ANCQ Questions.

[TheAspiringDoc]

Hi guys,
Couple of  questions from the 2006 intermediate ANCQ;

15)
Aluminium is a very reactive metal. It reacts easily with oxygen to form a coating of aluminium oxide on its surface, which seals the surface of the aluminium from the air. Many metals corrode easily by reacting with oxygen. Aluminium seems not to corrode because...
My answer: The oxide coating breaks down into aluminium and oxygen.
Their (correct) answer: The oxide coating protects the aluminium from corrosion.
Um?

18)
Rocket Fuel requires a very rapid chemical reaction in which solid or liquid fuel forms hot gases. One fuel used for rockets is liquid hydrazine, N2H4, which burns when it is mixed with liquid dinitrogen tetroxide, N2O4, in the engine of the rocket. Which of the following is the likeliest equation for the reaction of N2H4 with N2O4?
My answer: N2H4 (l) + 3N2O4 (l) -----> 4HNO3 (l) + 2N2 (g)
Their (correct) answer: 2N2H4 (l) + N2O4 (l) ------> 3N2 (g) + 4H2O (g)
Um um?

Could anyone explain these to me?
Thanks!

[IndefatigableLover]

Hi guys,
Couple of  questions from the 2006 intermediate ANCQ;

15)
Aluminium is a very reactive metal. It reacts easily with oxygen to form a coating of aluminium oxide on its surface, which seals the surface of the aluminium from the air. Many metals corrode easily by reacting with oxygen. Aluminium seems not to corrode because...
My answer: The oxide coating breaks down into aluminium and oxygen.
Their (correct) answer: The oxide coating protects the aluminium from corrosion.
Um?

18)
Rocket Fuel requires a very rapid chemical reaction in which solid or liquid fuel forms hot gases. One fuel used for rockets is liquid hydrazine, N2H4, which burns when it is mixed with liquid dinitrogen tetroxide, N2O4, in the engine of the rocket. Which of the following is the likeliest equation for the reaction of N2H4 with N2O4?
My answer: N2H4 (l) + 3N2O4 (l) -----> 4HNO3 (l) + 2N2 (g)
Their (correct) answer: 2N2H4 (l) + N2O4 (l) ------> 3N2 (g) + 4H2O (g)
Um um?

Could anyone explain these to me?
Thanks!

15):
Oxide coating doesn't break down into aluminium and oxygen as if they were to dissociate and even if it were to break down into simple aluminium and oxygen, it would react and corrode anyway since it's aluminium by itself with oxygen. The oxide coating protects the aluminium from corrosion as it has already "reacted" hence it cannot react with air anymore hence why it protects it.

18):
I would probably use intuition here in that it states that "Rocket Fuel requires a very rapid chemical reaction in which solid or liquid fuel forms hot gases" so I would assume that my products would all be gases which would be able to withstand high temperatures. As such, a liquid form for HNO3 would be highly unlikely as it would be in gaseous form (also if it were to be in liquid form then there would be some problems in storing it right)?

Just a little tip, sometimes intuition works better for some of these questions imo (well when I did it a while back in the day xD)