Lecture 16 Introduction to Gas Laws

# Lecture 16 Introduction to Gas Laws - Introduction to Gas...

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Introduction to Gas Laws B. A. Rowland 53750/53760 Phases of Matter There are three distinct phases of matter that we can observe on the Earth: solids, liquids, and gases. There are also two others: plasmas (ionized matter—very hot!) and Bose-Einstein condensates (when all atoms are behaving as an entire quantum system—very cold!). We will only focus on gases (this lecture) and solids and liquids (later in the course—these are known as the condensed phases of matter). Every bit of matter known has a preferred state of matter at a given temperature and pressure. You can change the state of matter for a substance by changing the temperature and/or the pressure (think condensation, evaporation, sublimation, etc). Solids are regularly ordered and have high intermolecular forces, liquids are semi-randomly ordered and have moderate intermolecular forces, and gases are completely chaotic and for some cases the intermolecular forces between particles can be ignored. Since gases are the focus of study, it is important that we define the variables we will be using to characterize gases: p is the pressure (arises from the collisions of gas molecules with the walls of the container), V is the volume (the amount of space occupied by the gas), n is the number of moles (how much gas you have), and finally T is the temperature (a measure of the average kinetic energy of the particles comprising the gas). Important Dates in the History of Gases Ancient Greece: Air is theorized to be one of the four fundamental elements. Van Helmont: discovers carbon dioxide through a chemical reaction. Finds that although carbon dioxide does behave like “air”, it has some very different properties as well (early 1600s). Torricelli: designed the first barometer, a pressure measuring device, circa 1643. Von Guericke: invents the vacuum pump, circa 1654. Boyle: first to quantitatively study gases, proposes Boyle’s law (late 1600s). Lavoisier, Priestly, and Scheele: co-discoverers of oxygen, 1770s. Charles: first solo balloon flight (he used hydrogen) and Charles’ Law, 1787. Gay-Lussac: Proposes Gay-Lussac’s law, 1802. Von Humboldt: Composition of atmosphere is uniform, 1805. Avogadro: Proposes Avogadro’s number, in 1811. Pressure Measurement Pressure in physics is defined as force per unit area. Mathematically, that is p = F/A , where F is

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given in terms of Newtons (N). In a gas, the force is the force imparted via the collisions of the particles comprising the gas and the walls of the container in which it is held. The SI unit of pressure is the Pascal (Pa) and this is defined as 1 Pa = kg m -1 s -2 . There are other units of pressure you should be aware of, and here are their conversions: 1 atm = 760 mm Hg = 760 torr = 101,325 Pa and 1 bar = 10 5 Pa Volume Volume is the amount of space occupied by a gas. It is usually given in some variant of the Liter (cL, kL, mL, L, etc). However, these are not SI units. The SI unit of volume is the cubic meter (m 3 ). To convert a volume from a unit expressed in a L-variant to one which is SI (cubic length), you must remember this relationship: 1 L = 1 dm 3 .
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