Assessment - Yr.13

8/16/2019 Assessment - Yr.13

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1. (a) (i) Ammonia is a base. What is meant by the term base?

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(ii) What feature of the ammonia molecule enables it to react as a base?

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(iii) Given that K b =

[OH NH

NH

4

− +![ !

[ ! calculate the "H of a #.$## mol %m & solution

of ammonia.

'he values of K b an% K are $.# * $# &+ mol %m & an% $.## * $# &$4 mol, %m &-

res"ectively.

(5)

(b) (i) /etch on the a0es belo the "H curve for the titration of ,+ cm of #.$ mol %m &

H1l ith #.$ mol %m & ammonia.

$ 4

2

#

" H

# $ # , # # 4 # + ## . $ m o l % m N H 3 c m

&

(ii) What in%icator oul% you use for this titration?

..........................................................................................................................(3)

(c) 'here is no suitable in%icator for the titration of ethanoic aci% ith ammonia.

Why is this?

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..........................................................................................................................(2)

Gateway International School (PVT) Limited 1


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(%) uest the con%itions un%er hich ammonia coul% be use% to convert 1H1H1l1O,H to

1H1H( NH,)1O,H.

....................................................................................................................................(2)

(e) 50"lain ho a solution of ,6amino"ro"anoic aci%7 1H1H(NH,)1O,H7 can act

as a buffer.

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....................................................................................................................................(3)

(Total 15 marks)

2. (a) /etch on the a0es belo the "H curve for the titration of ,+ cm of #.$ mol %m & H1l

ith #.$ mol %m & ammonia.

$ 4

2

#

" H

# $ # , # # 4 # + # # . $ m o l % m N H 3 c m &

(b) What in%icator oul% you use for this titration?

88..888...................888...............................................................................(Total 3 marks)



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3. When hy%rochloric or nitric aci% is reacte% ith so%ium hy%ro0i%e solution7 the enthal"y chane

is &+2.$ /9 mol &$. :f "ro"anoic aci% is use% instea% of the other aci%s7 the enthal"y chane is less

neative.

(a) uest hy this is so.

.......................................888.................................................................................

.......................................888.................................................................................(2)

(b) What oul% be the effect on the value of ∆ H for the reaction of usin ammonia solution

instea% of so%ium hy%ro0i%e ith "ro"anoic aci%?

.......................................888.................................................................................(1)

(Total 3 marks)

4. Aci%6base in%icators are ea/ aci%s7 hich can be re"resente% as H:n. :n a;ueous solution the

folloin e;uilibrium is set u"<

H:n(a;) H(a;) :n & (a;)

H:n an% :n & have %ifferent colours. 'he %issociation constant for the in%icator e;uilibrium is

calle% in.

"H is relate% to " K in by the e;uation

"H = " K in lo!H:n[

!:n[ −

(a) (i) >or bromo"henol blue in = $ * $# &4 mol %m &. 1alculate the "H of a solution in

hich [:n & ! = [H:n! for this in%icator.

(2)

(ii) 'he colour of :n & is seen hen [:n & ! is rouhly ten times [Hln!7 an% the colour of

H:n hen it is in tenfol% e0cess over :n & . @se this information to sho the

a""ro0imate rane of "H over hich in%icator chanes can be %etecte% visually.(2)

(b) 'he selection of a correct in%icator for a reaction %e"en%s on the nature of the reaction an%

the concentrations of the substances use%. 'he neutralisation curves for the a%%ition of

so%ium hy%ro0i%e solution to $## cm of hy%rochloric aci% of the same concentration are

shon for to %ifferent concentrations of aci% an% al/ali.



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(i) :t is often sai% that the titration of a stron aci% ith a stron al/ali can be

"erforme% ith any in%icator. :n the liht of the table an% titration curves iven7

comment on this statement.(2)

I n d i c a t o r

A e t h y l o r a n e

B r o m o " h e n o l b l u e

A e t h y l r e %

B r o m o t h y m o l b l u e

C h e n o l " h t h a l e i n

' h y m o l " h t h a l e i n

. 2

4 . #

+ . $

2 . #

D .

D . 2

" ? : n$ 4

$ ,

$ #

)

-

4

,

#

" H

# + # $ # # $ + # , # #

$ . # # m o l % m

# . # $ m o l % m

# . # $ m o l % m

$ . # # m o l % m

&

&

&

&

v o l u m e o f a l / a l i a % % e % 3 c m

'itration of $## cm of hy%rochloric aci% ith

so%ium hy%ro0i%e of the same concentration

(4)

(Total 6 marks)

5. 'he values of K a for a series of oranic aci%s at ,+ E1 are shon in the table belo<

Aci% >ormula a3mol %m &

ethanoic aci% 1H1OOH $.2+ * $# &+

chloroethanoic aci% 1H,(1l)1OOH $. * $# &

bromoethanoic aci% 1H,(Br)1OOH $.,- * $# &

io%oethanoic aci% 1H,(:)1OOH -.2- * $# &4

(a) (i) Give the e0"ression for K a for ethanoic aci%.



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(iv) What "ro"erty is shon by such a mi0ture?

8..88888888888888888888888888888..

8..88888888888888888888888888888..

8..88888888888888888888888888888..(2)

(b) (i) By consi%eration of bon% "olarity7 suest hy $6chloroethanoic aci% is a stroner

aci% than ethanoic aci%.

8..88888888888888888888888888888..

8..88888888888888888888888888888..

8..88888888888888888888888888888..

(2)

(ii) uest a reason for the tren% K a values shon in the table above.

8..88888888888888888888888888888..

8..88888888888888888888888888888..(1)

(iii) uest7 ith an e0"lanation7 hether $ 6fluoroethanoic aci% is li/ely to be a

stroner or a ea/er aci% than $6chloroethanoic aci%.

8..88888888888888888888888888888..

8..88888888888888888888888888888..

8..88888888888888888888888888888..(2)

(c) 5thanoyl chlori%e7 1H1O1l7 reacts violently ith ater7 hereas chloroethanoic aci%

reacts much more sloly in or%er to substitute the chlorine atom. uest hy this is so.

8..8888888888.888888888888888888888..

8..8888888888.888888888888888888888..

8..8888888888.888888888888888888888..(2)

(Total 15 marks)


Assessment - Yr.13

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