LIVING BY CHEMISTRY Unit 3: WEATHER Phase Changes and Behavior of Gases In this unit you will learn: • • • • • about proportional relationships about temperature scales and how thermometers work the effects of changing temperature, pressure, and volume on matter about the behavior of gases how to read weather maps and make weather predictions Section III: Concentrating Matter •Lesson 15 n is for Number •Lesson 16 STP •Lesson 17 Take a Breath •Lesson 18 Feeling Humid •Lesson 19 Hurricane! •Lesson 20 Stormy Weather Lesson 15: n is for Number •Pressure and Number Density ChemCatalyst •Look at the diagram of the Earth’s atmosphere. •Compare the atmosphere at sea level and at 34,000 ft, the altitude at which airplanes fly. 1. Describe at least three differences. 2. Explain why it is difficult to breathe at 34,000 ft. Key Question •How is the number of gas molecules in a sample related to pressure? You will be able to: • • • • define the number density of a gas describe the number density of the atmosphere as it relates to altitude explain the relationship between number density and gas pressure describe one way to measure gas pressure Prepare for the Lab •Work in groups of four. •Wear safety goggles at all times during the lab. Discussion Notes •The gas pressure increases as the number of gas molecules per unit of volume increases. n • P = k(V) •Number density: The number of gas particles per unit volume. Number density = n/V •The height of the water levels in a U-tube indicates differences in pressure. Discussion Notes (cont.) •In order to make the water levels uneven, it is necessary to seal off one end with a finger, a syringe, a cork, or a balloon. Discussion Notes (cont.) •Air pressure can be determined by measuring the difference in height of a liquid. Wrap Up How is the number of gas molecules in a sample related to pressure? • The composition of Earth’s atmosphere is not uniform. The density of the gas molecules in the air decreases with increasing altitude. This causes the pressure of the atmosphere to decrease with increasing altitude. • The number density of a gas is the number of gas molecules per unit of volume, n/V. • The pressure of a gas is directly proportional to the number of gas molecules per unit of volume. This relationship is written as P = k·(n/V). Check-in •A balloon is filled with helium, tied off, and then released. As it climbs into the air, its volume slowly increases. •Explain what is going on with the helium atoms inside the balloon and the air molecules outside the balloon in terms of number density and pressure. Lesson 16: STP •The Mole and Avogadro’s Law ChemCatalyst •There are two balloons. One is filled with helium, He, and the other with carbon dioxide, CO2. 1. Describe what happens when the balloons are released. 2. For the two balloons, state whether these properties are the same or different, and explain why: • pressure, P • number density, n/V • temperature, T • number, n • volume, V • mass, m • density, m/V Key Question •How do chemists keep track of the number of gas particles? You will be able to: • • • define a mole explain Avogadro’s law define standard temperature and pressure Prepare for the Activity •Work individually. •Chemists use a unit called a mole to describe the number of gas particles in a sample. •1 mole = 602,000,000,000,000,000,000,000 Discussion Notes •Standard temperature and pressure, STP: One atmosphere of pressure and a temperature of 273 Kelvin. •Equal volumes of gases contain equal numbers of gas particles if the temperature and pressure are the same. Discussion Notes (cont.) •There are exactly 602,000,000,000,000,000,000,000 particles in 22.4 L at STP. •Mole: A unit invented by chemists to count large numbers of gas particles. There are 602,000,000,000,000,000,000,000 particles in 1 mole. This is 602 sextillion. Wrap Up How do chemists keep track of the number of gas particles? • Avogadro’s law states that equal volumes of gases contain the same number of particles if they are at the same temperature and pressure. This holds true for all gases. • Gases are often compared at a standard temperature and pressure of 1 atm and 273 K. This is also referred to as STP. • At STP, any gas will occupy 22.4 L and consist of exactly 602 sextillion, or 602,000,000,000,000,000,000,000, particles. Check-in •One balloon contains 22.4 L of Ar, argon gas, and another balloon contains 22.4 L of Ne, neon gas. Both balloons are at 273 K and 1 atm. 1. Do the balloons contain the same number of atoms? Why or why not? 2. Will the balloons have the same mass? Why or why not? Lesson 17: Take a Breath •Ideal Gas Law ChemCatalyst 1. 2. 3. Describe how you can determine the volume of a breath of air. Name four factors that might affect the volume you measure. What do you need to know in order to determine the number of molecules in a breath of air? Key Question •How can you calculate the number of moles of a gas if you know P, V, and T? You will be able to: • • • define the ideal gas law define the universal gas constant, R complete calculations for finding n, using the ideal gas law Prepare for the Lab •Work in groups of four. •Wear safety goggles. Prepare for the Lab (cont.) •The equation for the ideal gas law is • PV = nRT •where R is equivalent to the proportionality constant, k, for this equation. • R= R = 0.082 L · atm/mol · K PV nT Discussion Notes •We should see differences in the volume of one breath of air from group to group. •There is more than one way to figure out the volume of air that was exhaled into the bottle. Discussion Notes (cont.) •The ideal gas law allows scientists to relate gas pressure, volume, moles of particles, and temperature. •Ideal Gas Law: The ideal gas law states that PV = nRT, where R, the universal gas constant, is equivalent to the proportionality constant, k, for this equation. • R = PV R = 0.082 L · atm/mol · K nT Discussion Notes (cont.) •Note that R is the same for all gases but the value of R does change depending on if the units change. •The number of moles can be converted to the total number of gas molecules by multiplying by 602 sextillion. •The ideal gas law can be used to solve for other variables besides n. Wrap Up •How can you calculate the number of moles of a gas if you know P, V, and T? • The ideal gas law relates volume, pressure, temperature, and the number of moles of a gas sample: PV = nRT, where R = 0.082 L · atm/mol · K. • R is a number that relates all the different units to one another in the ideal gas law. Its value does not change if the units don’t change. R is called the universal gas constant. Wrap Up (cont.) • The ideal gas law can be used to figure out P, V, n, or T when the other three variables are known. Check-in •You cap a 1.0 L plastic bottle on a mountaintop where the air pressure is 0.50 atm and the temperature is 298 K. 1. How many moles of gas are in the bottle? 2. What is the number density, n/V, of the gas inside the bottle on the mountaintop? 3. At sea level, the volume of the bottle becomes 0.50 L. What is the number density of the gas inside the bottle at sea level? Lesson 18: Feeling Humid •Humidity, Condensation ChemCatalyst 1. 2. Is there water vapor in the air right now? What evidence do you have to support your answer? What do you think humidity means? How does humidity depend on temperature? Key Question •What is humidity and how is it measured? You will be able to: • • • define humidity and relative humidity explain the relationship between humidity and phenomena such as cloud formation, fog, rainfall, and dew explain the relationship between water vapor density and air temperature Prepare for the Lab •Work in groups of four. •Humidity: The density of the water vapor in the air at any given time. Humidity is dependent on air temperature and pressure. Discussion Notes •Humidity is a measure of the amount of water vapor in the air. •The condensation procedure provides evidence that water vapor is present in the air. •The temperature at which water vapor condenses indicates how much water vapor is in the air. Discussion Notes (cont.) •Water Vapor Density Versus Temperature Discussion Notes (cont.) •There is an upper limit to the amount of water vapor that can be present in the atmosphere at a given temperature. •Humidity is sometimes expressed as relative humidity. •Relative humidity: The amount of water vapor in the air compared to the maximum amount of water vapor possible for a specific temperature, expressed as a percent. Wrap Up •What is humidity and how is it measured? • Humidity is a measure of the amount of water vapor in the air. It can be expressed as water vapor density or as relative humidity. • Water vapor density is affected by both air temperature and air pressure. • There is a limit to the amount of water vapor that can be present in the air at a given temperature. The maximum amount possible is called 100% relative humidity. Check-in •On a hot summer day, a firefighter records a dry-bulb temperature of 30°C and a wet-bulb temperature of 12 °C. What does this tell you about the relative humidity? Lesson 19: Hurricane! •Extreme Physical Change ChemCatalyst 1. 2. What is a hurricane? What characteristics does it have? Where do hurricanes form? What can you tell about a hurricane from the satellite image? Key Question •What are hurricanes and what causes them? You will be able to: • • • describe the meteorological conditions that result in a hurricane explain the role of phase change, air pressure, and temperature in hurricane formation define climate and global warming Prepare for the Activity •Work individually. Discussion Notes •Hurricanes are destructive storms characterized by strong winds and large amounts of rainfall. •Tropical depressions can build to tropical storms, which can build to hurricanes. •There are five categories of hurricane, with category 1 the least intense and category 5 the most intense. Discussion Notes (cont.) •Anatomy of a Hurricane Discussion Notes (cont.) •Tropical storms begin forming when a great deal of warm water evaporates into the atmosphere. •As the storm moves over areas of warmer water, evaporation increases. •Water vapor density is related to temperature. Discussion Notes (cont.) •Water Vapor Density Versus Temperature Discussion Notes (cont.) •Global Temperature Changes (1880-2000) Discussion Notes (cont.) •There is a great deal of scientific evidence supporting the idea that our planet is in a warming cycle, often referred to as global warming. •Global warming is causing climate change. Wrap Up •What are hurricanes and what causes them? • Hurricanes are intense tropical weather systems accompanied by strong winds, massive amounts of rain, and ocean flooding. They form around low-pressure systems and are characterized by spiraling clouds and winds. • Hurricanes form over the ocean in places where there is extremely warm, moist air. Wrap Up (cont.) • Rapid evaporation and subsequent condensation of moisture set up an air pressure differential that further feeds the evaporation and condensation. • At higher temperatures, small changes in temperature have dramatic effects on water vapor density, n/V. Global warming may increase the frequency and intensity of hurricanes on the planet. Check-in •Why do most hurricanes have their origins near the equator? Lesson 20: Stormy Weather •Unit Review ChemCatalyst 1. 2. Describe three different physical changes involved in weather. What physical changes of matter affect the weather? Key Question •What does chemistry have to do with weather? Prepare for the Activity •Work in pairs. Discussion Notes •Phase changes are a form of physical change. •It is necessary to understand the behavior of gases of you want to understand the weather. •The temperature of a gas must be converted to the Kelvin scale in order to utilize the gas laws. Discussion Notes (cont.) •Gas Laws V T •Charles Law k = Gay-Lussac’s law k = P T •Boyle’s Law k = PV Ideal gas law PV = nRT • or R = PV PV nT •Combined gas law k = T •Avogadro’s Law k = n V Discussion Notes (cont.) •The ideal gas law allows you to figure out problems that involve changes in the number of gas particles, n.