Name: Date: /Hour: Question What do chemical reactions usually do? What is a calori eter? What is the SI unit for temperature? What is heat? What is the SI unit for heat? What does the quantity of energy transferred depend upon? What are the units for specific heat? What is its symbol? Write the equation that is used to determine the quantity of energy gained or lost with a change in temperature. Where does the energy released from a reaction come from? What do we call a reaction that gives off heat? Sketch a reaction energy diagram (graph) for this type of reaction. What do we interpret the coefficients in a thermochemical reaction to be? The coefficients are never interprete as what? why? Whatis enthalpy? What is its symbol? How is it different than just plain heat? By using just the symbol for enthalpy, how do we show a reaction is exothermic? How do we show that it is endothermic? What concepts should we keep in mind when we use thermochemical equations?
Transcript
Name: Date: /Hour:
QuestionWhat do chemical reactions usually do?
What is a calori eter?
What is the SI unit for temperature?
What is heat? What is the SI unit for heat?
What does the quantity of energy transferred depend upon?
What are the units for specific heat? What is its symbol?
Write the equation that is used to determine the quantity of energy gained or lost with a change intemperature.
Where does the energy released from a reaction come from? What do we call a reaction that givesoff heat? Sketch a reaction energy diagram (graph) for this type of reaction.
What do we interpret the coefficients in a thermochemical reaction to be? The coefficients are neverinterprete as what? why?
Whatis enthalpy? What is its symbol? How is it different than just plain heat?
By using just the symbol for enthalpy, how do we show a reaction is exothermic? How do we showthat it is endothermic?
What concepts should we keep in mind when we use thermochemical equations?
What are the AH - for the elements? Why?
What do we know about elements with (+) AH values?
State Hess' law
What 2 factors allow chemist to predict spontaneity?
What is entropy? What sign does it have when its is spontaneous? What does that mean?
What is Gibbs Free Energy? What is free energy Change?
What does it ean when AG is (+)? (-)?
What is a reaction mechanism?
What are reaction intermediates?
Briefly explain collision theory.
What is activation energy?
In its brief existence, what is an activated complex like?
What are the four things that influence reaction rates?
Specifically what does a catalyst do to a reaction?
Chemist: Date: /Hour:
Some Like It Hot= mcAT Where q is th heat being give off or absorb d, it is measured in Joules, or J. m is the mass, measured in grams. <
the spec fic heat The specific heat is the amount of e ergy, J, needed to r ise the temperature of 1 g of a substance,l0C It is
measured m J/(g. C). Every subst ce has its own specific he t. Use table 17-1 on p.513. T is the chan e in temperature. AT =
Tj-Ti. It is measure in C
I. How much heat is required to change t e temperat re of 80.0 g of Alumin m by 4 °C.
2. By ow many degrees Celsius will the temperature of 200.0 g of ammonia increase if this substance absorbs
540.0 J of energy? The specific heat of ammonia is 2.189 J/g°C.
3. How uc energy is required to eat an iron nail it a mass of 7.0 g from 25°C until it becomes re hot at750°C?
4. A 30 g sam le of an unkno n metal is heated from 2.0°C to 59.2°C. During the process, 1000 J of energyis absorbed by the metal. What is the specific heat of metal? What is the metal?
Whe a 50.0 g piece of nickel absorbs 350.0 J of beat, the temperature of the nickel cha ges fro 20.0 °C to36.0 C What is the spec fic heat of nickel?
6. Calculate the a ou t of heat that must be removed from 200.0 g of ethyl alcohol to cool it from 25.0 °C to10.0 C
7 Calculate the quantity of heat needed to warm water from 12oC to 36oC to repare a cup of coffee. The cupholds 250 ml of water. For water 1 ml = 1 g.
8. If 5750 J of energy are added to a 455granite? f03
of gramte at 24.0 °C, What is the final temperature of the
9 A sol r heating specialist is considering parraffm as a possible solar heat cojlector. How many grams qfparaffin would be needed to collect as much energy as 4.78 * 103 kg of w ter 51
10. When 198 J are added to 5.0 g of methane gas at -10 °C, the temperature of the methane gas rises to 8 °C.Calcul te the specific heat of methane.
4.
5.
6.
7.
8.
Bond Energies
Bond Bond energy(kJ/mol)
Number ofbonds broken
Energyrequired (kJ)
Number ofbonds formed
EnergyReleased (kJ)
H-O 459 4 1836H-H 436 2 8720=0 498 1 498
Sum 1836 1370Result 466
Thus, this is an endothermic reaction (energy required) that absorbs 466 kJ.The thermochemical equation is 2H2O + 466 kJ -» 2H2 + 02
9. Calculate the heat of reaction for Hz + C!2 -> 2HCJ by completinga table similar to the one above. Write the thermochemical equation for this reaction.
