Essay Questions - Pcmac

Essay Questions1991

BCl3(g) + NHr(g) ) Cl3BNH3(s)

The reaction represented above is a reversible reaction.

(a) Predict the sign of the entropy change, AS, as the reaction proceeds to the right. Explain your prediction.(b) If the reaction spontaneously proceeds to the right, predict the sign of the enthalpy change, AFL Explain your

prediction.(c) The direction in which the reaction spontaneously proceeds changes as the temperature is increased above a

specifi c temperature. Explain.(d) What is the value of the equilibrium constant at the temperature referred to in (c); that is, the specific

temperature at which the direction of the spontaneous reaction changes? Explain.

1993 2 cqHrc.,)+ 13 ozk) -+ 8 co2(g) + r0 Hzo(/)

The reaction represented above is spontaneous at 25"C.Assume that all reactants and products are in their standard

s1ate.

(a) Predict the sign of ASo for the reaction and justifr your prediction.(b) What is the sign of AGo for the reaction? How would the sign and magnitude of AGo be affected by an increase

in temperature to 50"C? Explain your answer.

(c) What must be the sign of Alf for the reaction at 25"C? How does the total bond energy of the reactants

compare to that of the products?(d) **When the reactants are place together in a container, no change is observed even though the reaction is

known to be spontaneous. Explain this observation.

7994

2 H2S(g) + SOzk) + 3 S(s) + 2HzO(s)At298 K, the standard enthalpy change, A11o for the reaction represented above is -145 kilojoules.(a) Predict the sign of the standard entropy change, ASo, for the reaction. Explain the basis for youl prediction.(b) At298 K, the forward reaction (i.e.,toward the right) is spontaneous. What change, if any, would occur in the

value of AG' for this reaction as the temperature is increased? Explain your reasoning using thermodynamicprinciples.

(c) What change, if any, would occur in the value of the equilibrium constant, K,n, for the situation described in(b)? Explain your reasoning.

(d) The absolute temperature at which the forward reaction becomes nonspontaneous can be predicted. Write theequation that is used to make the prediction. Why does this equation predict only an approximate value for thetemperature?

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(a)

(b)

1-997

For the gaseous equilibrium represented below, it is observed that greater amounts of PCll and Cl2 are producedas the temperature is increased.

PCl5(g) ) PCI:(g) + Clz(s)

What is the sign of ASo for the reaction? Explain.

What change, if any, will occur in AGo for the reaction as the temperature is increased. Explain yourreasoning in terms of thermodynamic principles.

(") ** If He gas is added to the original reaction mixture at constant volume and temperature, what will happento the partial pressure of Cl2? Explain.

(d) **If the volume of the original reaction is decreased at constant temperature to half the original volume,what will happen to the number of moles of Clz in the reaction vessel? Explain.

1999

Answer the following questions in terms of thermodynamic principles and concepts of kinetic moleculartheory.

(a) Consider the reaction represented below, which is spontaneous at 298 K.

COzk) + 2 NH:(g) -+ CO(ltlHz)z(s) + H2O(l); LH"zgs: *134 kJ/mol

(i) For the reaction, indicate whether the standard entropy change, ASozsr, is positive, ornegative, or zero. Justifo your answer.

(ii) Which factor, the change in enthalpy, L H"2e8, or the change in entropy, ASozss, providesthe principal driving force for the reaction at 298 K? Explain.

(iii) For the reaction, how is the value of the standard free energy change, AGo, affected by anincrease in temperature? Explain.

(b) * * Some reactions that are predicted by their sign of AG o to be spontaneous at room temperaturedo not proceed at a measurable rate at room temperature.

(i) Account for this apparent contradiction.

(ii) A suitable catalyst increases the rate of such a reaction. What effect does the catalyst have onAc'for the reaction? Explain.

2000O:(g) + No(g) -+ OzG) + NOz(g)

Consider the reaction represented above.

(a) Refening to the data in the table below, calculate the standard enthalpy change, A.Ff, for the reaction at

25oC. Be sure to show your work.

fi3{sj N{X8i N{t:{ll}

Standanl *l{lr*lp1'rll*rma1i*n, Sff09. itt 25'1C

ikJ rrxrl 1.1

I jl3 i"rf] :33

(b) Make a qualitative prediction about the magnitude of the standard entropy change, ASo, for the reaction

at25"C. Justiff your answer.

(c) On the basis of your answers to parts (a) and (b), predict the sign of the standard free-energy change,

AG", for the reaction at25"C. Explain your reasoning.

(d) Use the information in the table below to write the rate-law expression for the reaction, and explain howyou obtained your answer.

