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Ch > Thermochem > Flashcards

Thermochem Flashcards

1
Q

thermodynamics

A

science of heat and work

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2
Q

system

A

part of the universe that we are interested in; it is separated from the rest of the universe (i.e. the surroundings) by a definite boundary

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3
Q

open systems

A

Allow heat, energy and matter to pass across the boundary into or out of the system

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4
Q

closed systems

A

Allow heat and energy to pass across the boundary into or out of the system but not matter

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5
Q

isolated systems

A

Allow no transfer of heat, energy or matter in either direction across the boundary of the system

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6
Q

isolated systems

A

only true example is the entire universe (assuming its not lying adjacted to another universe)

-sealed thermos flash makes for a good approximation

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7
Q

internal energy E of a system

A

sum of all its potential energy and kinetic energy

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8
Q

Change in internal energy of a system

A

Ξ”E = E𝒻ᡒₙₐₗ - Eα΅’β‚™α΅’β‚œα΅’β‚β‚— = Eβ‚šα΅£β‚’π’Ήα΅€π’Έβ‚œβ‚› - Eα΅£β‚‘β‚π’Έβ‚œβ‚β‚™β‚œβ‚›

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9
Q

Change in energy of a system

A

-accompanied by equal and opposite change in energy of surroundings

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10
Q

If Ξ”E < 0

A

system loses energy

energy transferred to surroundings

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11
Q

If Ξ”E > 0

A

system gains energy

energy absorbed from surroundings

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12
Q

energy transferred to/from a system occurs in two forms

A

heat (q)

work (w)

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13
Q

Heat (q)

A

energy transferred due to the difference in temperature between the system and its surroundings

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14
Q

temperature (T)

A

measures the average kinetic energy of the particles (atoms or molecules) in a system.

T measures the amount of heat in the system

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15
Q

β€’ The amount of heat transferred depends on the amount of material in the system and
on the temperature change

A

q ∝ mΞ”T

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16
Q

work

A

can be considered as the energy transferred when an object is moved by a force

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17
Q

work formula

A

w = mgh = Fd

m = mass
h = height
g = gravity
F = force
d = distance
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18
Q

other examples of work

A
  • expansion (Ξ”V) against an applied pressure P

- transfer of charge q through voltage V

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19
Q

expansion

A

w = -PΞ”V

P = pressure
Ξ”V = change in vol
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20
Q

transfer of charge through a voltage

A

w = qV

q = charge
V = voltage
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21
Q

if work is being done ON or BY the system

A
  • system does work: it transfers energy - equal to work done, w - to the surroundings
  • negative
  • done on a system: it will absorb energy - equal to work done, w - from the surroundings
  • positive
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22
Q

eg. of work done by a system vs. work done on a system

A

by a system: expansion of gas against applied pressure

on a system: compressing gas in a sealed container

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23
Q

total change in a system’s energy formula/change in internal energy formula

A

Ξ”E = q + w

(sum of energy transferred to/from surroundings as heat and/or work

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24
Q

first law of thermodynamics

A

total energy of universe is constant

Ξ”E(universe) = Ξ”E(system) + Ξ”E(surroundings) = 0

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25
Q

1 joule =

A

1 J = 1 kgm²s⁻²

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26
Q

state functions

A

properties that depend only on state of the system and not on path system takes to reach that state

eg. internal energy

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27
Q

extensive variables

A

those that are proportional to the amount of matter in the system

eg. V, mass, energy, q

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28
Q

intensive variables

A

those that are independent of the amount of material
– they have a constant value throughout any system in equilibrium

eg. P, T

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29
Q

Work done on a system under compression

A

w = F x d = Ξ”h x (Pβ‚‘β‚“β‚œ x A) = Pβ‚‘β‚“β‚œ x Ξ”V

for compression: Ξ”V negative
for expansion: Ξ”V positive

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30
Q

work done by a system

A

compression is work done on a system, expansion is work done by a system so

w = -Pβ‚‘β‚“β‚œ x Ξ”V

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31
Q

Boyle’s Law

A

V ∝ 1/P or PV = constant

n, T fixed

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32
Q

Charles’s Law

A

V ∝ T or V/T = constant

n, P fixed

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33
Q

Avogadro’s law

A

V ∝ n or V/n = constant

P, T fixed

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34
Q

Ideal Gas Law

A

PV = nRT

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35
Q

maximum work of isothermal expansion of an ideal gas at temp T

A

w = -nRT(ln (V𝒻/Vα΅’))

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36
Q

for reversible expansion

A

work done by system is maximised

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37
Q

change in enthalpy and change in internal energy formula

A

Ξ”E = qβ‚š + w

Ξ”E = qβ‚š - PΞ”V

Thus qβ‚š = Ξ”E + PΞ”V = Ξ”H

(qβ‚š is q at constant P)