10. Calculate the molar heat of combustion \f0r C2H5OH
Bond Bond energy(kJ/mol)
Number ofbonds broken
Energyrequired (kJ)
Number ofbonds formed
EnergyReleased (kJ)
C-C
C-H
c=oH-O
2. Which process releases energy: breaking a bond or forming a bond?
3. Which process requires energy: breaking a bond or forming a bond?
Define bond energy.If the energy used to break bonds is greater than the energy released in the formation of new bonds,is the reaction endothermic or exothermic?if the energy used to break bonds is less than the energy release in the formationof new bonds, is the reaction endothermic or exothermic?Look at the table of selected bond energies to the right.a. Which is the strongest single bond on this table?b. Which is the weakest single bond on this table?
(Note: This is not a question. It is an explanation of how to calculate the to al energy from areaction, based on bond energies. There are supposed to be e pty spaces in the table.)
Let s examine the electrolysis of water (fig 4 on page 587). The general reaction is2H20 -» 2H2 + C2 or H-O-H H-O-H -> H-H H-H + 0=0
The overall heat of reaction can be calculated as follows:
Bond
typeEnergykJ/mol
Br-Br 193C-Br 288C-C 348C=C 614G C 839C-Ci 330OF 488C-H 413
SHORT ANSWER Answer the following questions in the space provided.
1. For elements in th ir standard state, the value of Hf is
a
2. The formation and deco position of w ter can be represented by the following thermochemicalequations:
H2(s) + Oiig) H20(g) + 241.8 kJ/mol
H20(Z) + 241.8 kJ/mol -» H2(g) + o2(g)
a. Is heat being taken in or released s liquid H20 decomposes?
b. What is the ppropriate sign for the ent alpy ch nge in thisdecomposition reaction?
PROBLEMS Write the answer on the line to the left. Show all your work in the spaceprovided.
3. If 200. g of w ter at 20°C bsorbs 41 840 J of heat, what will its finaltemperature be?
4. Aluminum has a specific heat of 0.900 J/(g*°C). Ho much energy in kJ isneeded to raise the temperature of a 625 g bloc of aluminum from 30.7°Cto 82.1°C?
5. T roducts in a reaction have a total h at content of 458 kJ/mol and the react nts have total heatcontent of 658 U/moL
a. What is the value of AH for this reaction?
Name Date Class
SECTION 17-1 continued
b. Which is the more stable part of this syst m, the reactants or theproducts?
6. The heat of combustion of acetylene gas is 1301.1 kJ/mol of C2H2.
a. Write the balanced thennoche ical equation for the complete combustion of C2H2.
b. If 0.25 mol of C2H2 react according to the equ tion in part a, how muchheat is released?
c. How many grams of C2H2 are needed to react according to die equation i art a to release 3 00 kJ of heat?
7.. M, flOKT
hen 1 mol of AI2O3 is formedof heat ar liber ted. Determine the AH for the reaction between A1 andFe304 if the heat of formation for Fe304 is 1120.9 kJ/mol.
8Al(s) + 3Fe304(5) 4A1203( ) + 9Fe(r)
8. Use the dat in App ndix Table A-14 of the text to determine the AH of thefollowi g reaction.
2H202(/) -> 2H20(/) + 02(g)
Hess Law of Heat SummationName Hour
Determine the AH for the following reactions. If information is not provided, lookit up on a Thermodynamic Values information sheet.
1. Oxidation of FeO to Fe203. 2FeO(s) + % 02(g) -+ Fe203(s)
2. Production of ammonium fluoride. NH3(g) + HF NH4F(s)
4. Calcium oxide and sulfur trioxide. CaO(s) + S03(g) CaS04(s)
Use these equations to solve:H20(i) + S03(g) - H2S04(l) AH = -132.5 kJ/molH2S04(l) + Ca(s) CaS04 (s) + H2(g) AH = -602.5 kJ/molCa(s) + 1/2 02 (g) CaO(s) AH = -634.9 kJ/molH2(g) + 1/2 02 (g) H20(l) H = -285.8 kJ/mol
5. Sodium oxide and sulfur dioxide. Na20(s) SOaCg) Na2S03(s)
6. Oxidation of 1-butanol to make butanoic acid.C4H90H(I) + 02(g) -> C3H7COOH(l) + HzO(l)
Use these equations to solve:Combustion of butanol:C4H9OH(l) + 602(g) -> 4COz(q) + 5H20(I)Combustion of butanoic acid:
CsHyCOOHfl) + 502(g) 4C02(g) + 4H20(I)
AHC = -2675.9 kJ. of
AHC = -2183.6 kJ/mol
7. In locations where natural gas, methane, CH4, is not readily available, manypeople burn propane, CsHs, which is elivered by truck an stored s a liquidunder pressure.a. Write the che ical equations for the co plete combustion of 1 mol ofmethane, and for 1 mol of propane.
b. Calculate the enthalpy change for each reaction to determine the amount ofheat evolved by burning one ol of each fuel.
c. Using the molar heats of combustion you have calculated, determine the heatoutput per kilogram of each fuel. Which fuel yields more heat per unit mass?
Name Date Hour
HESS LAW
1) Calc. AH for Fe O j + 3 CO(g) -~> 2 Few + 3 CO Given AH
9 Determine the AHr for F2 (g) + H20 (1) 2HF ( ) + 02( )
10. Determine the AHr for Na20 (s) + S02 (g) Na2S03 (s)
11. Calculate the AHr for CaO(s) + S03( ) -> C S04(s)
ENTROPY Name
Entropy is the degree of randomness in a substance. The symbol for change inpy is AS.
