Required pracs P2 Flashcards
How to carry out flame tests
Dip wire into concentrated HCl then into sample
Place in blue Bunsen flame
Record the colour of the flame
Why dip wire in conc HCl
To remove any impurities that may affect the colour of the flame
Precautions to take when using a Bunsen burner
Don’t leave unattended
Tie back long hair
Keep flammable chemicals away from the flame
Lithium flame test
Crimson flame
Sodium flame test
Yellow flame
Potassium flame test
Lilac flame
Calcium flame test
Orange-red flame
Copper flame test
Green
Why can a flame test not be used when a compound contains a mixture of metal ions
The flame colours of some ions may be masked by the colours of other metal ions
Test for carbonate ions
Add a few drops of dilute HCl to sample
Pass the gas produced through lime water
Limewater turns cloudy (precipitate of type of carbonate)
How to test for sulphate ions
Add some dilute acid (HCl)
Then add barium chloride solution
White precipitate of barium sulphate formed
How to test for halide ions
Add some dilute nitric acid Then add silver nitrate Chloride - White precipitate (solid) Bromide - Cream precipitate (solid) Iodide - Yellow precipitate(solid)
How to carry out sodium hydroxide test
Add a little NaOH to a solution of the compound
followed by excess NaOH
Record colour
Result for copper - sodium hydroxide
Blue precipitate of copper hydroxide, Cu(OH)2
Iron II - sodium hydroxide
Green precipitate of iron (II) hydroxide, Fe(OH)2
Iron III - sodium hydroxide
Brown precipitate of iron (III) hydroxide, Fe(OH) 3
Calcium, magnesium, aluminium - sodium hydroxide
White precipitate of calcium hydroxide, Ca(OH)2
Magnesium Mg 2+ White precipitate of magnesium hydroxide, Mg(OH)2
Aluminium Al 3+ White precipitate of aluminium hydroxide, Al(OH)3
How to tell Ca, Mg and Al apart
To tell them apart, we add excess sodium hydroxide:
If white precipitate dissolves: Al 3+ ions are present
If the white precipitate does not dissolve, we have either
Ca 2+ or Mg 2+ ions in a compound, so we do a flame test to
decide:
no flame colour = Mg 2+ ions
orange-red flame = Ca 2+ ions
Sodium Thiosulphate-HCl Reaction
Measure 40 cm 3 of ‘thio’ into a conical flask & record solution temperature
Place it on paper that has a cross drawn on it
Measure 10 cm 3 of 0.100 moles/dm 3 HCl & record solution temperature
Add HCl to flask, start timer & swirl flask
Time disappearance of the cross
Repeat twice & calculate mean time
Repeat for different concentrations of sodium thiosulphate (dilute with water) but keep total volume constant
Sodium Thiosulphate-HCl Reaction
variables
Variables
IV concentration of ‘thio’
DV time for cross to disappear
CV concentration of HCl
Volume of HCl
Total volume of ‘thio’
Constant temperature
Improve accuracy sodium thiosulphate
To improve accuracy of measurements:
Use a pipette or burette to measure volumes
Use a light sensor connected to a datalogger to measure when light
intensity drops to a certain value (as precipitate forms)
Use a water bath set at a fixed temperature eg 20 0 C to keep
contents in flask at a constant temperature
Calcium Carbonate (Marble) or Magnesium-HCl Reaction
Measure 20 cm 3 HCl into a conical flask & record solution temperature
Measure 1 g of Mg or CaCO 3 using a balance
Add the solid to the flask, quickly place the gas syringe bung in flask & start timer, swirling flask
Time how long it takes to collect a fixed volume of gas
Repeat twice & calculate a mean time
Repeat for different concentrations of HCl (dilute with water) but keep total volume constant
Rate = total vol of gas / time
Calcium Carbonate (Marble) or Magnesium-HCl Reaction Variables
IV concentration of HCl
DV time for cross to disappear
CV Total volume of HCl
Same mass of solid
Constant room temperature
Improve accuracy of Calcium Carbonate (Marble) or Magnesium-HCl Reaction
Use a pipette or burette to measure volumes
Use a water bath set at a fixed temperature eg 20 0 C to keep
contents in flask at a constant temperature
