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Chemistry 2000 lab exam > Qualitative Analysis of Cations > Flashcards

Qualitative Analysis of Cations Flashcards

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1
Q

qualitative vs quantitative analysis

A

qualitative
- what is present

quantitative
- how much is present

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2
Q

group 1 is known as

A

the silver group

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3
Q

group 2 is known as

A

the copper group

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4
Q

group 3 is known as

A

the zinc-aluminum group

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5
Q

ions in group 1

A

Ag+
Hg2 +2
Tl+
Pb2+

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6
Q

ions in group 2

A

Hg2+
Bi3+
Cu2+
Sn2+
Sn4+

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7
Q

what group is Hg2+
apart of

A

group 2

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8
Q

what group is Bi3+
apart of

A

group 2

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9
Q

what group is Cu2+
apart of

A

group 2

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10
Q

what group is Sn2+
apart of

A

group 2

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11
Q

what group is Sn4+
apart of

A

group 2

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12
Q

what group is Ag+
apart of

A

group 1

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13
Q

what group is Hg2 +2
apart of

A

group 1

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14
Q

what group is Tl+
apart of

A

group 1

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15
Q

what group isPb2+
apart of

A

group 1

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16
Q

what ions are in group 3

A

Zn2+
Fe2+
Fe3+
Ni2+
Cr3+
Al3+

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17
Q

what group is Zn2+
apart of

A

group 3

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18
Q

what group is Fe3+
apart of

A

group 3

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19
Q

what group is Fe2+
apart of

A

group 3

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20
Q

what group is Ni2+
apart of

A

group 3

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21
Q

what group is Cr3+
apart of

A

group 3

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22
Q

what group is Al3+
apart of

A

group 3

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23
Q

what solution precipitates group 1

A

0.3 M HCl

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24
Q

what group does HCl precipitate

A

group 1

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25
Q

why does group 2 precipitate in HCl

A

these soft acids react strongly enough with a borderline base to precipitate in acid solution

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26
Q

what solution precipitates group 2

A

0.3 HCl containing H2S

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27
Q

what group precipitates with HCl containing H2S

A

group 2w

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28
Q

why does group 2 precipitate with H2S

A

because soft acids react with a very soft base

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29
Q

what solution precipitates group 3

A

alkaline solution of H2S

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30
Q

how can group 3 be distinguished into two different groups

A
  1. Fe+3, Cr+3 and Al+3 will redissolve if precipitate layered with weak acid
  2. Ni+2, Fe2+ and Zn+2 will not
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31
Q

why does Fe+3, Cr+3 and Al+3 redissolve if the precipitate is layered with a weak acid

A

these are hard acids that reacting with hard base (OH-) and as cations they precipitate when the Ph equals the Pka

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32
Q

what are the hard acids

A

Fe+3, Cr+3 and Al+3

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33
Q

why don’t Ni+2 and Zn+2 and Fe2+ dissolve if the precipitate is layered with a weak acid

A

they are borderline acids and bind to very soft base when Ph is adjusted to weaken the hydroxide hydrated cation

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34
Q

what are confirmatory tests

A

these that determine CONCLUSIVELY that a specific ion is present

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35
Q

are interfering ions left in a sample before a confirmatory test is done

A

NO, they are removed prior to running the test

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36
Q

what are separations

A

procedures that separate ions from other groups

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37
Q

what follows separations

A

confirmatory tests

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38
Q

what was important to due throughout testing cations

A

label ALL the test tubes

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39
Q

where were all solutions discarded after the lab

A

into the HEAVY WASTE bucket

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40
Q

is qualitative analysis precise and what does that mean

A

no it’s not = measurements don’t have to be exact

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41
Q

how are liquid reagents measured

A

in drops

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42
Q

how many drops equals 1 ml

A

20 drops

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43
Q

what techniques were used to mix solutions

A
  1. flicking the test tube when the volumes were small
  2. inversion when the volume was larger
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44
Q

what type of sample is centrifugation done

A

for a solution containing a solid and a liquid

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45
Q

why is centrifugation done

A

to separate liquid from a solid

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46
Q

what is the solid called after centrifugation andwhere is it found

A

precipitate - found at the bottom of the test tube

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47
Q

what is the liquid called after centrifugation and were is it found

A

supernatant - found above the solid

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48
Q

how is the supernatant and precipitate separated after centrifugation

A

supernatant poured carefully from the solid into a new test tube, LEAVING the solid in the original test tube

