Semester 2 Exam 1 Flashcards by David Coman

Know difference between a system and its surroundings

freelo

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endo vs. exothermic

endo: energy flows into system

exo: energy flows out of system

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What is Thermodynamics?

study of energy and its interconversions

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1st law of thermodynamics?

Energy of universe is constant

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2nd law of thermodynamics?

entropy of a closed system is always slowly increasing… this basically means that heat is lost as a product of work so no process is perfectly reversible

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equation for ΔE?

ΔE = q + w

change in energy of a system is the sum of heat and work

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Thermodynamic quantities’ signs are always in respect to the:

system

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Equation for work done by an expanding/compressing gas in perspective of the system?

units? convert to kJ

sign convention explanation?

interactive example 6.2 and 6.3

w = -PΔV

gives in Latm. 1Latm=0.1013kJ

this gives us the work required to expand a gas ΔV against an external pressure P
w and PΔV have opposite signs because when gas expands ΔV is positive and work flows into surrounds so from perspective of system it is negative. When a gas is compressed work is done on the system so w is positive but ΔV is negative

ALWAYS THINK ABOUT WHAT IS BEING DONE TO THE SYSTEM

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What is enthalpy?
equation?
equation for a rxn?

total heat of a system including its energy and the product of pressure and volume

H = E + PV
but deriving for E at constant pressure gives us

ΔH = q_p
change in enthalpy at a constant pressure is the flow of heat!

ΔH_rxn = H_products - H_reactants

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Calorimetry?
Heat capacity?

Science of measuring heat flow
C = heat absorbed/increase in temp

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Specific heat capacity?
Units?

amount of energy required to raise temp of an object by 1C
J/g*C

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State functions?

a property that depends on the current state, independent of how it got there.

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Intensive vs. Extensive properties

Extensive: depends on quantity of substance
ex: heat capacity, enthalpy, mass, volume, length

Intensive: independent of mass
ex: temp, density

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Specific heat capacity, heat capacity, and molar heat capacity

heat capacity: defined earlier, J/C
SHC: J/gC
MHC: J/molC

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Equation for flow of energy at constant pressure (q_p)

q_p = smΔT

s: specific heat
m: mass
ΔT: change in temp (K or C)

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what does a coffee cup calorimeter? Why is it good?

know what it looks like
-constant pressure!
-determine ΔH

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units of enthalpy?

J/mol or kj/mol

-all these units usually are measured in (per mol) so make sure to do that!

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Interactive example 6.4

…

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Bomb Calorimeter

what does it look like?

how does it work?

A device used to measure heat absorbed or evolved during a reaction

sample is placed in a small cup inside bomb, the bomb is sealed and pressurized w O2. Bomb is placed in an insulated container and convered with a measured quantity of water. Rxn starts by passing electrical current through a fine wire in contact w sample. Sample ignites and combusts and the H2O absorbs heat evolved. Constant volume and it determines heat of combustion!

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Equation for energy released in a bomb calorimeter:

ΔT * BC heat capacity (temp required to raise the water by 1C)

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How to calculate energy absorbed by calorimeter?
-equation and its variable meaning

do some practice problems for TS!!!!

q_cal = ΔT * C

C is the calorimeter constant!
J/C

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Is enthalpy a state function?

yes

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Hess’s Law:

if a rxn is carried out in a series of steps, ΔH is the sum of ΔH for each individual step!

on baby u gotta do some practice w this cause the canceling is hard…

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When considering the mass of a solution in a calorimeter you consider the ___ mass, not just the substance

mole of rxn is _____ to total moles of reactants but equal to ____ reactant
this is what you divide ΔH by.

entire

not equal
limiting

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standard enthalpy of formation? notation?

Used to calculate ΔH values for chemical rxns! ΔH*_f it is the change in enthalpy that accompanies the formation of one mole of compound from its elements with all other substances in their STANDARD STATES

what is standard state? elements in standard state ____ included in ΔHrxn calculations as ΔH of an element in SS is ___

for a gas: 1 atm of a liquid/solid: pure of a substance in solution: 1M of an element: form in which element exists at 1atm and 25C examples: SS of O2: 1atm SS of Na: Na(s) SS of Hg: Hg(l) ARE NOT; 0

How to calculate ΔH*_rxn using standard ehtalpies of formation example 6.10

ΔH*_rxn = ΔH*_f products - Δ H*_f reactants

entropy (S) is a ___ function. Spontaneous: occurs without ____ intervention entropy is the ____ ____ for spontaneous processes it is a measure of ___ and ___

state outside driving force molecular randomness and disorder

relate entropy of solid liquids and gases problem #39

Solid < liquid < gas

2nd law of thermodynamics (again) equation for Δ S_universe

entropy of universe is constantly increasing! ΔS_universe = ΔS_sys + ΔS_surr

To predict spontaneity of a process you must know sign of ___ + indicates: - indicates: 0 indicates:

ΔS_surr - increase of universe entropy and process is spontaneous in direction written -indicates process is spontaneous in opposite direction -process has no tendency to occur and sys is at equilibrium

How to determine spontaneity in ΔS_sys and ΔS_surr

if ΔS of the system and surr are both (+) then spontaneous, both (-) then not spontaneous. If one is (+) and the other is (-), then (+) one must have a larger magnitude in order for the rxn to be spontaneous

Effects of temp on spontaneity -ΔS surroundings will depend on ___ -ΔS surroundings is (+) for ____ rxns and (-) for ____ rxns.

direction of heat flow exothermic (heat flows into surroundings) endothermic(heat flows out of surroundings)

Magnitude of ΔS surroundings depends on ____, and depends directly on quantity of ____ transferred, and inversely on ____ at low temp magnitude is___ at high temp magnitude is ___ #48 txtbk and notebook

temperature heat temperature high low

Equation for ΔS_surr example 17.4

ΔS_surr = -(ΔH/T(kelvin)) at constant T and P

practice predicting sign of and calculating ΔS_surr

...

