Gases Flashcards
What is gas pressure
The force exerted by gas molecules colliding with the surface of objects.
P is the force (F) per unit area (A) $P=F/A$
Unit of Force = Newtons (N)
Unit of Area = meters squared (m$^2$)
Unit of Pressure = Pascals (N/m$^2$)
what is a Manometer
A Manometer is an instrument that measures the pressure of a gas sample, where the gas pushes a liquid like mercury up a U shaped tube with a vacuum on the other side.
The volume (V) of liquid in the tube is just related to the height of the column (h) and the cross sectional area of the tube (A):
V=Ah
F=dAhg
P=dAhg/A=dhg
so the height (h) of a known liquid is a direct measurement of the gas pressure
How does atmospheric pressure work
Gravity causes the atmosphere to press down on the earth’s surface, creating atmospheric pressure.
As you get higher in the atmosphere the pressure decreases.
What are the 5 spheres
- Troposphere
- Stratosphere
- Mesosphere
- Thermosphere
- Exosphere
how does the temperature change through the 5 spheres of earths atmosphere
Temperature drops during the Troposphere, increases in the Stratosphere, drops in the mesosphere, then increases in the thermosphere, and remains relatively constant in the exosphere.
what is atmospheric pressure
Atmospheric pressure at sea level = 1 atm = 760 Torr = 760 mmHg = 101,325 Pa = 101,325 N/m$^2$
what is a barometer
The barometer is an instrument that measures the atmospheric pressure
how does a barometer work
Gas from the atmosphere presses down on the liquid, pushing it up a vacuum tube.
The height of the column (h) depends on the pressure (p) and the density (d) of the liquid.
P=dhg
760mm Hg = 10774 mm H2O
Atmospheric pressure at the top of mount Everest = 38,200 Pa = 286 mm Hg
what are the properties of gases
We need to learn about the behaviour of gases to have a better understanding of our planet’s atmosphere.
A gas can be described through 4 properties:
- P → Pressure
- V → Volume
- T → Temperature
- n → amount
how is V related to n, T and P, and who showed each
V increases with n for constant P and T, proved by Avogadro’s experiment
V increases with T for constant n and P, proved by charles’ experiment
V decreases with P for constant n and T, proved by boyle’s experiment
what is ideal gas law and its two assumptions
PV=nRT
Pressure * Volume correlates to Moles * temperature
Assume that the volume of each molecule is negligible
Assume that there is no interactions between molecules (other than elastic collisions)
Under these two conditions, ideal gas law applies to any gas!
What are Boyle’s, Charle’s, and Avogadro’s law
At constant n and T, P is inversely proportional to V (Boyle’s Law)
At constant n and P, V is proportional to T (Charles Law)
At constant P and T, V is proportional to n (Avogadro’s Law)
what is the volume of a gas at STP
The volume of 1 mol of an ideal gas under standard conditions of Temperature and Pressure (STP) 273.15 K (0ºC) and 1 atm is 22.4 L/mol
what is R
The universal gas constant
R = 0.082 L atm /KM
R = 0.08206 atmL/Kmol = 8.314 J/Kmol = 62.36 mmHgL/Kmol
What can you calculate if the amount f gas doesn’t change
PV/T=nR is a constant for a specific sample (amount) of gas
so P1V1/T1=P2V2/T2
Meaning if the amount of gas doesn’t change, you can find an unknown value if you know the rest
What are partial pressure
Gases mix homogeneously
Total pressure = sum of partial pressures
For an ideal gas, the partial pressure of a gas in a mixture is the pressure that the gas would exert if it were alone (at the same T and V conditions)
what is the formula for pressure
P=nRT/V
why is the atmosphere composition not uniform
the atmosphere composition is not uniform because Gravity preferentially attracts heavier molecules towards earth’s surface, mixing is slow at the boundaries between layers
what is the atmospheric composition at sea level
Nitrogen, 78.084 %
Oxygen, 20.948%
Argon, 0.948%
Carbon dioxide, 0.0382%
Neon, 0.001818
Helium, 0.000524%
Methane, 0.0002%
Krypton, 0.000114%
Hydrogen, 0.00005%
Nitrous Oxide, 0.00005%
Xenon, 0.0000087%
At constant T, P, and V, what defines stoichiometry
Volumes of ideal gases are proportional to numbers of moles
n=VP/RT
At a constant T and P, V is proportional to n, so the volume of the gas defines reaction stoichiometry
How do you calculate mole fraction
Xa = na/nt
What are the 5 assumptions of the Kinetic Molecular theory
- Particles move randomly in straight-lines
- There are no attractive or repulsive forces between particles
- Particles collide without energy exchange (elastic collisions). No friction
- The particle volume is negligible when compared to total volume of the gas sample
- Particle kinetic energy is proportional to the temperature
how does volume affect pressure
Increasing the volume at constant T increases the distances particles travel before colliding with a wall, decreases the number of collisions with walls, so: Increase in V = decreases P
How does temperature affect pressure
Increasing T at constant V increases average particle speed and increases number of collisions with walls, so: decrease in T = decreases in P
how do you calculate the average kinetic energy for a mole of monatomic gas
For a monoatomic gas, the average kinetic energy <KE> is</KE>
<KE> = 3/2 RT
for R = 8.314 J/Kmol (SI)
</KE>
When does the ideal gas law fall apart
Ideal gases obey PV = nRT, but this does not apply to real gases at low temperature or high pressure.
