3 replies to this topic

[Andy-1872]

Having trouble identifying the redox equation since there are no signs (2+ , 3- etc)

27. Which of the following is a redox reaction?
A NaOH + HCl -> NaCl +H₂0
B Zn + 2HCl -> ZnCl₂ + H₂
C NiO + 2HCl -> NiCl₂ + H₂O
D CuCO₃ + 2HCl -> CuCl₂ + H₂O + CO₂

Taken from 2002 Higher Chemistry past paper.
Any help would be appreciated, thanks.

[Marcus]

Andy-1872, on 24 May 2011 - 07:23 PM, said:

Having trouble identifying the redox equation since there are no signs (2+ , 3- etc)

27. Which of the following is a redox reaction?
A NaOH + HCl -> NaCl +H₂0
B Zn + 2HCl -> ZnCl₂ + H₂
C NiO + 2HCl -> NiCl₂ + H₂O
D CuCO₃ + 2HCl -> CuCl₂ + H₂O + CO₂

Taken from 2002 Higher Chemistry past paper.
Any help would be appreciated, thanks.

Then let's rewrite the options with signs
Just ignore any ions that do not change
A OH^- + H⁺ -> H₂0 (chlorine and sodium stay as ions so ignore)
B 2H⁺ -> H₂ (This time the zinc and chlorine stay as ions but the hydrogens become a molecule ie ions to a molecule)
C O^2- + 2H⁺ -> H₂O (again the nickle and chlorine stay the same it is the hydrogens and water combine to form water)
D CO₃^2- + 2H⁺ -> H₂O + CO₂ (Sim. to above but slightly more complex)

Then look in data book to find the equations that is a combination of both oxidation and reduction reactions

[Andy-1872]

Marcus, on 24 May 2011 - 09:12 PM, said:

Andy-1872, on 24 May 2011 - 07:23 PM, said:

Having trouble identifying the redox equation since there are no signs (2+ , 3- etc)

27. Which of the following is a redox reaction?
A NaOH + HCl -> NaCl +H₂0
B Zn + 2HCl -> ZnCl₂ + H₂
C NiO + 2HCl -> NiCl₂ + H₂O
D CuCO₃ + 2HCl -> CuCl₂ + H₂O + CO₂

Taken from 2002 Higher Chemistry past paper.
Any help would be appreciated, thanks.

Then let's rewrite the options with signs
Just ignore any ions that do not change
A OH^- + H⁺ -> H₂0 (chlorine and sodium stay as ions so ignore)
B 2H⁺ -> H₂ (This time the zinc and chlorine stay as ions but the hydrogens become a molecule ie ions to a molecule)
C O^2- + 2H⁺ -> H₂O (again the nickle and chlorine stay the same it is the hydrogens and water combine to form water)
D CO₃^2- + 2H⁺ -> H₂O + CO₂ (Sim. to above but slightly more complex)

Then look in data book to find the equations that is a combination of both oxidation and reduction reactions

Apparently the answer is B but I still don't understand how. I thought B was just reduction.

[donaldc50]

I am sorry I have just read this after the exam so it will only help in future. Redox reactions by definition have two parts. One is an oxidation and the other a reduction. Remember OILRIG. In answer B the Zinc starts as an element and ends up in a compound. Metals have a tendency to lose electrons during this process becoming positive ions, and so are oxidised.. The hydrogen in the acid starts as a positive ion and ends up as hydrogen gas element. It has gained electrons and so has been reduced. If you could not follow which ion is which, don't panic. This reaction was an example of a DISPLACEMENT reaction where one element is displaced by another. This is why we don't refer to a metal reacting with an acid as a neutralisation. The other options were all neutralisations. Notice each started with an acid and a metal oxide or hydroxide, and made a salt plus water. The metal carbonate also made carbon dioxide. These are all classic neutralisations from standard grade.