UNIT 3 Laboratory Chemistry T

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AS Chemistry – Revision Notes
Unit 3 – Laboratory Chemistry
Warren Rieutort-Louis
Topic 3.1B – Laboratory chemistry

Tests for simple ions:
Name of
ion
Formula
Carbonate
CO32-
Hydrogen
carbonate
HCO3-
Sulphate (VI)
SO42-
Test

Add dilute HCl(aq)



Add to boiling water
Test for CO2
Add few drops of HCl
(remove unwanted ions)
Add few drops of
Barium chloride
solution
Add dilute HCl.
Test gas with filter paper
soaked in acidified
potassium dichromate
solution.
Dissolve the solid in pure
water.
Add dilute nitric acid
(removes other ions)
Add silver nitrate
Dissolve the solid in pure
water.
Add dilute nitric acid
(removes other ions)
Add silver nitrate
Dissolve the solid in pure
water.
Add dilute nitric acid
(removes other ions)
Add silver nitrate
Heat solid strongly.
Test the gas.
In solution, add Al and
NaOH  damp red
litmus paper blue



Sulphite
SO32
Chloride
Cl-



Bromide
Br-



Chloride
Nitrate
I-

NO3-




Result
Test gas with lime water and turns
milky
No gas with boiling water.
Turns limewater milky.
White precipitate
Ba2+(aq) + SO42-(aq)  BaSO4 (s)
Insoluble in HCl
Turns dichromate orange to green.
SO2 is produced.
or same test as above but HCl 2nd :
White ppt. soluble in HCl.
White precipitate
Ag+ (aq) + Cl-(aq)  AgCl (s)
Dissolves in dilute ammonia
Cream precipitate
Ag+ (aq) + Br-(aq)  AgBr (s)
Dissolves in conc. ammonia
Yellow precipitate
Ag+ (aq) + I-(aq)  AgI (s)
Insoluble in dil. or conc. ammonia
With Na or K, solid melts, oxygen
gas liberated
With Li or G2 metals: brown gas
(NO2)+ glowing splint relights
(O2)
Ammonium
Magnesium
NH4+
Mg2+

Heat with dilute NaOH
Ammonia is given off  indicator
paper blue

Add dilute NH3 until
excess
or add NaOH(aq) until
excess
White precipitate is formed

Flame Tests
Lithium
Li+
Sodium
Na+
Crimson red flame



+
Potassium
K
Calcium
Ca2+
Strontium
Sr2+
Barium
Ba2+


Place some solid on
clock glass.
Place c. HCl In another
clock glass
Clean the tip of a
platinum wire with the
acid by putting it in the
flame until no colour.
Put tip in acid, then solid,
then in the flame.
Yellow flame
Lilac flame
Brick-red flame
Scarlet red flame
Apple-green flame
Tests for simple gases:
Gas
Formula
Hydrogen
H2

Use a lighted splint
Squeaky ‘pop’
Oxygen
O2

Use a glowing splint
Relights
Carbon
dioxide
CO2

Bubble through lime
water
Turns milky

Use damp red litmus
paper
Place near HCl
 Turns red litmus blue
 Produces white fumes

