enthalpy answers.pdf

8
gChemistry Name 2.3,1 Enthalpy Changes Exam Questions 1. The standard enthalpy change of formation of hexane is -199 kJ mol-1. Using the axes below, show the enthalpy profile diagram for the formation of hexane. On your diagram label the enthalpy change of reaction, LH, and the activation energy, Ea enthalpy [otal3 marks] Alkanes are important hydrocarbons since they are used as fuels in homes and in industry. lt is important that the enthalpy changes involved in alkane reactions are known. (i) Define the term enthalpy change of formation of a compound. aar.A.+ff . Qfft.v!€ /,/Hilt/- / nzzzt+-.. ..... . . . .5 tt* 5.2. Arue. €. . . . . . #. . . . {.*?Ken) FE c>14 . . . lf. 5 . . . ez.9???su X, ./A/-..7/r'e.8...s.f,miap.,:ru,2- ...9A72--5... (:(..2#-. 97"vt/'P!!'%O QrzY i)t7t OntS I2j (ii) Write the equation, including state symbols, that accompanies the enthalpy change of formation of hexane, C6H14(l). 6C,ri r pJ,ry 64,u (iii) What conditions of temperature and pressure are used when measuring the standard enthalpy change of formation? temperatur " .......... 2f..9. * t1l f-otal5 marksl 2. t2l reaction pathway Greenhead College pressure /471ezPg,a/P{

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Page 1: Enthalpy answers.pdf

gChemistry Name

2.3,1 Enthalpy Changes Exam Questions

1. The standard enthalpy change of formation of hexane is -199 kJ mol-1.

Using the axes below, show the enthalpy profile diagram for the formation of hexane.

On your diagram label the enthalpy change of reaction, LH, and the activation energy,Ea

enthalpy

[otal3 marks]

Alkanes are important hydrocarbons since they are used as fuels in homes and inindustry. lt is important that the enthalpy changes involved in alkane reactions areknown.

(i) Define the term enthalpy change of formation of a compound.

aar.A.+ff . Qfft.v!€ /,/Hilt/- / nzzzt+-.. €...... . . .5 tt* 5.2. Arue. €. . . . . . #. . . . {.*?Ken) FE c>14

. . . lf. 5 . . . ez.9???su X,

./A/-..7/r'e.8...s.f,miap.,:ru,2- ...9A72--5... (:(..2#-. 97"vt/'P!!'%O

QrzY i)t7t OntS I2j

(ii) Write the equation, including state symbols, that accompanies the enthalpychange of formation of hexane, C6H14(l).

6C,ri r pJ,ry 64,u(iii) What conditions of temperature and pressure are used when measuring the

standard enthalpy change of formation?

temperatur "

.......... 2f..9. *

t1l

f-otal5 marksl

2.

t2l

reaction pathway

Greenhead College

pressure /471ezPg,a/P{

Page 2: Enthalpy answers.pdf

gChemistry Name..........

3. The combustion of butane is shown in the equation below.

c+Hro(g) * 6 + oz(g) -+ 4co2(g) + 5H2o(t)2

(i) The standard enthalpy change of combustion of butane is -2877 kJ mol-1. Whatdoes sfandard mean in this context?

7f,91, , / ezmb%./ed_

t1l

(ii) Define the term enthalpy change of combustion.4 -z-/ mezg_ dsaSszt^/?r7

knzpAU€rc5 (e4vpz+v{ mz&t-#Z/m: /N- &tv: r//Rz ..s'7(#.p.ezD 5:7,€7€s utut2c# sz."fuD&22(ontpzT/M/J

(iii) Complete the enthalpy profile diagram for the combustion of butane. Label theactivation energy, Eu, and the enthalpy change, AH.

enthalpy

progress of reaction

t2l

t3I

fl-otal6 marks]

4. ln an experiment to determine the standard enthalpy change of combustion of propan-

Greenhead College

C+Hro(g) + 6102(9)

/+tOt +;4O

Page 3: Enthalpy answers.pdf

gChemistry

1-ol, C3H7OH, a student used the apparatus shown below.

propan-1-ol

(a) Define the term enthalpy change of combustion.