Solids are very ordered and have low entropy. Liquids and aqueous ions have moreentropy because they move about more freely, and gases have an even larger amountof entropy. According to the Second Law of Thermodynamics, nature is alwaysproceeding to a state of higher entropy.
Determine whethe the folio ing reactions sho an increase or decrease in entropy.
1. 2KC103(s) 2KCI(s) + 302(g)
2. H20(l) H20(s)
3. N2(g) + 3H2(g) 2NH3(g)
4. NaCi(s) -» Na+(aq) + Cl'(aq)
5. KCI(s) KCI(i)
C02(s) C02(g)
7. H+(aq) + C2H302-(aq) HC2H303(i)
8. C(s) + 02(g) C02(g)
9. H2(g) + Ci2(g) 2HCi(g)
10. Ag+(aq) + Cl (aq) -> AgCI(s)
11. 2N205(g) 4N02(g) + 02(g)
12. 2AI(s) + 3I2(s) 2AI!3(s)
13. H+(aq) + OH-(aq) H20(!)
14. 2NO(g) N2(g) + 02(g)
H Cg) H20(l)
Chemistry IF8766
POTENTIAL ENERGY DIAG AM
A + B ** C + D + energy
Answer the questions using the graph above.
1. Is the above reaction endothermic or exothermic?
2. What letter represents the potential energy of the reactants?
3. What letter represents the potential energy of the products?
4. What letter represents the heat of reaction (AH)?
5. What letter represents the activation energy of the forward reaction?
6. What letter represents the activation energy of the reverse reaction? _
7. What letter represents the potential ene gy of the activated complex?
8. Is the reverse reaction endothermic or exothermic?
9. If a catalyst were added, what letter(s) would change?
Chemistry 1F8766
Nam Date Class
CHAPTER 17 REVIEW
Reaction Energy and Reaction Kinetics
SECTION 17-2
SHORT ANSWER Answer the following questions in t e space provided.
1. For the following examples, state hether the change in entropy favors the for ard or reversereaction
a. HCKO HCKs)
b. CsH OftCa r)
c. 2NH, -) N v> +
d. 3C2H4(A )*±Ct,H|J(/)
2. a. Wiite an equation that \haw.s the relationship between enthal .entropy, and free energy.
b. For a reaction to occur pontaneously, the sign f C h uldbe
3. Consider the following reaction NH,(g) + H,0(/) 2 NHj (oc/) + + heat energy
a. The enth l y factor (as s the forward reaction. True or False
b. The sign of T S is negative This eans the entrop factorfavors the
c. Given that AG for the above reaction is positi e, hich term is gre ter m a nitude andtherefore predomin tes, T S or //?
4. Consi er the following equation for the vaporization of water
H20{/) (± HjO(g) W = +40.65 kJ/mol at 100 C
a Is the foi ward reaction exothermic or endothermic?
b. Does the enthalpy factor favor the for rd or reversereaction?
c. Does the entropy factor favor the for ard or reversereaction0
MODERN CHEMISTRY SECTION 17-2 REVIEW 139HRW mieml copynjhttd under neucc appearing earlier i s wort.
Name D te Class
SECTION 17-2 continued
P OBLEMS Write the answer on the line to the left. Sho all your ork In the spaceprovided.
5. Halogens c n combine with other halogens to form several unstable compounds.Consider the following equ tion tCs) + CijCg) *2 2ICl(g)A/// for the formation of 1C1 +I80kJ/ ol andAG°= -5.4 kJ/mol
a Is t e forwar or reverse reaction favored b theenthalpy factor0
b. Will the for ard or reverse reaction occurspontaneously t standard con itions0
c. Is the forwar or reverse reaction favored b theentrop factor?
d Calculate the value of 7AS for this system
e. Calcul te the value of 5 for this system at 25°C
. Calculate the free energy change for the re ctions below Determine whether eac reaction ill be
spontaneous or nonspontaneous
a. CM + 2H2(g) -» CH4(g)
5° = -80.7 J/(mol'K), AW = -7 0 J/mol,7== 298 K
b. 3Fe203(r)-»2Fe30„(s) + 502(g)
5° = 134.2 J/(mol»K), H0 = 235 8 kJ/mol,7 = 298 K
140 SECTION 17-2 REVIEW MODERN CHEMISTRYH W maicnai copyiigtued under no ce ep euing ewller In ihis work
Name Date Class
CHAPTER 17 EVIEW
Reaction Energy and Reaction Kinetics
SECTION 17-3
SHORT ANSWE Answer the following questions in the space provided.
1. Refer to the energy diagram at the bottom of this page to answer the following questions
a. Which letter represents the e ergy of the activated complex?
(a) A (c) C(b) B (d) D
b. Which letter represe ts the energy of the reactants?
(a) A (c) C(b) B (d) D
c. Which of the following choices represents the quantity of activation energy for thefor ard reaction?
