4 (2.3) Group 2 and 7 test tube reactions

Question 1

Test for grp 2 metal cations?

Answer 1

-group 2 compound on test tube
-add dilute NaOH dropwise into test tube, observe ppt forms
-keep adding NaOH until in excess

Question 2

Results from magnesium (mg2+) with dilute sodium hydroxide solution

Answer 2

Initial- colourless solution
With OH- : slight white ppt
With excess OH- : white ppt

Question 3

Observation of calcium ion (Ca2+) with dilute sodium hydroxide solution

Answer 3

Initial- colourless solution
With OH- : slight white ppt
With excess OH- : slight white ppt

Question 4

Observation strontium ion (Sr2+) with dilute sodium hydroxide solution

Answer 4

Initial- colourless solution
With OH- : slight white ppt
With excess OH- : slight white ppt

Question 5

Observation of barium (Ba2+) with dilute sodium hydroxide solution

Answer 5

Initial- colourless solution
With OH- : colourless solution (no change)
With excess OH- : colourless solution (no change)

Question 6

Flame tests for group 2 ions?

Answer 6

Dip nichrome wire loop in conc HCl
Dip wire loop in unknown compound
Hold loop in clear blue part of Bunsen burner flame
Observe colour change of flame

Question 7

Flame colour of each metal ion?

Answer 7

Ca2+ = brick red flame
Sr2+ = red flame
Ba2+ = pale green flame

Question 8

Test for ammonium ions?

Answer 8

-add dilute NaOH to unknown solution + gently heat (ammonium ions will react with OH- ions to produce ammonia gas and water NH4+ (aq) + OH- (aq) —> NH3 (g) + H2O(l)
Ammonia gas- alkaline
-damp red litmus paper turn blue if ammonium ions present

Question 9

Test for hydroxide ions?

Answer 9

-OH- ions make solution alkaline
-dip damp red litmus paper in solution- turn blue if hydroxide ions present

Question 10

Test for carbonate (CO3^2-) ions

Answer 10

-add dilute hydrochloric acid
-solution containing carbonate ions will fizz (give carbon dioxide)
-test CO2 by bubbling gas through limewater - turn cloudy if carbonate ions

Question 11

Test for sulfate ions (SO4^2-)

Answer 11

-add dilute hydrochloric acid, followed by barium chloride solution
-white ppt of barium sulfate forms if sulfate ions present

Question 12

What is order of testing ions?

Answer 12

Carbonate —> Sulfate —> halide

-prevents false positive results occuring (ie unexpected insoluble ppt such as Ag2SO4, Ag2CO2)

Question 13

How to test for group 7 ions (halides)

Answer 13

-add dilute nitric acid (HNO3)
-add silver nitrate solution (AgNO3)

Question 14

Test for halides results?

Answer 14

Chloride (Cl-) = white ppt of silver chloride
Bromide (Br-) = cream ppt of silver bromide
Iodide (I-) = yellow ppt of silver iodide

Question 15

Further test for group 7 halides?

Answer 15

-add dilute ammonia then conc ammonia to samples of dolution

Question 16

Results with further halide testing?

Answer 16

Cl- - white ppt dissolves in dilute NH3 (solution go colourless)
Br- - cream ppt dissolves in conc NH3 (solution go colourless)
I- - yellow ppt remains in conc NH3

Question 17

Equations for carbonate ion test?

Answer 17

Na2​C​ O3​​ +2HCl→2NaCl+H2​O​ +CO2

Ca(OH)2​ ​ + CO2​ ​ → CaCO3​ ​ + H2​O

Question 18

Test for group 7 ions: solid salts

Answer 18

-add few small spatulas of solid potassium halide salt
-add few drops conc sulfuric acid
-test any gas evolved with damp blue litmus paper

Question 19

Observations for group 7 union solid salts test

Answer 19

KCl- white, steamy fumes
KBr- orange fumes
KI- purple fumes + purple/black solid
For all- turn damp blue litmus paper red

Question 20

Safety for test of potassium halide salts

Answer 20

-conc sulfuric acid
-gases produced toxic & corrosive

Question 21

Test for sulfate ions equation?

Answer 21

MgSO4 + BaCl2 —> BaSO4 (s) + MgCl2 (aq)

Question 22

Why shake?

Answer 22

Ensure reactant combine properly

Question 23

Simplest Ionic Equation for formation of Mg(OH)2 (s)

Answer 23

Mg2+ (aq) + 2OH-(aq)—>Mg(OH)2 (s).

Question 24

Results for group 3 + OH-?

Answer 24

Aluminium salts will form a white precipitate of aluminiumhydroxidewhichwilldissolveinexcess sodium hydroxide to form a colourless solution

Question 25

Ionic equation for Al + OH-

Answer 25

[Al(H2O)6]3+(aq) + 3OH- (aq) —> Al(H2O)3(OH)3 (s) + 3H2O (l) Al(H2O)3(OH)3(s) +OH-(aq)—>[Al(OH)4]-(aq)+3H2O(l)