Ex1:eriurcnlNutrtL*r

lnili*l l{}3llm*rlL-1j

l:iitinl fli*'l. ,., _1 .llllot t- '1

lnili;rl Rat* olF*un:rti*rt *l' N{-}r

lrlol L*l l;-l'j

I 0_{}{}t {l {1.{xl 1 {} ,1'

: il.0ill{:} {.}.ilii3il ?r:

^) il.{i{}:{) {i.{}t}1 it fx4 0.00?{) {1.{i*:il 4.r

(e) The following three-step mechanism is proposed for the reaction. Identiff the step that must be the

slowest in order for this mechanism to be consistent with the rate-law expression derived in part (d).

Explain.

Step I: 03 + NO ---+ O + NO:

Step II: O + O: ---+ 2 Oz

Step III: NO3 + NO ---' 2 NOz

2003

Answer the following questions that relate to the chemistry of nitrogen.

(a) Two nitrogen atoms combine to form a nitrogen molecule, as represented by the following equation.2 N(g) -+ Nz(s)

Using the table of average bond energies below, determine the enthalpy change, AH, for the reaction.

Bond Average Bond Energy (kJ mol-l)

N_NN=NN=N

160

420

950

(b) The reaction between nitrogen and hydrogen to form ammonia is represented below.

Nz(g) + 3 Hz(s) -) 2 NH:(g) Allo: -92.2kJ

Predict the sign of the standard entropy change, ASo, for the reaction. JustiS'your answer.

(c) The value of AG" for the reaction represented in part (b) is negative at low temperatures but positive at hightemperatures. Explain.

(d) **When Nzk) and H2(g) are placed in a sealed container at a low temperature, no measurable amount ofNH:(g) is produced. Explain.

2004B

Nz(g) + 2Hz@) ) N2Ha@) AH 2eg: +95,4 kJ mol-l; ASo zss : -176 J K-l mol-l

Answer the following questions about the reaction represented above using principles of thermodynamics.

(a) On the basis of the thermodynamic data given above, compare the sum of the bond strengths of thereactants to the sum of the bond strengths of the product. Justify your answer.

(b) Does the entropy change of the reaction favor the reactants or the product? Justifu your answer.(") For the reaction under the conditions specified, which is favored, the reactants or the product? Justify

your answer.(d) Explain how to determine the value of the equilibrium constant, K,n, for the reaction. (Do not do any

calculations.)(e) Predict whether the value of K"n for the reaction is greater than l,equal to 1, or less than 1 . JustiS' your

prediction.

(

Problems

1_990

Standard Free Energies of Formation at 298 K

Substancec2HaCl2@)C2H5Cl(g)HCI(g)clz(g)

@1*80.3*60.5*95.3

0

Average Bond Dissociation Energies at298K

BondC-HC-CC-CICI-CIH-CI

Energli. kJ mol-l414

341317243431

The tables above contain information for determining thermodynamic properties of the reaction below.

CzHsCl(s) + Clzk) + C2HaCl21g) + HCI(sr)

(u) Calculate the A1l for the reaction above, using the table of average bond dissociation energies.

(b) Calculate the ASo for the reaction at298 K, using data from either table as needed.(c) Calculate the value of K"n for the reaction at298 K.(d) What is the effect of an increase in temperature on the value of the equilibrium constant? Explain

your answer.

r992Clz(g) + 3Fzk) -+ 2ClF3@)

ClF3 can be prepared by the reaction represented by the equation above. For CIF: the standard enthalpy offormation, L,Hy",is-163.2 kilojoules/mole and the standard free energy of formation, AG1o, is -123.0kilojoulesimole.

(a) Calculate the value of the equilibrium constant for the reaction aI298K.(b) Calculate the standard entropy change, ASo, for the reaction at298K.(c) If ClF3 were produced as a liquid rather than as a gas, how would the sign and the magnitude of AS for

(d) ffffillf"'"13ffi:f3],ffi1f;clz(g) and crF3(g ) are222.e6 joules per mole .Kelvin and2Bt.s0joules per mole ' Kelvin, respectively.(i) Account for the larger entropy of ClF3k) relative to that of Cl2(g).(ii) Calculate the value of the absolute entropy of F2(g) at 298K.

1995

Propane, c:Hr, is a hydrocarbon that is commonly used as fuel for cooking.(a) Write a balanced equation for the complete combustion of propane gas, which yields COz(g) and H2O(f .

(b) Calculate the volume of air at 30"C and I .00 atmosphere that is needed to burn completely i 0.0 grams ofpropane. Assume that air is 21.0 percent Oz by volume.

(c) The heat of combustion of propane is -2,220.1kJ/mol. Calculate the heat of formatio n, AHyo of propanegiven that LHy" of H2O(f : -285.3 kJ/mol and A,H1" of CO2(g) : -393.5 kJ/mol.

