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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: lacoste on September 25, 2009, 06:23:02 pm
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Question 6
English scientist, Sir Humphry Davy, first isolated potassium in 1807 by electrolysis of
molten potassium hydroxide.
a.
i. Write balanced half equations for the reactions that will occur at the anode and the
cathode in this cell.
anode __________________________________________________________
cathode __________________________________________________________
ii. What is the polarity of each electrode?
anode ______________________ cathode ___________________________
2 + 2 = 4 marks
b. Give two reasons why this reaction was a very dangerous one.
________________________________________________________________________
________________________________________________________________________
2 marks
c. A current of 6.20 amps flows for 4.00 minutes. Calculate the mass of potassium that will
be produced.
________________________________________________________________________
________________________________________________________________________
2 marks
d. This potassium hydroxide electrolysis cell is operated for about 15 minutes. The power
supply is then removed and replaced with an ammeter. The ammeter indicates that a
current is flowing.
i. Explain why a current is flowing even though the power supply has been removed.
____________________________________________________________________
____________________________________________________________________
ii. Give the half equation of the reaction occurring at the anode.
____________________________________________________________________
iii. What is the polarity of the anode?
A lot of parts here, but I'm only stuck and unsure about 'd'? the entire theory and what the answer is
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I think its because if the products of an electrolysis reaction remain in contact with the electrodes, they can react spontaneously as they would in a galvanic cell. I think its similar to the recharge-discharge process in a lead-acid car battery, only the order is reversed. The half reactions of the galvanic cell should just be those of the electrolysis reaction in reverse, but the cathode reaction in the electrolysis reaction will reverse and occur at the anode, and vice versa. For iii), just remember PRC and NOA, anode is negative as it is now a galvanic cell.
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Another question about the TSSM 2008 paper:
2b. The solutions say that an increase in pressure (which will cause a back reaction) will decrease K?????
Is this a typo, because i learnt that the only thing that will change K is temperature
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Another question about the TSSM 2008 paper:
2b. The solutions say that an increase in pressure (which will cause a back reaction) will decrease K?????
Is this a typo, because i learnt that the only thing that will change K is temperature
You are probably correct. What is full question?
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This is the question:
Question 2
Ethyne, C2H2, is a gas used in industry to produce a high temperature flame. The flame is used
to cut through metal. Ethyne can be produced from ethane in a reversible reaction.
C2H6(g) ⇔ C2H2(g) + 2H2(g) ΔH = + 355 kJ mol
b. At mild conditions of temperature and pressure, the yield of ethyne is low. Chemical
engineers change the conditions to improve the yield. Complete the table below to
predict the impact on this process of each change listed.
For each space, choose from decreases, doesn't change or increases
ii. Increase in pressure --------------- value of K
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mm you are correct. increase in pressure does not change the value of K.
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As electrolysis is a non-spontaneous reaction, it follows that when the products come in contact with each other, they will react in a spontaneous reaction, producing an electric current.