Laude Exam 3- thermo
Phosphine has a delta H of vaporization of 14.6 kj*mol and a delta S of vaporization of 78.83 J*mol*K. What is the normal boiling point of phosphine?
-87.8 degrees celsius use delta S= delta H/T equation
Is there a temperature above which the reaction of N2 (g) with H2 (g) to form NH3 (g) at 1 atm pressure would be non-spontaneous?
...
Which statements is/are true about the first law of thermo? 1) the internal energy (U) of the universe is conserved 2) the internal energy of a system plus that of its surroundings is conserved 3) the change in internal energy (delta U) of a system and its surroundings can have the same sign
1 and 2 are true because the first law states that the internal energy of the universe is conserved and since the system plus the surroundings is the universe, their sum is also conserved. Statement 3 is false because if the change in internal energy of the system and the surroundings had the same sign that the internal energy of the universe would be increasing or decreasing which violates the first law.
Which statements about the 1st and 2nd law of thermo are true: 1) energy of universe is always conserved 2)energy of a system is always conserved 3) energy of a system always increases 4) entropy of the universe always increases 5) entropy of the universe is always conserved 6)entropy of the system is always conserved
1 and 4 are true
3 definitions of internal energy:
1) the enthalpy after the correction for pressure volume work 2) Delta E= q+w 3) the difference between the final internal energy and the initial internal energy
5 features of the bomb calorimetry apparatus used to measure the internal energy of a reaction:
1) the large heat capacity of water is beneficial in measuring heat released by combustion reactions. 2) the volume of the reaction vessel is held constant to eliminate energy released as work 3) the heat capacity of the calorimeter should be known to accurately correct for any heat lost to it 4) the thermometer is placed in the water that surrounds the reaction vessel 5) large quantities of water surrounding the reaction vessel absorb the majority of the heat loss
What are the rotational and vibrational energies, respectively, for 10 molecules of I3-?
10 KT, 20 KT
The value of the W term in the Boltzmann equation is determined by which of the following? 1) Temp 2) number of orientations 3) number of atoms of molecules
2 and 3
Which of the following compounds has an experimental value for the absolute entropy that most deviates from the value predicted by the Boltzmann equation S=klnW? 1) C6H6 2) H2O 3) O2 4) CO 5) CO2
2) H2O because water has the greatest intermolecular forces (H-bonding) which will therefore create more order and that is not considered in the Boltzmann equation.
How much internal energy is associated with just the vibrational motion of 10 molecules of CO2?
20 KT. Because 9-3 for translational and then 6-2 for rotational since its linear so you are left with 4 which you multiply by 1/2 kT and then you get 2 which you multiply by the 10 molecules to get 20 kT.
Calculate Delta S of the universe for the condensation of methanol at 10 degrees celsius if the molar entropy from methanol (g) and methanol (l) are 240 J*mol and 127 J*mol respectively. Assume the standard boiling point of methanol is 65 degrees.
22 J*mol*K
What is the maximum positional entropy at absolute zero for 2 moles of CF3Cl?
23 J/K
What are the translational and rotational energies, respectively, for a mole of I3-?
3/2 RT, RT
Calculate the boiling point of a liquid for which Delta H of vaporization= 25 kJ/mol and Delta S of vaporization= 83 kJ/mol*K. Answer in units of K.
301 K
The standard enthalpy of formation if ammonium perchlorate (NH4ClO4) at 289 K is -295.31 kJ*mol. In the balanced equation, what is the sum of the coefficients of the reactants?
5
What is the residual entropy of 0.5 moles of CH3Cl?
5.76 mol*K use S=KlnW
The enthalpy of vaporization of acetone is 31.3 kJ/mol, and its boiling point is 56.3 C. What is the entropy change for the vaporization (boiling) of acetone?
95 J/mol K
Define formation reaction
A chemical reaction where only elements in their standard states are sued as reactants and exactly ONE mole of the compound is formed as the product.