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49
Q

decanting

A

technique for separating solid from liquid

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50
Q

how is decanting done

A

let the solid settle at the bottom of the test tube and CAREFULLY pour off the liquid WITHOUT disturbing the solid

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51
Q

precipitation is most often done to

A

remove ions

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52
Q

how to test for completeness of precipitation

A

add a drop or two of the precipitating reagent to the supernatant

if more precipitate forms than there are still ions in the solution and need to centrifuge again and combine precipitates

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53
Q

what is desirable while doing precipitation

A

to make sure all ions are removed from the solution OR precipitation is complete

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54
Q

how were test tubes heated

A

in a water bath with the cork REMOVED and the opening of the tube pointing AWAY from you

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55
Q

when is evaporation done

A

when the volume of a solution needs to be concentrated or reduced

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56
Q

how can evaporation be done

A

transfer the desired solution to an evaporating dish and use tongs to GENTLY move the container back and forth over the hot plate

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57
Q

most common cause of false observations leading to incorrect conclusions

A

cross contamination

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58
Q

how are ions in groups related

A

based on similar chemical and analytical properties

59
Q

how are ions in groups NOT related

A

based on the periodic table

60
Q

why are some ions omitted from experiments

A

because of their toxicity, expense or rarity

61
Q

what group 1 cation was not tested and why

A

Tl - toxicity

62
Q

how are cations differentiable among and between groups

A

based on differential precipitation properties between groups and among members of a group

63
Q

how can group 1 cations be separated from other groups

A

they form slightly soluble chlorides with HCl

64
Q

are there other groups besides group 1 that precipitate with HCl

A

NO - others will remain in solution

65
Q

precipitate reaction of silver

A
66
Q

precipitation reaction of mercury +2

A
67
Q

precipitation reaction of lead 2+

A
68
Q

what are the colours of the precipitates of group 1 with HCl

A

all are white precipitates

69
Q

what was the first step of group 1 analysis

A

adding 0.5 ml (10 drops) of 6M HCl and stirring well

70
Q

what is done after adding HCl to group 1 sample

A

centrifuge and decant the supernatant into another test tube and test for completeness

71
Q

what might appear if HCl is added to the unknown sample and what does that mean

A

a white precipitate forms which indicates at least ONE group 1 cation

72
Q

what prevents the precipitation of Bi3+ if it is present in the sample

A

adding 6 M HCl to the sample because it becomes slightly acidic

73
Q

what is done to the precipitate in group 1 after decanting the supernatant

A

add 5 ml of RO water and heat the contents for 5 mins WHILE STIRRING

after heating centrifuge any precipitate and decant the supernatant into another tube

74
Q

what can the heated sample of group 1 cations reveal

A
  1. the saved precipitate can contain Ag+ or Hg2 + 2
  2. Pb2+ ONLY cation soluble in water (must test supernatant for lead)
75
Q

what is the ONLY cation of group 1 that is soluble in water

A

LEAD

76
Q

steps for the confirmatory test for lead in the supernatant of group 1

A
  1. add 2 to 3 drops of 6 M acetic acid (Ch3COOH)
  2. add 3 to 4 drops of 1.5 M K2CrO4
  3. a yellow precipitate will form in presence of lead
77
Q

what is the yellow precipitate that confirms presence of lead in sample

A

PbCrO4

78
Q

what is added to group 1 to test for Ag+ and Hg2 +2

A

NH4OH

79
Q

what group 1 cation is a part of a disproportionation reaction

A

Hg2 2+

80
Q

reaction for confirmation of Ag+

A
81
Q

reaction for confirmation of Hg2 + 2

A
82
Q

what is a disproportionation reaction

A

The reaction in which one reactant gets oxidized and the same reactant gets reduced