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38
Q

enthalpy (H)

A
  • the heat content - equal to heat absorbed at constant pressrue
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39
Q

Change in enthalpy (Ξ”H)

A

change in internal energy (E) plus the product of pressure (P) and change in volume (Ξ”V)

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40
Q

formula for change in enthalpy

A

Ξ”H = Ξ”E + PΞ”V

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41
Q

If Ξ”H < 0

A

system loses heat, or heat is a product

exothermic process

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42
Q

If Ξ”H > 0

A

system gains heat, or heat is a reactant

endothermic process

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43
Q

specific heat capacity (c)

A

amount of heat req to raise temp of unit mass of substance by 1K (or 1Β°C)

44
Q

heat released or absorbed formula

A

q = mcΞ”T

45
Q

molar heat capacity (C)

A

amount of heat required to raise the

temperature of a mole of a substance by 1K

46
Q

measuring heat absorbed/released by chemical/physical process

A

calorimeter:
for
-constant pressure or
-constant volume

47
Q

constant pressure

A
  • coffe-cup calorimeter
  • assume heat generated absorbed by water

qβ‚š = Ξ”H

48
Q

constant volume

A
  • bomb calorimeter
  • useful for combust reactions

Ξ”E = q + w = qα΅₯ + 0 = qα΅₯

49
Q

thermochemical equations

A
  • balanced chemical equations
  • physical states specified
  • associated enthalpy change is added
50
Q

enthalpy and forward/backwards reactions

A

as enthalpy is a state function, forward and reverse reaction will be of same magnitude but opposite sign

51
Q

Hess’s Law

A

enthalpy change for overall process is sym of enthalpy changes of its indiv steps

if change can be brough about in more than 1 way, ent change is same for each way

52
Q

standard enthalpy change

A

Ξ”HΒ°

Ξ”H usually specified as a standard ent change under a set of particular conditions called standard states

for any pure substance, stand state refers to most stable form

53
Q

standard state for metals and for molecular elements

A

for metals - solid form

for mol elements - the molecular form eg. O2

54
Q

Ξ”HΒ° obtained

A

at a standard T of 25Β°C (298 K)

for a gas, standard state is pressure of 1 atm

for aw solution standard state is 1M concentration

55
Q

standard enthalpy of a reaction ( Ξ”HΒ°α΅£β‚“β‚™)

A

enthalpy change of a reaction measured at the standard state

56
Q

standard enthalpy of formation (Ξ”H°𝒻)

A

ent change associated with formation of a compound from its elements in their standard states

-zero for elements/molecular elements in their standard forms (eg. Al, Fe, O2, etc)

57
Q

standard enthalpy of combustion (Ξ”HΒ°π’Έβ‚’β‚˜α΅¦)

A

associated with combusting a pure substance in its standard state

58
Q

Ξ”HΒ°α΅£β‚“β‚™ formula

A

ΣΔ𝐻°𝒻 (π‘π‘Ÿπ‘œπ‘‘π‘’π‘π‘‘π‘ ) βˆ’ ΣΔ𝐻°𝒻 (π‘Ÿπ‘’π‘Žπ‘π‘‘π‘Žπ‘›π‘‘s)

59
Q

Δ𝐻°𝒻 and telling us if comp is exo or endo thermic

A

If Ξ”H°𝒻 < 0 Exothermic compound

If Ξ”H°𝒻 > 0 Endothermic compound

60
Q

standard molar enthalpy of fusion/melting (Δ𝐻°𝒻ᡀₛ)

A

enthalpy change associated with melting one mole of the substance at the normal melting point

61
Q

standard molar enthalpy of fusion/melting (Δ𝐻°α΅₯β‚β‚š)

A

enthalpy change associated with vaporising 1 mole of a substance at the normal boiling point

62
Q

change in enthalpy with molar heat cap at constant pressure

A

Ξ”H = Cβ‚šΞ”T

at constant P

63
Q

spontaneous change

A

change that occurs under specified conditions without a
continuous input of energy from outside the system

If a change is spontaneous in one direction it will not be spontaneous in the other
direction

64
Q

how to know if process is spontaneous

A

The dispersal of energy seems to act as an indicator for spontaneous change

eg. phas change, solid > liquid, or liquid > gas
eg. dissolution of salt, ions are separated + spread out, so energy of motion more dispersed

sign of Ξ”H tells us nothing about if reaction is spontaneous or not

65
Q

entropy

A

a measure of the dispersal of energy in a the system

66
Q

Second law of thermodynamics (Kelvin)

