CPrac L - QA

1. Add BaCI2(aq)

2. Add HCI (aq) 

Look out for…

Add BaCI2(aq) Add HCI (aq)

Test for SO42‒ , CO32‒ and SO32‒  (white ppt) Add dilute acid  White ppt insoluble in excess = SO42-  Ppt. dissolves with gas evolved SO32- => SO2 gas NO2‒ => brown NO2 gas CO32‒ => CO2 gas

Leave mixture to stand for a few minutes 

Look out for…

Leave mixture to stand for a few minutes

Test for Fe2+ or Mn2+ (turns dirty off-white on standing due air oxidation)

Equations for Zn2+ and H+

1. Add H+ 

• [Zn(OH)4​]2− + 2H+ (ANY acid can be used) → Zn(OH)2​ + 2H2​O

2. Excess H+ 

• The basic Zn(OH)2 dissolves as it undergoes acid-base reaction to form ZnCl2 solution

• Zn(OH)2(s) + 2HCl(aq) → ZnCl2(aq) + 2H2O(l)

How to take down observation

At aqueous NaOH slowly with shaking until no further change is seen. Leave to stand for a few minute

1.  (3 observations!)

• First observation: light brown ppt produced 

• After adding excess NaOH: ppt insoluble in excess 

• Left to stand: Upon standing, brown ppt observed

Add equal volume of hexane (POSS observation and deduction?)

Add chlorine (mix of sodium chlorate(I) and ethanoic acid)  Add equal volume of hexane

Observation: Orange solution formed  Deduction: Br- oxidised to Br2  Orange-red solution is formed, Br2 is present  Deduction: Br- is present

Add KI (aq) (sometimes KI is swapped for CuSO4, unknown is I-)  -> add Na2S2O3 to mixture

What is the observation

White precipitate in brown solution -> Brown solution decolourised

State deduction

Green precipitate, turning brown on contact with air, insoluble in excess NaOH

Green precipitate, turning brown on contact with air, insoluble in excess NaOH

Fe2+ present Fe2+ is oxidised to Fe3+ by air

How do you ensure that reagent bottle is not contaminated 

• Do not let tip of squeeze bottle touch mouth of test-tube 

• If dropper is used, do not leave the dropper on the bench or allow tip to touch anything except reagent 

When given a solid sample, why is it necessary to dissolve it form a solution 

• To carry out a test involving formation of precipitate or colour change of solution -> identity of ion(s) present 

Why do you need to filter 

To test for ions in residue and filtrate separately 

What is the chemistry involved in reactions (Zn form ppt when NH3 added → excess Nh3 added → filtrate added to HCI → excess HCI added)

• Aqueous NH is a weak base which dissociates partially in water as follows: NH3 (aq) + H2O(l) ⇌ NH4(aq) + OH- (aq)

• Hydroxide is precipitated when aq. NH3 is added dropwise

• Zn2+(aq) + 2OH- (aq) ⇌ Zn(OH) (s) —- (2) (and one for Fe) 

1. When excess NH3(aq) is added

• [Zn(NH3)4]2+ complex ion is formed as shown: Zn2+(aq) + 4NH3(aq) ⇌ [Zn(NH3)4]2+ (aq) — (4)

• The formation of [Zn(NH3)4]2+ causes [Zn2+] to decrease and by LCP, equilibrium position in reaction (2) shifts left, hence Zn(OH)2 dissolves

2. When the filtrate is added to HCI (or any acid) 

• [NH3] decreases due to acid-base reaction between NH and HCl, NH3(aq) + HC(aq) → NH4C/(aq)

• By Le Chatelier's Principle, equilibrium position in reaction (4) shifts left. 

• This would cause [Zn2+] to increase, hence equilibrium position of reaction (2) will shift right and so a white ppt. of Zn(OH)2 is formed (talk abt Ksp too)

3. Excess acid added 

• In excess sulfuric acid, OH‒ ions react with H+ to form water, causing [OH‒] to decrease

• Equilibrium position of reaction (2) shifts left, leading to the dissolution of Zn(OH)2

Why should damp red litmus paper avoid contact with mouth of test tube 

• Litmus paper might be contaminated by bases on mouth of the test tube, giving rise to a false positive test 

1. Mixture of two cations

• One forms hydroxide insoluble in excess NaOH(aq), the other forms hydroxide soluble in excess NaOH(aq)

• There are two solutions, one of them contains only Mg2+ ions while the other contains both Mg2+ and Zn2+ ions

• Suggest a method to distinguish the two solutions 

• Add excess NaOH(aq) to both FA1 and FA2 -> Filter both mixtures -> Add H2SO4(aq) dropwise to both filtrates -> The one that has a white ppt. reformed contains Zn2+

• Zn2+ in FA2 reacts with NaOH to form Zn(OH)2 which is soluble in excess to form [Zn(OH)4]2-, so it is found in the filtrate -> then when u add H2SO4 it neutralizes the OH-⁻, so Zn(OH)2 is re-formed as white ppt

Looks like displacement BUT it isn’t (bruh) 

FA1 (Zn) reduces Cu2+ to Cu forming reddish-brown Cu solid 

Use your observations in the above tests to suggest the identities of the anions present in FA1 with reasons 

• Observations = chemistry (reaction type) + observation 

• Ans: Br- 

• Reason: In test (f), Cl2 oxidised Br‒ to Br2 which gave an orange colour in aqueous solution and an orange-red colour in hexane.

• Ans: NO3- 

• Reason: In test (g), Al with NaOH reduced NO3‒ or NO2‒ to NH3 gas which turned damp red litmus blue. NO2‒ is absent since there is no brown fumes of NO2 when HCl(aq) is added to FA1 in test C

Why the test-tube needs to be moved about the flame

• To ensure that the temperature of the test tube is not changed drastically from room temperature to a high temperature -> break test-tube

• Ensures that heating of the solid is gradual -> temperature of the test-tube to be gradually increased

• Ensure even heating of all the solid in the test-tube

Explain why excess nitric acid is added to S before carrying out QA test 

• Ensure all carbonate ions are removed to prevent Ag2CO3 and BaCO3 from forming (also why nitric acid is used and NOT H2SO4 of HCI which will give insoluble salts）

Explain how impurities can be removed from a tablet 

Add excess water and stir well to dissolve all the compound , filter the mixture and collect the filtrate 

Amt of KMnO4	Amt of H+	Amt of Zn	Observation
Excess			Purple solution remains
Excess			Brown solid in purple solution
Limited			Purple solution decolourised (expected)

FILLL

Amt of KMnO4	Amt of H+	Amt of Zn	Observation
Excess	Excess	Limited	Purple solution remains
Excess	Limited	Excess	Brown solid in purple solution
Limited	Excess	Limited	Purple solution decolourised (expected)