chem practical (Inorganic QA)

Question 1

when adding soln into test tube, why tip of bottle/dropper should not touch the sides of test-tube

Answer 1

avoid contamination

Question 2

how to stir/mix small and large volumes of solns/ solns with solids

Answer 2

small: tap bottom of test-tube against your palm

large: use clean glass rod

Question 3

how to heat solids?

Answer 3

1. heat gently first (remove water vapour)
2. heat strongly (ensure complete decomposition of solid / no further changes)

Question 4

what should i do if solid gives off water when heated?

Answer 4

1. tilt test-tube slightly downwards, almost horizontal (prevent condensed water at the mouth of test tube from running back to hot part of test tube, breaking glass)

Question 5

Possible observations for:
heating blue or green solids

Answer 5

black residue of Cu/Ni present

Question 6

Possible observations for:
residue appears reddish-brown when hot, appears brown when cold

Answer 6

Fe compounds present
cold-> Fe2O3 formed

Question 7

Possible observations for:
heating pink solids

Answer 7

black residue of Mn compounds present
Mn2+ undergo [o]

Question 8

Possible observations for:
residue appears yellow when hot, white when cold

Answer 8

Zn compounds present

Question 9

How should i heat solutions

Answer 9

Dont hold at a fixed positon or heat only at bottom of test tube (may cause hot contents to spill unexpectedly)

Question 10

why need to wet filter paper with deionised h2o o when filtering mixture?

Answer 10

reduce loss of liquid mixture as some might get absorbed by filter paper

Question 11

strength of NaOH / NH3

Answer 11

NaOH: strong base
NH3: weak base

Question 12

what to do when no ppt seen when NaOH added

Answer 12

test for NH3(g). might have NH4+

Question 13

colours of Fe3+ and Mn2+ at higher [ ]

Answer 13

Fe3+: orange-brown (normal: pale-yellow)
Mn2+: pale pink (normal: colourless)

Question 14

list acidic cations

Answer 14

Al3+, Cr3+, Fe3+, NH4+

cuz they are acidic, can use NaCO3 to distinguish them

Question 15

Why ppt is Al(OH)3 and not Al2(CO3)3 when Al3+ (aq) reacts with NaCO3?

Answer 15

This explanation is applicable for Cr³⁺ and Fe³⁺

By le chatelier’s principle, removal of H₃O⁺ by CO₃²⁻ (to form CO₂ + H₂O) causes poe (1) to shift right, producing more H₃O⁺ and [Al(H₂O)₅(OH)]²⁺. Continual removal of H₃O⁺ results in poe of (2) and (3) to shift right, eventually forming Al(H₂O)₃(OH)₃ = Al(OH)₃

(1): [Al(H₂O)₆]³⁺ +H₂O ⇌ [Al(H₂O)₅(OH)]²⁺ + H₃O⁺

(2): [Al(H₂O)₅(OH)]²⁺ +H₂O ⇌ [Al(H₂O)₄(OH)2]⁺ + H₃O⁺

(3): [Al(H₂O)₄(OH)₂ ]⁺ +H₂O ⇌ [Al(H₂O)₃(OH)₃] (s) + H₃O⁺

Question 16

who can form amphoteric metal hydroxide w OH- and complex ions?

Answer 16

Al3+, Cr3+, Fe3+

Question 17

Al³⁺ (aq) =

Answer 17

[Al(H₂O)₆]³⁺

Question 18

common reducing agents u need to know

Answer 18

KI (test for Cu2+ and Fe3+), SO2,

Question 19

ppt reformation from soluble complex ions:

REFORMING Zn(OH)2 FROM [Zn(OH)4]2-
1. Place test-tube containing filtrate on test-tube rack.

2. Add 2 to 3 drops of nitric acid. Do not shake the drops of nitric observe for precipitate reformation. (Observation: Some white precipitate reforms)
3. Continue to add more nitric acid to filtrate until no further change. Observe the solution followed by shaking the test tube.
4. (Observation: More white precipitate reforms. After shaking, the white precipitate dissolves in excess acid to give a colourless solution)

Answer 19

1. avoids the test-tube from shaking and allows observation of any ppt
2. Adding nitric acid provides H+ which removes OH-. By Le Chatelier’s poe shifts left, reforming Zn(OH)₂ (s).

Zn(OH)₂ (s)+ 2OH- (aq) ⇌ [Zn(OH)₄]²⁻ (aq)

3. Shaking the test-tube initially will prevent clear observation of the reformed ppt cuz excess OH present will dissolve the reformed ppt. As more nitric acid is added, excess OH present will be removed (neutralisation occurs). More Zn(OH)₂ (s) will be reformed.
4. In excess volumes of nitric acid, Zn(OH)₂ (s), being amphoteric, undergoes acid-base rxn to form Zn(NO₃)₂ (aq), which is colourless.

Zn(OH)₂(s) + 2HNO₃(aq) → Zn(NO₃)₂ (aq) + 2H₂O(l)

Question 20

what are diluted acids (HCl, H2SO4, HNO3) used to test for

Answer 20

any of the 3: CO₃²⁻, NO₂⁻, SO₃²⁻

cuz they can form co2, no2+no, so2 respectively

Question 21

Take note for NH3 liberated on heating with OH- and Al foil:

Answer 21

dont record effervescence observed cuz this effervsecence is H2(g) from a side rxn when Al (s) + 2OH- –> 2[Al(OH)4]- + H2(g)

Question 22

what to take note when reagents and procedure only mention solid and heat?

Answer 22

to do thermal decomposition of ammonium, nitrate or carbonate salt.

gas to be tested: NH3(g) , CO2(g) , O2(g) w brown NO2(g)

Question 23

why test tube containing limewater needs to be removed first

Answer 23

to prevent suck back of limewater into mixture

Question 24

“until no further change is seen” means

Answer 24

add reagent until in excess

Question 25

“add an equal depth of dilute sulfuric acid” means

Answer 25

add dropwise 1st, observe, shake + observe, then add in ex

Question 26

potential types of rxn they might ask

Answer 26

LIGAND EXCHANGE REACTION, acid-base, hydrolysis, redox, [o], [r], precipitation (rxn only got ppt)