Required Practical 6 - Carry out simple test-tube reactions to identify: • cations – Group 2, NH4 + • anions – Group 7 (halide ions), OH–, CO3 2–, SO4(2-)

How would you test for group 2 metal ions?

Add 10 drops of 0.1 mol dm-3 metal ion solution in a test tube. Add 10 drops of 0.6 mol dm-3 of sodium hydroxide solution. Continue to add NaOH until its in excess.

What are the results for this test?

White precipitate - Magnesium hydroxide. Slightly alkaline

White precipitate (but needs more NaOH than MgOH) - Calcium hydroxide. More alkaline

Strontium and barium do not form hydroxides due to their high solubility.

What is the trend that can be seen?

Group 2 Hydroxides become more soluble as you go down the group

What will you see for group three metal ions?

Aluminium salts will form a white precipitate of aluminium hydroxide, which will dissolve in excess NaOH to form a colourless solution.

Al(H2O)(OH)3 --> [Al(OH)4]-

What will you see in transition metals?

Copper - blue ppt

Iron(II) - Green ppt

Iron (III) - Brown ppt

M(H2O)4(OH)2 (but 3 and 3 for Fe 3+)

How do you test for ammonium ions?

Adding dilute alkali (NaOH) and gently heating (using a water bath) produces ammonia gas which turns damp red litmus paper blue

Which group 2 metal ions will from ppts with sulfate ions and why?

Strontium and barium will form white precipitates, as goup 2 sulfates become less soluble as you go down the group.

How do you test for sulfate ions?

Add dilute HCl followed by barium chloride solution. A white preciptate will form.

Why do we add the HCl? Why can't we use H2SO4?

To react with carbonate impurities that are often found in salts which would form a white precipitate and give a false positive. You can't use H2SO4 as the SO4 will react with the BaCl2 and give a false positive.

How can you test for halide ions?

Add dilute nitric acid and then silver nitrate solution

What are the results?

Flourides - No precipitate

Chlorides - white precipitate

Bromides - Cream precipitate

Iodides - Pale yellow precipitate.

What can you do to further diffrenciate the results?

Treat with ammonia solution.

Silver chloride in dilute ammonia - colourless solution

Silver bromide in concentrated ammonia - colourless solution

Silver iodide - does not react with ammonia (too insoluble)

how do you test for presence of carbonate ions?

Add any dilute acid and see if there is any effervescence. Then, bubble the gas through limewater to test for CO2. Limewater will turn cloudy.

How do you test for hydroxide ions?

turns solution alkaline so use a PH indicator i.e red litmus paper will turn blue

What happens to the reducing power of halides as you go down the group?

Increases as the ions get bigger so it is easier for the outer electrons to be given away, as the pull from the nucleus becomes smaller, so it becomes easier to donate electrons.

What happens when flouride and chloride react with H2SO4?

Not strong enough reducing agent, so no redox reaction happens. H2SO4 acts as an acid. White steamy fumes of HF/HCL are produced with NaHSO4

What happens when bromide ions react with H2SO4?

Stronger reducing agent so NaHSO4 and SO2 produced. White steamy fumes of HBr. Red fumes of Bromine

What happens when iodide ions react with H2SO4?

Strongest reducing agents - NaHSO4, SO2, S and H2S produced. White steamy fumes of HI. Black solid and purple fumes of Iodine. Yellow solid sulphur. H2S - gas with bad egg smell.