(d) Assuming that all ofthe heat evolved in buming 30.0 grams ofpropane is transferred to 8.00 kilograms ofwater (specific heat : a.18 J/g'K), calculate the increase in temperature of water.

1998

C6H5OH(s) + 7 Oz(d-+ 6 CO2(g) + 3 H2O(/)

When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely bumed according to the equationabove, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questionsthat follow.

Calculate the molar heat of combustion of phenol in kilojoules per mole at25"C.

Calculate the standard heat of formation, AIf 1, of phenol in kilojoules per mole at25'C.

Calculate the value of the standard free-energy change, AGo for the combustion of phenol at 25.C.

If the volume of the combustion container is 10.0 liters, calculate the final pressure in the containerwhen the temperature is changed to 110"C. (Assume no oxygen remains unreacted and that allproducts are gaseous.)

(a)

(b)

(c)

(d)

Standard Heat ofFormation, A,If1,

25"C (kJlmol

Absolute Entropy, S",at25"C (J/mol-K)

-285.85

5OH(s)

20038

In an experiment, a sample of an unknown, pure gaseous hydrocarbon was analyzed,. Results showed thatthe sample contained 6.000 g of carbon and I.344 g of hydrogen.

(a) Determine the empirical formula of the hydrocarbon.(b) The density of the hydrocarbo n at 25" C and 1 .09 atm is 1 .96 g L-r .

(i) Calculate the molar mass of the hydrocarbon.(ii) Determine the molecular formula of the hydrocarbon.

In another experiment, liquid heptane, CzHro (/), is completely combusted to produce COzk) and H2O(/),as represented by the following equation

CzHro(/)+ 1l Ozk) -+ 7 CO2(g) + 8 H2O(f)

The heat of combustion, AHo,o^6,for one mole of C7H16(f is -4.85 " 103 kJ.

(") Using the information in the table below, calculate the value of LlffiorCzHro (f in kJ mol-1.

C*tn1t*ntril ,!ffi {kJ rnr.rl*11

c{ }.{s) *39i.5

H:t]{4 *3i'5.X

(d) A 0.0108 mol sample of C7H16 (f is combusted in a bomb calorimeter.

(i) Calculate the amount of heat released to the calorimeter.

(ii) Given that the total heat capacity of the calorimeter is 9.273 kJ 'C -1, calculate the temperaturechange of the calorimeter.

2004

32 Fe(s) * ; Or(g) --+ FezO:(s) LH't : -824 kJ mol-l

Iron reacts with oxygen to produce iron(Il| oxide, as represented by the equation above. A 75.0 g sampleof Fe(s) is mixed with 1i.5 L of Ozk) at2.66 atm and 298 K.

(a) Calculate the number of moles of each of the following before the reaction begins.

(i) Fe(s)(ii) oz(s)

(b) Identiff the limiting reactant when the mixture is heated to produce Fe2O3(s). Supporl your answerwith calculations.

(c) Calculate the number of moles of Fe2O3(s) produced when the reaction proceeds to completion.

(d) The standard free energy of formation, AGoy, of Fe2O3(s) is -740. kJ mol at298K.

(i) Calculate the standard entropy of formation, A,Soy, of Fe2O3(s) at298 K.Include units withyour answer.

(ii) Which is more responsible for the spontaneity of the formation reaction at298 K, the standardenthalpy of formation, AIfy, or the standard entropy of formation, AS.T? Justifu your answer.

The reaction represented below also produces iron(Ilf oxide. The value of AFf for the reactionis -280. kJ per mole of FezO:(s) formed.

2Feo(s) + iOrfsl --- FezO:(s)

(e) Calculate the standard enthalpy of formation, AH"1 of FeO(s).

20068

Answer the following questions about the thermodynamics of the reactions represented below.

Reactionx, 1Ir(r) *1 ctr(g) ) ICl(g) LIqJ:18kJmol-l, ASo:pa: TgJK*] mol-l

Reactiony: llr(s) *lgrr(l) ) IBr(g) alfl:41 kJmol-l, ASo2e5: t24JK-1 mol-ltai-l

(a) Is reaction X, represented above, spontaneous under standard conditions? Justifu your answer with acalculation.

(b) Calculate the value of the equilibrium constant, Keq, for reaction X at25oC.

(c) **What effect will an increase in temperature have on the equilibrium constant for reaction X ?

Explain your answer.

(d) Explain why the standard entropy change is greater for reaction Y than for reaction X.

(e) Above what temperature will the value of itre equilibrium constant for reaction Y be greater than i.0 ?Justifu your answer with calculations.

(f) For the vaporization of solid iodine, I2(s) --- Iz(s), the value of LIf 2s6 is 62 kJ mol-r. Using thisinformation, calculate the value of LHo 2e6 for the reaction represented below.

Iz(s)+Clz(g)a2IC1(g)