the object has the same amount of matter as one kilogram of water. Even if we do not know what the object is made of, we can measure how much matter it has. Air is very light, but a cubic meter of air at sea level has a mass of about 1 kg. Put your hand out of the window in a moving car and you can feel the effect of the mass of the air against your hand. mass: the amount of matter, measured in units of grams or kilograms (SI). weight: a force (push or a pull) that results from gravity acting on mass, measured in newtons in the SI system of units and pounds or ounces in the English system of units. kilogram: SI unit of mass. gram: unit of mass equal to 1/1,000 kg. 8 A NATURAL APPROACH TO CHEMISTRY

6 Volume The definition of volume The units of volume Volume is an amount of space having length, width, and height. A large object, like a box truck, takes up a lot of space and has a large volume. A tiny object, like a mouse, has a relatively tiny volume. In your chemistry lab, most of your measurements of volume will be in liters (L) or milliliters (ml). One liter is the volume of a cube 10 centimeters on a side. One milliliter is the volume of a cube one centimeter on a side. A milliliter is a fairly small volume; there are 5 ml in a standard teaspoon. The chart below shows several volume units and their equivalent in liters and milliliters. Volume units are derived from length units The graduated cylinder Fundamentally, the unit for volume comes from the unit for length. A centimeter (cm) is a unit of length. The volume of a cubic centimeter is 1 cm 1 cm 1 cm = 1 cm 3. A milliliter has the same volume as a cubic centimeter. One liter, the volume of a soda bottle, is the same as 10 cm 10 cm 10 cm = 1,000 cm 3. Empty a one liter bottle and it just fills a 10 centimeter cube. Cubic meters (m 3 ) are used for larger volumes. There are 1,000 liters in a cubic meter. The graduated cylinder is a tool for measuring the volume of liquids. Most graduated cylinders have markings that read in milliliters. The example shows a volume of 75 ml. Notice that you read the graduated cylinder at the flat (lowest) part of the surface. Water adheres slightly to glass and plastic, so the surface creeps up around the edges. This shape is called a meniscus. To read a graduated cylinder, you read the lowest point of the meniscus, not the highest point. volume: an amount of space having length, width, and height. liter: an SI unit of volume equal to a cube 10 centimeters on a side, or 1,000 cm 3. milliliter: a unit of volume equal to a cube 1 cm on a side, or 1 cm 3. graduated cylinder: an instrument used to measure volume. A NATURAL APPROACH TO CHEMISTRY 9

7 Density Equal size does not mean equal mass You come into contact with many kinds of matter every day. Some matter is solid and hard, such as iron and plastic. Some matter is liquid, such as water, and some is a gas, such as air. Even within the category of solid matter, there are big differences. Think about the differences among plastic, iron, and glass. Imagine you had a solid cube of each substance, and all the cubes were the same size and painted black. Does each contain the same amount of matter? Could you tell which was plastic, iron, or glass? Equal size does not mean equal mass Polethylene plastic Glass Iron Density describes the mass per unit volume The density of water and air A block of plastic and a block of steel may be the same size, but one has much more mass than the other. Because of the difference, plastic floats in water and iron sinks. Whether an object floats or sinks in water is related to its density. Density describes how much mass is in a given volume of a material. The units of density are mass divided by volume, often grams per cubic centimeter (g/cm 3 ). Iron has a high density; it contains 7.8 grams of mass per cubic centimeter (7.8 g/cm 3 ). A one centimeter cube of plastic contains only 0.94 grams of matter (0.94 g/cm 3 ). 1. Density is a property of matter that is independent of size or shape. 2. Density is mass per unit volume. Solids range in density from cork (0.12 g/cm 3 ) to platinum, a precious metal with a density of 21.5 g/cm 3. The density of water is about one gram per cubic centimeter. The density of air is much lower, about grams per cubic centimeter. density: a property of a substance that describes how much matter the substance contains per unit volume; typical units are grams per cubic centimeter (g/cm 3 ). 10 A NATURAL APPROACH TO CHEMISTRY

