ionic equilibrium extra question

8/13/2019 Ionic Equilibrium Extra Question

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Ionic Equilibrium Extra Questions

1 N2005/II/1 Water, H2O, covers about two-thirds of the Earth’s surface and is vital to life. About 0.005% of water

olecules consist of an o!"#en ato bonded to two atos of the h"dro#en isoto$e, deuteriu, 2.

euteriu o!ide, 2O, is &nown as 'heav" water’ and is used for research into cheical reactions

because deuteriu atos react less (uic&l" than noral h"dro#en atos,

)

H.

*i&e H2O, $ure 2O is wea&l" ionised.22O ⇌ +O O-

or 2O we can use the ter instead of w and $ instead of $H.

(a) efine the followin# ters/ and $. 21

(b) or $ure 2O, $ )3.46. 7alculate the values of the followin#/ +1

(i) [ 1.35 x 10-15 mol 2 dm-6 ] (ii) +O1 [ 3.67 x 10-8 mol dm-3 ] (iii) $ [7.43]

(c) When $ure H2O and $ure 2O are i!ed, e!chan#e of H and atos ta&es $laceand the followin# e(uilibriu is established.

2O8l9 H2O8l9 ⇌ 2HO8l9c +.5 at 264

:f a i!ture containin# +0 # of 2O and 2; # of H2O is ade u$, how an" oles of

HO will be $resent at e(uilibriu at 264 < [ 1.46 mol] 31

(d) raw the dot-and-cross dia#ra of the +O ion and state its sha$e and bond an#le. +1

2(a)

=sin# the data in the table below, deterine whether the dissociation of water is e!otheric orendotheric.

Temperature / C ! / 10"1# mol2 dm"$

0 0.))3

)0 0.26+

20 0.4)

+0 ).3;)

30 2.6)

50 5.3;

%1&

(b) Heav" water, 2O, ioni>es in the sae wa" as water. ?he ionic $roduct of 2O at 25 °7 is 6.4 @ )0-

) ol2 d-. 7alculate the $ at this te$erature. [7.5] %2&

(c) euteroaonia, +, is a wea& base, with $b 3.6 at 25 °7.

7alculate the $ of the solution that contains 0.)0 ol of deuteroaonia and 0.+0 ol ofdeuteroaoniu chloride in ).0 d-+. [9.56] %2&

' 7alculate the $H of the followin#/ 8ien *ata+ $a of H3 6.25, $b of 7H+7O2

- 6.239 %,&

1



(a) a solution -1 $re$ared b" dissolvin# )0.; # of aoniu chloride in ).00 d+ of 0.)00 old-+ a(ueous aonia [8.95]

(b) a solution -2 $re$ared b" i!in# 0.50 d+ of 0.)0 ol d-+ sodiu h"dro!ide with 0.50 d+

of 0.+0 ol d-+ ethanoic acid [4.46]

(c) a solution -' $re$ared b" addin# 0.50 d+ of 0.)0 ol d-+ h"drochloric acid to 0.50 d+ of0.+0 ol d-+ a(ueous sodiu ethanoate [5.06]

(d) a solution -# $re$ared b" addin# 0.50 d+ of 0.)0 ol d-+ ethanoic acid to 0.50 d+ of 0.20ol d-+ a(ueous sodiu ethanoate [5.06]

ien *ata+ $a of H3 6.25, $b of 7H+7O2

- 6.23

# H"dra>ine, 2H3, is one of the coon h"drides of nitro#en.

(a) 8i9 raw the dot-and-cross dia#ra of h"dra>ine. )1

8ii9 escribe the sha$e of the h"dra>ine olecule. )1

8iii9 Btate the HCC bond an#le in the h"dra>ine olecule. )1

(b) H"dra>ine is a wea& diacidic base with the followin# b values/b) 4.5 ! )0-; ol d-+ and b2 4.6 ! )0-) ol d-+.

8i9 Write e(uations to show how h"dra>ine ionises in water. )1

8ii9 E!$lain the difference in the b values. )1

(c) ?he followin# solutions were $re$ared/

Bolution . is 0.)0 ol d-+ 2H3 solution.Bolution is 0.20 ol d-+ H7l solution.

25.0 c+ of solution . was titrated with solution until )2.50 c+ of solution D was added.

7alculate the followin#/ 1

8i9 the initial $H of solution ., statin# an" assu$tions ade [10.46]

8ii9 the $H of the solution when .25 c+ of solution was added to solution . [7.93]

8iii9 the $H of the solution when )2.50 c+ of solution was added to solution . [4.55]

5 ?he water of *a&e a&uru in the en"an rift valle" contains dissolved sodiu carbonate and sodiuh"dro#encarbonate.

8a9 D" the use of e(uations, e!$lain how such a i!ture acts as a buffer on the addition of either a saaount of acid or al&ali. +1

8b9 ?he $H of *a&e a&uru is )0.+ and the ratio of 7O+2C 8a(91 to H7O+

C 8a(91 is 0.654. Write an e!$ression fothe e(uilibriu constant for the dissociation of h"dro#encarbonate to for carbonate and calculate it

value. [4.80 x 10 − 11 mol dm− 3 ] +1

8c9 Bu##est a wa" to decrease the ratio of 7O+2C 8a(91 to H7O+

C 8a(91. )1

$ Dlood is buffered ainl" b" the H27O+ C H7O+ C s"ste, in which the ratio of H27O+ 8a(91 to H7O+

C 8a(91 isabout )/20.