.dS Cr)u Z ffiz /tt)

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(b) write the equation for the standard enthalpy change of combustion ofpropan-1-ol, C3H7OH.

KJ

entharpy.n"nsJ...'n"

t ot ^o,'

t2l

l2l

&ilnr;1/r,) t Scz Tz 3Co .* t &l/, o pl

(c) The student measured 50.0 cm3 of water into the beaker and lit the burner. Whenthe temperature of the water had gone up by 12.8 'c, he found that 0.100 g ofpropan-1-ol had been burnt.

(i) Calculate the energy, in kJ, produced by burning 0.100 g of propan-1-ol.The specific heat capacity of water is 4.1 8 J g-' K t.

q)=r%f -) :-e,)< L,tg , l7,g energy = ..2.:.6.8.-./.--teytO /ooa(ii) Calculate the number of moles of propan-1-ol in 0.100 g.

,rL'/(%9 = Q' I number of moles =o oo l6Y

6c(iii) Calculate the enthalpy change of combustion, in kJ mol-1, of propan-1-ol.

26V-,--e

i) 'oef Mv

(d) The student then calculated the error of the 50cm3 measuring cylinder used. The

Greenhead College

tl I

Page 4: Enthalpy answers.pdf

gChemistry Name..........

maximum error of the measuring cylinder was +- 1cm3. Calculate the % error inthis piece of apparatus.

c/o etolcn = + */L)o '= ? cySr) A /o(e) The student looked in a text book and found that the actual value for the standard

enthalpy change of combustion of propan-1-ol was more exothermic than theexperimental value.

Suggest two reasons for the difference between this value and the one heobtained experimentally.

1 . . . . . . . . . .P ffi7 . . . . /:e2 .2. . . .T€'. . . . . 7//q I u,r tzo'tz:ru,p2 NS

t11

2 $@71am/ H (*" zta7s7\J

(f) The student told one of her friends and she suggested using a 10cm3 measuringcylinder 5 times as its maximum error was +- 0.5cm3

State and explain whether this method would reduce the error in measuring50cmo of water.

/uu&,7/Pt€ us€t h trrwesp7/%fl uJ/(-L' B€ lylutTtPZll?D 5 rtr"t b

(-7t) -= o5.:'___-=- x S x/OC:/o = 25% €zzaszpz rl/e/ltt U)

(g) The student repeated the experiment using the same apparatus. The waterstarted at 60oc. The student was surprised to find the value was less

t21

l1l

exothermic. Explain why.

Tate /"J47gz /S e7. ' . i1'I),ry€ //gzg7 /,-,// /

t rt?."Z fi=,zz€ y /d//1erf 7tr'F h/.47H

FPAlfuYvf UPF',' 7//€Pe57e'6rY * / - t>z ti u ;€rv/9ratteW

Greenhead College

/-,'J lF iaft)Zp,S tf

,1 /-/t€t/t?zt3€ /csr Ta

11I

7t<z'w@87u/?6-fil€ -9etzEawx,//>/L$

/J"lr,/€ Et/4?Or€zl7cb 545v744{

(h) Other students in the class got very similar enthalpy changes of combustion eventhough their temperature changes and masses of propan - 1 - ol burnt weredifferent. Explain this in terms of energy and moles. t1l

[Total 15 marks]

7r lffiz rasfft?z4v&ti€$ rl/,ffi/*'e{ /S

0Mt /u"r/L B{

Page 5: Enthalpy answers.pdf

gChemistry Name

5. A student carries out an experiment to determine the enthalpy change of combustion ofglucose.

,z tt ll,u Otln the experiment, 0.831 g of glucose is burned. The energy released is used to heat100 cm3 of water from 23.7 'C to 41 .0 'C.

(i) Calculate the energy released, in kJ, during combustion of 0.831 g glucose.

The specific heat capacity of water = 418 J g-' K '.Density of water = 1.00 g cm-3.