8 Doing calculations with density Measuring density To find the density of a material, you need to know the mass and volume of a solid sample of the material. Mass is measured with a balance or a scale. For simple shapes you can calculate the volume. For irregular objects, the displacement method is used to find the volume. Suppose you want to know the volume of five steel nuts. First you record the volume of water in the graduated cylinder before inserting the nuts. Then you gently drop the five nuts in and record the volume again. The volume of the nuts is the measured change in volume. Densities of common substances TABLE 1.2. Densities of common substances Material Density (g/cm 3 ) Material Density (g/cm 3 ) Platinum 21.5 Nylon plastic 2.3 Lead 11.3 Rubber 1.2 Iron 7.8 Liquid water 1.0 Titanium 4.5 Polyethylene plastic 0.94 Aluminum 2.7 Ice 0.92 Glass 2.7 Oak (wood) 0.60 Granite 2.6 Pine (wood) 0.44 Concrete 2.3 Cork 0.12 If 45 g of titanium are added to a graduated cylinder containing 50 ml of water, what will the cylinder read after the titanium has been added? Before going on, discuss with your classmates what is asked and what is given. Asked: Volume reading after adding 45 g of titanium Given: 45 g of titanium, density of titanium d = 4.5 g/cm 3, 50 ml of water Relationships: d = m/v Solve: d = m V; therefore V = m d V = 45 g 4.5 g/cm ml / cm 3 = 10 ml. The titanium adds 10 ml to the cylinder, which now reads 60 ml. Answer: 60 ml Discussion: This is an example of measurement using the displacement method. A NATURAL APPROACH TO CHEMISTRY 11

9 Pressure Force and fluids Force Think about what happens when you pump a bicycle tire. As you push down on the pump, you squeeze air into the tire. The tire expands and gets firm. The firmness of the inflated tire is an example of pressure. Pressure is a force per unit area exerted by matter, often when it is restrained from moving or expanding. For example, suppose the bicycle tire is inflated to a pressure of 60 pounds per square inch (60 psi). Each square inch of the inside of the tire feels a force of 60 pounds from the trapped air in the tire. A force is an action, like a push or a pull, that has the ability to make things move (or stop them). In the SI system, force is measured in newtons. However, not many people use newtons in ordinary life. It takes about four and a half newtons to make a pound (4.448 N = 1 lb). In chemistry, we are not so interested in forces; we are more interested in the ratio of force per unit area, which is pressure. Pressure is most important in the understanding of gases (such as air) or liquids (such as water). For example, consider the water in a hose. If there is pressure in the water, the pressure exerts forces in all directions on any surface touching the water. You can prove this is true by poking a hole in the hose. The water squirts out no matter whether you poke the hole in the top, bottom, or side of the hose. Pressure in a liquid or gas pushes equally in all directions You know that liquids flow to take the shape of a container. This happens because liquids have weight. Weight pulls down, but in a liquid or gas, the downward force of gravity adds to the pressure, which pushes equally in all directions. Pressure pushes the boundary of a liquid outward until it fills its container in the precise shape that gets the most liquid closest to the ground. pressure: force per unit area with units of Pa (N/m 2 ) or psi (lb/in 2 ) or atm; acts equally in all directions within a liquid or a gas. force: an action, such as a push or a pull, that has the ability to change the motion of an object, such as to start it moving, stop it, or turn it. 12 A NATURAL APPROACH TO CHEMISTRY

10 Air pressure Is an empty bottle really empty? Air is matter The weight of air creates air pressure Demonstrating air pressure Units of pressure Hold up an empty plastic water bottle and think about what is inside. What comes to mind? Nothing? Is an empty bottle really full of nothing? To answer the question, consider what happens when you put the cap tightly on the empty bottle and try to squeeze it flat. You can t do it something in the empty bottle prevents you from squeezing it flat. If you open the cap, you can squeeze the bottle easily. Whatever is inside an empty bottle comes out if the cap is off, but it is trapped inside if the cap is on. Air is matter! Air is transparent, and its mass is spread so thin that you move through it easily. Put your hand out a moving car window and you can instantly feel that air has mass because it takes force to push it out of the way. In fact, a cubic meter of air has a mass of about one kilogram, or about the same as a one-liter bottle of water. Air has weight. The weight of the atmosphere creates pressure that acts equally in all directions. A board with an area of one square meter feels a force from air pressure of 101,325 newtons on one side. That is an incredible 22,780 pounds! The board doesn t move because air pressure acts equally in all directions, so the other side of the board feels an equal force in the opposite direction. The board doesn t move, but it s not because air is nothing! The board doesn t move because the air pressure is equal on all sides. To demonstrate air pressure, watch what happens when you suck the air out of a bottle. Why does the bottle collapse? You did not apply any force to it. The bottle collapses because removing some air from the inside makes the pressure inside lower than the pressure outside. The pressure of the outside air is what crushes the bottle. If you have a force of 1 newton distributed over an area of 1 square meter, you have a pressure of 1 pascal (Pa). The pascal is the SI unit of pressure, but it is very small. Many chemistry applications use atmospheres as a unit of pressure. One atmosphere (1 atm) is 101,235 Pa, and that is the average pressure of the air at sea level. pascal (Pa): SI unit of pressure, equal to one newton of force per square meter of area (Pa = 1 N/m 2 ). atmosphere (atm): a large unit of pressure equal to 101,325 Pa; the average pressure of air at sea level; also equal to 14.7 pounds per square inch (14.7 psi). A NATURAL APPROACH TO CHEMISTRY 13

11 Accuracy and precision Precision Accuracy Measurements are never perfect Why accuracy and precision are important When we take a number of measurements, precision tells us how close the measured values are to each other. A balance that can read to 0.1 grams has a precision of 0.1 g. Many of your lab experiments will be done with balances that have this precision. The word accuracy tells us how close a measurement is to the true value. For example, a meter stick that has been stretched can make a measurement of length that is precise to one millimeter. However, measurement will not be accurate because the meter stick is no longer one meter long. Measurements of real quantities in experiments are never exact. For example, you cannot determine that something has a mass of exactly 10 g. Why not? Because measurements of mass are made with instruments, such as balances. All real instruments have a limit to how small a quantity they can measure. Suppose you have an accurate balance that can measure mass to a precision of 0.1 g. You claim the mass is exactly 10.0 g because your balance shows However, suppose the real mass was g. Your balance rounded the measurement off to One way to show this is to write your measurement as 10.0 ± 0.1 g. This tells a reader that the actual mass could have been anything between 9.9 and 10.1 g. For many of your lab experiments, two masses of g and g can be considered the same because they differ by only 0.02 g. This is smaller than the balance can measure. When analyzing the observations you make in the lab, you must consider both the accuracy and the precision of your measurements before making a conclusion. A measurement that is not accurate may lead you to the wrong conclusion. With a measurement that is not precise you may not be able to tell the difference between agreement and disagreement. accuracy: a term that describes how close a measurement is to the true value. precision: a term that describes how close measured values are to each other. 14 A NATURAL APPROACH TO CHEMISTRY

12 Significant figures How many decimal places do you record? Significant figures tell the precision of a measurement Certain rules must be followed when calculating with significant figures Suppose you measure 10.0 g of sugar on a balance. The balance has a precision of 0.1 g. In your data table do you write down 10 or 10.0 g? Does it make a difference? The answer is yes, it does! Scientists use the rule of significant figures as a way of recording data that tells the precision of a measurement without writing ± after each value. The rule is that the last written digit is uncertain by plus or minus the value of the uncertainty. For example, suppose you write the measurement as 10 g. According to the rule of significant figures, you can trust the first digit (1) and assume the second digit (0) is plus or minus one, or ± 1 g. The actual mass of sugar could have been anything between 9 and 11 g. The uncertainty is of 1 out of 10, or 10% in the measurement! If you write it as 10.0 g, it now has three significant figures. You can trust the first two digits (10) and the uncertainty is only plus or minus one tenth, or ± 0.1. The uncertainty is now 1 out of 100, or 1%. In numbers that do not contain zeros, all digits are significant. When zeros are contained in a number, it is significant if it is between other digits or at the end to the right of a decimal point. Leading zeros are not significant. When rounding, if the first digit to the right of the last significant figure is less than 5, that digit and all others following are dropped. If that digit is greater than 5, the last signficant figure is increased by one and the rest of the digits dropped. When adding or subtracting, the answer should have the same number of decimal places as the quantity with the smallest number of decimal places. When multiplying or dividing, the answer should have the same number of significant figures as the quantity with the smallest number of significant figures.. What value should be recorded for the volume measurement in the picture? Asked: The value with the correct number of significant figures Given: You can estimate to a tenth of the graduation of a cylinder or ruler. Relationships: The last digit on the right is assumed to be ± 0.1. Solve: The meniscus is right on 18, so estimate 18.0 ml. Answer: 18.0 ml Discussion: The real value is confidently known to be between 17.9 and 18.1 ml. significant figures: a way of writing data that tells the reader how precise a measurement is. A NATURAL APPROACH TO CHEMISTRY 15