(a) the a ount of energy at C minus the mount of energy at B(b) the amount of energy at D minus the amount of ener y at A(c) the amount of energ t D minus the amount of energy at B(d) the amount of energy at D ra us the mount of energy at C
d. hich of the follo ing choices represents the qu ntity of activation energy for thereverse reaction?
(a) the amount of energy at C minus the amount of energy at B(b) the amou t of energy t D minus the mount of energy at A(c) the amo nt of energy at D minus the mount of energ at B( ) the amount of energy t D minus the amount of energy t C
e. Which of the following choices represents the quantity of the heat of reaction for theforward reaction?
(a) the amount of energy t C minus the amount of energy at B(b) the amount of energ t B mmus the amount of energy at C(c) the amount of energ t D minus the amount of energy t B(d) the amo nt of energy t B minus the mount of energ at A
MODERN CHEMISTRY SECTION 1 7-3 REVIEW 141HRW materiftl co ynghled under nolic appearing earlier in (his work.
Name Date Class
SECTION 17-3 continued
2. For the re ction + B - X, the activation energy for the forw rd direction equals 85 kJ/mol andthe ctivation energy for the reverse direction equals 80 kJ/mol
a. hich side has the gr ter energy content, thereactants or the roduct?
b. Wh t is the heat of reaction in the forward direction7
c. The heat of reaction m the reverse direction is e ual inmagnitude but opposite m sign to the heat of re ctionra the forward irection True or F lse?
3. Below Is an incomplete energy diagram
a. Use the follo ing dat to complete the diagram' Ea = +50 kJ/mol, = 10 J/molLabel the reactants, pro ucts, AS, Ea, and the ctivate complex
b. What is the value of E ?
4. It is proposed th t ozone undergoes the follo ing two-step mechanism in our up er atmosphere
03(g) -+ 02(g) + 0( )03(g) + 0(g) -> 202( )
a. Identif any intermediates formed in the above equ tions.
b. rite the net equation
c If Atf is neg tive for the reaction m part b, which is the more stableform of o ygen, 03 or 037
142 SECTION 17-3 REVIEW MODERN CH MIST YHR matenaJ co yrighted under node* ap earing earlier In this work.
Name: Date' Hour
Ener y Worksheet:
1. List the following in order of least entropy to t e most entrop : gas, liquid, soli
2. Do the following reactions show an increase (I) or a decrease (D) in entropy?a. Mg(s) + 2H20 (i) --> Mg(OH)2 (I) + H2 (g)b. 2MgO (s) --> 2Mg(s) + 02(g)c. a piece of paper is burned to form carbon, car on dioxide, and water vapor
. AS = 10 kJ/moi Ke. S = -10 kj/mol K
3. Label a-e in question 2 as spontaneous (S) or not spontaneous (NS) based on their entropychanges.
4. Are the following endothermic (END) or exothermic (EXO)?a. burning woodb. cooking a steakc. elting iced. N2 + 3H2-¦> 2NH3 AH = -92 kJ/mole. 2N2 + 02 - 2N20 AH = 82.1 kj/mol
5. Label a-e in question 4 as spontaneous (S) or not spontaneous (NS) based on their enthalpychanges.
6. Both endothermic and exothermic reactions require activation ener y. Explain why exothermicreactions, such as burning, are considere s ontaneous even though energy must be added tostart the reaction.
7. Nature moves toward energy and entropy.
8. Do the following conditions indicate if a reaction is spontaneous (S) or not spontaneous (NS)?a. enthalpy increases (+) and entropy decreases (-)b. enthalpy decreases and entropy increases
9 Fill in the following table:symbolSpontaneous when + or -
entropy
enthalpy
Gibb s free energy
What it means
q
10. Solve the following problems using the appropriate equations:AG= AH-TAS or q= mXcXAT
How much heat is absorbed when a 5.0 g piece of metal is heated from 0°C to 25°C? Thespecific -leat of the metal is 0.75 J/g °C.
b. How much heat is released when 10.0 g of water are cooled from 100°C to 0°C? Thespecific heat of water is 4.18 J/g°C.
c. Is a reaction spontaneous when AH= -510.0 kJ/mol, AS= 0.10 kJ/mol K and the temperatureis 10°C?
d. d. Is a reactions spontaneous when AH= -110.0 kJ/mol, AS= -1.5 kJ/mol K and thetemperature is 25°C?
11. A + B react to form AB. The reactants have an energy of 500 Joules (J). The product, AB, hasan energy of 200 J. The reactions requires an activation energy of 300 J.
a. Draw an energy diagram for this reaction. Include the activation energy and label all parts ofyour diagram.
b. is this reaction exothermic or endothermic?
c. Does this reaction absorb energy or release it? How much?
d. How much activation energy is required to start this reaction?
e. Using a dotted line, show how adding a catalyst would affect the energy diagram.
TktrWiftj tM .QUESTIONS AND PROBLEMS
KEY WORDS
Entropy (S),Gibbs free energy,Second law of
thermodynamics. Standard free-energychange (AG°).
Standard free energy offormation (AG/0;),
QUESTIONS AND PROBLEMS
SPONTANEOUS PROCESSES
Review Questions
19.1 Explain what a spontaneous process is. Give twoexa ples each of spontaneous a d nonspontane-
us processes.