2



8i9 What would $H of blood be if this is the onl" buffer< 8a of H27O+ is 3.2 ! )0 C; ol d C+9 [7.68] )18ii9 With a$$ro$riate e(uations, e!$lain how such a buffer wor&s. +18iii9 ost acidic buffers are ade u$ of about e(ual aounts of the wea& acid and its anion but for the H27O+ C

H7O+ C buffer s"ste in blood, the aount of the wea& acid 8H27O+9 is uch less than its anion 8H7O+

C9.Five a $lausible reason wh" this is so. )1

uested olutions to Ionic Equilibrium Extra Questions

1(a) K is the the ionic product o 3ea4 !ater *26 or c3emical equilibrium constant or t3e sel"ionisation o *26. :t has units of mol2 dm 7$. :ts e!act value de$ends on temperature.

7heical e(uation for self-ionisation of heav" water,22O ⇌ +O O- thus K D +O

1 O C 1,

$ is the ne#ative lo#arith 8to base )09 of the deuteriu ion concentration 8in ol d -+9 of thesolution, $ l#+O1.

1(b)(i)

)0−$ )0−)3.46 ).+5 ! )0-)5 ol2 d-

(ii) +O1 O C 1 8).+5 ! )0-)5 ol2 d-90.5 +.; ! )0-4 ol d-+

(iii) $ −lo#8+.; ! )0-49 ;.3+

(c) 2O 8l9 H2O 8l9 2HO 8l9

:nitial atGol ).5 ).5 -

∆ in atGol −! −! 2!

E( atGol ).5−! ).5−! 2!

*et the volue be d+.

c ]][[

][

22

2

O DO H

HDO 56.3

)5.1

(

)2

(

2

2

=−

V

x

V

x

! 0.;2

E( at of HO 2! 280.;2 ol9).3 ol

(d) Bha$e/ ?ri#onal I"raidal, bond an#le)0;.5°

ot and cross dia#ra is siilar to H+O ion.

2(a) Accordin# to *7I, when te$erature increases, the endotheric reaction is favoured to absorb thee!cess heat as I.O.E. shifts to the ri#ht when w increases.Hence, the dissociation of water isendotheric.

(b) +O1 O C 1 86.4! )0-) ol2 d-90.5 +.)+ ! )0-4 ol d-+

$ −lo#8+.)+ ! )0-49 ;.50

(c)