what happens at low temp with gases
At low temperature, molecules are slower and attractive intermolecular forces decrease the magnitude of the collisions with surfaces so that the observed pressure is less than the ideal pressure.
To correct: P ideal = P observed + a*n^2 / v^2
what happens to gases at high pressure
At high pressure, the volume of the gas molecules is not negligible, therefor the observed volume is greater than the idea V.
To correct: V ideal = V observed - nb
what is the Van Der Waals equation
a and b are constants that can be found in tables for each specific gas
Van der waals equations: $(P+an^2/v^2)(V-nb)=nRT$
How do greenhouse gases work and influence climate change
The atmosphere is important for maintaining Earth’s Temperature. The earth is warmed by solar radiation (all wavelengths) the earth emits energy as infrared light.
Natural greenhouse effect: CO2 in the atmosphere absorbs IR radiation emitted by the earth, releasing heat gradually over time. This keeps Earth’s average temperature at 15º C instead of -15º C (based on distance from the sun).
CO2 vibrates back and forth making a dipole when it absorbs IR radiation.
what are joules
Joules (SI unit) for energy, KE, or work
= Force * distance
= mass * distance^2 / time^2
= Kgm^2/s^2
What are the 5 main energy units and their conversions
Energy Units and Conversions
1 calories (cal) = 4.184 joules
1 joule (J) = 1 coulomb volt (CV) = 1 watt second (Ws)
1 Calorie (Cal) = 1000 calories (cal)
1 kilowatt hour (kWh) = 3.6 x 10^8 joules (J)
what is Kinetic energy
Energy of Motion
(<> = average)
<KE> = m<v>^2 / 2 = 3RT/2
for a monoatomic gas.
</v></KE>
what is potential energy
stored energy due to position
PE = mgh
Gravity is an attractive force (F) - by definition, negative forces are attractive forces.
F = -mg, it pulls an object down and transforms PE into KE.
Remember that energy is conserved always in an transformation.
what is electrical potential energy
plays a key role in chemical interactions.
PEel correlates to Q1Q2/d
Electrostatic interaction goes to zero for very large values of d
PEel < 0 → attractive interactions
PEel > 0 → repulsive interactions
what is internal energy
the internal energy (U) is the sum of the kinetic (KE) and potential energy (PE) of all components of the system
KE of each particle due to their individual motions relative to each other
PE of each particle due to their individual position relative to each other
what do we normally determine instead of the internal energy
It is very difficult to know the absolute value of the internal energy (U).
Normally we determine the change in internal energy
∆u = ∆products - ∆reactants
Since the total energy must be conserved, heat might be released in this process (exothermic), or absorbed (endothermic)
what are the two ways to transfer energy, and what is another definition of internal energy
Two ways to transfer energy: heat and work
Another definition of internal energy (u): the capacity of a system to do work or transfer heat
what is work and heat
Work (w): energy that causes a mass to move by applying a force. w = f * d
Heat (q): energy that causes an increase in temperature
what do the signs of q and w indicate
The values and signs of ∆u, q, and w refer to the system:
convention:
- q < 0 , system transfers heat to surroundings
- q > 0 , surroundings transfers heat to system
- w < 0 , system does work on surroundings
- w > 0 , surrounds do work work on system
what is the formula for ∆u
∆ u = q + w
the change in the internal energy of a system is the sum of energy transferred to/from the surroundings in the form of heat and work
what are the different types of system
Open systems: Exchanges matter and energy with the surroundings
Closed systems: Exchanges only energy with the surroundings
Isolated systems: Neither matter nor energy are exchanged with the surroundings
what is heat
Heat (q) , Heat transfer is proportional to temperature change , q ∝ ∆t
When a cold object is placed in contact with a hot object heat will flow from the hot object into the cold object.
