thermodynamics2 - internal energy

Question 1

what is the purpose of thermodynamics?

Answer 1

thermodynamics explains how energy is converted from 1 form to another, and how much useful energy there is in a system - what is good to burn, eat, etc

Question 2

what are the 2 approaches to thermodynamics?

Answer 2

classical - based on observable + quantifiable laws
statistical - based on mathematical models of average behaviour of individual molecules

Question 3

state function definition + 5 examples

Answer 3

properties whose value doesn’t depend on how the path it took to reach that specific value
e.g. enthalpy, entropy, free energy, internal energy, temperature

Question 4

what are the 4 types of systems?

Answer 4

isolated system
open system
closed system
adiabatic system

Question 5

what is an isolated system?

Answer 5

a system in which there can be no exchange of matter or energy between the system and its surroundings

Question 6

what is an open system?

Answer 6

a system where things can be added to/taken from the system and any form of energy can be transferred - free movement of matter and energy

Question 7

what is a closed system?

Answer 7

a system where there can be no exchange of matter but energy can be transferred in the form of heat or work

Question 8

what is an adiabatic system?

Answer 8

a system where there can be no heat transfer, only energy transfer is in the form of energy

Question 9

adiabatic definition

Answer 9

no heat is able to flow

Question 10

diathermic defintion

Answer 10

heat is able to flow

Question 11

describe the state of thermal equilibrium

Answer 11

the state of a system split by a diathermic barrier which allows heat flow between the 2 separated systems in the absence of an overall flow/charge/transfer
- this means the 2 systems are at the same temperature
- this can apply to a continuous number of systems, not just 2
- this is also the basis of the zeroth law

Question 12

how is the relationship between molecular energy/speed and temperature described?

Answer 12

by boltzmann curve + energy levels between molecules, and root mean squared speed
- low temp means lower RMS speed and increased population of lower energy states/population spread across only a few energy levels (+ vice versa)

Question 13

internal energy, U definition

Answer 13

the total energy, kinetic + potential, of a system/the constituent particles in a system, spread across all energy levels
- a state function

Question 14

how is internal energy related to temperature?

Answer 14

as temperature increases, more higher energy states are occupied and internal enegry increases

Question 15

what is the first law of thermodynamics?

Answer 15

‘the internal energy of an isolated system is constant’
- this is obvious as isolated systems are detached from surroundings, and no heat/work can be done

Question 16

energy definition

Answer 16

a measure of a systems ability to do work

Question 17

work definition

Answer 17

the use of energy to move against an opposing force = an organised action

Question 18

explain the derivation of the work equation

Answer 18

work is done on a mass by raising it in a gravitational field, and if that mass is allowed to fall it can do work
this gives w = Fd => dw = Fdz => w = FΔz
where w = work, F = force, z = displacement
- essentially, all work is the equivalent of raising a mass in a gravitational field

Question 19

how is work related to energy?

Answer 19

high energy systems can do more work than low energy systems, the internal energy of the system will increase if work is done on the system, as this increases the systems ability to do work

Question 20

expansion work definition

Answer 20

work done when a gas expands against an external pressure

Question 21

give the equation describing the relationship between expansion work and work done equation

Answer 21

as the system expands work is done on its surroundings causing the internal energy of the system to decreases
w = -Δ(p.ex*V)
where w = work done
p.ex = external pressure
V = volume

Question 22

what are the 2 types of expansion work?

Answer 22

reversible and irreversible work

Question 23

explain reversible work?

Answer 23

theoretically, the maximum possible expansion work is possible from a system which is at a constantly changing external pressure, always in mechanical equilibrium such that it is only ever infinitesimally higher than internal pressure
therefore p.ex = p.int + Δp
and w = -nRT ln(Vf/Vi)
internal and external pressure are effectively equal at all points in the expansion

Question 24

what factor affects reversible expansion work?

Answer 24

temperature - work required for the isothermal reversible expansion of an ideal gas increases as temp increases
- isothermal = constant temp

this can also be true for irreversible expansion work in some respects

Question 25

explain irreversible work

Answer 25

w = -p.exΔV at constant external pressure, this is realistic the piston moves through a distance against as an external pressure which applies a force opposing the expansion of p.ex of the gas

Question 26

how can the expressions for reversible and irreversible expansion work combine to provide an expression that allows work to be quantified?

Answer 26

when gas is produced in a system, the change in internal energy depends on whether or not volume expands, as this determines the effect on pressure at higher pressures, more work has to be done to push back the surrounding atmosphere, and thermodynamic work will only be done if there is a change in volume therefore any reaction involving gas production will have to do work against the atmosphere, and so using the ideal gas law we can determine how much work is done - combining expressions: w = -p.exΔV and w = -Δn.gasRT => pv=nRT

Question 27

what is the effect of work on the internal energy of a system?

Answer 27

all forms of work change the internal energy of the system, non expansion work is related to gibbs free energy

Question 28

heat definition

Answer 28

a mode of energy transfer that achieves random motion in the surroundings, e.g. vibrations

Question 29

what is the direction of heat flow?

Answer 29

heat flows from high to low temps

Question 29

what if a system and its surroundings are different temperatures?

Answer 29

energy will be transferred between the 2 until the system and surroundings reach thermal equilibirum

Question 30

exothermic reaction definition

Answer 30

reactions that give out heat, meaning the system temperature increases and energy is released in the form of heat, promoting random motion outside the system surrounding temp rises

Question 31

endothermic reaction definition

Answer 31

reactions that take in heat, meaning the system temperature decreases, energy is absorbed by the reaction in the form of heat causing less random motion in the surroundings surrounding temp falls

Question 32

explain the first law using ideas about equilibrium reactions

Answer 32

if isothermal expansion is reversible, like a reversible reaction, ΔU = 0 and q = -w = heat exchange this gives q = nRT ln(Vf/Vi) therefore the first law can be considered as ΔU = w + q, meaning that internal energy of a system can be accounted for by any work done/heat transferred = the only possible forms of energy transfer

Question 33

how does a bomb calorimeter work?

Answer 33

a bomb calorimeter burns a substance in an O2 rich environment with an isochoric volume via complete combustion upon combustion heat is transferred to the bomb and surrounding water bath until thermal equilibrium is achieved, any change in ΔU of the system has to be accounted for b y heat as no work is possible

Question 34

isochoric definition

Answer 34

a fixed volume

Question 35

enthalpy definition

Answer 35

enthalpy is related to internal energy by how much expansion work the system needs to do H = U + pV where p is approx, constant - a state function