Thermodynamics Flashcards
What is thermodynamics?
study of energy transfer during chemical reactions
What is a system?
what you are studying. A collection of reactions and processes
What are the surroundings?
environment around the system (local)
What is the universe?
the system and the surrounds, everything else
What is chemistry?
the study of matter and its energy changes
What is potential energy?
energy of position
What is kinetic energy?
energy of motion
What are the 3 types of energy?
enthalpy, entropy, free energy
What is enthalpy?
H
heat moved/transferred during a reaction
What is entropy?
S
Measures as “disorder” - measure of randomness in a system
What is free energy?
G
maximum amount of work that can be derived from a system
What is a state function?
function that is independent of the pathway. Only depends on initial and final positions. 𝚫H, 𝚫S, 𝚫G
Does a state function care about the pathway in between initial and final positions?
no
What does 𝚫 mean?
change in
final-initial
What are non-state functions?
functions that are dependent on the pathway. Heat (q) and work (w)
What is energy?
capacity to do work or transfer heat
What is 𝚫u?
equivalent to heat of a system plus the work
What is q sub p?
heat and constant pressure=H (enthalpy)
What does it mean when q is greater than 0?
heat is being absorbed (+)
What does it mean when q is less than 0?
heat is being released (-)
What is the SI unit for energy?
Joule (J)
What does 1 J equal?
kgm^2/s^2
What is a calorie (cal)?
amount of energy required to raise 1.00 grams of water 1 degree C or 1K
What does 1 Calorie (with a capital C) equal?
1kcal = 1000 cal
How many joules are in 1 cal?
4.184 J
What is Specific Heat (c) and heat capacity (C)?
How much energy is required to increase a substances temperature
What are the units of specific heat (variable mass) (c)
J/g celsius or J/g kelvin
What are the units for heat capacity (fixed mass) (C)?
J/celsius or J/kelvin
What does mass multiplied by c(specific heat) equal?
C (heat capacity)
What is calorimetry?
measuring heat transfer
What does calorimetry determine?
how many calories (energy) that food provides
What are the calorimetry equations?
q=mc𝚫T or q=C𝚫T
What do the letters in q=mc𝚫T mean?
m is mass, c is specific heat, 𝚫 is Tf-Ti,
What do the letters in q=C𝚫T mean?
C is heat capacity
Heat lost equals…
heat gained
What does q sub p equal?
enthalpy change (𝚫H)
What are the calorimeter constants (heat capacity)?
Mass times c = C sub heat capacity
What is the 1st Law of Thermodynamics?
Energy cannot be created or destroyed
What is the 1st law of thermodynamics equation?
e=mc^2
What do the letters in e=mc^2 mean?
m is mass, e is energy, c is speed of light (3.00x10^8)
What is the symbol for internal energy?
U
What does U(internal energy) equal?
q + w (heat+work)
Does the change in internal energy of the system equal the change in internal energy of the surroundings?
yes
What is the enthalpy equation?
H=U+PV
the internal energy of the system plus PV(pressure x volume)
What does exothermic mean?
heat given off
What does endothermic mean?
heat absorbed
Do the products in an exothermic reaction have more or less energy than the reactants?
less
Do the products have more or less energy than the reactants in an endothermic reaction?
more
What does sign mean?
direction of enthalpy (heat) flow
What kind of reaction is it when 𝚫H is positive and greater than 0?
endothermic reaction (energy in)
What kind of reaction is it when 𝚫H is negative and less than 0?
exothermic (energy out)
What does magnitude mean?
proportional to the amount of stuff reacted
What do all reactions have a value of?
a change in enthalpy
What is Thermochemical equivalence?
𝚫H and coefficients are proportional
What is Hess’s Law?
the enthalpy change of an overall reaction or process is the sum of the enthalpy changes of its individual steps
What happens to the sign of a reaction if you reverse the reaction?
the sign changes
What happens to 𝚫H if you change the coefficients by a given factor?
𝚫H changes by the same factor
What is pressure at the Thermodynamic Standard State?
1 atm=760 torr=760mmHg
What is temperature at Thermodynamic Standard State?
25 degrees Celsius=298K
What are the Standard Conditions for Solutions at the Thermodynamic Standard State?
1.000M
What is the symbol for nonstandard conditions?
𝚫H
What is the symbol for standard conditions?
𝚫H°
What is 𝚫H° sub f?
heats of formation
The amount of energy (heat) given off absorbed by forming 1.000 mole of a compound from its elements
Can absolute enthalpy be measured directly?
no
Is there a way to measure a zero-point?
no
Do you have to define a zero point for heats of formation?
yes
Define zero enthalpy
the elements in their standard form under standard conditions
What is another version of Hess’s law?
𝚫H°rxn = Σn𝚫H°f products - Σn𝚫H°f reactants
What is bond enthalpy?
Using lewis dot structures and enthalpies of bond breaking and forming to determine enthalpy changes of reactions
Another method for calculating 𝚫H°rxn
Is bond breaking endothermic or exothermic?
endothermic because it requires energy
Is bond formation endothermic or exothermic?
exothermic because it gives energy back
Is Hess’s Law or Bond Enthalpy more accurate? Why?
Hess’s Law because it is created for individual molecules and does not have averages unlike bond enthalpy
How many moles of a product are always in formations?
1 mol
What does spontaneous mean?
reaction or process that occurs without input of energy
eg. An ice cube melting on a surface without you putting in any energy
Water running down a hill
What does non-spontaneous mean?
reaction or process that occurs only with an input of energy
eg. Puddle of water after melting an ice cube does not spontaneously freeze
Water does not spontaneously flow up hill
Have to put in energy to have that happen
Does exothermic always mean spontaneous?
no
Is spontaneity determined by enthalpy?
no
Bonfire–combustion of cellulose
Endothermic/exothermic?
Spontaneous/non-spontaneous?
Exothermic
spontaneous
Cell decay: tree falls over in wood, dies, tree cells decay
Endothermic/exothermic?
Spontaneous/non-spontaneous?
Spontaneous
exothermic
H2O freezing at -10 C
Endothermic/exothermic?
Spontaneous/non-spontaneous?
Spontaneous
Exothermic
Because cold ice is lower energy and gave away extra energy to be a liquid
Gone from a place of higher energy (liquid) to a place of lower energy (solid)
Ice melting at 25 C
Endothermic/exothermic?
Spontaneous/non-spontaneous?
Spontaneous
Endothermic
Gone from a place of lower energy (solid) to a place of higher energy (liquid)
NaCl dissolving in water
Endothermic/exothermic?
Spontaneous/non-spontaneous?
Spontaneous
Endothermic
Because NaCl takes energy to dissolve in water
What is heat formation?
The heat required to make 1 mole of a compound from its elements in their standard states.
Can a spontaneous reaction be spontaneous in the reverse direction?
no
What is the 2nd law of thermodynamics?
entropy of the universe is always increasing
𝚫S sub universe>0
What does entropy measure?
Measure of the amount of energy that becomes unavailable to us during a reaction or process
What is low entropy?
very little randomness/disorder
What is high entropy?
a lot of randomness/disorder
What is the 3rd law of thermodynamics?
the entropy of a perfect crystal at 0K is zero
What is the absolute entropy scale?
0 → infinity
Does any substance about 0K has an entropy value greater than 0?
yes
Are all absolute entropy values positive?
yes
If something is going up in entropy…
something else is going down
Is it more favourable to go up or down in entropy?
up
more random = more options
Is it true that the more atoms are in a molecule, the more randomness there is (higher entropy)?
yes
What are the entropy units?
J/Kmol
How many times smaller is entropy than enthalpy values?
1000 times smaller
Entropy is related to…
temperature
What does this sign convention mean?
Enthalpy: 𝚫H=-
exothermic
Products have lower energy than reactants
More favourable
What does this sign convention mean?
Enthalpy: 𝚫H=+
Endothermic
Products have higher energy than reactants
Less favourable
What does this sign convention mean?
Entropy: 𝚫S=-
Going down in disorder
Products are less disordered than reactants
Unfavourable
What does this sign convention mean?
Entropy: 𝚫S=+
Going up in disorder
Products are more disordered than reactants
favourable
Predict the entropy change for Temperature changes:
Tsystem increases, Ssystem increases 𝚫S=+
Tsystem decreases, Ssystem decreases 𝚫S=-
Predict the entropy change for Physical states and phase changes:
Solid → liquid → gases
Gases are high entropy
Solids are low entropy
Predict the entropy change for Dissolution of a solid or liquid:
Solid → solution/solute
Low S higher S
𝚫S=+
Predict the entropy change for Increase/decrease in the number of moles of gas:
N2(g) + 3H2(g) → 2NH3 (g)
4 mol of gas → 2 mol of gas
𝚫S=-
Predict the entropy change for Isopropyl alcohol evaporating:
Increase: 𝚫S=+
Going from a liquid to a gas
Predict the entropy change for Humidity condensing in cold air:
Decrease: 𝚫S=-
Steam goes from a gas to a liquid
Predict the entropy change for A pond freezing in winter:
Decrease: 𝚫S=-
Going from a liquid to a solid
Predict the entropy change for Silver chloride precipitates from a solution:
Decrease: 𝚫S=-
Solution to a solid
What is Hess’s Law - entropy equation?