A

No process is possible in which sole result is absorption of heat from a reservoir + its complete conversion into work

67
Q

Second law of thermodynamics (in terms of entropy)

A

There is a state function called the entropy (S); the entropy of an isolated system increases in the course of every spontaneous change

Ξ”Sβ‚œβ‚’β‚œ = Ξ”Sₛᡧₛ + Ξ”Sβ‚›α΅€α΅£α΅£ > 0
or

Ξ”Sα΅€β‚™α΅’α΅₯ = Ξ”Sₛᡧₛ + Ξ”Sβ‚›α΅€α΅£α΅£

68
Q

number of microstates

A

number of ways it can disperse its kinetic energy among the various motions of all its particles

69
Q

Boltzmann’s formula for relating no. of microstates to entropy

A

S = k ln(W)

k = Boltzmann's constant
W = no. of microstates
70
Q

change in entropy of system

A

W𝒻ᡒₙₐₗ/Wα΅’β‚™α΅’β‚œα΅’β‚β‚— = 2ᴺᡃ

S = k ln(W)

βˆ†π‘†π‘ β‚›α΅§β‚› = 𝑆𝒻ᡒₙₐₗ βˆ’ π‘†α΅’β‚™α΅’β‚œα΅’β‚β‚—

= π‘˜ ln(π‘Š)𝒻ᡒₙₐₗ βˆ’ π‘˜ ln(π‘Šα΅’β‚™α΅’β‚œα΅’β‚β‚—)

= π‘˜ ln(π‘Šπ’»α΅’β‚™β‚β‚—/π‘Šα΅’β‚™α΅’β‚œα΅’β‚β‚—)

= π‘˜ ln(2ᴺᡃ)

Subbing in the values for K, Nₐ and ln(2), we get
=5.76 J/k

71
Q

entropy of solids liquids and gases

A

S(solid) < S(liquid) < S(gas)

72
Q

changes to systems that increase entropy (or no. of microstates)

and in reverse reduces entropy

A

β€’ Increasing amount of gas - through
chem reactions or by vaporising liquids or solids

  • Breaking down larger molecules into smaller ones
  • Melting solids
  • Increasing V of a gas (at constant T)
  • Mixing pure substances
  • For a given substance, S increases with increasing temp
73
Q

third law of thermodynamics

A

perfect crystal has zero entropy at absolute zero

74
Q

standard molar entropy (SΒ°)

A

is defined for diff substances in units of
J/mol/K

-have non-zero values for atomic and molecular elements

75
Q

standard entropy change of reaction (Ξ”SΒ°α΅£β‚“β‚™)

A

entropy change of a given reaction that occurs when all reactants and products are in their standard states.

76
Q

Ξ”SΒ°α΅£β‚“β‚™ formula

A

Ξ”SΒ°α΅£β‚“β‚™ = Σ𝑆°(π‘π‘Ÿπ‘œπ‘‘π‘’π‘π‘‘π‘ ) βˆ’ Σ𝑆°(π‘Ÿπ‘’π‘Žπ‘π‘‘π‘Žπ‘›π‘‘π‘ )

77
Q

change that decreases entropy of the system

A

A change that decreases the entropy of the system, may occur spontaneously – but only if it is outweighed by an increase in the entropy of the surroundings

78
Q

exothermic process summed up

A
  • Heat is released by system + absorbed by surroundings
  • More heat in surroundings increases dispersion of energy
  • qₛᡧₛ < 0, qβ‚›α΅€α΅£α΅£ > 0, Ξ”Sβ‚›α΅€α΅£α΅£ > 0
79
Q

endothermic process summed up

A
  • The surroundings release heat which is absorbed by system
  • Energy in surroundings less dispersed
  • qₛᡧₛ > 0, qβ‚›α΅€α΅£α΅£ < 0, Ξ”Sβ‚›α΅€α΅£α΅£ < 0
80
Q

entropy change and temp: at low T

A
  • Little motion in surroundings - little energy dispersion

* Transfer of heat causes relatively large change in dispersal of energy

81
Q

entropy change and temp: at high T

A
  • Greater number of microstates – large amount of energy dispersed
  • Transfer of heat causes relatively small change in dispersal of energy
82
Q

Ξ”Sβ‚›α΅€α΅£α΅£ formulas

A

Ξ”Sβ‚›α΅€α΅£α΅£ = qₛᡧₛ/T

Ξ”Sβ‚›α΅€α΅£α΅£ = βˆ’Ξ”π»β‚›α΅§β‚›/𝑇

83
Q

Gibbs Free Energy formula

A

Δ𝐺ₛᡧₛ = Δ𝐻ₛᡧₛ βˆ’ 𝑇Δ𝑆ₛᡧₛ

84
Q

change in the free energy (Ξ”G)

