Chem Exam

[99.95_goal]

Yeah - so, the hydrogen gas one could be something they accept, and the tutoring agency that shared their answers wrote that as well. For me, the first thing I notice is that there's so much lithium around. Lots and lots and lots of lithium, which is an alkali metal. Pretty much any alkali metal + water = highly exothermic reaction, which is the big thing that I'd be cautious of. Presumably - they accept both.

it was a three mark question, so what would the marks generally be located for? Because I feel like there wasn't too much to explain in this question

[keltingmeith]

it was a three mark question, so what would the marks generally be located for? Because I feel like there wasn't too much to explain in this question

Yeah, good question, I missed that. Since they mentioned you should use an equation, you 100% need to list the lithium hydrolysis equation:

\[
\ce{
2Li_{ (s)} + 2H2O_{ (l)}\to 2LiOH_{ (aq)} + H2_{ (g)}
}
\]

(you may also write it with the lithium hydroxide as individual ions, that's fine, both are correct)

That'll get you one mark, and you should have been able to create this equation from looking at the electrochemical series. The next two will be about discussing problems with the cell. There are two safety concerns with the above equation:

1. Lithium hydroxide is a strong base, and so will be caustic (you should still be able to catch this in the ionic equation, as you'd have free hydroxide, which is the definition of a strong base - ionised hydroxide ions)
2. Hydrogen gas is generated, which is dangerous when uncontrolled

This would get you all three marks. However, if you also listed things such as:

3. Lithium hydrolysis is highly exothermic, and therefore dangerous

Or anything else I can't think of (ask me if unsure!), then they should each constitute one mark, as well.

VCAA can get a bit weird, so it might even be 3 marks distributed as - 1 for concern, 1 for equation, 1 for linking the two. But, my money would be on 1 for relevant equation, 1 for first concern, 1 for second concern.

EDIT: Now that I've said that, I reckon it is actually 1 for concern, 1 for equation, 1 for linking the two, because they haven't prompted you to give more than 1 reason, just to highlight why it might be unsafe with an equation. We'll know for sure when the assessor's report comes out, I guess

[zoharreznik]

Yeah, good question, I missed that. Since they mentioned you should use an equation, you 100% need to list the lithium hydrolysis equation:

\[
\ce{
Li_{ (s)} + H2O_{ (l)}\to LiOH_{ (aq)} + H2_{ (g)}
}
\]

(you may also write it with the lithium hydroxide as individual ions, that's fine, both are correct)

That'll get you one mark, and you should have been able to create this equation from looking at the electrochemical series. The next two will be about discussing problems with the cell. There are two safety concerns with the above equation:

1. Lithium hydroxide is a strong base, and so will be caustic (you should still be able to catch this in the ionic equation, as you'd have free hydroxide, which is the definition of a strong base - ionised hydroxide ions)
2. Hydrogen gas is generated, which is dangerous when uncontrolled

This would get you all three marks. However, if you also listed things such as:

3. Lithium hydrolysis is highly exothermic, and therefore dangerous

Or anything else I can't think of (ask me if unsure!), then they should each constitute one mark, as well.

VCAA can get a bit weird, so it might even be 3 marks distributed as - 1 for concern, 1 for equation, 1 for linking the two. But, my money would be on 1 for relevant equation, 1 for first concern, 1 for second concern.

EDIT: Now that I've said that, I reckon it is actually 1 for concern, 1 for equation, 1 for linking the two, because they haven't prompted you to give more than 1 reason, just to highlight why it might be unsafe with an equation. We'll know for sure when the assessor's report comes out, I guess

Would I lose a mark for making a mistake with balancing the equation in that q? Or they won’t care if I have that written down

[keltingmeith]

Would I lose a mark for making a mistake with balancing the equation in that q? Or they won’t care if I have that written down

You likely would lose the mark for writing the equation, because VCAA can be very finicky about that. However, if you used the equation as evidence, you would likely still get the evidence mark, because you have demonstrated that you understand what reaction is taking place. (having said that, I just looked up and noticed I forgot to balance it - oops. See now for properly balanced equation, if you based whether yours was balanced on mine)

[zoharreznik]

You likely would lose the mark for writing the equation, because VCAA can be very finicky about that. However, if you used the equation as evidence, you would likely still get the evidence mark, because you have demonstrated that you understand what reaction is taking place. (having said that, I just looked up and noticed I forgot to balance it - oops. See now for properly balanced equation, if you based whether yours was balanced on mine)

I honestly can’t remember, I may have possibly written Li(OH)2 lmao (and also with a solid state by accident too) so I’ll probs lose the mark for that