Find Delta H of formation at standard conditions for HCl (g) given H--Cl bond enthalpy = +431 kJ/mol. Delta H of formation at standard conditions for H= +217.9 kJ/mp; Delta H of formation at standard conditions for Cl= +121.4 kJ/mol
Add up 217.9 and 121.4 and then subtract 431 to get -91.62 kJ/mol
If the standard enthalpy of combustion of octane (C8H18 (l)) at 298 K is -5471 kJ/mol, calculate the standard enthalpy of formation of octane. The standard enthalpies of formation of carbon dioxide and liquid water are -393.51 and -285.83 kJ/mol
Because it is talking about a combustion reaction we must remember that O2 will be on the left of the equation and CO2 will be on the right of the equation. It gives us the standard enthalpies of water and we know that must go on the right since there are hydrogens on the left. Then balance the equation to that it looks like this: C8H18 (l) + 25/2 O2 (g) --> 8CO2 (g) + 9H2O (l). Then subtract summation of the change in enthalpies of the reactants from products to get -249.55 kJ/mol.
For what reaction is Delta H at standard conditions referred to as the standard molar enthalpy of formation of carbon monoxide?
C (graphite) + 1/2 O2 (g) --> CO (g)
Methanol has a standard free energy of formation of -166 kJ*mol and Cyclohexane has a standard free energy of formation of 6.4 kJ*mol. Which of these is thermodynamically unstable?
Cyclohexane because it has a positive standard free energy of formation.
Delta H is often nearly equal to
Delta E
Delta H is sometimes exactly equal to
Delta E
the thermodynamic quantity most easily measured in a coffee cup calorimeter is
Delta H
Name 3 definitions of enthalpy or change in enthalpy.
Delta H= H final-H initial A state function that influences the spontaneity of a reaction Delta H= Delta E + P Delta V
For the reaction H2(g) + Cl2(g)-->2HCL(g) at constant pressure and temp, we would expect
Delta H=Delta E Because to find Delta n you do 2-2 which is zero which cancels out the second part of the equation in Delta E= Delta H-Delta n (RT). You are left with Delta E=Delta H
For heat of formation questions, what must be true?
Everything must be in its standard condition and the result must only be one mole. **remember Rebecca's analogy Have (g) No (g) Fear (g) Of (g) Ice (s) CoLd BeeR (l)
Ture or false: For a given transfer of energy, a greater change in disorder occurs when the temp is high.
False. Absolute entropy is greater at high temps, but the change in entropy is less at high temps. Think about the coughing at a concert analogy. You can also see this in the equation Delta S=q/T. Since T is in the denominator, the smaller the number the greater Delta S will be.
Methanol explodes in an exothermic combustion reaction to make carbon dioxide and steam (water vapor). Would more heat be generated by the system in a coffee cup or in a bomb calorimeter.
First balance the equation and the find Delta n which is 6-3 which equals +3 and therefore work is negative because the greater the number of molecules the to more work is being done on the surroundings. The coffee cup calorimeter therefore does work while bomb calorimeters do not do any work so heat output is maximized in a BOMB CALORIMETER.
Assuming the gases are ideal, consider the work done on the system in each of the following reactions: a) the conversion of one mole of brown nitrogen dioxide into colorless dinitrogen tetroxide at 30 degrees celsius: 2 NO2 (g)-->N2O4 (g) b) the decomposition of one mole of an air pollutant, nitric oxide, at 300 degrees celsius: 2 NO (g)--> N2 (g)+O2 (g) calculate the work done on (a) and the work done on (b).
For work calculations remember that work = (-P)deltaV = (-deltaN)RT. A) Temperature is 303 Kelvin because you add 273 to the stated 30 degrees. R is Ridberg's constant which is 8.31. So -(-0.5)(8.31)(303)=1.26 kJ. B) Delta N is zero so since it is multiplied by RT, the total work is zero.
Which of the compounds will form a perfect crystal at 0 K? H2, HCl, CCl4, CO, O3, CO2
H2, CCl4, CO2
In order for an endothermic reaction to be spontaneous
In an endothermic reaction heat is gained by the system. Delta S of the universe equals Delta S of the system plus Delta S of the Surroundings. We know that Delta S of the system will be positive because it is gaining heat so in order for the reaction to be spontaneous (+Delta S of the universe) the Delta S of the surroundings which will be negative because it is losing heat must be smaller.
Hot coffee in a vacuum flask (thermos) is an example of what type of system?
Isolated system because neither energy nor matter is transferred to the surroundings.
Which of the following have a heat of formation equal to zero? O2(g), O2(l), H2(g), H2(l), H2O(g), H2O(l)
O2(g) and H2(g) because molecules in their native state at STP have a heat of formation of zero.
The change in enthalpy for a process is equal to what?
The amount of heat absorbed at constant pressure.
