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Chem Y12 > Mod 8.1 > Flashcards

Mod 8.1 Flashcards

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1
Q

How are flame tests conducted?

A
  1. Clean a platinum or nichrome (nickel chromium alloy) wire by dipping it into concentrated HCl. Dilute acid can also be used but doesnt produce a strong colour
  2. Hold the wire in a hot Bunsen burner flame and observe its colour. This removes any substances from the wire and sterilises it
  3. Repeat the above process until wire doesnt produce any colour in the flame. If using a nichrome wire, expect a trace of orange in the flame. Could chop the used portion of the wire, if the wire has traces of orange that doesnt disappear
  4. When the wire is clean, moisture it again with some acid and dip it into a small amount of the ionic solution
  5. Put the moistened wire with the solution into the blue flame of the Bunsen Burner and observe the colour of the flame
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2
Q

What is the flame colour of lithium?

A

Carmine (dull red)

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3
Q

What is the flame colour of sodium?

A

Yellow

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4
Q

What is the flame colour of Potassium?

A

Light purple (lilac)

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5
Q

What is the flame colour of Calcium?

A

Brick red (orange red)

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6
Q

What is the flame colour of Barium?

A

Pale green (apple green)

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7
Q

What is the flame colour of Copper?

A

Blue-green

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8
Q

What are the rules regarding solubility?

A

All of group 1 is soluble
Nitrates are soluble
Acetates are soluble
Ethanoates (CH3COO-) are soluble

Chlorides (Cl-), bromides (Br-), iodides (I-) are soluble EXCEPT for Silver(Ag+) and Lead (Pb2+)

Sulfates (SO4) are soluble EXCEPT for Silver (Ag), Lead (Pb), Barium (Ba), Strontium (Sr), Calcium (Ca)

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9
Q

What are the rules regarding insolubility?

A

Carbonates (CO3 2-), sulfites (SO3 2-), phosphates (PO4 3-) are insoluble EXCEPT for Group 1 and ammonium (NH4+)

Hydroxides (OH-), oxides (O 2-) are insoluble EXCEPT for Group 1, ammonium, Barium, strontium and Calcium

Sulfides (S 2-) are insoluble EXCEPT for Group 1, 2 and ammonium (NH4+)

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10
Q

What is a complex ion?

A

A complex ion is an ion formed when one or more small molecules attach themselves to a central cation to form a combined ion that has properties that are quite distinct from the original ions or molecules

I.e. the tetraamine copper (II) complex ion; Cu(NH3)4 2+ (the ligands are the four NH3 molecules)

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11
Q

What is a ligand?

A

The molecules or ions that attach to the central metal cation is called a ligand

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12
Q

What are the four methods of testing for a Pb 2+ cation?

A
  1. With chloride Cl- (such as adding NaCl)
  2. With iodide I- (such as adding NaI)
  3. With CrO4 2- (such as adding potassium chromate, K2CrO4)
  4. Use flame test
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13
Q

How can Cl- be used to test for the presence of a Pb 2+ cation?

A

When Pb2+ reacts with Cl−, it forms a white precipitate if the concentration of the Pb2+ is more than 0.05 M.

Pb 2+(aq) + Cl− (aq) –> PbCl2 (s) (White)

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14
Q

How can I- be used to test for the presence of a Pb 2+ cation?

A

When Pb2+ reacts with I−, it forms a yellow precipitate:

Pb 2+ (aq) + I- (aq) –> PbI2 (s) (Yellow)

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15
Q

How can CrO4 2- be used to test for the presence of a Pb 2+ cation?

A

When Pb2+ reacts with CrO4 2−, it forms a yellow precipitate:

Pb 2+ (aq) + CrO4 2- (aq) –> PbCrO4 (s) (Yellow)

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16
Q

How can flame test be used to test for presence of a Pb 2+ cation?

A

Gives a white/gray flame colour

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17
Q

What are the 3 methods of testing for a Ba2+ cation?

A
  1. With sulfate SO4 2- (such as adding Na2SO4 or H2SO4)
  2. With chromate CrO4 2- (Such as adding K2CrO4)
  3. Use flame test
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18
Q

How can SO4 2- be used to test for presence of a Ba2+ cation?

A

When Ba2+ reacts with SO4 2−, it forms a white precipitate:

Ba2+ (aq) + SO4 2- (aq) –> BaSO4 (s) (white)

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19
Q

How can SO4 2- be used to test for presence of a Ba2+ cation?