/i9.2 Vhich of the following processes are spontane¬ous and which a e nonspont neous? (a) Dissolv¬ing t ble salt (NaCl) in hot soup, (b) climbing Mt.Everest, (c) spreading ragrance in a roo by
.-removing the cap fro a perfume bottle.
19.3/Which of the following processes are spontane-ous and which are non pontaneous at a giventemperature?
H O- » NaN03(ag) saturated soln(a) NaNOjCQ
(b) NaN03(s)
(c) NaN0 (s)
H-.O
H-O
NaN03(fl<7)unsatu aled soln
NaN03(fl#)supers tu ated soln
Problem
19.4 Referring to Figure 19.1(a), c lculate the p ob¬ability of all the molecules in the same flask if thenu ber is (a) 6, (b) 60, (c) 600.
ENTROPY; SECOND LAW OF THERMODYNAMICS
Revie Questions
19 Define entropy. What re the units of entropy?Sl te the second law of ther odynamics in wo ds
ProMemsni9.7yFor each pair of substances listed here, choose
the one haring the larger standa d entropy at25°C. The s me olar amount is used in the com¬parison. Explain the basis for your choice.( ) Li(s) or Li(/); (b) C2H5OH(/) or CH3OCH3(/)(Hi t: Which molecule c n for hydrogenbonds?); (c) Ar(g) or Xe(g); (d) CO(g) or C02(g);(e) graphite or dia ond; (f) N02(g) or N204(g).
9.8 Arrange the following substances (1 mole each) ino der of increasing entropy at 25°C: (a) Ne(g),(b) S02(g), (c) Na(s), (d) NaCl(s), (e) NH3(g). Givethe reasons fo your arrangement.
LS Using the data in Appendix 2, calculate the stan¬dard entropy chan e for the following reactionsat 25°C: •
19.11 Without consulting Appendix 2, predict whetherthe ent opy change is po itive or negative for thefollo ing eactions. Explain your pre ictions.(a) 2KC104(s) 2KC103(s) + 02(g)(b) H20(g) > H20(/)(c) S(s) + 02(g) > SO?(g)(d) 2Na(s) + 2H->0(/) 2NaOH(nr/) + H2(g)(e) CH4 g) + 202(g) C02( ) + 2H20(/)(f) N2( ) 2N( )(g) 2LiOH(a<7) + C02( ) - Li2C03(a(?) + H20(/)
19.12 State whether the sign of the entropy change ex¬pected for each of the following processes ill bepositive or negative and explain your predictions.(a) PC13(/) + Cl2( ) > PCl5(s)(b) 2HgO(s) 2Hg(/) + 02( )(c) H2( ) * 2H( )(d) U(s) + 3F2( ) > UF6(s)
FREE ENERGY
Review Questions
19.13 Define free energy. What are its units?
19.14 Why is it more convenient to predict the directionof a reaction in terms of AGsy* rather than Suni ?Under what conditions can GSVS be used to pre¬dict the spont neity of a reaction?
19.23 Consider the following reaction at 25
Fe(0H)2(5) Fe2+(ag) + 20H (a<?)
Calculate AG° for the reaction. for Fe(OH1.6 x IC 14.
19.24 Calculate G and Kp for the equilibrium reac
2H20(g 2H2(g) + 02( )
Problems( 19 l5 alculate A for the ollowing reactions at 25°C;
(a) U2(g) + 02(g) 2NO(g)(b) H30(/) -> H20(g)(c) 2C2H2(g) + 502(s) 4C02(g) + 2H20(/)(Hint: Look up the standard fr e energies offormation of the reactants and products in Ap-jen ix 2.)
alculate AG0 for the following reactions at 25°C:(a) 2Mg(s) + 02( ) -»¦ 2MgO(s)(b) 2S02(g) + 02(g)-2S03(g)(c) 2C2H6(g) + 702(g) 4C02(g) + 6H20(/)See Appendix 2 for thermodynamic data.
19.17 From the follo ing H and AS values, pre ictwhether each of the reactions would be spontane¬ous at 25°C If not, at what temperature might thereaction become spontaneous? Reaction A AH 10.5 kJ, AS = 30 J/K; reaction B. H=1.8kJ,AS = 113 J/K.
19.18 At what tem eratures would reaction with thefollowing AH and S values become spontane¬ous? (a) = -126 kT, S = 84 J/K; (b) =-11.7 kJ, AS = -105 J/K
t 25°C
19.25] (a) Calculate AG° and KP for the follo ing equrium reaction at 25°C. The AG/ values are CCl2(g), —286 kJ/mol for PCUfg), and —325 kJ,for PCI5(g).