The 2nd law of thermo requires that spontaneous processes generate entropy, wither in the system or in the surroundings. What is the thermodynamic driving force for dissolving a solid in a liquid of it is an endothermic process (which reduced the entropy of the surroundings)?
The driving force is the increase of the entropy of the system. Since the reaction happens the entropy of the universe must be positive therefore since the entropy of the surroundings is reduced, the entropy of the system must increase.
What is the internal energy associated with the translation of 6 water molecules?
The energy is 3/2 kT per mole, so for six water molecules 6(3/2 kT) = 9 kT
Calculate the enthalpy change for the reaction 2 SO2 (g) + O2 (g) --> 2 SO3 (g) Delta H of formation for SO2 (g) = -16.9 kJ/mol Delta H of formation for SO3 (g) = -21.9 kJ/mol
To calculate the Delta H of reaction: Summaton of (n)(delta H of formation of products)- summation of (n)(delta H of formation of reactants). So plugging in: (2 mol)(-21.9)-(2 mol)(-16.9)-(1 mol)(0) = -10 kJ/mol rxn
5 grams of hydrocarbon fuel is burned in a calorimeter that contains 207 grams of water initially at 25 degrees Celsius. After combustion, the temp is 26.55 degrees Celsius. How much heat is evolved per gram of fuel burned? The heat capacity of the calorimeter (hardware only) is 92.3 J/C. and the specific heat of water is 4.184 J/gC.
To find the amount of heat evolved by the reaction you must add up the Delta H of the water and the Delta H of the calorimeter. To solve for the Delta H of the water you do (Specific Heat)(mass of water)(change in temp). So.. (4.184)(207)(1.55)= 1342 J. Then to solve Delta H of the calorimeter you do (Specific Heat)(change in temp). So.. (92.3)(1.55)= 143 J. Then since the total heat evolved is Delta H of the water+Delta H of the calorimeter, 1342+143= 1485 J. The question asks for the heat evolved PER GRAM, so since there are 5 grams of hydrocarbon burning, we divide 1485 by 5 and get 279 J/g as the final answer.
Consider the following specific heats: copper- 0.384 J/g C lead- 0.159 J/g C water- 4.18 J/g C glass- 0.502 J/g C Which substance once warmed will keep you warm at a long cold football game?
Water has the highest specific heat of the substances listed so it has the capacity to emit the largest quantity of heat with minimal temperature loss; the emitted heat keeps you warm. The substance continues to warm you until its temperature is at or below your body temp.
When will a reaction always happen?
When Delta H is negative and Delta S is positive.
When are Delta H and Delta E close in value?
When pressure volume (expansion) work is small
Definition of work:
Work is organized molecular motion since it is displacement of a solid against a force.
A thermodynamically stable compound is a compound with..
a negative standard free energy of formation
A thermodynamically unstable compound is a compound with..
a positive standard free energy of formation
water changing from a solid to a liquid is____.
an endothermic process because heat is required for melting.
C2H2 (g) + H2 (g) --> C2H4 (g) at constant pressure. Work is done on the _____.
because work=-(delta n)RT and delta n=-1, the sign of work must be positive indicating hat work is done on the system.
No work is done in a reaction occurring in a
bomb calorimeter
Ethane is C2H6. Consider the reaction: 2C graphite (s) + 3 H2 (g) --> C2H6(g) and the delta G of reaction=7.86 kj*mol. The ______ of ethane is ______ and therefore ethane is ____.
decomposition; spontaneous; unstable
If heat absorbed for a process at constant pressure is negative, the process must be:
exothermic
A charcoal briquette burns. this process is:
exothermic. the briquette releases heat as it burns.
Which physical state has the greatest entropy?
gas
heat flow is considered positive when heat flows ___ a system; work is considered positive when work is done ___ a system.
into; on
Exothermic processes end with ___ energy that they began with; the energy was transferred to the ____.
less; surroundings
A measurement of the heat produced when a known amount of sugar burns in a constant volume calorimeter would enable us to first calculate the...
molar internal energy change for the combustion of sugar. Delta E= q+w. w=0 for a comb calorimeter because Delta V=0. So Delta E=q
in the absence of any external source of light or energy, the production of a sugar as in the photosynthesis reaction, would have what kind of value for delta S?
negative because many gaseous CO2 molecules are used to make few sugar molecules so the randomness of the system decreases.
if a process is carried out at constant pressure and the volume of the system decreases, then delta V is ____ and the work is _____.
negative; positive. because work=-PdeltaV and V is negative and a negative times a negative leaves us with positive work.