A

When Ba2+ reacts with CrO4 2−, it forms a yellow precipitate:

Ba2+ (aq) + CrO4 2- (aq) –> BaCrO4 (s) (White)

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20
Q

How can flame tests be used to test for presence of a Ba 2+ cation?

A

Gives a pale green (apple green) flame colour

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21
Q

What are the 3 methods of testing for Ca2+ cation?

A
  1. With sulfate SO4 2- (such as adding Na2SO4 or H2SO4)
  2. With fluoride F- (Such as adding NaF)
  3. Use flame test
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22
Q

How can SO4 2- be used to test for presence of a Ca2+ cation?

A

When Ca2+ reacts with SO4 2−, it forms a white precipitate if the concentration of the Ca2+ is more than 0.05 M.

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23
Q

How can F- be used to test for presence of a Ca2+ cation?

A

When Ca2+ reacts with F−, it forms a white precipitate.

Ca2+ (aq) + SO4 2- (aq) –> CaSO4 (s) (White)

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24
Q

How can flame tests be used to test for presence of a Ca2+ cation?

A

Gives a brick red (dark red) flame colour

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25
Q

What is the method of testing for Mg2+ cation?

A
  1. With phosphate PO4 3- (such as adding ammonium phosphate)
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26
Q

How can PO4 3- be used to test for presence of Mg 2+ cation?

A

When Mg2+ reacts with PO4 3−, it forms white precipitate.

2Mg 2+ (aq) + PO4 3- (aq) –> Mg3(PO4)2 (s) (White)

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27
Q

What is the method of testing for Ag + cation?

A
  1. With chloride Cl- (such as adding NaCl)
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28
Q

How can Cl- be used to test for presence of Ag+ cation?

A

When Ag+ reacts with Cl−, it forms a white precipitate

Ag+ (aq) + Cl- (aq) –> AgCl (s) (White)

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29
Q

What are the 3 methods of testing for Cu 2+ cations?

A
  1. With hydroxide, OH- (such as adding NaOH)
  2. With NH3
  3. Use flame test
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30
Q

How can OH- be used to test for presence of Cu 2+ cation?

A

When Cu2+ reacts with OH−, it forms a blue precipitate.

Cu2+ (aq) + OH- (aq) –> Cu(OH)2 (s) (Blue)

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31
Q

How can NH3 be used to test for presence of Cu 2+ cation?

A

Dissolves to form a deep blue solution which contains a complex ion:

Cu2+ (aq) + 4NH3 (aq) –> Cu(NH3)4 2+ (aq)

The complex ion here is Cu (NH3)4 2+, which is deep blue in solution

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32
Q

What are the 3 methods of testing for Fe2+ cation?

A
  1. With OH- (such as adding NaOH)
  2. With acid and dilute KMnO4 solution
  3. With ammonium Sulfide (NH4)2 S
33
Q

How can OH- be used to test for presence of Fe2+ cation?

A

When Fe2+ reacts with OH−, it forms a green or white precipitate which slowly turns brown when left exposed to the atmosphere.

Fe 2+ (aq) + 2OH- (aq) –> Fe(OH)2 (s) (Brown)

34
Q

How can acid and dilute KMnO4 solution be used to test for presence of Fe2+ cation?

A

Add Fe2+ solution with potassium permanganate, KMnO4 which has a distinct purple colour. Then add an acid, and if the solution becomes clear, then
the substance is Fe2+.

5Fe2+ (aq) + MnO4- (aq) + 8H+ (aq) –> 5Fe 3+ (aq) + Mn2+ (aq) + 4H2O (l)

35
Q

How can ammonium Sulfide ((NH4)2S) be used to test for presence of Fe2+ cation?

A

Forms a black precipitate which is readily soluble in dilute acids

Fe 2+ (aq) + S2- (aq) –> FeS (s)

36
Q

What are the 2 methods of testing for presence of Fe3+ cations?

A
  1. With OH- (Such as NaOH)
  2. With thiocyanate (SCN-)
37
Q

How can OH- be used to test for presence of Fe3+ cations?

A

When Fe3+ reacts with OH-, it forms a brown precipitate, Fe (OH)2

Fe3+ (aq) + 3OH- (aq) –> Fe(OH)3 (s) (Brown)

38
Q

How can thiocyanate (SCN-) be used to test for presence of Fe3+ cations?

A

When Fe3+ reacts with thiocyanate, SCN−, forms a deep red gelatinous (like jelly) which forms a complex ion

Fe3+ (aq) + SCN- (aq) –> (FeSCN) 2+ (aq)

39
Q

Memorise flowchart

A

Memorise flowchart

40
Q

Why do we need to monitor the environment?