PCI5(g) PCl3(gJ + Cl2(g)
(b) Calculate AG for the reaction if the papressures of the initial mixture are P 0.0029 atm, pci, 0-27 atm, and Pci, = 0.40
19.26 The equilibrium constant (Kp) for the reac
H2(g) + C02(g) =± H20( ) + CO( )
is 4.40 at 2000 K. (a) Calculate Q for the ition. (b) Calcul te AG for the reaction w>partial pressures are Ph, = 0 25 atm, .0.78 atm, Ph,o = 0.66 atm, and Pco 1-20 ai
19.27 Consider the decomposition, of calcium c rate.
CaC03(s) CaO(s) + C02( y
Calculate the pressure in atmospheres of CCan equilibrium process ( ) at 25°C and (t800°C. Assume that H° 177.8 kJ and 160.5 J/K
free energy AND chemical. EQUILIB IUM 19.28 The equilibrium constant Kp for the reactioi
Review Questions
19.19 Explain why Equation (19 ) is of great impor¬tance in chemistry.
19.20 Explain clearly the difference between AG andG0.
Problems
19.21 For the re ction
H2( ) + I2( ) 2HI( )G° = 2.60kJ. Calculate KP for the reaction t
25QC.
19.22 For the autoionization of water
CO(g) + Cl2(g) =± COCl2(g)
is 5.62 x 1035 at 25°C. Calculate A ?for COC25 C. :
19.29 At 25°C, ° for the process
H2O(0 H20(g)
is 8 6 kJ. Calculate the equilibriu consta tth process.
19.30 Calculate G° fo the process
C(diamond) » C(graphite)
Is the reaction spontaneous at 25° If so,don t diamonds become graphite on standin
H,O(0 .+(aq) + OH (a9>
Kn is 1.0 x I0~14. hat is AG° for the p ocess?
MISCELLANEOUS PROBLEMS
19.31 Explain the following nur er rh me in le t)
Specific Heat Lab
Purpose: Determine the specific heat of several metals and compare the experimental value to thetheoretical value.
Prelab activity: Look up (internet or text) the value for the Specific Heat of four metals (Units shouldbe J/g°C)
Fe ; Pb ; Al and Brass
Procedure:
The metals may be preheated for you
1. Fill a medium sized beaker (250 ml size) about V2 full of tap water and heat to boiling on the hotplate. Keep the water boiling gently through the lab (you may need to add additional water eventuallyif it boils down. There needs to be enough water to completely cover the metal block).
2. Get a preheated metal sample and record its type (Fe, Cu, Pb, Al, etc) in the data table along withthe mass.
3. Put the metal block into the boiling water and let it sit for 4-5 minutes.
4. Measure the boiling water temp and record this as the INITIAL TEMP for the METAL.
5. Get a calorimeter cup set up and add 100 0 mL of fresh, cool water. Measure the temp of thewater and record as the INITIAL TEMP for the WATER
6 Put the metal block in the calorimeter (with the cool water). Record the equilibrated temperatureas the final value for both the metal and the water
Return the metal block and repeat with a new sample.
20 Pts. Chemist:
Partner: Date: /Hour:
Calorimetry Data SheetShow ALL work with proper u its and sig figs!!!
Metal Water
Mass of metal: Mass of water
Init. T (metal) Initial temp
Final T (metal) Final temp
AT: AT:
Calculate heat absorbed by water (= heat released by metal):
Calculate the Specific Heat of the Metal:
Theoretical Value of Specific Heat ofMetal: Percent Error:
Metal Water
Mass of metal: Mass of water:
Initial T( etal): Initial temp:
Final T(metal): Final temp:
AT: AT:
Calculate heat absorbed by w ter (= heat released by metal):
Calculate the Specific Heat of the Metal:
Theoretical Value of Specific Heat ofMetal: Percent Error:
Metal Water
Mass of metal: Mass of water:
Initial T (metal): Initial temp:
Final T (metal): Final temp:
AT: AT:
Calculate heat absorbed by water (= heat released by metal):
Calculate the Specific Heat of the Metal:
Theoretical Value of Specific Heat ofMetal: Percent Error:
Conclusion Questions:
1 lie specific heat of silver is 0.24 J/g.°C. Express t is in calories per gr m degree Celsius (calorie/g °C)
2. T e specific eat of gold is 0.128 J/g°C. The specific eat of krypton is 0.247 J/g°C. W ich loses more heatwhen cooling down from 50 °C to 25°C? W ich has more kinetic energy at 25°C? Explain.
3. How much energy in Joules is needed to raise the temperature of TOOL of aluminum (Cp= 0.89 J/g°C, Density2.70 g/ml) from -40.0 °C to 300.0°C?
4. Hydrogen gas releases 120. J/g when burned in oxygen. How many kilojoules of energy are released when5.0x102 g of hydrogen gas is bu ed?
5. 30.00g of Au (Cp 0.128 J/g°C) and 30.00g ofPd (Cp= 0.245 J/g °C) are placed in a dry test tube. The testibe and contents is heated in a boiling water bath to 100.°C. The mixture of gold and palladium metal is
placed into 125g of water at 21.5°C. To what final temperature is the water heated by the metals?(Note: You should find that you are only working with one variable, T2)
Laboratory
The Heat of Reaction
Background:
In this experiment, you will measure and comparethe quantity of heat involved in three chemicalreactions. The reactions are:
1) Reaction i: Solid sodium hydroxidedissolves in water to form an aqueous
solution of ions:
NaOH (s) Na+(aq) + OH'(aq) +xi joules
AHl = - i joules
2) Reaction 2: Solid sodium hydroxide reactswith an aqueous solution of hydrogenchloride to form water and an aqueous
3) Reaction 3: An aqueous solution ofsodium hydroxide reacts with an aqueoussolution of hydrogen chloride to for m waterand an aqueous solution of sodium chloride.
