| Gas | Test and Test Results |
| Ammonia (NH3) | turns damp red litmus paper blue |
| Carbon dioxide (CO2) | gives white ppt with limewater, ppt dissolves with excess CO2 |
| Chlorine (Cl2) | bleaches damp litmus paper |
| Hydrogen (H2) | produces "pop" sound with lighted splint |
| Oxygen (O2) | relights a glowing splint |
| Sulfur dioxide (SO2) | turns aqueous acidified potassium dichromate (VI) from orange to green |
Test for Anions
| Anion | Test | Test result |
| Carbonate (CO32-) | Add dilute acid | Effervescence, carbon dioxide produced |
| Chloride (Cl-) (in solution) | Acidify with dilute nitric acid, then add aqueous silver nitrate | White ppt |
| Iodide (I-) (in solution) | Acidify with dilute nitric acid, then add aqueous lead(II) nitrate | Yellow ppt |
| nitrate (NO3-) (in solution) | Add aqueous sodium hydroxide, then aluminium foil, warm carefully | Ammonia produced |
| Sulfate (SO42-) (in solution) | Acidify with dilute nitric acid, then add aqueous barium nitrate | White ppt |
Colours of some metal hydroxides
| Metal hydroxide | Colour |
| calcium hydroxide | white |
| copper(II) hydroxide | light blue |
| iron(II) hydroxide | green |
| iron(III) hydroxide | red-brown |
| lead(II) hydroxide | white |
| zinc hydroxide | white |
Source: SEAB
Testing for cations
- Cations can be identified by their reactions with aqueous sodium hydroxide and aqueous ammonia
- A precipitate is an insoluble solid.
- When testing for cations, these precipitates only form when a metal ion in solution joins with hydroxide ions in solution to form an insoluble metal hydroxide
- eg Cu2+ + 2OH- --> Cu(OH)2 (blue copper(II) hydroxide precipitate)
| Cation | Add dilute NaOH (5 drops) | Add excess dilute NaOH | Add dilute aqueous NH3 (5 drops) | Add excess dilute aqueous NH3 |
| Al3+ | white ppt of aluminium hydroxide | ppt dissolves colourless solution | white ppt of aluminium hydroxide | ppt insoluble |
| Ca2+ | white ppt of calcium hydroxide | ppt insoluble | no reaction | no reaction |
| Cu2+ | blue ppt of copper(II) hydroxide | ppt insoluble | blue ppt of copper(II) hydroxide | ppt dissolves deep blue solution |
| Fe2+ | dirty green ppt of iron(II) hydroxide | ppt insoluble | dirty green ppt of iron(II) hydroxide | ppt insoluble |
| Fe3+ | red-brown ppt of iron(III) hydroxide | ppt insoluble | red-brown ppt of iron(III) hydroxide | ppt insoluble |
| Pb2+ | white ppt of lead(II) hydroxide | ppt dissolves colourless solution | white ppt of lead(II) hydroxide | ppt insoluble |
| Zn2+ | white ppt of zinc hydroxide | ppt dissolves colourless solution | white ppt of zinc hydroxide | ppt dissolves colourless solution |
| NH4+ ammonium | ammonia gas is produced on warming with dilute NaOH. This gas has a pungent smell and turns moist red litmus paper blue | - | no reaction | - |
[Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.]
1. Which statement about salts is not correct?
a, salts are made by neutralising alkalis with acids
b. salts contain anions and cations
c. salts are made by dissolving metal oxides in acids
d. salts always contain water of crystallisation
2. An example of a salt which can be prepared by precipitation is
a. lead(II) nitrate
b. sodium carbonate
c. silver chloride
d. magnesium sulphate
3. A way of distinguishing dilute hydrochloric acid from dilute sulphuric acid is to
a. add universal indicator
b. add aqueous barium nitrate
c. add a metal carbonate
d. add magnesium ribbon
4. Which of these statements about solubility is true?
a. all sulphates are soluble in water except calcium and lead sulphate
b. all nitrates are insoluble in water except sodium and potassium nitrate
c. most metal oxides are soluble in water except those of Group I and II
d. most metal carbonates are soluble in water
5.