13 Very large numbers Describing large and small quantities Scientific notation for large numbers Atoms are so small that describing them requires extreme numbers. A single grain of sand contains 200,000 million million atoms. This number written out the usual way is 200,000 million million = 200,000,000,000,000,000 Don t worry if you cannot get your mind around this huge number no one can without a reference. Fortunately, there is a shorthand method for writing and calculating with extremely large or small numbers. The method is called scientific notation. Scientific notation works by expressing very small or very large numbers as the product of two smaller numbers. The first number is called the mantissa. The second number is a power of 10. Any number can be represented as a mantissa times a power of 10. As an example consider the number 1,500: 1,500 = The number 15 is the mantissa. The number 100 is the power of = 10 squared and is usually written as So 1,500 = The small superscript number 2 in 10 2 is called the exponent. Writing the base The mantissa is usually written with only one digit in front of the decimal point. For example, 1,500 would be written because 10 3 is 1,000. To make 1,500 into 1.5, you have to move the decimal point by three places so the correct power of 10 is For the number 1,500, scientific notation does not seem to be very beneficial. However, 200,000 million million in scientific notation is This number is much easier to write and calculate with when expressed in scientific notation. The number is very important in chemistry and it is very nicely presented in scientific notation. Convert 34,500 to scientific notation. Asked: The number in scientific notation Given: 34,500 as a decimal number Relationships: 10,000 = 10 4 Answer: scientific notation: a method of writing numbers as a base times a power of 10. mantissa: a decimal number that multiplies the power of 10 in scientific notation. exponent: the power of 10 for a number written in scientific notation, such as the 2 in the number = A NATURAL APPROACH TO CHEMISTRY

14 Very small numbers Chemistry may use very small numbers Scientific notation for small numbers Many of the important ideas in chemistry involve what happens on the size of individual atoms, the particles of matter. Atoms are extraordinarily small. A grain of sand is only 0.01 centimeters across. A single atom is a million times smaller, or centimeters across. Fortunately, scientific notation allows us to work with extremely small numbers as easily as with large numbers. Powers of 10 that are negative mean numbers smaller than one. Consider the number Let s write it in scientific notation: = The number is 1 1,000 = = = in scientific notation It is important to remember that a negative sign on the exponent of 10 does not mean the whole number is negative! Negative exponents just mean that the value is less than one. Convert to scientific notation. Asked: The number in scientific notation Given: as a decimal number Relationships: = 10 4 Answer: Using a calculator Scientific calculators can work with numbers in scientific notation, if you know how to enter them! On the specific calculator in the diagram, you use the following keystrokes to enter the number The keystrokes may be different on other calculators; however, the key labeled EE is usually the one that allows you to enter the exponent. A NATURAL APPROACH TO CHEMISTRY 17

15 Converting between units Two measurements can only be compared if both are in the same units Suppose you drive to Mexico and gas at one filling station costs $1.25 per liter. Across the border in Texas, gas is $4.05 per gallon. Which is less expensive? The answer is not obvious. Think of the different systems of units as different languages. To compare two measurements, both need to be in the same units. That means you must translate between the language of dollars per liter and that of dollars per gallon. To translate Spanish to English you need a Spanish/English dictionary. To translate between units you need conversion factors. Conversion factors are ratios of two units. An example is that one gallon = L. To convert the gas prices, we use the conversion factor. Convert $1.25 / L to dollars per gallon. Asked: A value in dollars per gallon Given: $1.25/L Relationships: 1 gallon = L Solve: $ L L 1 gallon = $4.73 gallon Answer: $4.73/gallon Discussion: $1.25/L is more expensive than $4.05/gallon. Converting from one unit to another To do a conversion, arrange the conversion factors as ratios and multiply them so that unwanted units cancel out. You should be left with only the units you want. There are many conversion factors on the inside back cover of this book. Another word for units is dimensions. The process of converting between units with conversion factors is also called dimensional analysis. Dimensional analysis is a very powerful tool and it is used extensively by scientists and engineers. It is also a very useful tool in our daily lives. conversion factor: a ratio of two different units that has a value of 1. dimensional analysis: technique of using conversion factors to convert between units. 18 A NATURAL APPROACH TO CHEMISTRY