*Note that the energy term is shown both as athermochemical equation and separately with the
AH notation
You will calculate the amount of energyafter measuring the temperature change of aknown quantity of water. An expanded
polystyrene cup will serve as the calorimeter inthis experiment. Assume that the heat of reactionwill be used to change the temperature of theaqueous solution only. Neglect small losses to thesurroundings.
You do not need to find the mass of waterused since 1.0 mL water has a mass of 1.0 g (for
general purposes). It is appropriate to measurevolume to the nearest 0.1 mL.
Procedure:
Part I: Determination of the Heat ofReaction 1.
Caution: Sodium hydroxide is extremelycaustic to the skin and can cause blindness.
Avoid all skin contact.
a. Put 200.0 mL of cool tap water into the
expanded polystyrene cup. Stir carefully witha thermometer until a constant temperature
is reached. Measure and record this
temperature as accurately as possible.
b. Weigh out approximately 2 grams ofN OH pellets (1.9-2.1) as precisely aspossible. Record the exact mass. Since
sodium hydroxide becomes moist as it isbeing weighed in open air, weigh quickly andim ediately recap the NaOH container whenfinished.
c. Pour the weighed NaOH into the expandedpolystyrene cu . Place the thermometer into
the solution and stir gently but continuouslyuntil the sodium hydroxide is dissolved.Record the extreme temperature reached.
Before proceeding to Reaction 2, discard thesolution in the sink and rinse the cupthoroughly.
Part II: Determination of the Heat ofReaction 2.
a) Repeat steps a, b, and c of Part I, butsubstitute 200 mL of 0.25 M HCl solution forthe tap water in step a.
b) Discard the solution and rinse the cupbefore proceeding to Reaction 3.
Part III: Determination of the Heat ofReaction 3.
a) Measure 100 mL of 0.50 M HCl in thepolystyrene cup and 100 mL of 0.50 M NaOHsolution into a beaker. Both of these solutionsshould be at, or slightly below, roomtemperature. Check this with a thermometer,
(rinse and dry the thermometer beforechanging from one solu ion to the other) andrecord the initial temperature of thesolutions.
b) Add the NaOH solution to the HCl in thepolyst rene cup. Mix quickly and record theextreme temperature reached.
c) Clean up and return all materials
Calculations and Results - showyour work and make a table to summarize theresults.
l. For each reaction, calculate:
a) the temperature change
b) the amount of heat absorbed by thesolution/water
c) the number of moles of NaOH used
d) the amount of heat evolved per mole ofNaOH.
2. E press the results found in id. as heats ofreactions: AHi, AH2, and AH3 (includethe appropriate sign.
3. Write the net ionic equations for reactions
2 and 3.
4. In reaction 1, AHi represents the heat
evolved as solid NaOH dissolves. Look at thenet ionic equations for reactions 2 and 3 andmake a similar statement as to what AH2 andAH3 represent.
5. a) Compare AH2 with (AHi + AH3) andexplain in terms of your answer to question 4.
b) Calculate the % difference between AH2and (AHi + AH3). Assume that the AH2value is correct (accepted).
6. Suppose you had used 8.0 grams ofNaOH(s) in reaction 1.
a) How would this have affected thetemperature chan e?
b) What would the number of Joules ofenergy evolved have been?
c) Would this have changed the valuedetermined for AHi, the heat evolved ermole?
Table 1 A. Data recording sheet for Reactions 1 and 2
Reaction 1
NaOH (s) -> Na* + OH'
Reaction 2
NaOH (s) + H+ + Cr -+ Na+ + Cl' + H20
Volume H20 or HCI used (ml)
Mass of solid + container (g)
Mass of empty container (g)
Mass of solid used (g)
Initial water/HCI temperature(°C)
Final water/HCI temperature
( C)Change in temperature (°C)
Table 1B. Data recording sheet for Reaction 3.
Reaction 3
Na* + OH' + H* + CP -»¦ Na+ + Cl' + H20
Volume NaOH solution (mU M
Volume HCI solution fmL) M
Total solution volume (ml)
Initial temperature HCI solution (°C)
Initial temperature NaOH solution (°C)
Average temperature (°C)
Final te perature of mixture (°C)
Change in temperature (°C)
1. Exothermic reactions are reactions which release heat energy (heat is
produced) and can be shown as :
A + B > C + 2.2 kJ
where the heat energy is shown as 2.2 kJ.
Endothermic reactions absorb heat energy and can be shown as :
A + 2.2 kJ > B + C
2. Thermochemical equations are chemical equations which show heat
energy being absorbed or released. The two equations shown above are
thermochemical equations.
3. The standard conditions for thermochemical equations are 25 °C and
1 atm of pressure.
