HSC Chemistry Question Thread

[itssona]

That causes confusion with NO.

NO is nitrogen monoxide
N2O is dinitrogen monoxide.

ohhhhhhhhhhhhh okay, thank you

[Kekemato_BAP]

Hey! There is a rule that a metal + water will result in a metal oxide + hydrogen gas. So, in this case

Doesn't a metal + water reaction produce a metal hydroxide + H2?
Calcium should react with water to produce calcium hydroxide and hydrogen.
Ca+ 2H2O -> Ca(OH)2 + H2

[jakesilove]

Doesn't a metal + water reaction produce a metal hydroxide + H2?
Calcium should react with water to produce calcium hydroxide and hydrogen.
Ca+ 2H2O -> Ca(OH)2 + H2

That is absolutely correct, no idea why I went with a Metal oxide rather than a Metal hydroxide.

[itssona]

hey so I think I missed on learning half equations and I understand the basics like Sulfur + 2e- --> S2- which is common logic

but can someone explain the half equations for aluminium + dilute nitric acid?
I understand the aluminium part but then we have this? 2H+ + 2e– → H2

also please tell me any rules like for acids and others, thank you!

[anotherworld2b]

thank you for your help
I was wondering how could I further explain why the equivalence point is in the acidic region?
I have been trying to write a procedure to find the acidity of white wine. We are required to write a step by step method, which allows someone to determine the total acidity of wine. Expressing the final determination of total acidity of the wine in grams per litre (g.L-1) Include the proposed calculations in this plain, using x and multiples of x to represent unknown value/s. Assume the main and only contribution of the acidic properties of wine is tartaric acid.
 
This is what I have so far but I'm not sure if I'm doing it correctly or validly.

Materials:
•White wine
•200ml o a standardised hydroxide solution
•Calibrated pH meter
•50mL burette
•Retort stand and burette clamp
•150mL beaker
•250mL conical flask or beaker
•1 small label to label the 150mL beaker
•Wash bottle with distilled water

Procedure:

1.Rinse the 150mL Beaker with a  small amount of sodium hydroxide solution, empty it, label it and fill it with about 100mL of the sodium hydroxide solution
2.Prepare the burette, then ill it with the sodium hydroxide solution
3.Rinse the reaction vessel (conical flask or 250 mL beaker) with water
4.Weigh the reaction vessel
5.Pour about X mL of white wine into the reaction vessel and reweigh it
6.Add about YmL of water to the reaction vessel so the depth is sufficient to cover the glass electrode o the pH meter
7.Place the pH meter in the reaction vessel and record the pH
8.Place the reaction vessel under the burette and begin the titration, stopping after each milliliter of sodium hydroxide has been added, to record the pH.
9.Continue adding sodium hydroxide until the PH is fairly constant in the basic region
10.Repeat the titration a couple more times with more frequent sampling of pH near the large change in ph

 

Hey, sorry without looking at the full original question, Question 2 just seems like nonsense (since it looks like they gave you that n(HCl)=0.05 moles?)

The equivalence point depends on the titration itself. If we are titrating a strong acid and a weak base (as here), the equivalence point will be in the acidic zone (ie. pH around 4-5). 

We were titrating a strong acid and a weak base, so the equivalence point should be a lower pH. Instead, using Phenolphathein will require the pH to be raised a lot more than necessary. So, LESS acid would be used (as you've noted).

If we use LESS acid to neutralise the same amount of base, but THINK that there are more moles in that small quantity of acid, then we will calculate that the acid is stronger (less volume, but same number of moles, equals higher concentration).

[jakesilove]

hey so I think I missed on learning half equations and I understand the basics like Sulfur + 2e- --> S2- which is common logic

but can someone explain the half equations for aluminium + dilute nitric acid?
I understand the aluminium part but then we have this? 2H+ + 2e– → H2

also please tell me any rules like for acids and others, thank you!

Hey! Basically, all you have to think about is which species is oxidising, which species is reducing, and which species is a spectator ion. So, let's look at aluminium and nitric acid



Just as a side note; Aluminium will barely react with Nitric acid. But, that's beside the point.

Now, we look at the equation as a whole. What changes? What stays the same?

Well, we can see that the NO3 ions stays EXACTLY the same. It is in aqueous form, it has a change of -1. So, we can REMOVE it, as it is a 'spectator ion'



Now that we have the net ionic equation, we can easily find the half equations. Clearly, Aluminium oxidises (as the oxidation number increases, it loses electrons).



So, Hydrogen must reduce



And there you have it! Not too difficult, hey?

