A few chemistry questions

• anpanman

  4 Nov 08, 17:17

  I did an experiment in school today and these few questions have been posed by my teacher. If possible, please tell me why they are so. Thanks.

   

  1.When you add zinc to dilute HCl and a few crystals of copper(II) sulphate into test tube to test for H2 gas, what is the purpose of the CuSO4? what is the reddish brown substance formed from this reaction?

   

  I have no idea but may I know whether it acts as a catalyst?

   

  2.I am sure all of you know when CO2 is passed into limewater, white ppt form. But when CO2 is passed CONTINUOUSLY into limewater, the white ppt disappears.

   

  is there a reaction behind all this, may I ask? i simply have no clue why it disappears because i didn't see such results in my experiment.

   

  3. when you test for CO2, why must we remove the test tube of limewater before cooling the apparatus?

   

  is it because the Ca(OH)2 would continue to form white ppt if it is not detached from the test tube containing CO2 and so we wouldnot be able to cool the apparatus.

   

  I'm really intetested to know about all this. So do help if you can!

• Craappiboy

  4 Nov 08, 19:59

  3. i tink if never detach, den somehow the limewater will be sucked in i tink.

• arigatoast

  4 Nov 08, 21:29

  Q1)  Zn + HCl ----> ZnCl2 + H2   metal + acid reaction

  ZnCl2 + CuSO4 ----> ZnSO4 + Cu ( reddish brown substance) metal displacement .

  CuSO4 is  not a catalyst in this experiment.

   

  Q2)  CO2 is an acidic oxide, so there will be an reaction with limewater Ca(OH)2. it will give CaCO3 as the white ppt.

  Ca(OH)_{2 (aq)} + CO_{2 (g)} → CaCO_{3 (s)} + H₂O _(l)

  Futher addition of CO2... think of this... acid + carbonate reaction.. it will give calcium hydrogencarbonate salt, which is soluble.

  CaCO₃(s) + CO₂(g) + H₂O(l) Ca(HCO₃)₂(aq)

  

   

   

   

• UltimaOnline

  4 Nov 08, 21:54
• Craappiboy and arigatoast are correct. Well done!  

 

Q1) Write the ionic equation Zn(s) + Cu2+(aq) --> Zn2+(aq) + Cu(s)

 

Q2) Thermal contraction of gas will lower pressure causing 'suckback' effect.

 

Q3) Calcium cation and carbonate anion would rather be (ionically) attracted ('stuck') to each other, instead of having favourable (ion-dipole) interaction with surronding water molecules, hence calcium carbonate is insoluble. But when further carbon dioxide is bubbled into water and forms carbonic acid which releases some protons (CO2 + H2O --> H2CO3 --> H+ + HCO3-); these protons (H+) protonate the carbonate anion (from calcium carbonate solid) to form monohydrogen carbonate ion (H+ + CO3 2- --> HCO3-), which only then is able to let go of the calcium cation (old boy/girlfriend), and thereafter both partners (calcium cation and newly fashioned monohydrogen carbonate anion) are willing to move on to seek other life partners around them (ie. having favourable ion-dipole interactions with the surrounding water molecules), and the white ppt of CaCO3 dissolves.