rp4 - cations and anions

results of halide ions w/ conc sulfuric acid in halide ions fcs

describe how we can use dilute NaOH to identify group 2 cations:

• add 6 drops of dilute NaOH solution to test tube containing metal ion solution (e.g. magnesium sulfate, Mg²⁺ etc.) and observe precipitate (if any) formed

• keep adding dilute NaOH until it is in excess - record any changes you see

give 3 ways we can identify group 2 metal cations:

• flame test (not in RP4)

• by adding dilute NaOH

• by adding H₂SO₄

what is seen for each of the group 2 metal cations when dilute NaOH is added?

what would be the initial observation for a group 2 metal compound?

colourless solution

how can we test for ammonium (NH₄⁺) ions?

• add approx 10 drops of substance you are testing to test tube

• add approx 10 drops NaOH solution and shake

• warm gently using a water bath

• ammonia should be released - test fumes released w/ damp litmus paper

• if ammonium ions present, damp litmus paper turns from red → blue

give the symbol eqn for the ammonium ion test:

NH₄⁺ _(aq) + OH^- _(aq) → NH_{3 (g)} + H₂O _(l)

describe how we can use H₂SO₄ to identify group 2 metal cations:

• add 10 drops of H₂SO₄ solution to test tube containing metal ion solution (e.g. magnesium sulfate, Mg²⁺ etc.) and observe precipitate (if any) formed

• keep adding H₂SO₄ until it is in excess - record any changes you see

what is seen for each of the group 2 metal ions when H₂SO₄ is added?

give 2 ways we can test for OH^- ions:

• using litmus paper

• using ammonia solution and litmus paper

how can we test for OH^- ions with just red litmus paper?

• dip a piece of red litmus paper into the solution

• if OH^- ions are present, the paper will turn blue

how can we test for OH^- ions using ammonia solution and litmus paper?

• take 5 drops of ammonia solution and place on filter paper

• place this inside a petri dish w/ a lid

• dampen a piece of red litmus paper w/ deionised water and place on other side of petri dish

• replace lid and observe over a few minutes - if OH^- ions present, litmus paper should turn blue as ammonia vapours produced

how does the test for OH^- ions and litmus paper work?

• we add deionised water as OH^- ions form when NH₃ comes into contact w/ water

• this turns red litmus paper blue as ammonia solution vapours are alkaline

how can we test for CO₃^2- ions in aqueous solution?

• add 2 cm³ Ca(OH)₂ (limewater) to a test tube

• to a different test tube, add 3 cm³ Na₂CO₃ and an equal volume of dilute HCl

• immediately place a delivery tube from this to the limewater test tube

• if CO₃^2- ions present, limewater goes cloudy

explain how HCl can help detect CO₃^2- ions and give the eqn:

• limewater turns cloudy if carbonate ions present as CO₂ is produced in the reaction w/ dilute HCl (H⁺ ions)

• CO₃^2-_(aq) + 2H⁺ _(aq) → CO_{2 (g)} + H₂O _(l)

how can we test for sulfate ions in aqueous solution?

• add dilute HCl and BaCl₂

• white precipitate of BaSO₄ forms if SO₄^2- ions present

give the eqn for the test for sulfate ions in aqueous solution:

Ba²⁺ _(aq) + SO₄^2- _(aq) → BaSO_{4 (s)}

how can we test for halide ions in aqueous solution?

• add 10 drops of substance you are testing to a clean, dry test tube

• add approx 5 drops of dilute nitric acid and shake

• add 10 drops silver nitrate solution and record observations

• add dilute ammonia (or ammonia solution if testing for I^- ions and if so, work in a fume cupboard) and record further observations

why do we add nitric acid during the test for halide ions?

to remove CO₃^2- and OH^- ions, which would also form precipitates and interfere w/ the test

give the general eqn for the reaction of halide ions w/ silver nitrate:

Ag⁺ _(aq) + X^- _(aq) → AgX _(s)

give the results of the silver nitrate test:

AgCl - white ppt

AgBr - cream ppt

AgI - yellow ppt

give the results of the silver nitrate halide ions test when aqueous ammonia/ammonia solution is added:

AgCl - redissolves

AgBr - redissolves slowly and needs a lot of ammonia

AgI - does not dissolve

how can we test for halide ions in solid salts?

carry out in a fume cupboard and wear gloves:

• place a small spatula of the solid you are testing into a clean, dry test tube

• slowly add a few drops of concentrated H₂SO₄ and record observations

• test gas w/ moist blue litmus paper and record observations

• (repeat w/ acidified K₂Cr₂O₇ / lead (II) nitrate solution)

give the observations for the tests for halide ions in solid salts: