Year 11 Chem

[squance]

My sister is going to start yr 11 chem next year and she was just reading ahead in her textbook..

She has some questions that she is stuck on....

I did chem but I forgot the majority of the stuff so I was wondering if some of you guys can help me....

"When 2.8g of magnesium reacts with 50g of HCl solution, the remaining solution weighs 52.6g. Why does the law of conservation of mass seem not to apply in this case?"

and....

This reaction...

NH3(g) + HCl(g) -----> NH4Cl (s)
Is it a precipitation, combination, or neutralisation reaction?? Explain why..

Thanks

[Collin Li]

When 2.8g of magnesium reacts with 50g of HCl solution, the remaining solution weighs 52.6g. Why does the law of conservation of mass seem not to apply in this case?

Hydrogen gas is formed, which will just escape into the atmosphere, so although there is less mass in the end, it is because 0.2g of hydrogen gas has been formed and escaped.

[Collin Li]

NH3(g) + HCl(g) -----> NH4Cl (s)
Is it a precipitation, combination, or neutralisation reaction?? Explain why.

It is both a combination reaction and a neutralisation reaction.
* combination reaction: two reactants have combined to form one product
* neutralisation reaction: an acid, hydrogen chloride, and a base, ammonia gas, react to form a neutral ionic compound.

It is not a precipitation reaction, because the solid has not been formed from solution.

[squance]

Thank you !

[bec]

I'm doing yr 12 chem next year and I've got a question too...
in a reaction between iron (III) oxide and carbon monoxide, how do we know what the products are?
initially i put:
Fe₂O₃ + CO --> 2Fe + CO₂ + O₂

but in the solutions it had it as:
Fe₂O₃ + 3CO --> 2Fe + 3CO₂

Why was my answer wrong? Any explanations about this would be really appreciated because there's obviously something I'm missing here!
thanks!

edit: hahaha i just realised i'd written that i'm doing yr 11 chem next year.. living in the past!! i meant year 12...which means i'm also in the wrong thread...sorry!

[Collin Li]

bec, something like that is pretty ambiguous. You wouldn't be expected to know the products of the reaction between iron (III) oxide and carbon monoxide (not for VCE).

[bec]

ok thanks for that coblin, good to know!

i've got another question...it's kind of nitpicky but some of you smarties out there can probably help:

"write a full equation for the reaction of ammonium hydroxide and aluminium nitrate"
i just assumed (correctly as it turns out) that it would be:
3NH₄OH + Al(NO₃)₃ --> 3NH₄NO₃ + Al(OH)₃
but since NH₄⁺ and OH^- aren't on the electrochemical series sheet we're given, how do we know that a reaction will take place at all? is there another way to predict the "feasibility" of a reaction?

[Collin Li]

but since NH₄⁺ and OH^- aren't on the electrochemical series sheet we're given, how do we know that a reaction will take place at all? is there another way to predict the "feasibility" of a reaction?

Oh. You have a flaw in your thinking here. The electrochemical series tells you if reduction or oxidation will occur (see how all the half equations involve the transfer of electrons?). In this reaction, no such thing is happening. It is not a redox reaction, it is just a rearrangement of the ions.

You won't really need to know if a reaction proceeds. You'll learn more about the kinetics of the reaction in Unit 4, I think.

[bec]

oh yeah, redox! it's coming back to me now...
thank you!
so basically, the only use (for a pre- unit 3/4 chem student anyway) of the electrochemical series is to determine if a redox reaction will occur and if so, which will be the oxidant etc?

[Collin Li]

Yeah, pretty much

[Mao]

oh yeah, redox! it's coming back to me now...
thank you!
so basically, the only use (for a pre- unit 3/4 chem student anyway) of the electrochemical series is to determine if a redox reaction will occur and if so, which will be the oxidant etc?

mmm the electrochemical series gives you information on how the redox reaction occurs, what how is the oxidant reduced, how is ther reductant oxidised, their half equations, if acid/base is produced (in case water is involved), and if an acidic environment is required, the voltage generated, blah blah blah

a more useful tool for determining if a redox reaction occur is oxidation numbers, its simple, quick, easy enough to work in your head, and pretty accurate, you dont even need the full equation, so long as you know the reactants and products

[Collin Li]

I think you're being confusing there.

a more useful tool for determining if a redox reaction occur is oxidation numbers, its simple, quick, easy enough to work in your head, and pretty accurate, you dont even need the full equation, so long as you know the reactants and products

You should be saying: "to identify if a given chemical reaction is a redox process" instead of "if a redox reaction occur [sic]." Oxidation numbers are useful to show that a particular reaction is a reduction-oxidation process, but they don't tell us if the reaction will occur.

You do need the electrochemical series to know if a redox equation occurs (spontaneously).