A

There can be trace elements in our natural water system such as Lead II, copper, Iron and other metallic species as a result of factory waste. Environmental bodies inspect companies that use poisonous substances, and provide limitations for such companies which release such waste into the environment

There can also be trace elements in our atmosphere which causes a need to monitor the environment. A number of compounds are destructive to both the environment and people. SO2 (sulfur dioxide) is released when coal is combusted. This is because sulfur is present in all types of coal.

The reason why sulfur dioxide is harmful to the environment and people is that it causes respiratory problems in humans, and when it starts to rain, the gaseous SO2 reacts with water to form sulfurous acid and sulfuric acid.

S8 (g) + 8O2 (g) –> 8SO2 (g)

SO2 (g) + H2O (l) –> H2SO3 (aq)

2H2SO3 (aq) + O2 (g) –> 2H2SO4 (aq)

This is a form of acid rain. The problems of acid rain include:

-Damage to natural landscape such as water, this will disrupt the ecosystem and kill plants and animals

-Damage to buildings and various valuable landmarks - that are made with marble (CaCO3) which is a common stone used. This is because:

CaCO3 (s) + H2SO4 (aq) –> CaSO4 (aq) + CO2 (g) + H2O (l)

Another atmospheric pollution requiring monitoring is the burning of nitrogen gas, which combusts with Oxygen but in places where oxygen gas is low in supply. For example, in the confines of car engines.

2N (g) + O2 (g) –> 2NO2 (g)
2NO2 (g) + H2O (l) –> H2NO3 (aq)

As we use private transport, engine exhaust releases poisonous gases, such as CO, which is poisonous for humans and all living creatures. This is because CO expels O2 from our lungs if CO gas is inhaled –> humans and living creatures suffocate

Thus, monitoring the environment is essential to prevent these trace elements within our atmosphere from harming us and other living creatures

41
Q

What is a precipitation titration?

A

It is a volumetric analysis that involves a titration where the analyte and titrant react to form a precipitate.

A common precipitating reagent used in precipitation titrations is silver nitrate

The difficulty in precipitation titrations is determining the end point as it is difficult to visually tell when a precipitate stops forming

42
Q

What are 3 methods that could be used to determine the end point of a precipitation reaction?

A

Mohr’s method
Volgard’s method
Fajan’s method

43
Q

What is Mohr’s method?

A

Mohr’s method quantifies chloride, bromide and cyanide ions using direct titration with a standard silver titration solution:

Ag + (aq) + X- (aq) –> AgX (s)

Yellow potassium chromate is used as the indicator

Silver preferentially precipitates with halide ions X-. When excess silver is added, it will react with chromate ions to produce a reddish brown precipitate (Silver chromate)

Mohr’s titration should be performed at a netural or weak basic solution of pH 6-9. This is because high pH results in formation of silver hydroxide precipitate, low pH causes chromate to form chromic acid which reduces concentration of chromate ions and delay formation of precipitate

44
Q

What are the limitations of Mohr’s method?

A

Presence of multiple anions in a sample can lead to invalid results. Carbonate and phosphate ions will precipitate with silver

Conical flask must be thoroughly stirred during titration to ensure that Ag+ ions are evenly distributed

titration must be conducted at a pH 6.5-9

Can’t analyse iodide or thiocyanate ions, ONLY Cl or Br ions

Large amounts of Ag+ needs to be added. This excess silver required to reach the end point leads to systematic error, which can be corrected using a blank titration

45
Q

What is a blank titration in Mohr’s method?

A

The same quantity of chromate indicator ti be used in the titration is added to a suspension of calcium carbonate (which imitates the white AgX precipitate)

AgNO3 is added to the solution until a colour change occurs.

The volume of AgNO3 required to cause the colour change in the blank is the excess volume required. This volume is subtracted from the titration volumes

46
Q

What is volhard’s method of titration?

A

It uses back titration to determine concentrations of ions such as Cl, Br I, CN, PO4, CR2O7, S, CO3 in acidic solutions. However, a direct titration can be used for Ag+

The titration uses Fe3+ as the indicator to detect the end point

47
Q

Explain the steps of Volhard’s method

A

A known excess quantity of AgNO3 is added to sample to precipitate all analyte anions

The initial precipitate must be filtered off before titrating with SCN- to prevent any other reactions occurring

Excess AgNO3 is determined by back titration against KSCN standard solution

A small amount of Fe3+ is added as the indicator. When excess SCN- is present, there will be a permanent colour change

48
Q

What are the limitations of Volhard’s method?

