practicals - paper 1

Question 1

test for halide ions

Answer 1

method:
- take 5cm^3 of unknown solution
- add 3-5 drops of nitric acid followed by 3-5 drops of silver nitrate solution

positive result:
- chloride = white precipitate
- bromide = cream precipitate
- iodide = yellow precipitate

Question 2

test for ammonium ion

Answer 2

method:
- take 5 cm^3 of unknown
- add 1cm^3 of sodium hydroxide solution and warm the mixture
- test any gas evolved with damp red litmus paper

positive result:
- litmus paper turns blue

Question 3

qualitative test for halide ions

Answer 3

• make solution of halide
• acidify with dilute nitric acid
• add a few drops of silver nitrate solution
• precipitate of silver halide will be formed

silver chloride: white precipitate
silver bromide: cream precipitate
silver iodide: yellow precipitate

Question 4

qualitative test for halide ions pt 2

Answer 4

add ammonia as the white, cream and yellow precipitates can look very similar in colour
- add dilute NH3
- if precipitate dissolves in dilute ammonia, unknown halide is chloride
- if nothing happens, add concentrated NH3
- if precipitate dissolves in concentrated NH3, unknown halide is bromide
- if precipitate doesn’t dissolve in either NH3, then unknown halide is iodide

Question 5

qualitative test for carbonates

Answer 5

• add small amount of dilute hcl into test tube
• equal amount of sodium carbonate solution added to test tube
• as soon as sodium carbonate solution is added, bung with delivery tube should be attached to the test tube
• delivery tube should transfer gas which is formed into a different test tube that contains a small amount of limewater
• carbonate present = effervescence and white precipitate formed

Question 6

qualitative test for sulfates

Answer 6

• add dilute hcl to sample and add a few drops of barium chloride
• sulfate present = white precipitate
• or use barium nitrate solution

Question 7

qualitative test for ammonium

Answer 7

• react sample with warm aqueous sodium hydroxide
• ammonium gas present = pungent smell/ turns red litmus paper blue

Question 8

preparing standard solution

Answer 8

• Solid weighed + dissolved in beaker using less distilled water than needed to fill volumetric flask
• Transfer to volumetric flask + last traces rinsed into flask with distilled water
• Add distilled water drop wise until bottom of meniscus matches up with mark
• Flask inverted slowly several times to mix, if not titration results will be
  inconsistent

Question 9

acid-base titration procedure

Answer 9

• Add measured volume unknown solution to conical flask w/ pipette
• Add known solution (standard solution) to burette, record initial reading
• Add few drops indicator (phenolphthalien or methyl orange) to conical flask
• Run solution from burette into conical flask, swirling it, until it reaches the end
  point
• Record final reading.
• 1st titre carried out quickly to get approx, then repeat accurately adding solution
  dropwise as end point approached. Carry out until two accurate titres are concordant (within 0.1 cm3)

Question 10

determining enthalpy change of neutralisation

Answer 10

• measure 25cm3 hydrochloric acid using a 25cm3 measuring cylinder and add it to a polystyrene cup
• place the cup in a 25cm3 glass beaker
• construct a suitable table to record the temperature of the acid at minute intervals for up to 10 minutes
• measure 25cm3 of sodium hydroxide into a 25cm3 measuring cylinder
• start the timer and record the initial temperature of the hydrochloric acid in the cup
• continue to record the temperature each minute for 3 minutes
• at fourth minute, add the sodium hydroxide to the cup, do not record the temperature
• at 5th minute, continue recording the temperature up until 10 minutes
• stir solution in the cup each time each temperature is recorded.
• plot graph of temperature against time