The objects will exchange heat until they reach thermal equilibrium - this means that they will have the same final temperature (T final)
what is work
Work: force * distance = work
Pressure = F / A
F = PA
W = -F∆h = -PA∆h
W = -P∆V
we need the minus sign to follow the sign convention
what are state functions
They are path independent. The overall change depends only on the initial and final status
∆PE = PEf - PEi
Potential energy is an example of a state function.
T, P, V, and u are also examples of state functions. State functions are properties of the system.
are u, w, and q state functions
Change in internal energy is a state function, and it is the same not matter how much is converted to heat or work
q and w are not state functions. They are path functions (properties of path from initial to final state).
what is the first law of thermodynamics
The total energy of the universe is conserved.
Energy is not created nor destroyed. Any energy lost by the system is gained by the surroundings
how does heat exchange at constant volume work
Heat in chemical process at constant volume ($q_v$)
Constant volume, ∆V = 0 (isochoric conditions), so no work done
∆u = $q_v$
The heat exchanged measure at constant volume is equal to the change in internal energy
This is useful, since U is a state function. But measuring, chemical processes at constant V requires special equipment.
what is a bomb calorimeter
Bomb calorimeter measures changes in internal energy of reactions (∆u rxm)
Heat flows between reaction (rxn) and the calorimeter (cal)
q cal = - q rxn
q _c_a_l=C_c_a_l∆T_c_a_l
Ccal is a constant; determined independently using a standard compound.
In a calorimeter experiment, the chemical reaction is the system; the calorimeter is part of the surroundings.
what is a coffee cup calorimeter
Coffee cup calorimeter measures heat flow at constant pressure (q_p)
heat flows between reactions (rxn) and solution (soln)
the heat from the reaction (system) will change the temperature of a certain mass of solution (surroundings)
∆u ≠ q_p
Easier and cheaper experimet than using a bomb calorimeter.
q soln = (Cs of solution)(mass of solution)(∆T soln)
Cs of solution = specific heat capacity
what is enthalpy
Chemical changes typically take place at constant pressure, and heat flow is easy to measure in these conditions.
We therefore define a new thermodynamic quantity for the heat exchange at constant pressure ($q_p)$ called enthaly H
Enthalpy is a state function (since U, P and V are state functions)
we define: H = U + PV
what is the change in enthalpy at constant pressure
∆P = 0 (isobaric conditions)
the heat exchanged measured at constant pressure is equal to the change in enthalpy
Since P is constant:
∆H = ∆u + P∆V
Recall the equations for ∆U and for w:
∆u = q + w
w = -P∆V
∆H = q - P∆V + P∆V
∆H = qp
What is specific heat capacity
Specific Heat Capacity is the amount of heat needed to raise the temperature of 1 gram of a substance by 1ºC or K
Units: J /gK
q = Cs * m * ∆t
what is molar heat capacity
Molar heat capacity is the amount of heat required to raise the temperature of 1 mole of a substance by 1º C or K
units: J / mol K
1 = Cm * n * ∆t
what types of materials have what levels of heat capacities
metals have low heat capacities and are easy to heat up
Non metals have higher
Water has a high heat capacity and can store a lot of heat
summarize how constant volume vs pressure give differences in measure heat exchange
In summary, the measured heat exchange is equal to changes in different state functions of the system (e.g. a chemical reaction), depending on the conditions:
- Heat flow at constant volume gives change in internal energy. ∆u
- Heat flow at constant pressure gives the change in enthalpy, ∆H
Enthalpy is a useful state function for chemists since we prefer to measure chemical rxns for constant pressure
what are phase changes
Changes in physical state, with no change in composition. Each phase change involves a change in the energy of the system.
which phase changes are endothermic vs exothermic
Vaporization, Fusion, and Sublimation are Endothermic
Condensation, Freezing, and Deposition are exothermic
what represents the enthalpies of phase changes
The phase changes are represented by ∆H_sub, fus, vap, etc
∆H for exothermic reactions are the negative of the correlated endothermic reaction. ∆Hcond = ∆Hvap
what are the conditions of enthalpies of phase changes
The Enthalpy changes for different state changes represent the change in enthalpy for a material changing at its temperature of change and 1 atm.
Tabulated enthalpies of phase changes are molar quantities (units of KJ/mol) enthalpy changes per mole of the substance
ex H20 6.007 for fusion and 40.66 for vaporization