A

It must be performed in low pH to prevent the precipitation of Fe3+ indicator as Fe (OH)3. At low pH, there is a relatively low oh- , which prevents the precipitate from forming

It is less valid if the first precipitate formed is more soluble than AgSCN (which has a kSP of 1.03*10^-12)

49
Q

What is Fajan’s method?

A

This is a direct titration, where the end point is determined by an adsorption indicator (thats not spelt wrong)

The indicator/dye adsorbs onto the surface of the precipitate at the end point, causing the colour change. A specific adsorption indicator is needed for a specific pH

50
Q

What are the limitations of Fajan’s method?

A

The concentration of the species being analysed cannot be too low, as the amount of precipitate will not be enough for the indicator to change colour

A significant amount of non reacting ions can cause the indicator to coagulate with it, resulting in no visible colour change

It also relies on a large precipitate surface area to allow sufficient dye to adsorb so the colour change is easily visible

51
Q

What is a gravimetric analysis? What are the steps?

A

This is a quantitative analytical technique used to determine the proportion, by mass, of a particular chemical substance in a compound or mixture.

The substance to be analysed should be separated from the mixture, typically through precipitation. This precipitation is collected by filtration, then washed, dried and weighed. Then, the % composition of substance can be determined

52
Q

What are the assumptions for gravimetric analysis? (to ensure validity)

A

Only the target ion precipitates out (use of selective reagent, remove or prevent the precipitation of other salts)

All the target ion precipitates out (selective reagent (highly insoluble precipitate, really small Ksp), added in excess)

All the precipitate is collected and in a pure form (Filtration, wash precipitate with warm distilled water multiple times, ensure it is dry when measuring its mass)

53
Q

What are the advantages of gravimetric analysis?

A

It is very accurate, if the gravimetric analysis is carefully designed (assumptions + validity) and conducted properly

It doesnt require specialty equipment to perform

It does not require the construction of a calibration curve, which can be time consuming

54
Q

What are the disadvantages of gravimetric analysis?

A

The target ion of interest must be able to form a sufficiently insoluble precipitate for accurate results (A limitation of the technique)

The precipitate must be highly pure

It is extremely time consuming compared to modern analytical techniques

Concentration of the target ion must be high enough to produce a precipitate that can be accurately weighed

55
Q

What is the method for testing for Cl- anions?

A

React with AgNO3 and then add ammonia to confirm presence of AgCl

56
Q

How can reacting Cl- with AgNO3 and then adding ammonia be used to test for presence of Cl- anions?

A

First we acidify the Cl- solution then add AgNO3, which produces a white precipitate.

Ag+ (aq) + Cl- (aq) –> AgCl (s) (white)

Adding ammonia to silver chloride will cause the precipitate to dissolve, and therefore confirms the presence of silver chloride:

AgCl (s) + 2NH3 (aq) (reverse reaction) Ag(NH3)2+ (aq) + Cl- (aq)

Also, precipitate will darken in sunlight

57
Q

What is the method for testing for Br- anions?

A

Reaction with AgNO3

58
Q

How can reacting Br- with AgNO3 be used to test for presence of Br- anions?

A

Add AgNO3 to the Br- solutions which produces a creamy white precipitate

Ag+ (aq) + Br- (aq) –> AgBr (s) (creamy white)

59
Q

What is the 2 method for testing for I- anions?

A

React with AgNO3 to an I- solution

Acidify solution with HNO3 and then add Pb(NO3)2

60
Q

How can reacting I- with AgNO3 be used to test for presence of I- anions?

A

React with AgNO3 to an I- solution and it produces a pale yellow precipitate.

Ag+ (aq) + I- (aq) –> AgI (s) (pale yellow)

61
Q

How can acidifying I- with HNO3 and then adding Pb(NO3)2 be used to test for presence of I- anions?

A

First we acdifiy the iodide solution with nitric acid, and then we add Pb(NO3)2 to form PbI2, precipitate which has a very distinct yellow colour.

Pb2+ (aq) + I-(aq) –> PbI2 (s) (Bright yellow)

62
Q

If only reacting with AgNO3, how do you differentiate between Br-, Cl-, and I- ions?

A

If it is Cl- , there is a white precipitate which darkens when exposed to sunlight and dissolves when NH3 is added

To differentiate between Br- and I- ions which look similar as precipitates, add NH3, if the precipitate dissolves then it is likely to be Br-, but if it doesnt dissolve then it is likely to be I-

To further confirm I- Pb(NO3)2, and see if a precipitate forms

To further confirm Br-, add chloroform and chlorine water, shake, observe reddish colour in organic layer

63
Q

What are the 2 method for testing for CO3 2- anions?