83
Q

why is the reaction of Hg2Cl2 disproportionate

A

because the Hg2Cl2 oxidizes and reduces itself to produce Hg and Hg(NH2)Cl

84
Q

what product of Hg2Cl2 reaction is black

A

Hg

85
Q

what product of Hg2Cl2 reaction is white

A

Hg(NH2)Cl

86
Q

what is the precipitate if Hg2 +2 is present in the sample

A

grey/black precipitate

87
Q

what is present if the chloride precipitate dissolves with Nh4OH

A

Ag+

88
Q

how can we be sure there is silver in the sample if the precipitate turns from white to grey (indicating presence of mercury)

A

test with HNO3 to the supernatant after decanting from precipitate

89
Q

what indicates presence of silver in the group 1 sample

A

if the supernatant has a white precipitate form AFTER adding HNO3

90
Q

what is the reaction causing the white precipitate of Ag to form

A
91
Q

how did we ensure the sample in group 1 was free of lead

A

wash the precipitate that came after heating the sample with 2ml of HOT RO water and filter through Whatman paper

92
Q

when testing for confirmation of silver, is the solution acidified or basic

A

acidified with HNO3

93
Q

if red litmus turns blue the solution is

A

basic

94
Q

if blue litmus turns red the solution is

A

acidic

95
Q

are group 1 soluble or insoluble chlorides

A

slightly INSOLUBLE chlorides

96
Q

are group 2 soluble or insoluble chlorides

A

soluble

97
Q

what cations in group 2 precipitate with H2S on 0.3 H+

A

HgS
Bi2S3
CdS
CuS
SnS
SnS2

98
Q

will other sulfides in other groups precipitate out with H2S at 0.3 H+

A

no

99
Q

what are the colors of precipitates of groups 2 cations with H2S

A

HgS - black
Bi2S3 - brown
CdS - Yellow
CuS - Black
SnS - Brown
SnS2 - Yellow

100
Q

what is the colour of Bi+3 precipitate with H2S

A

brown

101
Q

what is the colour of Hg2+ precipitate with H2S

A

black

102
Q

what is the colour of Cu2+ precipitate with H2S

A

black

103
Q

what is the colour of Cd2+ precipitate with H2S

A

yellow

104
Q

what is the colour of Sn2+ precipitate with H2S

A

brown

105
Q

what is the colour of Sn4+ precipitate with H2S

A

yellow

106
Q

what is the only group 2 sulfide that is SOLUBLE in ammonium sulfide

A

Tin

107
Q

how is Sn4+ reduced to Sn2+

A

with IRON FILLINGS and adding HgCl2

108
Q

how can HgS, Bi2S3, PbS, CdS and Cus be separated in group 2 sample

A

by differential solubility

109
Q

what is the only group 2 cation NOT soluble in nitric acid (HNO3)

A

HgS (mercury(

110
Q

what happens to the cations as the sample is with HNO3 (group 2)

A

HgS will remain as a precipitate

BUT

all other cations are in the solution

111
Q

what DOES HgS dissolve in and can confirm its presence

A

the aqua regia (HCl and HNO3)

112
Q

how can Bi3+ be removed from Cu2+ and Cd2+

A

adding NH4OH to the solution

113
Q

how does Cu2+, Cd2+ and Bi3+ react to NH4OH

A

Bi3+ = forms a white precipitate (Bi(OH)3)

Cu2+, Cd2+ remain in solution

114
Q

how is Cu2+, Cd2+ separated

A

the reduction of Cu2+ to elemental copper using sodium dithionite as reducing agent

115
Q

what colour precipitate is produced when cadmium reacts with thioacetamide

A

yellow precipitate

116
Q

what is IMPORTANT for steps containing thioacetamide

A

all steps are done in a fume hood because it is highly toxic

117
Q

what must be done to precipitate HgS, Bi2S3, CdS, CuS, SnS2

A

make the solution 0.3 M H+ (make slightly acidic)