Barium sulphate is insoluble in water. It is used in a 'barium meal' to
allow X-ray studies of the intestines. It can be prepared by a
precipitation reaction between two aqueous solutions. Which two
substances would be suitable for preparing barium sulphate for use in
X-ray radiography?
a. barium carbonate and sulphuric acid
b. barium chloride and sodium sulphate
c. barium oxide and potassium sulphate
d. barium nitrate and calcium sulphate
6. Which of these salts is best prepared by reaction with an acid and a base?
a. barium sulphate
b. copper(II) carbonate
c. magnesium sulphate
d. silver chloride
7.
A solution of substance X gave a white precipitate when aqueous NaOH
was added. However, when lead(II) nitrate solution was added to an
acidified solution of X, a yellow precipitate formed. What is the
correct identity of X?
a. calcium chloride
b. magnesium sulphate
c. sodium bromide
d. zinc iodide
8.
Iron(III) hydroxide is precipitated out of solution when aqueous sodium
hydroxide solution is added to iron(II) chloride solution.
Fe3+ (aq) + 3OH- (aq) --> Fe(OH)3 (s)
What is the minimum volume of 2 mol/dm3 aqueous NaOH required to precipitate the maximum amount of iron(III) hydroxide from 20cm3 of 1 mol/dm3 iron(III) chloride solution?
a. 10 cm3
b. 20 cm3
c. 30 cm3
d. 60 cm3
9.
After acidification with dilute nitric acid, a colourless solution X
reacts with aqueous silver nitrate to give a yellow precipitate. What
could X be?a. calcium iodide
b. copper(II) chloride
c. iron(II) iodide
d. sodium chloride
10.
An element reacts with steam but not with cold water. Its oxide can be
reduced by heating it with carbon. When it is placed in a solution
containing iron(II) ions, a grey deposit is formed. The element is most
likely to be
a. lead
b. magnesium
c. zinc
d. copper
MCQ Answers
1. d 2. c 3. b 4. b 5. b 6. c 7. d 8. c
9. a 10. c
Structured Questions Worked Solutions
1. A student was given an aqueous solution analyse. It contains copper(II) chloride and aluminium nitrate.a. Describe how he could detect the presence of chloride ions in the aove solution.b. Name the precipitate(s) formed when excess aqueous ammonia is added to the above solution.
Solution
1a.
1a.
- Add dilute nitric acid followed by aqueous silver nitrate.
- A white precipitate will be seen in the presence of chloride ions
OR
- Add dilute nitric acid followed by aqueous lead(II) nitrate.
- A yellow precipitate will be seen in the presence of chloride ions
2.
W is an alkali and X is a salt. When the two solutions were mixed
together, a reddish-brown precipitate Y was obtained. When a salt Z was
added to solution W and heated, a pungent gas which turned moist red
litmus blue was evolved. Suggest what could W, X, Y, and Z be.
Solution
W: Sodium hydroxide X: Iron(III) chloride/sulphate/etc Y: Iron(III) hydroxide Z: Ammonium chloride/nitrate/etc
W: Sodium hydroxide X: Iron(III) chloride/sulphate/etc Y: Iron(III) hydroxide Z: Ammonium chloride/nitrate/etc
3. Give the name and formula of the ions present in each of the solutions X, Y and Z below - a. Solution X gives a white precipitate when dilute hydrochloric acid and aqueous barium chloride are added to it.b. An alkaline gas is given off when sodium hydroxide solution is added to the colourless solution Z and the mixture heated.
Solution
3a. sulphate ion (SO42-) 3b. Ammonium ion (NH4+)
4. A similar reagent is added to zinc carbonate and sample S to initiate both reactions A and B.
ai. Name the reagent(s) required for Reaction A.aii. Write down the chemical equation (with state symbols) for Reaction A.bi. Give a possible identity of Sample S.bii. Based on your answer in bi, write a chemical equation with state symbols for Reaction B.
ai. Name the reagent(s) required for Reaction A.aii. Write down the chemical equation (with state symbols) for Reaction A.bi. Give a possible identity of Sample S.bii. Based on your answer in bi, write a chemical equation with state symbols for Reaction B.