Use damp blue litmus
paper
Blue litmus turns red then
bleached

Use damp blue litmus
paper
Turns red

Use orange dichromate
paper
Turns orange to blue/green
Ammonia
Test
NH3

Chlorine
Cl2
Nitrogen
dioxide
NO2
Sulphur
dioxide
SO2
Result

Volumetric analysis:
o Preparing a solution of known concentration (Standard solution):
 Place beaker on balance and weigh between x and y grams of solid
(after checking 0 error).
 Make a note of mass added
 Add about 100 cm3 of distilled water to beaker and dissolve solid with
glass rod.
 Use a funnel to transfer this to volumetric flask.
 Rinse the beaker and glass rod thoroughly with distilled water into
the volumetric flask .
 Use wash bottle to add water to standard flask until bottom of
meniscus is on the 250-cm3 mark on the standard flask.
 Place stopper and make homogeneous by shaking and inverting
the flask several times
o Performing a titration:
 Use a rinsed pipette and safety filler to place 25 cm³ of acid or alkali into
several conical flasks.
 Add a few drops of indicator into the conical flask. Methyl orange if alkali
in flask. Phenolphthalein if acid in flask.
 Rinse and then fill a burette with acid or alkali up to the 0 mark.
 Add the acid or alkali slowly, swirling the flask constantly until indicator
turns (phenolphthalein colourless - pink /methyl orange  orange - red)
 Stop adding the acid or alkali and record the volume of H2SO4 added.
 Repeat the whole experiment until 3 consecutive results within 0.2 cm3 are
obtained.
 Calculate the concentration of the alkali or acid.
o Errors in titrations:
 Percentage error = (actual error / measurement) x 100
 Error in pipette = 0.06 cm³
 Error in burette = 0.1 cm³
 Measurements involving difference: twice the error.

Enthalpy change measurements:
o Weigh a spirit lamp (containing a liquid alcohol) using a balance accurate to 3
decimal places. Record the mass measured.
o Use a measuring cylinder to put 100 cm3 of distilled water into a small beaker and
clamps this at a fixed height above the spirit lamp (about 2 cm).
o Record the initial temperature of the water using a thermometer.
o Light the lamp using a burning splint.
o Heat the water using the spirit lamp until the temperature has gone up by about
10C. Stir the water with the thermometer the whole time.
o Put a cap on the spirit to stop the alcohol burning. The lid stops also stops further
evaporation of the liquid alcohol.
o Reweigh the spirit lamp and record the mass.
o Calculate the enthalpy change
Possible sources of error:
 There may be heat loss due to the apparatus used and heat may have dissipated
through the insulating material  should use a polystyrene cup and insulation like a
lid.




The specific heat capacity and density of water are used (and not of HCl).
The masses of solid added to the acid are ignored.
It is assumed that the specific heat capacity of the polystyrene cup is negligible.
Some heat is lost when the hydrogen or carbon dioxide are evolved in the reactions.

Distillation and heating under reflux:

To check purity of distillate: measure boiling point and
compare with data book.
Often, under reflux, there are side reactions and reactions do
not go to completion.
Generally reactants are added slowly  exothermic reactions.
Fractional distillation can only be used when substances have
sufficiently different boiling points.




Organic tests:
o Collect 10 cm³ of the samples.
o Test the samples in the following order
 Alkenes – bromine water  decolourises ? 
alkene
 Alcohols – Spatula of solid PCl5. Test fumes
with damp litmus paper  litmus red? White
fumes near ammonia?  alcohol
 Halogenoalkane – Add NaOH, ethanol as solvent. Shake and warm for
3 minutes. Cool and add nitric acid + silver nitrate.  white = chloride;
cream = bromide; yellow = iodide.  Confirm with ammonia
 Alkane, the substance left is the alkane.

Sulphate solubility
group 2 ion in solution
Mg2+
Ca2+
Sr2+
Ba2+

Hydroxide solubility
group 2 ion in solution
Mg2+
Ca2+
Sr2+
Ba2+

effect of adding a sulfate solution
no precipitate MgSO4 is soluble
white precipitate of CaSO4
white precipitate of SrSO4
white precipitate of BaSO4
effect of adding a hydroxide solution
faint white precipitate of Mg(OH)2
faint white precipitate of Ca(OH)2
faint white precipitate of Sr(OH)2 on standing
no precipitate, Ba(OH)2 is soluble
Heating Carbonates and nitrates:
substance
lithium and all group 2
carbonates
sodium and potassium
carbonates
sodium and potassium
nitrates
lithium and all group 2
nitrates
effect of heat
carbon dioxide detected
no effect (except water of crystallisation may be given off)
oxygen only gas evolved
nitrogen dioxide and oxygen evolved
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