Gases Flashcards

Question

how do you calculate the average kinetic energy for a mole of monatomic gas

Answer 1

For a monoatomic gas, the average kinetic energy is = 3/2 RT for R = 8.314 J/Kmol (SI)

Answer 2

Ideal gases obey PV = nRT, but this does not apply to real gases at low temperature or high pressure.

Answer 3

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

Answer 4

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

Answer 5

a and b are constants that can be found in tables for each specific gas Van der waals equations: $(P+an^2/v^2)(V-nb)=nRT$

Answer 6

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.

Answer 7

Joules (SI unit) for energy, KE, or work = Force * distance = mass * distance^2 / time^2 = Kgm^2/s^2

Answer 8

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)

Answer 9

Energy of Motion (<> = average) = m^2 / 2 = 3RT/2 for a monoatomic gas.

Answer 10

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.

Answer 11

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

Answer 12

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

Answer 13

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)

Answer 14

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

Answer 15

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

Answer 16

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

Answer 17

∆ 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

Answer 18

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

Answer 19

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)

Answer 20

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

Answer 21

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.

Answer 22

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).

Answer 23

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

Answer 24

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.

Answer 25

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.

Answer 26

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

Answer 27

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

Answer 28

∆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

Answer 29

Specific Heat Capacity is the amount of heat needed to raise the temperature of 1 gram of a substance by 1º C or K Units: J /gK q = Cs * m * ∆t

Answer 30

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

Answer 31

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

Answer 32

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: 1. Heat flow at constant volume gives change in internal energy. ∆u 2. 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

Answer 33

Changes in physical state, with no change in composition. Each phase change involves a change in the energy of the system.

Answer 34

Vaporization, Fusion, and Sublimation are Endothermic Condensation, Freezing, and Deposition are exothermic

Answer 35

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

Answer 36

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

Gases Flashcards

(60 cards)