When no heat is exchanges and pressure-volume work os done on the system, Delta E is--
positive
When a sugar cube dissolves in a cup of coffee (an endothermic process), entropy changes of the sugar plus the water, the surroundings, and the universe are, respectively...
positive, negative, positive delta s of the system is positive because it is becoming more disordered as it becomes a mixture as the sugar dissolves. the delta s of surroundings is negative because it is giving heat to the system (losing heat) so it is becoming more ordered. the delta s of the universe is positive because the reaction is spontaneous-- second law of thermo. (in spontaneous changes, the universe tends toward a state of greater disorder)
In an endothermic reaction the change in energy is ____ and heat is ____.
positive; absorbed
At constant volume what is Delta E for a system equal to?
q
A 0.10 g piece of chocolate cake is combusted with oxygen in a bomb calorimeter. the temp of 4,000 g of water in the calorimeter raised by 0.32 K. (The specific heat of the water is 1.0 cal/gK and the heat of vaporization of water is 540 cal/g). What is Delta E for the combustion of chocolate cake? Assume no heat is absorbed by the calorimeter.
q= (1)(4000)(0.32) = 1280 cal released. Since it is released it is negative. There was only 0.10 grams of cake so (-1280 cal/0.10 g)(kcal/1000 cal)= -12.8 kcal/g <-- the answer
Sublimation
results in a significant increase in the volume of the system, allowing it to do work on its surroundings, i.e. the pressure-volume work function is negative. Sublimation is also an endothermic process, making heat positive. Since the process described "happens" as a given problem, the change in free energy must be negative. Change in entropy must be positive since a solid is becoming a gas.
which of the following statements is true? 1) if a reaction has deltaG of reaction= -275 kj/mol, it must proceed rapidly toward equilibrium 2) if a reaction is spontaneous, it is not spontaneous in the reverse direction 3) all endothermic reactions are non spontaneous 4) a spontaneous reaction for which the entropy change is negative is entropy driven. 5) the value of delta G of reaction is not dependent on temperature
statement 1 is false because a negative Delta G means the reaction is spontaneous which means the reaction can occur but does not indicate how rapidly. statement 5 is false because to predict spontaneity use Delta G=Delta H- T(Delta S). Delta G is clearly dependent on temperature. statement 3 is false because an endothermic reaction can be spontaneous if delta S is positive and T is high enough so that T(delta S) dominates the value of delta G. statement 4 is false because if entropy change (delta s) is negative, the only way delta G can be negative to be spontaneous is if delta H is very negative to dominate. therefore it is enthalpy driven, not entropy driven. statement 2 is true because if a reaction is reversed, then delta g, delta h and delta s are all reversed in sign.
Which statements is/are true about the first law of thermo? 1) the internal energy of the universe is always increasing 2) internal energy lost by the system is always gained by the surroundings 3) the universe is an isolated system
statement 1 is false because the energy of the universe is constant. statement 2 is true because in order to keep the energy of the universe constant, the system and the surroundings must counteract each other. statement 3 is true because the universe conserves both energy and matter.
Delta U (change in internal energy) for a process is equal to:
the amount of heat absorbed at constant volume.
At a certain elevated temperature and pressure, diamond and graphite are in equilibrium. When graphite changes to diamond under these conditions...
the change in molar Gibbs free energy is zero
total motational energy is a measure of
the internal energy-- not enthalpy
The weaker the dipole moment...
the more closely it will be to the predicted residual entropy because it will land more random orientations and will not be influenced to land in a particular orientation
2 SO2 (g) + O2 (g) --> 2 SO3 (g) has a reaction enthalpy of -10 kJ/mol rxn. predict the spontaneity.
the reaction is spontaneos only at low temperatures since delta H is negative and delta S is negative so delta G can be negative at low temps.
When given a list of reactions, how do you determine which has the largest increase in entropy?
the reaction with the greatest positive value for delta n gas will have the greatest value of delta S of reaction.
If Delta n for gases is zero then..
the work done in the process occurring at constant pressure is zero.
Bond Energies are approximate because
they are averages over a number of molecules. The C--H bonds in CH4 an CH3OH are not the same and require different amounts of energy to break.