4. Calorimetry is the measuring of changes in heat energy in units of calories.
Heat is the total kinetic energy of the particles of a substance. This cannot be
measured directly, but its effects on the temperature of water can be observed
and measured. A calorie is the amount of heat energy it takes to raise 1 gram
of water one °C. A substance is put into a calorimeter and allowed to undergo
an energy change (in the case of foods they are usually burned to release heat
energy) and the energy that is released or absorbed changes the temperature
of the water in the calorimeter and the number of calories can then be
calculated by the equation :
Q = (m)(c)(A T)
( q.i8'T=
where m = mass of water, c = the specific heat of water (1.00 cal/g x °C), and
AT = change in temperature
For example if 5.0 g of cocoa puff cereal were put into a calorimeter and burned
completely and the surrounding 15.0 g of water were raised 20 0 °C we can
then calculate the number of calories per gram of cocoa puff cereal: (values
are not accurate -just made up for purposes of illustration)
5. Enthalpy is a measure of the hea content of a substance at a given
te perature and pressure. The symbol fo enthalpy is H and for change in
enthalpy: A (reads "delta H").
6. The enthalpy of a substance is easure by the amount of energy abso be
or released during a chemical reaction. The standard conditions for measuring
enthalpy are 25 °C and 1 atm of pressure
7 The heat of reaction is the a ount of heat energy absorbed or released by a
che ical reaction and is equivalent to AH.
8 Exother ic reactions always have a negati e H because the enthalpy of
the products is less than that of the reactants since heat is being released.
Endothermic reactions always ha e a positive AH because the enthalpy of the
products is greater than that of the reactants since heat is being absorbed.
9. Heat of co bustion is the amount of heat released when one ole of a
substance is completely burned.
10. Standard heat of formation is the H for the formation of one mole of the
compound from its elements at standard conditions and is designated as H°/
11. To deter ine the AH value for a chemical reaction that is not in the form of a
standard, there are two methods that can be used. One is called Hess's Law of Heat
su mation and is rather cumbersome, but technically very nice for showing the
inter ediate concepts in the process. The other technique is shorter.
The standard heat of formation alues(AHV) for the substances are multiplied by the
number of moles (balanced equation) and then the total AHreactaris is subtracted from
the total Hproducts
Calculation of total number of calories :
Calculation of calories per gram of cocoa puff cereal:
HESS LAW OF HEAT SUMMATION
When a reaction can be broken down into a series of simpler reactions, thevalue is simply the sum of the AHf of all the parts.
Heat of formation values can be used and rearranged to replicate the originalequation. It is also possible to use given reactions and their AH values.
Example'
Determine the AH for this reaction: CaC03 (s) - CaO(s) + C02(g)
1. Look up the heat of formation for each substance. Multiply by coefficientsfrom original equation IF NEEDED.AHf CaCOs (s) = 1207 kj/mol Ca(s) + C(s) + 3/2 02(g) CaC03 (s)AHf CaO (s) = - 635 kJ/mol Ca(s) + 1/2 02 CaO(s)AHf C02 (g) =-393.5 kJ/mol C(s) + 02(g) C02(g)
2. Rewrite the equations, reversing the equation if necessary to duplicate theoriginal equation. Remember to reverse the signs at the same time.
Ex.
CaC03 (s) Ca(s) + C(s) + 3/2 02(g) AH =+1207 kJ
Ca(s) + % 02 CaO(s) AH = - 635 kJ
C(s) + Ch(Q) ¦> CO?(q)AH = - 393.5 kJ
CaC03 (s) - CaO(s) + C02(g) Total AH = 178.5 kJ
GIBBS FREE ENERGY
Two factors control the spontaneity of chemical reactions, the enthalpychange A H and the entropy change AS These factors may work together,as in the case of the de olition of a building In other cases the processesoppose each other, as would be seen in the melting of ice
In the late 1800's J Willard Gibbs propose the concept of free energywhich would show the combined relationship of enthalpy and entropy.
G = H - TS
For a change at constant T and P then'
AG = AH ~ TA S
This value A G is called the Gibbs free energy change and is a composite ofthe H and S values
The sign of G indicates the spontaneity of a reactionIf AG is negative a reaction is spontaneousIf AG is positive a reaction is nonspontaneous
The following table illustrates the combinations of H and S and theresulting effect on AG
AH AS Outcome of reaction******** ********* ***********************
_ + Spontaneous at all te peraturesNonspontaneous at all temperatures
+ + Spontaneous only at high temperatures- - Spontaneous only at low temperatures
G = £A G prod. -£A G react
iw/ The concept of free energy" does not cs n that the energy is without cost,actually, it is the maxi um amount of energy released in a reaction that isavailable to perform useful work
Maxi um work is the amount of work that is a ailable from a reversibleprocess This is a goal. Real changes do not follow reversible paths so theamount of work attained is less than t e amount specified from GKnowing G allows us to calculate the efficiency of a process
Example' Living system harness only about 40 % of the free energyavailable in glucose oxidation. The other 60% is lost as heat which is wh living systems require temperature control sys ems. Mechanical system arefrequently even less efficient.
THIRD LA OF THERMODYNAMICS:AT ABSOLUTE ZERO, THE ENTROPY OF ANYPURE CRYSTALLINE SUBSTA CE IS ZERO.
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