A

measure of spontaneity of a process and the useful energy available from it

85
Q

Ξ”Sα΅€β‚™α΅’α΅₯ and spontaneity

A

Ξ”Sα΅€β‚™α΅’α΅₯ > 0 for spontaneous process (energy goes from being more conc to more dispersed

Ξ”Sα΅€β‚™α΅’α΅₯ < 0 for non spont process

Ξ”Sα΅€β‚™α΅’α΅₯ = 0 for process at equilibriym

86
Q

Ξ”G and spontaneity

A

Ξ”G < 0 for spont process

Ξ”G > 0 for non-spont process

Ξ”G = 0 for process at equil

87
Q

Ξ”G and work

A

Ξ”G = maximum amount of useful work that can be done by a system during a spont process at constant pressure + pressure

Ξ”G = wβ‚˜β‚β‚“

88
Q

determining Δ𝐺°ₛᡧₛ

A

Δ𝐺°ₛᡧₛ = Δ𝐻°ₛᡧₛ βˆ’ 𝑇Δ𝑆°ₛᡧₛ

89
Q

Δ𝐺°ᡣₓₙ =

A

ΣΔ𝐺°𝒻 (π‘π‘Ÿπ‘œπ‘‘π‘’π‘π‘‘π‘ ) βˆ’ ΣΔ𝐺°𝒻 (π‘Ÿπ‘’π‘Žπ‘π‘‘π‘Žπ‘›π‘‘s)

90
Q

standard free energy of formation (Δ𝐺°𝒻)

A

free energy change associated with formation of a compound from its elements in their standard states

91
Q

rules for standard free energy of formation

A
  • zero for element in its standard state
  • reversing reaction changes it sign
  • state function, so apply Hess’s law to find for a given process
  • reaction coefficients multiply their contribution when working it out
92
Q

Ξ”H, Ξ”S, and Ξ”G

spontaneous

A

If Ξ”H < 0 and Ξ”S > 0 β†’ Ξ”G always < 0

reaction spontaneous at all T

93
Q

Ξ”H, Ξ”S, and Ξ”G

nonspontaneous

A

If Ξ”H > 0 and Ξ”S < 0 β†’ Ξ”G always > 0

reaction nonspontaneous at all T

94
Q

Ξ”H and Ξ”S have same sign

A

the spontaneity of the reaction will depend on temperature

This can be determined by setting Ξ”G = 0 (i.e. at equilibrium)

βˆ†πΊ = βˆ†π» βˆ’ π‘‡βˆ†π‘† = 0

𝑇 = βˆ†π»/βˆ†S

95
Q

equilibrium

A

dynamic

96
Q

what happens at equilibrium

A

concentrations of reactants and products are constant
because a change in one direction is balanced by a change in the other

rateπ’»π“Œπ’Ή = rateα΅£β‚‘α΅₯

97
Q

The equilibrium constant, K

A

K = kπ’»π“Œπ’Ή/kα΅£β‚‘α΅₯

98
Q

equilibrium constant formula

A

𝐾 = [𝐢]ᢜ[𝐷]ᡈ/[𝐴]ᡃ[𝐡]ᡇ

99
Q

what value of k tells us

A
  • K > 1 equilibrium favours products
  • K < 1 equilibrium favours reactants
  • K β‰ˆ 1 at equilibrium have about the same concentration of reactants and products
100
Q

very large K

A

Very little reactant remaining – the reaction effectively goes to completion

101
Q

very small K

A

Very little products formed – effectively no reaction

102
Q

reaction quotient (Q)

A

used when particular reaction is not at equilibrium

103
Q

Reaction quotient (Q) formula

A

Q = [𝐢]ᢜ[𝐷]ᡈ/[𝐴]ᡃ[𝐡]ᡇ

104
Q

formula relating Q, K and Ξ”G

A

βˆ†πΊ = 𝑅𝑇𝑙𝑛(𝑄/𝐾) = 𝑅𝑇𝑙𝑛(𝑄) βˆ’ 𝑅𝑇𝑙𝑛(𝐾)

T = temp in K
R = gas constant
105
Q

βˆ†πΊ away from standard states

A

βˆ†πΊ = βˆ†πΊΒ° + 𝑅𝑇𝑙𝑛(q)

106
Q

Van’t Hoff Equation

A

𝑙𝑛(𝐾₂) βˆ’ 𝑙𝑛(𝐾₁) = βˆ’βˆ†π»Β°/𝑅 x (1/𝑇₂ - 1/T₁)

if we know Ξ”HΒ° and K at one temperature we can calculate K at any other temperature

Ch (3 decks)

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