A

React with Dilute HNO3

pH with universal indicator

64
Q

How can reacting CO3 2- with diliute HNO3 be used to test for presence of CO3 2- anions?

A

Carboantes react with acids to produce a salt, carbon dioxide and water.

Co3 2- (aq) + 2 H+ –> CO2 (g) + H20 (l)

To test the presence of carbon dioxide, we conduct the lime water test. The reaction between the carbonate and acid will bubble due to CO2. Then with the lime water test, the clear lime water will turn milky white due to the presence of calcium carbonate, and will settle and sediment to the bottom of the test tube

65
Q

How can placing CO3 2- into the universal indicator test for presence of CO3 2- anions?

A

Carbonates have a pH in the 8-11 range, so if it shows up with that, then it is a carbonate (I think its blue)

66
Q

What are the 2 method for testing for SO4 2- anions?

A

Reaction with Ba(NO3)2

Reaction with PB(NO3)2

67
Q

How can reacting SO4 2- with Ba(NO3)2 be used to test for the presence of the anion?

A

To an acidified sample of sulfate, SO4 (2-), we add barium nitrate to get barium sulfate precipitate

SO4 2- (aq) + Ba 2+ (aq) –> BaSO4 (s)

68
Q

How can reacting SO4 2- with Pb(NO3)2 be used to test for the presence of the anion?

A

By adding to an acidified sample of nitrate, NO3 - we add lead(II) nitrate to get lead (II) sulfate precipitate.

SO4 2- (aq) + Pb2+ (aq) –> PbSO4 (s)

69
Q

What are the 3 methods for testing for PO4 3- anions?

A

React with NH3, then Ba(NO3)2

React with Mg (NO3)2

Acidification with HNO3, and then add (NH4)2 MoO4

70
Q

How can reacting PO4 3- with NH3 and the Ba (NO3)2 be used to indicate the presence of the anion?

A

Addition of ammonia followed by barium nitrate produces a white precipitate:

2PO4 3- (aq) + 3Ba2+ (aq) –> Ba3 (PO4)2 (s)

71
Q

How can reacting PO4 3- with Mg(NO3)2 be used to indicate the presence of the anion?

A

If we add magnesium nitrate, we get magnesium phosphate precipitate.

PO4 3- (aq) + Mg 2+ (aq) –> Mg3 (PO4) 2 (s)

72
Q

How can the acidifcation of PO4 3- with HNO3 before adding (NH4)2 MoO4 be used to identify the presence of the anion?

A

Acidification with HNO3, followed by addition of ammonium molybdate solution (NH4)2 MoO4 produces a yellow precipitate, ammonium phosphomolybdate, (NH4)3 PO4 . 12MoO4. Warming the mixture for a few minutes may be necessary.

PO4 3- (aq) + 12 (NH4)2 MoO4 (aq) –> (NH4)3 PO4 . 12MoO4 (s)

73
Q

What are the 3 methods for testing for OH - anions?

A

Reaction with CuSO4

Reaction with AgNO3

With universal indicator

74
Q

How can reacting OH- with CuSO4 be used to indicate the presence of the anion?

A

The Cu2+ (aq) would react with OH- to form Cu(OH)2 precipitate with a distinctly blue-ish green colour

Cu2+ (aq) + 2OH - (aq) –> Cu(OH)2 (s)

75
Q

How can reacting OH- with AgNO3 be used to indicate the presence of the anion?

A

The Ag+ (aq) would react with OH- to form AGOH precipitate which is a light brown colour:

Ag+ (aq) + OH- (aq) –> AgOH (s) (Light brown)

76
Q

How can using OH- with the universal indicator be used to indicate the presence of the anion?

A

The indicator should turn violet

77
Q

What are the 3 methods for testing for CH3COO- anions?

A

Reaction with Cu2+ or Zn2+ or Pb2+ (not really counted as a method)

Add NaCl and use universal indicator

Neutralise solution and add FeCl3

78
Q

How can neutralising the CH3COO- solution before adding FeCl3 be used to indicate the presence of the anion?

A

When acetate is added with FeCl3 in a neutral environment, it produces (CH3COO)3 Fe, which is a distinct red solution that is partially soluble in water.

3CH3COO- (aq) + FeCl3 (aq) –> (CH3COO)3 Fe (aq) + 3Cl - (aq)

Chem Y12 (16 decks)

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