11 — QA Flashcards
Similarities and differences betw NaOH and NH3
Similarity: both r alkalis, both dissociates in water to give OH- ions
Differences:
NaOH is a strong alkali which dissociates fully in water to give a high conc of OH- ions
NH3 is a weak alkali which dissociates partially in water to give a low conc of OH- ions
NH4+
NaOH few drops:
No precipitate formed. On warming, effervescence observed and gas evolved turns moist red litmus paper blue. Ammonia gas is produced.
Cu2+
Few drops of NaOH:
Light blue precipitate of Cu(OH)2 formed.
Excess NaOH:
Precipitate is insoluble in excess NAOH
Few drops of NH3:
Light blue precipitate of Cu(OH)2 formed
Excess NH3:
Precipitate is soluble in excess NH3, resulting in formation oof a dark blue solution
Fe2+
Few drops of NaOH:
Green precipitate of Fe(OH)2 formed
Excess NaOH:
Insoluble in excess NaOH
Few drops of NH3:
Green precipitate of Fe(OH)2 formed
Excess NH3:
Insoluble in excess NH3
Fe3+
Few drops of NaOH:
Red-brown precipitate of Fe(OH)3 formed
Excess NaOH: insoluble in excess
NH3:
Red-brown precipitate of Fe(OH)3 formed
Excess NH3:
Insoluble in excess
Ca2+
NaOH:
White precipitate
Excess NaOH:
Insoluble in excess
NH3:
No precipitate formed
Zn2+
NaOH:
White precipitate of Zn(OH)2 formed
Excess:
Soluble in excess NaOH(aq), resulting in the formation of a colourless solution
NH3:
White precipitate of Zn(OH)2 formed
Excess NH3:
Precipitate is soluble in excess NH3, resulting in the formation of a colourless solution
Al3+
NaOH:
White precipitate of Al(OH)3 formed
Excess NaOH:
Precipitate soluble in excess NaOH resulting in the formation of a colourless solution
NH3:
White precipitate of Al(OH)3 formed
Excess NH3:
Insoluble in excess
Why r zinc and aluminium hydroxides soluble in excess NaOH
They have amphoteric characteristics and r able to react w both strong alkali n strong acid to form a stable compound
Why is there no precipitate observed when aq ammonia is added to samples containing calcium ions?
Unlike NaOH, aq ammonia is a weak alkali which dissociates to give a low conc of OH- ions. Hence only a small amt of Ca(OH)2 is formed. Since calcium hydroxide is sparingly soluble, no obvious precipitate will be observed.
CO3^2-
Add dilute acid. Bubble the gas given off into limewater.
Obsv:
Effervescence observed. Gas given off forms white precipitate (CaCO3) in limewater. CO2 gas is produced
NO3-
Add aq NaOH then add apiece of aluminium. Warm the mixture carefully. Test the gas given off w a piece of damp red litmus paper.
Obsv:
Effervescence observed. Gas given off turns damp red litmus paper blue. Ammonia gas is produced.
Cl-
Add dilute nitric acid then add aq silver nitrate
Obsv:
White precipitate of AgCl formed
I-
Add dilute nitric acid then add aq silver nitrate
Obsv:
Yellow precipitate of AgI formed
SO4^2-
Add dilute nitric acid then add aq barium nitrate
Obsv:
White precipitate of BaSO4 formed
Why the addition of dilute nitric acid before adding aq silver nitrate or barium nitrate to Cl-,I-/SO4^2-?
To acidify the salt samples. Dilute nitric acid is added to remove interfering ions such as carbonates and hydroxides as they r mostly insoluble. Presence of interfering ions could lead to false positive results.
Dilute nitric acid is used as its salts r alw soluble.
H2 gas
Colour and odour:
Colourless and odourless gas
Test:
Place a burning splint at the mouth of test tube
Obsv:
Burning splint is extinguished w a ‘pop’ sound
O2
Colour and odour:
Colourless and odourless gas
Test:
Lace growing splint at the mouth of test-tube
Obsv:
The growing splint rekindled
CO2
Colour and odour:
Colourless and odourless gas
Test:
Bubble the gas given off into limewater
Obsv:
White precipitate is formed in the limewater
CL2
Colour and odour:
Yellow green gas w pungent smell
Test:
Place a piece of damp blue litmus paper at the mouth of test-tube
Observations:
Damp blue litmus paper turns red and is then bleached
SO2
Colour and odour:
Colourless gas w pungent smell
Test:
Place a piece of filter paper soaked w acidified potassium manganate (VII) at the mouth of test-tube
Observations:
Purple acidified potassium manganate (VII) solution turns colourless
NH3
Colour and odour:
Colourless gas w pungent smell
Test:
Place a piece of damp red litmus paper at the mouth of test tube
Observations:
Damp red litmus paper turns blue
Explain why aqueous ammonia can only be used to identify one of the ions in the solution containing Zn2+, Pb2+ and Al3+ ions. [2]
Zn ions form a white precipitate which dissolves in excess ammonia to give a colourless solution when AQUEOUS ammonia is added to the solution.
When aqueous ammonia is added, lead and aluminium ions gives the same observations where white precipitate is formed which is insoluble in excess aqueous ammonia.
Hence aqueous ammonia can only be used to identify zinc ions.
Sea water often contains large percentage of NaCl. Suggest a method to measure the amount of chloride ions present in a sample of sea water
Add excess silver nitrate, dry and weigh the precipitate formed
Based on the graph above, the student concluded that the anion is sulfate ion, but
not carbonate ion.
Do you agree with the student?
Explain your answer with reference to the graph.
Graph depicts that on adding aq BaCl2, precipitate is formed but no precipitate is formed when HNO3 is added.
Steps:
1 Add aqueous barium chloride to a test tube containing solution T.
2 Measure the height of precipitate formed after 5 minutes.
3 Add excess dilute nitric acid to the above mixture.
4 Measure the height of the precipitate formed after 5 minutes.
Don’t agree with student.
Upon adding of barium chloride, ppt formed could be due to sulfate or carbonate ions. (✓)
Height of ppt decreases/ppt dissolves upon adding nitric acid (✓) and this means that the ppt reacted with nitric acid. (✓)
Hence the ppt could be BaCO3 which reacted with acid since BaSO4 (✓) cannot react with acid.
4 (✓) – [3]; 2 – 3 (✓) – [2], 0 – 1 (✓) – [1]