Solution
ai. Hydrochloric acid aii. ZnCO3 (s) + 2HCl (aq) ---> ZnCl2 (aq) + CO2 (g) + H2O (l) bi. Zinc bii. Zn (s) + 2HCl (aq) ---> ZnCl2 (aq) + H2 (g)
ai. Hydrochloric acid aii. ZnCO3 (s) + 2HCl (aq) ---> ZnCl2 (aq) + CO2 (g) + H2O (l) bi. Zinc bii. Zn (s) + 2HCl (aq) ---> ZnCl2 (aq) + H2 (g)
5.
a. Identify A to F.b. Write an ionic equation (with state symbols) for the formation of the yellow precipitate.c. Describe a test for the pungent gas R.d.
Pungent gas R dissolves in water to form a solution. Describe the
observations when a few drops of copper(II) sulphate solution is added
to the solution.
a. Identify A to F.b. Write an ionic equation (with state symbols) for the formation of the yellow precipitate.c. Describe a test for the pungent gas R.d.
Pungent gas R dissolves in water to form a solution. Describe the
observations when a few drops of copper(II) sulphate solution is added
to the solution.
Solution
a. A: lead(II) carbonate B: dilute nitric acid C: ammonia gas D: carbon dioxide E: lead(II) nitrate F: lead(II) hydroxide
a. A: lead(II) carbonate B: dilute nitric acid C: ammonia gas D: carbon dioxide E: lead(II) nitrate F: lead(II) hydroxide
b. Pb2+ (aq) + 2I- (aq) ---> PbI2 (s)
c. Place two pieces of damp red and blue litmus papers at the mouth
of the test tube. If the gas is ammonia, the damp red litmus paper will
turn blue. A pungent gas will also be detected.
d. Blue precipitate is formed which dissolves in excess aqueous ammonia to form a dark blue solution.
6.
In the experiment shown below, the gas X produced by the action of
dilute sulphuric acid on the zinc granules was passed over two heated
metallic oxides. A colourless liquid W was collected and the excess gas X
was burnt off at Y.
a. What is gas X? Write the ionic equation for the formation of the gas.b. State what is observed of:i. zinc oxideii. copper(II) oxideWrite equation(s) for any change observed.c. Explain your observation made in bi and bii.d. Give a chemical test to identify liquid W.e. Suggest a suitable drying agent to be placed inside the drying bulb.f. Why was the excess gas X burnt off at Y?g. What precautions should be taken before the excess gas was lit?
a. What is gas X? Write the ionic equation for the formation of the gas.b. State what is observed of:i. zinc oxideii. copper(II) oxideWrite equation(s) for any change observed.c. Explain your observation made in bi and bii.d. Give a chemical test to identify liquid W.e. Suggest a suitable drying agent to be placed inside the drying bulb.f. Why was the excess gas X burnt off at Y?g. What precautions should be taken before the excess gas was lit?
Solution
a.hydrogen gas Zn + 2H+ ---> Zn2+ + H2
bi. Zinc oxide turns from white to yellow. bii. Copper(II) oxide turns from black to pink CuO + H2 ---> Cu + H2O
a.hydrogen gas Zn + 2H+ ---> Zn2+ + H2
bi. Zinc oxide turns from white to yellow. bii. Copper(II) oxide turns from black to pink CuO + H2 ---> Cu + H2O
c. Zinc is above hydrogen in the reactivity series so zinc oxide
will not be reduced by hydrogen to zinc. Upon being heated, zinc oxide
will change its colour from white to yellow. Copper is below
hydrogen in the reactivity series so copper(II) oxide will be reduced to
form pink copper.
d. W is tested with
anhydrous cobalt(II) chloride paper. If W is water, the cobalt(II)
chloride paper will turn from blue to pink.
e. fused calcium chloride
f. Because hydrogen gas is flammable and a mixture of hydrogen and air is very explosive.
g. Ensure that there is no leakage in the apparatus.
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