𝚫S° subreaction = ∑nS° subproducts-∑nS° subreactants
Is it 𝚫H or 𝚫S that determines spontaneity?
Both enthalpy and entropy guides to spontaneity
What is the enthalpy and entropy change of Melting of ice at 25 C?
Spontaneous
𝚫H= endothermic (+)
𝚫S= + (more disorder)
Spontaneous due to entropy: entropically driven
What is the enthalpy and entropy change of Freezing of water at -10 C?
Spontaneous
𝚫H=exothermic (-)
𝚫S= - (less disorder)
Spontaneous due to enthalpy: enthalpy driven
What is Gibbs Free energy 𝚫G?
Total energy of the system
Maximum amount of work that can be derived from a system
What is Gibbs Free Energy equation?
𝚫G = 𝚫H - T𝚫S
Is Gibbs free energy temperature independent or temperature dependent?
dependent
What are the units of 𝚫G = 𝚫H - T𝚫S?
𝚫G is in units of kJ/mol
𝚫H is in units of kJ/mol
T is units of K
𝚫S is in units of J/molK
What does 𝚫G determine?
spontaneity
What does it mean when 𝚫G is less than 0 (negative value)?
exergonic, spontaneous
What does exergonic mean?
gives off free energy
What does it mean when 𝚫G is greater than 0 (positive value)?
endergonic, non-spontaneous
What does endergonic mean?
takes in energy
Is free energy a state function?
yes
Does a 0 point need to be defined in standard free energy of formation?
yes
What is the free energy formation symbol?
𝚫G°f
What value is the elements in their standard form in free energy of formation?
0
What is Hess’s Law -free energy equation?
𝚫G°reaction = ∑n𝚫G°products - ∑n𝚫G°reactants
or
𝚫G°reaction = 𝚫H°reaction - T𝚫S°reaction
What are Temperature-independent systems (opposite signs of 𝚫H and 𝚫S) for spontaneous at all temperatures? 𝚫H<0 (negative) and 𝚫S>0 (positive)
Both enthalpy and entropy favor a spontaneous reaction
What are Temperature-independent systems (opposite signs of 𝚫H and 𝚫S) for non-spontaneous at all temperatures?
𝚫H>0 (positive) and 𝚫S<0 (negative)
Both enthalpy and entropy oppose spontaneity
What are Temperature-dependent cases (same signs of 𝚫H and 𝚫S) for spontaneous at higher temperatures?
𝚫H>0 (positive) and 𝚫S>0 (positive)
Entropy favours spontaneity but enthalpy does not
Going up in disorder
Can drive the 𝚫G below 0, making it spontaneous
Entropy driven process
What are Temperature-dependent cases (same signs of 𝚫H and 𝚫S) for spontaneous at lower temperatures? 𝚫H<0 (negative) and 𝚫S<0 (negative)
Enthalpy favours spontaneity but entropy does not
Only works below 0, so if 𝚫G is driven above 0, it is not going to work
What does it mean if the signs are the same for temperature-dependent cases?
the relative sizes of the 𝚫H and T𝚫S terms decide whether or not the process is spontaneous
What is the ∆G when ∆H is negative and ∆S is positive? What is the spontaneity?
negative
always spontaneous
What is the ∆G when ∆H is positive and ∆S is negative? What is the spontaneity?
positive
always spontaneous
What is the ∆G when ∆H is negative and ∆S is negative? What is the spontaneity?
negative when T∆S < ∆H
positive when T∆S > ∆H
spontaneous at low T
nonspontaneous at high T
What is the ∆G when ∆H is positive and ∆S is positive? What is the spontaneity?
negative when T∆S > ∆H
positive when T∆S < ∆H
spontaneous at high T
nonspontaneous at low T
What happens to the spontaneity when 𝚫G goes above 0?
it turns from spontaneous to non-spontaneous
When both signs of 𝚫H and 𝚫S are the same, is the spontaneity temperature dependent or independent?
dependent
When is a reaction entropically driven?
when both 𝚫H and 𝚫S are positive