118
Q

how is it known if group 2 cations are present in the sample

A

if there is a precipitate formed after adjusted to 0.3 M in respect to H+

119
Q

describe solubility of group 2 cations

A
  1. DO NOT form slightly soluble chlorines (like group 1)
  2. DO NOT form sulfides by H2S in 0.3 M H+
120
Q

what group 3 cations are soluble in H2S when more basic than 0.3 M

A
  1. Zn2+
  2. Fe2+/Fe3+
  3. Ni2+
  4. Cr3+
  5. Al3+
121
Q

what colour sulfide precipitate does Zn2+ form when H2S is more basic

A

white

122
Q

what colour sulfide precipitate does Ni2+ form when H2S is more basic

A

Black

123
Q

what colour sulfide precipitate does Fe2+ form when H2S is more basic

A

Black

124
Q

what colour sulfide precipitate does Fe3+ form when H2S is more basic

A

reddish-brown

125
Q

what colour sulfide precipitate does Cr3+ form when H2S is more basic

A

blue-green

126
Q

what colour sulfide precipitate does Al3+ form when H2S is more basic

A

white

127
Q

what cations are dissolved when precipitate is treated with SO4-2 / HSO4 0 buffer

A

Fe2+ /3+
Cr3+
Al3+

128
Q

what cations are NOT dissolved when precipitate is treated with SO4-2 / HSO4 0 buffer

A

Ni2+
Zn2+

129
Q

where will Ni2+ and Zn2+ be found in group 3

A

in precipitate (after treated with buffer)

130
Q

where will Fe2+/3+, Cr3+, Al3+ be found in group 3

A

in supernatant (after treated with buffer)

131
Q

what happens to the Ni2+ and Zn2+ in the precipitate after it is treated with NaOH

A

Ni2+ will form a green precipitate

Zn2+ is in solution (dissolved)

132
Q

what is the equation for Zn2+ reacting with OH

A
133
Q

what is the equation for Ni2+ reacting with OH

A
134
Q

how is the supernatant (after treated with NaOH) tested for presence of Zn

A

testing with H2S will form a white precipitate

135
Q

how is the precipitate (after treated with NaOH) tested for presence of Ni

A

tested with dimethylglyoxime should form a red precipitate

136
Q

the supernatant with Al3+, Fe2+/3+ and Cr3+ is treated with NaOH and H2O2, what happens to each ion

A

Iron
- oxidization of Fe2+
- forms reddish-brown precipitate

Cr3+
- oxidized
- forms blue-green precipitate

Al3+
- remains in solution

137
Q

what happens when Fe reacts with Potassium thiocyanate

A

dark red solution

138
Q

what indicates the presence of chromium

A

if the solution containing the aluminum, chromium OR iron is adjusted to proper H+ and a green solution occurs (Cr6+) indicating chromium ions might be present

139
Q

confirmatory test for iron

A
  1. in a separate test tube add 1 ml 6 M HCl, 2 ml H20 and a few drops of 1 M KSCN
  2. add the solution from step 1 to the precipitate that formed after adding H2O2 and heating
  3. appearance of dark red colour confirms presence of iron
140
Q

confirmatory test for aluminum

A
  1. collect the filtrate through a filter paper after the solution was treated with H2O2 and heated (centrifuge to separate supernatant from precipitate)
  2. add 6M HCl until just acidic to BLUE litmus paper
  3. add 3 M NH4OH till the solution is JUST basic to red litmus paper
  4. warm in water bath before centrifuging to collect the precipitate AND the supernatant (test for chronium)
  5. a white precipitate indicates Aluminum
141
Q

confirmatory test for chromium

A
  1. solution tested for aluminum should be YELLOW/ORANGE
  2. acidify the supernatant from testing Aluminum with 6 M HCl
  3. add a few drops of H2O2
  4. appearance of dark blue complex confirms the presence of Cr
142
Q

confirmatory test for nickel

A
  1. a pale green precipitate that forms after adding 6 M NaOH till basic to red litmus paper INDICATES nickel is present
  2. dissolve the precipitate in 6 M HCl and make solution alkaline to red litmus paper with 6 M NH4OH
  3. add few drops of dimethylglyoxime
  4. red solution will form IF NICKEL PRESENT
143
Q

confirmatory test(s) for zinc

A
  1. after treating the solution with 6 M NaOH till basic to red litmus paper and centrifuge the supernatant might hold zinc
  2. divide the supernatant into two parts
  3. to one part, add 15 drops thioacetamide and heat in water bath for 5 mins. white precipitate will form if Zinc present
  4. acidify other portion of supernatant using 3 M HCl till solution JUST acidic to blue litmus paper. white precipitate will form and redissolve if zinc present
144
Q
A

Chemistry 2000 lab exam (5 decks)

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