$O $b l#][

][

3

4

ND

ND +

3.6 l#

]0.1

10.0[

]0.1

30.0

[

3

3

dm

mol

dm

mol

5.33

$ $− 5.33 −l#86.4! )0-) ol2 d-9 − 5.33 )5 − 5.33 6.5

' (a)$OH $b l#

][

][

3

4

NH

NH +

3

~~~THE END~~~



)3-$H8)3-6.259l#

]0.1

10.0[

]0.1

20.0[

3

3

dm

mol

dm

mol

$OH5.05, $H )3 − 5.05 4.65

'(b) 7H+7OOH 8a(9 aOH 8a(9 → 7H+7OOa 8a(9 H2O 8l9

:nitial atGol 0.)5 0.05 - -∆ in atGol −0.05 −0.05 0.05 -inal atGol 0.)0 - 0.05 -

$H $a l#][

][

3

3

COOH CH

COOCH −

$H8)3-6.259l#

]0.1

10.0[

]0.1

05.0[

3

3

dm

mol

dm

mol

3.3

'(c) 7H+7OOa 8a(9 H7l 8a(9 →

7H+7OOH 8a(9 a7l 8l9:nitial atGol 0.)5 0.05 - -∆ in atGol −0.05 −0.05 0.05 -inal atGol 0.)0 - 0.05 -

$H $a l#][

][

3

3

COOH CH

COOCH −

$H8)3-6.259l#

]

0.1

05.0[

]0.1

10.0[

3

3

dm

mol

dm

mol

5.05

'(d)$H $a l#

][

][

3

3

COOH CH

COOCH −

$H8)3-6.259l#

]0.1

05.0[

]0.1

10.0[

3

3

dm

mol

dm

mol

5.05

#(a)(i) 2(a)(ii) 2(a)(iii)

tri#onal $"raidal with )0;

o

res$ect to each ato

#(b)(i)H2CH28a(9 H2O8l9 ⇌ H2CH+

8a(9 OH-8a(9H2CH+

8a(9 H2O8l9 ⇌ 2H28a(9 OH-8a(9

#(b)(ii)?he first ionisation involves the addition of H to the neutral 2H3 while the second ionisation involves theaddition of H to the $ositivel" char#ed 2H5

.

4

Ax

Hx

x

xx x

H x

A H

H



Bince it is easier to add a $ositivel" char#ed H to a neutral s$ecies than to a $ositivel" char#ed s$ecies due toore re$ulsion in the latter reaction, the first ionisation occurs ore readil", resultin# in a bi##er b).

#(c)(i)Bince b) JJ b2, it can be assued that the OH - fro the second ionisation is ne#li#ible and can be i#noredand all the OH- in the solution coes fro the first ionisation.

2H38a(9 H2O8l9 ⇌ 2H58a(9 OH-8a(9

At e(uilibriu, let OH-1 " ol d-+.?hen 2H5

1 " ol d-+ and 2H31 80.)0 C "9 ol d+

b 2H51 OH-1G2H31 4.5 ! )0-; ol d-+

"2G80.)0 C "9 4.5 ! )0-;

Bince 2H3 is a wea& base with a sall b value, it can be assued that " KK 0.)0 such that 80.)0 C "9 ≈ 0.)0.

∴

"

2

G0.)0 4.5 ! )0

-;

" 2.6)5 ! )0-3

OH-1 2.6)5 ! )0-3 ol d-+

$OH Cl#OH-1 Cl#82.6)5 ! )0-39 +.53

∴$H )3 C +.53 )0.3

#(c)(ii)

2H3 H7l → 2H57l-

aount 80.025980.)09 80.0025980.209usedGol 2.50 ! )0-+ ).25 ! )0-+

aountreactedGol ).25 ! )0-+ ).25 ! )0-+

aount$resent at ).25 ! )0-+ 0 ).25 ! )0-+

the endGol

?otal volue 0.0+)25 d+

?he resultant solution contains 2H3 and 2H5of e(ual concentration at e(uilbriu.

or this al&aline buffer,$OH $b) l#82H5

1G2H319 -l#84.5 ! )0-;9 l# )

.0;$H )3 C .0; ;.6+

#(c)(iii)

2H3 H7l → 2H57l-

aount 80.025980.)09 80.0)25980.209usedGol 2.50 ! )0-+ 2.50 ! )0-+

aountreactedGol 2.50 ! )0-+ 2.50 ! )0-+

aount$resent at 0 0 2.50 ! )0-+

5



the endGol

?otal volue of solution 0.0+;5 d+

?he resultant solution contains 2H5 which under#oes acidic h"drol"sis.

2H58a(9 H2O8l9 ⇌ 2H38a(9 H+O

:nitial conc. .; ! )0-2

Gol d-+

e(uilibriu .; ! )0-2

C " " "conc.Gol d-+

a of 2H5 wGb) of 2H3 ).00 ! )0-)3G4.5 ! )0-;

).)4 ! )0-4 ol d-+

a of 2H5 2H31H+O

1G2H51 ).)4 ! )0-4

"2G8.; ! )0-2 C "9 ).)4 ! )0-4

Bince 2H5 is a wea& acid with a sall a, it can be assued that " KK .; ! )0-2

such that 8.; ! )0-2 C "9 ≈ .; ! )0-2.

∴ "2G8.; ! )0-29 ).)4 ! )0-4

" 2.4) ! )0-5

H+O1 2.4) ! )0-5 ol d-+

$H Cl#H+O1 Cl#82.4) ! )0-59 3.55

58a9On addition of an acid to the i!ture,

7O+2− 8a(9 H 8a(9 → H7O+

−

8a(9 )1

?hus, H1 does not increase si#nificantl" and $H is aintained.

When soe al&ali is added to the buffer,

H7O+−

8a(9 OH− → 7O+2−8a(9 H2O89 )1

?hus, OH−1 does not increase si#nificantl" and $H is aintained.

?he lar#e reservoir of 7O+2− and H7O+

−

in the buffer solution sufficientl" co$es with the addition of sall aounts o

acid and al&ali. )1

58b9H 8a(91 )0−$H )0−)0.+ 5.0)2 ! )0−)) ol d−+. )1

a 7O+2C 8a(91H 8a(91

H7O+

−

8a(91 )1

0.654 ! 5.0)2 ! )0−)) 3.40 ! )0−)) ol d−+. )1

58c9Addition of an acid. )1

8i9 $H of blood $a lo# H7O+ C 8a(91G H27O+ 8a(91 ;.4 )1

8ii9 On addition of a sall aount of acid, H 8a(91 is &e$t a$$ro!iatel" constant as H7O+ C reacts with H/ H 8a(9

H7O+ C 8a(9 H27O+

8a(9 )1

6



On addition of a sall aount of a base, OH C 8a(91 is &e$t a$$ro!iatel" constant as H27O+ reacts with OH C/

H27O+ 8a(9 OH C 8a(9 H7O+ C 8a(9 H2O 8l9 )1

*ar#e reservoir of both the acid and its anion sufficientl" co$es with the addition of sall aounts of acids orbases to &ee$ $H constant. )1

8iii9 an" of the reactions in the bod" $roduce acids so that a lar#er aount of the anion is needed to co$e with the$roduction of H )1or en>"es in the blood function best at $H about ;.;. ?he hi#h aount of H7O+

C allows $H of blood to beaintained at this wea&l" al&aline $H.

7

ionic equilibrium extra question

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