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Ch. 12 Behavior of Gases

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Gases Gases expand to fill its container, unlike solids or liquids Easily compressible: measure of how much the volume of matter decreases under pressure

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Variables that describe a gas Pressure (P) – Measured in kilopascals, kPa – Pressure and number of molecules are directly related increase molecules = increase pressure – Gases naturally move from areas of high pressure to low pressure, due to the available space to move into

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Variables that describe a gas Volume (V) – Measured in Liters, L – Volume and pressure are inversely related As volume decreases, the pressure increases Smaller container = less room for movement, therefore molecules hit sides of container more often

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Variables that describe a gas Temperature (T) – Measured in Kelvin, K – The temperature and pressure are directly related Increase in temp = increase in pressure Volume must be held constant Molecules hit the walls harder (due to increase in K.E.) and more frequently. Think about a tire in hot weather…

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Variables that describe a gas Amount – Measured in moles, mol – Moles and pressure are directly related Increase in # of moles = increase in pressure Ex: Inflating a balloon is adding more molecules. Temperature must remain constant

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Gas Laws Describe how gases behave Change can be calculated Know the math and the theory!!

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Boyles Law (1662) Gas pressure is inversely related to volume (as volume increases, pressure decreases) Temperature is constant P 1 V 1 = P 2 V 2

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Ex: The pressure of a 2.5L of gas changes from 105 kPa to 40.5 kPa. What will be the new volume?

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Charless Law (1787)

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Ex: A sample of Nitrogen occupies a volume of 250 mL at 25 o C. What volume will the gas occupy at 95 o C?

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Gay-Lussacs Law (1802)

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Ex: A gas has a pressure of 710 kPa at 227 o C. What will the pressure be at 27 o C, if the volume does not change?

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Combined Gas Law

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Ex: 3.0 L of Hydrogen gas has a pressure of 1.5 atm at 20 o C. What would the volume be if the pressure increased to 2.5 atm at 30 o C?

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Ideal Gas Law

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A gas behaves ideally if it conforms to the gas laws – Gases do not usually do this – Real gases only behave this way at: 1.High temps (molecules move fast) 2.Low pressure (molecules are far apart) This is because gases will stay a gas under these conditions – Molecules are not next to each other very long so attractive forces cant play a role b/c molecules are moving too fast – Ideal Gases do no exist because: 1.Molecules do take up space 2.There are attractive forces between molecules otherwise no liquid would form. (Molecules slow down to become liquids)

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Ex: What volume will 2.0 mol of N 2 occupy at 720 torr and 20 o C?

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Daltons Law of Partial Pressures Used for mixture of gases in a container If you know the P exerted by each gas in a mixture, you can calculate the total gas pressure It is particularly useful in calculating pressure of gases collected over water. P total = P 1 + P 2 + P 3 … *P1 represents the partial pressure or the contribution by the gas

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Ex: Helium, Nitrogen, and Oxygen exist in a container. Calculate the total pressure of the mixture for the following partial pressures: He = 200 kPa N= 500 kPa O= 400 kPa

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Grahams Law of Effusion

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