Many of you in Singapore will be taking your End-of-Year School Exams or GCE O-Levels Exams in less than 5 weeks time.
Let’s try out some questions for revision.
Question 1:
Describe what you would see in each of the following reactions. Name the reaction product (s) which is responsible for each observation.
(a) Ethene is bubbled into aqueous bromine
(b) Magnesium is added into Copper(II) sulfate solution
(c) Solutions of silver nitrate and sodium iodide are mixed
(d) Zinc is added to sulfuric acid
(e) Calcium carbonate is heated in a test tube
(f) Solutions of sodium hydroxide and iron (II) chloride are mixed
Go try it out and leave your answers in the “Comment” section below.
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1) red brown colour of bromine water will decolourise. product: 1,2-dibromoethane
2)displacement reaction will take place. mg being reactive than cu displace it from so4. a white solution off mgSO4 can be seen with the pink deposit of cu metal.
3)with a displacement reaction a white solution of sodium nitrate will form with yellow precipitate of silver iodide which later turns to white
4) a white solution of zinc sulphate will form with a colorless gas hydrogen which extinguishes a lighted splint witha pop sound
5) thermal decomposition of CaCO3 will take place with the formation of white precipitate of CaO and a clourless odourless gas CO2 which when passed through limewater turn it milky.
6)a green precipitate of Fe(OH)2 can be seen with a white solution of NaCl
please correct my mistakes and mark the answer if possible..
thank you very much sir for your question . It will be grateful if you provide such more questions.
(a). The Bromine water turns from brown to colourless due to the double bonds present in ethene.
(b). Copper is displaced by magnesium and hence pink deposits of copper are observed.
(c). Sodium Nitrate and Silver Iodide are formed. Only the yellow colour of silver iodide is observed.
(d). Zinc Sulphate is the salt formed together with hydrogen gas which gives a ‘pop’ sound when tested with a lighted splinter.
(e). Calcium Carbonate, when heated, forms calcium oxide and carbon dioxide. Due to carbon dioxide, effervescence occurs and it also turns limewater milky when bubbled through it.
(f). Dirty-green precipitate is formed due to the formation of Iron(2) hydroxide.
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Regards,
Sean Chua
Author SimpleChemConcepts.com
Very good answers!
Keep it up,
Sean Chua
Hi Lee,
Observations is required as per the question.
Refer to my answers above.
All the Best to your Chemistry,
Sean Chua
Hi Michael,
Good answers.
However, please refer to your answer in part (f). Iron (II) hydroxide precipitate will be formed first which has a dirty green colour. It is only after standing that the iron (II) hydroxide is oxidised in the air to form iron (III) hydroxide reddish-brown precipitate.
Hope this helps! Keep learning!
Sean Chua
I agree with Steven in terms of what is required for students to answer.
In exams, they usually do not mark you wrong for giving extra info, its just that you will be wasting time for writing something extra that is not required (i.e. marks are not given for the extra info) and you might run out of time to do other questions.
Hope this helps. Keep learning and sharing Chemistry!
Sean Chua
Good attempt. However, do note that question was also asking for the observations. You need to mention about the observations being made.
Check Garrick’s answer above as well as my comments.
All the Best to your Chemistry,
Sean Chua
Good attempt Garrick!
However, do check your answer for part (D)
When carbon dioxide is passed into limewater, it will form an insoluble white precipitate which is Calcium Carbonate, CaCO3(s) and NOT calcium hydrogen carbonate (which is soluble in water),
It is only when excess amount of carbon dioxide is passed into limewater, that the calcium carbonate changes into calcium hydrogen carbonate.
Hope this is clear.
Sean Chua
ty for correcting my part (a) and (f) mistakes, but for (d) and (e),
the question only ask for observation.
hence we should only give what we observed.
i am looking for chemistry tips which is releted to the drugs industry.
(a) Addition reaction–Bromination reaction. Since the is a c-c double bond, then ethene would be able to react with bromine to form a saturated compound. Since bromine has been fully reacted, reddish brown aqueous bromine is decolourised.
(b) Metal displacement reaction. Since magnesium is higher than copper in the metal reactivity series, it will displace copper and form magnesium sulfate. Thus, pockets of pinkish metal (copper) is formed on the surface of magnesium.
(c) Silver iodide will be formed. This is a method to prepare the insoluble salt silver iodide, which is by precipitation. A white solid is formed at the bottom of the beaker.
(d) It will form zinc sulfate, a soluble salt. Effervescence due to liberation of hydrogen is observed. This is the reaction of metal and acid.