Q- Y:4:4T /oox/'/9*lY3iocc) 6

a. tzenergy = .....T....(.J...... f<.t

(ii) Calculate the amount, in moles, of glucose that is burned.

flle69-= o Y^3i

/3oamount = O oc*'62

(iii) Calculate the enthalpy change of combustion of glucose.Give your answer to three significant figures.

t2t

t2t

4/J- Y23 : -is6b? + -is7o aoQ;6'Z AH6 = ... kJ mol-1

6. Some reactions of H2O2 are exothermic. Use ideas about the enthalpy changes thattake place during bond breaking and bond making to explain why some reactions areexothermic.

rw(w6 a/e.U€.F_.- /: zez(ffiW /sr/:4../1/t#u'-&was E&. m ...Z//'.ffi/''ru{€.P-u&?{ y E&}st64DZ) f&g6#/4 'ft/€ //u/v/aL- {-*wDS

l2l

[otal6 marks]

Greenhead College

lf otal2 marksl

Page 6: Enthalpy answers.pdf

gChemistry

The equations for the combination of gaseous atoms of carbon and hydrogen to formmethane, CHa, ?nd ethane, CzHo, are shown below.

7.

C(g) + 4H(g) -+ CHa(g) LH = -1652 kJ mot-1

2c(g) + 0H(g) -+ c2H6(g) LH = -2825 kJ mol-1Use this data to calculate:(i) the bond enthalpy of a C-H bond,

bond enthalpy = ......... kJ mol-1

i6 s2--4

(ii) the bond enthalpy of a C-C bond.

bond enthalpy = ......... kJ mol-1

ht S t t,ruu/''

lEezaer% "8a' 7t crtsLttlt 8€ F-U,.kV/&Ur(45 &ntPS4Pd&Qw.cn:

u k)YA L/4LL//-'S

t1l

- ?aa CO, '* LU, A

_ .--,r /standard enthalpy change I ?...... kJ mot-1

(azalErts'udrt'l . ^ . fitfllfcv'EDCe4Al

(A-r-r,f) .\f-q z zszs f '-c = 78zs * 2t+19

zAYg + (c*c) =zszeJ = M1'[otal 3 marks]

Enthalpy changes can be calculated using enthalpy changes of combustion. The tablebelow shows some values for standard enthalpy changes of combustion.

Use these values to calculate the sta;rdard enthalpy change of the reaction below.@@C(s) + 2Hz@) -+ CHa(s)

LIJ- *146 4 9Qosqo)+ Ylo

LHcelkJ mol-1

Greenhead College

[Total3 marks]

Page 7: Enthalpy answers.pdf

gChemistry

Enthalpy changes of combustion can be used to determine enthalpy changes offormation.

(i) Write the equation for the standard enthalpy change of formation of butane,C+Hro. lnclude state symbols in your answer.

ACtil ! SA*A- ep,or,)

(ii) Use the following data to calculate the standard enthalpy change of formation ofbutane.

l2l

standard enthalpy chanse .f(ffi@fkJ mot-1

carbon -394

hydrogen -286

butane -2877pC rr\ -t 9t

.F

-->llr'9)

* tuoA I.to*answer kJ mol-1

"' , fWecz.spo^t tt/

kCTq o.. S

ht-3qA=-rs;,\"-J, -2gA'-l4=.\-- -. 'f e zsYY)

s[/"-D +7gYv-,t

- zao| -+ 2 v7+ = /2? /-J/4d-!otarSmarksl

Greenhead College

Page 8: Enthalpy answers.pdf

gChemistry

10. The standard enthalpy change of combustion of glucose can also be determinedindirectly.

Calculate the standard enthalpy change of combustion of glucose using the standardenthalpy changes of formation below.

substance6{wmor-1

C6H12O6(s) -1250

coz(g) -394

H2O(r) -286

e) aDCeHrzOa(s) + 6Oz(q)-- 6COr(q) + 6H2O(l60z(g)-- 6COz(9) + 6Hzo(l)

Evw/#/ciu'^ Vv2vaanf

uP"

' $ \, .A/ 6^ - 4?4' = - !36+(tzsa)\.. ., I g,-ze6=-tilb

\ \- -.t tzgo 6cst r gtl t eo, - +-o8b

+ / ^so

+' 40ga

answer = ....*..2.9.'fu . kJ mor-1

fl-otal3 marks]

Greenhead College