# Gases

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 Author: hellosleepy ID: 158834 Filename: Gases Updated: 2013-04-13 15:47:18 Tags: MCAT Folders: Description: MCAT General Chemistry Show Answers:

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1. STP conditions
An ideal gas at STP occupies 22.4L at 1atm and 273K/0C
2. Gas mixtures
• Gases can mix with one another regardless of polarity differences but in lower temperatures, denser gases settle below less dense gases
• This is why hot air rises – because it is less dense than cold air
3. Dalton's Law of Partial Pressures
The amount of pressure contributed by a gas in a mixture is:

Xg*Ptot = Pg
4. Mole fraction
Moles of a substance/total moles of the compound
5. Graham's Law
• Effusion - Spreading of one gas from a high to low pressure through a pinhole
• Thegas with the higher velocity will be more prone to finding the pin hole because it makes more collisions

• Diffusion
• The spreading of one gas into another or into empty space
• The product will form closer to the gas with the slower diffusion rate

6. What influences the rate constant of a reaction?
• 1. Catalyst presence
• 2. Temperature
7. What influences reaction rates?
• 1. Temperature
• 2. [reactants]

Do not confuse reaction rates with Keq
8. RDS if not first step
If the first step is a fast step, then use the intermediate product of the first fast step (the product) to contribute to the rate law along with the slow step!

Use the equilibrium constant for any intermediates because we can assume that the intermediate reaches and stays at equilibrium
9. Effect of solvent on reaction rate
Solvent-reactant bonds must first be broken in order for a reaction to take place
10. What is the rate of a reaction at Keq?
Zero...because reactants and products are being produced at the same rate
11. Keq

• Point of greatest entropy
• DO NOT USE PURE LIQUIDS OR SOLIDS IN EQ.
12. The reaction quotient
If Q > 1, then reaction will move towards reactant side

If Q<1, then reaction will move to products side
13. Le Chatlier's Stresses
• 1. Addition or removal of a product or reactant
• 2. Temperature changes
• 3. Pressure changes (but not if nonreactive gas like He is added)
14. Haber process
• All gas, exothermic reaction
• Raising heat will push the reaction to the left, toward the reactants side
15. Chracteristics of an ideal gas
• 1. Molecules have ZERO VOLUME
• 2. Molecules have ZERO ATTRACTION to one another
• 3. Energy is conserved in collisions because molecules collide elastically with one another and walls of the container
• 4. KEavg = 3/2RT
16. Real vs Ideal Gases
Vreal > Videal

Ideal gas molecules are thought to have no volume while real gas molecules obviously have volume

Preal < Pideal

Attraction between real gas molecules, causing them to slow down before they hit the walls of the container and exert less force/area

The closer together the gas molecules are, more apparent deviations from ideal situations become

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