What is Gibb's energy? (G) It is a ___ function The equation for G and ΔG

G is the maximum amount of energy present in a thermodynamic system that can be used to perform work at a constant temperature and pressure state G = H - TS ΔG = ΔH - TΔS *temp in Kelvin

Equation for ΔS_univ in terms of ΔG and T in kelvin

ΔS_univ = -ΔG/T(K)

Free energy of a chemical reaction ____ until a minimum value is reached. When this min is reached, an ___ state exists

decreases equilibrium

A process at constant T and P is spontaneous in direction in which ΔG ___ and ΔS_univ ____

decreases increases

Signs of ΔG and spontaneity:

(-): rxn is spontaneous forwards (+): rxn is nonspontaneous in forward direction and work must be supplied from surroundings 0: rxn is at equilibrium

The sign of ΔG will depend on sign and magnitude of ___ and ___. Value of T directly affects magnitude of ____

ΔH, TΔS ΔS

Using equation for ΔG using ΔH and ΔS, predict spontaneity: +ΔS and -ΔH +ΔS and +ΔH -ΔS and -ΔH -ΔS and +ΔH

ΔG = ΔH - T(K)ΔS spontaneous at all temps because ΔG will be - no matter what spontaneous at high temps because magnitude of ΔS > ΔH (exothermicity doesn't matter) Spontaneous at low temps because magnitude ΔH > ΔS (exothermicity is dominant) Not spontaneous at any temp (but reverse process is spontaneous at all temps)

will you ever need to calculate standard enthalpy or standard entropy of a system?

no! these values will always be provided for you :D

Main concept of 17.5 ____ is not enough to predict solubility; we must also consider ____ effects example given?

enthalpy; entropic Since dissolution of ionic solids produces heat in most cases we know ΔH will be positive so ΔS will usually overcome this to make ΔG negative. However this isn't always the case for example dissolution of LiF in water has a negative value of ΔS as the ions become surrounded by groups of water molecules: an order producing process

As we know, entropy changes in a system are determined by ____ probability. Because of this, ___ molecules have much higher entropy than both ____ and ____ So in general when a rxn involves ____, the change in ___ entropy is determined by relative # of molecules of ___ reactants and products Explain how to use this to predict sign of ΔS of a reaction

positional gas, liquids, solids gases, positional, gases if moles of gases increase, then ΔS will usually be positive. If moles of gases decrease, then ΔS will usually be negative. THIS CAN ONLY PREDICT, you can only know for sure from calculation.

#55 textbook

know how to explain answers

Third law of thermodynamics

the entropy of a perfect crystal at 0k is 0.

The more complex the molecule, the ____ the standard entropy

higher example 17.8

What is standard free energy change? ΔG* is it measurable? the more ___ the ΔG*, the further the reaction will go the right to reach equilibrium Example 17.9 and #67

change in free energy that will occur if reactants in their standard states are converted to products in their standard states. not measurable, only calculatable. negative

Know how to calculate *ΔG_f through standard values

easy af example 17.10 #73

At ΔG<0 a rxn is spontaneous or nonspontaneous? ΔG>0? There are several ways to force nonspontaneous rxns...

spontaneous nonspontaneous applying external energy or coupling reactions

Equilibrium is a _____ of two processes, one ____ of the other ____ of the rxn is related to the equilibrium

balance, opposite stoichiometry

What is the equilibrium constant for aA + bB ⇌ cC + dD

......[C]^c*[D]^d k = --------------- [A]^a*[B]^b

equilibrium constant stays the same when the ___ and ___ are the same

reaction and temperature

Equilibrium point occurs in ____ value of free energy available to a rxn system

lowest fig 17.8 and 17.9

second equation for ΔG? know what variables are

ΔG = ΔG* +RT(ln(Q)) R: 8.314J/k*mol T: temp (kelvin) Q: reaction constant

how are the reaction constants Q and K different?

K is at equilibrium, Q is not

What is not considered in calculation of Q or K?

pure liquids or solids on one side of the equation

How does free energy depend on pressure?

Equilibrium constant! Q can be calculated from the respective partial pressures instead of the concentrations... Example 17.13 #79

If ΔG = 0 and Q = k (this is equilibrium, as stated earlier), then solve for G* at ΔG* = 0 and k = 1, equilibrium contains... ΔG* < 0 and k>1 ΔG* > 0 and k<1

ΔG* = -RT*ln(k) note: notice how if k> 1 ΔG* will be negative and positive if k<1 (negative times negative) approx same amount of reactants and products mostly products mostly reactants figure 17.10 example 17.14 and 17.15 problem #83

at Equilibrium ΔG* = -RT*ln(k) = ΔH* - TΔS* solve for ln(k) ln(k) vs. ___ will be linear with ___ as the slope and ___ as the intercept value of k for a given reaction can be determined at various ___

ln(k) = (ΔH*/-R)(1/T)+(ΔS*/R) 1/T; ΔH*/-R; ΔS*/R temperatures problem #95

Semester 2 Exam 1 Flashcards

6.1-6.4, 17.1-17.9 (62 cards)