AP Chemistry Start this over break? Due Next Class: Determining Gas Law Constant Pre-lab & Read your lab! Don’t Be a wet Blanket! Upcoming Due Dates: Ch. 10 MC 11/29 AF 12/3 Determination of the Gas Law Constant Due 12/6 Ch. 11 MC 12/8 AF 12/14 Chromo Lab 12/14 Penny 12/18 Chapter 10-13 Test 12/20

Which gas below has the smallest density at STP? Ar Kr Ne Xe Answer: c

Lecture 10.6-10.9

Dalton’s Law of Partial Pressures If two gases that don’t react are combined in a container, they act as if they are alone in the container. The total pressure is the sum of the pressures that each would exert if it were present alone. In other words, Ptotal = p1 + p2 + p3 + …

Mole Fraction Relating partial pressures to mole ratios… That ratio of moles of a substance to total moles is called the mole fraction, χ.

The partial pressure of each gas in a mixture of gases is proportional to the _______ of the gas. mass molecular weight square root of the molecular weight mole fraction Answer: d

Kinetic-Molecular Theory Laws tell us what happens in nature. Each of the gas laws we have discussed tell us what is observed under certain conditions. Why are these laws observed? We will discuss a theory to explain our observations.

Main Tenets of KMT Gases consist of large numbers of molecules that are in continuous, random motion. The combined volume of all the molecules of the gas is negligible relative to the total volume in which the gas is contained. Attractive and repulsive forces between gas molecules are negligible.

Energy can be transferred between molecules during collisions, but the average kinetic energy of the molecules does not change with time, as long as the temperature of the gas remains constant. The average kinetic energy of the molecules is proportional to the absolute temperature.

How Fast Do Gas Molecules Move? Temperature is related to their average kinetic. Individual molecules can have different speeds of motion. Three different speeds: ump is the most probable speed (most molecules are this fast). uav is the average speed of the molecules. urms, the root-mean-square speed, is the one associated with their average kinetic energy. (this one is associated with temperature too!)

urms and Molecular Mass At any given temperature, the average kinetic energy of molecules is the same. If KE = ½ mv2 is the same for two gases at the same temperature. A gas that has a low mass, has a greater velocity than for a heavier molecule.

Effusion vs. Diffusion Effusion is the escape of gas molecules through a tiny hole into an evacuated space. Diffusion is the spread of one substance throughout a space or a second substance.

Graham’s Law Describes Diffusion & Effusion Graham’s Law relates the molar mass of two gases to their rate of speed of travel. The “lighter” gas always has a faster rate of speed.

Effusion in the real world https://youtu.be/KHsotHcTLSw

Neon gas undergoes effusion _______ krypton gas does. slower than at the same rate as twice as fast as four times as fast as Answer: c

The mean free path is the total distance traveled by a gas molecule. the distance between two gas molecules. the distance between a gas molecule and the container wall. the average distance traveled between molecular collisions. Answer: d

Real Gases In the real world, the behavior of gases only conforms to the ideal-gas equation at relatively high temperature and low pressure. Even the same gas will show wildly different behavior under high pressure at different temperatures.

Deviations from Ideal Behavior The “ideal” assumptions break down at high P and/or low T. Aka- when volume approaches 0

Corrections for “Nonideal” van der Waals equation The pressure adjustment is due to the fact that molecules attract and repel each other, a. The volume adjustment is due to the fact that molecules occupy some space on their own, b.

practice

Water is decomposed by electrolysis and the products are collected separately. If 1 L of H2 is produced, ___ L of O2 will be produced. ½ 1 2 some other amount Answer: a

A gas is most likely to exhibit ideal behavior at (X) temperature and (Y) pressure. X = low, Y = low X = low, Y = high X = high, Y = low X = high, Y = high Answer: c

According to its van der Waals constants, which gas below best approximates ideal behavior? Ne Ar Kr Xe Answer: a

Which gas deviates most from ideal behavior? Cl2 CO2 NH3 H2 Answer: a

Estimate the fraction of molecules at 100 °C with speeds less than 300 m/s. A)1/10 B) 1/3 C)1/2 D) 2/3

Which of these gases has the largest molar mass? Which has the smallest? a. O2; H2 b. H2; O2

Because pressure and temperature are constant in this figure but volume changes, which other quantity in the ideal-gas equation must also change? R n Answer: b

True or false: Nitrogen gas behaves more like an ideal gas as the temperature increases. b. False

Increase Decrease c. No change How would you expect the pressure of a gas to change if suddenly the intermolecular forces were repulsive rather than attractive? Increase Decrease c. No change Answer: a