HSC Chemistry Question Thread

[jakesilove]

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In this haloalkane when naming it. Is it 2,3-dibromo-4-chloropentane or 3,4-dibromo-2-chloropentane?

As the first one has a lower sum for the bromine, but my teacher gave us a rule to give the most electronegative halogen  (Cl>Br) the lowest numbers which would be the second one?

Both would be accepted in the HSC; the rules are complicated, and not taught well. A multiple choice question will always stick with the lowest sum rule, but if you write out the answer and use the electronegativity rule (and justify it) you'll get the marks

[J.B]

Both would be accepted in the HSC; the rules are complicated, and not taught well. A multiple choice question will always stick with the lowest sum rule, but if you write out the answer and use the electronegativity rule (and justify it) you'll get the marks

Ok, Thank you

[seventeenboi]

Because we're adding it to a buffer.

A buffer is made by mixing certain concentrations of an acid with its conjugate base. However the purpose of doing so is that a system is created whereby small changes in amounts of acids and bases added do NOT produce a strong effect on the system's pH. This is essentially how the pH in our blood is regulated should extra acidic/basic contents be added; the buffer will withstand the acid/base for a fair while before it starts breaking down.

If, on the other hand, we added it to something like water, then C would be valid.

ohhhh makes sense :O
thanks so much!

[Bubbly_bluey]

Hi! I got this question about naming this halogen incorrect. I got 1,1,2-tribromo-1,2,2-trifluroethane
But the answer was1,2,2-tribromo-1,1,2-trifluroethane. Is it something to do with adding something up? I'm not sure what to add up though.
Thanks

[bsdfjnlkasn]

Hi! I got this question about naming this halogen incorrect. I got 1,1,2-tribromo-1,2,2-trifluroethane
But the answer was1,2,2-tribromo-1,1,2-trifluroethane. Is it something to do with adding something up? I'm not sure what to add up though.
Thanks

Hey there,

So what the answers have done is given the lowest numbers to the more electronegative element (fluorine) whilst naming the whole compound in alphabetical order (so bromine first then fluorine).

Hopefully this helped!

[seventeenboi]

Hi!
Just 2 questions about galvanic cells:
'If a galvanic cell was allowed to run for 6 hours, describe what changes you would expect to see.'
'Over time cell voltages will fall. Identify 2 possible reasons for this.'

Thank you!

[sirecg02]

Ok so with testing for an unknown anion you would firstly add nitric acid and if it bubbles CO3^2- is present. Next you would add dilute calcium nitrate but after neutralising solution w/ ammonia. So if I was to add 10 drops of the unknown solution to a test tube would I add 10 drops of ammonia? or would I add more or less? 

[f_tan]

I'm having trouble answering the second part of the question - what is the impact of decreasing the temperature on the equilibrium constant K?

[kiwiberry]

I'm having trouble answering the second part of the question - what is the impact of decreasing the temperature on the equilibrium constant K?

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When temperature is decreased, by Le Chatelier's Principle, equilibrium will shift to favour the reaction which produces more heat, ie the reverse exothermic reaction (as the forward reaction is endothermic). Thus, the concentration of the products will decrease, and the concentration of the reactants will increase. As K=[products]/[reactants], the equilibrium constant will therefore be lower.
Note that temperature is the only factor that affects the equilibrium constant - the decrease in volume will only have an effect on the position of equilibrium, but K will remain the same.

[anotherworld2b]

Hi i was just wondering if someone could have a look at my answers please.

[MisterNeo]

Hi i was just wondering if someone could have a look at my answers please.

This is my attempt at the question for part B.
I'm a bit skeptical of your final value though...135 tons of SO3 from 1 ton of Sulfur.
Part c/d:

This is what I got for part c/d. I may be wrong though, so answers would be nice.
Hope this helps

[ellipse]

I think the answer is 2.37*10^6 grams for part B. I dont think the mole ratios carry over between the steps, I guess it's just used to show the ratios of moles between the reactants/products in a single equation, not between. Cause step 2 could also be written as 1 SO2 + 1/2 O2 --> 1 SO3 or even 10 SO2 + 5 O2 --> 10 SO3 which kinda messes it up.  The 2's in the 2nd step just signifies the ratios between the reactants/products (so if 2 moles of SO2 reacts, 1 moles of O2 is needed and 2 moles of SO3 is produced).

So back to the question, if 1 tonnes of sulfur reacts than that is equal to 31181 moles of S. This produces 31181 moles of SO2 (as its 1:1).
This 31181 moles of SO2 reacts with 31181/2 moles of O2 (cause its SO2:O2 is 2:1) to produce 31181 moles of SO3 (cause SO2:SO3 is 1:1)
Taking the 95% eficiency, this equates to 2.37*10^6 grams.

I think the rest of the question works similarly
Hope someone could check over this, I might be completely wrong

[anotherworld2b]

thank you for your help
I don't have answers to this question

[chloe9756]

What are some points about accuracy, validity and reliability that can be discussed in the anion and cation precipitation tests? 

[MisterNeo]

What are some points about accuracy, validity and reliability that can be discussed in the anion and cation precipitation tests?

Hi!!
Validity: Is the experiment actually suitable for your aim?
Eg. I want to determine if sulfate is in this solution. What I would do is add a barium nitrate solution. What I would not do is conduct a flame test because that is only for cation testing. Am I adding two nitrate solutions together? Lol that would be pointless.
Reliability: Do you get identical results upon repetition?
Eg. Some salts are slightly soluble such as calcium hydroxide. Sometimes it will produce a notable precipitate, sometimes it will not. So you would need to repeat this experiment multiple times to ensure that the precipitate either stays or goes away fully.
Accuracy: How close does it come to the true result or other people's results?
Eg. If I put two soluble solutions together (barium niitrate+sodium chloride) and somehow get a precipitate, this will mean either two things: Someone labelled the chemical wrong, or someone contaminated the solution. This will need to be taken into account if something doesn't appear right compared to other people's results and the actual result.

Hope this helps