(e) Calcium Carbonate will decompose into Calcium Oxide and the by-product would be carbon dioxide. A white solid (calcium oxide) is found in the test tube after heating.
(f) Dirty green precipitate due to formation of iron(ii) hydroxide. Sodium chloride is also formed, though it is in aqueous state.
a) The solution decolorise
b) Copper(II) is displaced by Magnesium
c)Sodium Nitrate and Silver Iodide is formed
d)Zinc Sulfate and Hydrogen gas is produced
e)Carbon Dioxide is produced
f)Reddish-Brown ppt produced, iron (II) hydroxide and sodium chloride forme
(a) Bromination reaction will occur. Ethene will rapidly decolourise the aqueous Bromine Solution (yellow when diluted, red when conc.) and a layer of oil is formed.
(b) Displacement reaction will occur. Blue solution turns colourless (AKA Copper (II) Sulfate solution becomes Magnesium Sulfate solution), and reddish-brown precipitate of Copper precipitate will be formed.
(c) Yellow precipitate of Silver Iodide will be formed.
(d) Bubbles of hydrogen gas (effervescence) will be produced.
(e) Calcium Carbonate will decompose to form Calcium Oxide (Solid) and Carbon Dioxide gas.
(f) Reddish-brown precipitate of Iron (III) Hydroxide, or rust, will be formed as well as aqueous sodium chloride.
Correct me if i am wrong, thanks! =]
In general you’re correct.
(a) bromine is not purple. It can be described as brown, reddish brown, orange depending on its state and the solvent used. Iodine is purple/violet in organic solvents.
(d) and (e) you have to include the positive tests for the gases evolved.
(f) Correct but insufficient as green ppt will turn brown subsequently as Fe(OH)2 is readily oxidised by air to form Fe(OH)3.
(a) Ethene is bubbled into aqueous bromine
decolourises the purple aq bromine
ethene + bromine -> 1,2-dibromethane
(b) Magnesium is added into Copper(II) sulfate solution
pinkish-brown droplet(copper) appeared
Magnesium(s) + copper(II) sulfate(aq) -> magnesium sulfate(aq) + copper(s)
(c) Solutions of silver nitrate and sodium iodide are mixed
yellow solid(sliver iodide) is formed
Sliver nitrate(aq) + Sodium iodide(aq) -> Sodium nitrate(aq) + Sliver iodide(s)
(d) Zinc is added to sulfuric acid
effervescence(hydrogen) is observed
Zinc + sulfuric acid -> zinc sulfate + hydrogen gas
(e) Calcium carbonate is heated in a test tube
white solid(calcium oxide) is formed
Calcium carbonate -> Calcium oxide + Carbon dioxide
(f) Solutions of sodium hydroxide and iron (II) chloride are mixed
green ppt(Iron hydroxide) is formed
Sodium hydroxide(aq) + iron(II) chloride(aq) -> sodium chloride(aq) + iron hydroxide(s)
Am not good, just trying! Please correct me ~
(a) Aq bromine decolorised
(b) Magnesium displaces copper
(c) Silver iodide+sodium nitrate
(d) Zinc sulfate
(e) Carbon dioxide produced
(f) Iron (II) hydroxide+sodium chloride
(A) Addition Reaction of Alkenes
C2H4(g) + Br2(aq) –> C2H4Br2(aq)
Observations: Reddish-brown aqueous bromine decolourises into a colourless solution.
(B) Displacement Reaction of Metals
Mg(s) + CuSO4(aq) –> MgSO4(aq) + Cu(s)
Observations: Blue solution of Copper (ll) Sulfate decolourises into a colourless solution of Magnesium Sulfate, Pink solid which turns reddish brown formed on the bottom of the reaction mixture as precipitate.
(C) Reaction between Acid & Metal
Zn(s) + H2SO4(aq) –> ZnSO4(aq) + H2(g)
Observations: Colourless & odourless effervescence observed which extinguishes a lighted splint with a characteristic “Pop” sound.
(D) Thermal Decomposition of Metal Carbonates
CaCO3(s) –> CaO(s) + CO2(g)
Observations: Heat given off (Exothermic Reaction); Colourless & odourless effervescence observed which reacts with limewater (Calcium Hydroxide) to form a white insoluble precipitate of Calcium Hydrogen Carbonate.
(E) Ionic Displacement
FeCl2(aq) + 2NaOH(aq) –> Fe(OH)2(s) + 2NaCl(aq)
Observations: Dirty green precipitate of Iron (ll) Hydroxide formed in colourless solution which becomes reddish brown if left in the open. (Rusting)