[jakesilove]

thank you for your help
I was wondering how could I further explain why the equivalence point is in the acidic region?
I have been trying to write a procedure to find the acidity of white wine. We are required to write a step by step method, which allows someone to determine the total acidity of wine. Expressing the final determination of total acidity of the wine in grams per litre (g.L-1) Include the proposed calculations in this plain, using x and multiples of x to represent unknown value/s. Assume the main and only contribution of the acidic properties of wine is tartaric acid.
 
This is what I have so far but I'm not sure if I'm doing it correctly or validly.

Materials:
•White wine
•200ml o a standardised hydroxide solution
•Calibrated pH meter
•50mL burette
•Retort stand and burette clamp
•150mL beaker
•250mL conical flask or beaker
•1 small label to label the 150mL beaker
•Wash bottle with distilled water

Procedure:

1.Rinse the 150mL Beaker with a  small amount of sodium hydroxide solution, empty it, label it and fill it with about 100mL of the sodium hydroxide solution
2.Prepare the burette, then ill it with the sodium hydroxide solution
3.Rinse the reaction vessel (conical flask or 250 mL beaker) with water
4.Weigh the reaction vessel
5.Pour about X mL of white wine into the reaction vessel and reweigh it
6.Add about YmL of water to the reaction vessel so the depth is sufficient to cover the glass electrode o the pH meter
7.Place the pH meter in the reaction vessel and record the pH
8.Place the reaction vessel under the burette and begin the titration, stopping after each milliliter of sodium hydroxide has been added, to record the pH.
9.Continue adding sodium hydroxide until the PH is fairly constant in the basic region
10.Repeat the titration a couple more times with more frequent sampling of pH near the large change in ph

 

Your method looks good! Perhaps make it a bit more formal; instead of saying 'a couple more times', say 'Repeat the procedure outlined above 3 times' or something like that. However, you look like you've got the right idea.

Re equivalence point; this is actually something I didn't know myself, so had to look up! With a strong acid and strong base, the equivalence point will be neutral; that just makes sense. But, with a strong acid and a weak base, the reaction forms an ACIDIC SALT. This acidic salt brings down the pH, even as the substances themselves are being neutralised. So, the overall pH will be acidic!

[itssona]

Hey! Basically, all you have to think about is which species is oxidising, which species is reducing, and which species is a spectator ion. So, let's look at aluminium and nitric acid



Just as a side note; Aluminium will barely react with Nitric acid. But, that's beside the point.

Now, we look at the equation as a whole. What changes? What stays the same?

Well, we can see that the NO3 ions stays EXACTLY the same. It is in aqueous form, it has a change of -1. So, we can REMOVE it, as it is a 'spectator ion'



Now that we have the net ionic equation, we can easily find the half equations. Clearly, Aluminium oxidises (as the oxidation number increases, it loses electrons).



So, Hydrogen must reduce



And there you have it! Not too difficult, hey?

OMG I FINALLY GET IT AHHHHHHHHHHHHHHHH

THANK YOU JAKE

[anotherworld2b]

thank you for your help
Would my method be able to allow someone to find the acidity of wine?
What kind of method could you use if you wanted to use a titration to find the acidity of wine? Would it be possible?

Your method looks good! Perhaps make it a bit more formal; instead of saying 'a couple more times', say 'Repeat the procedure outlined above 3 times' or something like that. However, you look like you've got the right idea.

Re equivalence point; this is actually something I didn't know myself, so had to look up! With a strong acid and strong base, the equivalence point will be neutral; that just makes sense. But, with a strong acid and a weak base, the reaction forms an ACIDIC SALT. This acidic salt brings down the pH, even as the substances themselves are being neutralised. So, the overall pH will be acidic!

[anotherworld2b]

I was also wondering what could other reasons be?
Q3. Why was the anhydrous sodium carbonate placed in the oven?In brief:
The primary standard, Na2CO3 (s) is made anhydrous by heating it in an oven at 260-270 degrees celsius. To prevent the anhydrous Na2CO3 (s) from reabsorbing water vapour from the air, it is kept in a desiccator. Silica gel crystals inside the dessicator keep the air dry by absorbing water vapour.

[Aaron12038488]

is it possible if I post up a past preliminary paper with my answers, and receive marking?

[jakesilove]

thank you for your help
Would my method be able to allow someone to find the acidity of wine?
What kind of method could you use if you wanted to use a titration to find the acidity of wine? Would it be possible?

Yep, your method will do the trick! To find the acidity of a substance, you need to titrate it. You also need to know the chemical making the wine acidic, so that you can write out the chemical formula. From there, you carry out the usual mathematical steps to find the concentration (and therefore pH) of the wine.

As for your second question, you haven't really explained WHY it is placed in an oven; merely how it is heated up/kept dry etc. Obviously, you know the answer; it is to ensure the substance doesn't absorb water, affecting the purity and the way in which you measure to mass/moles used to create a standard solution. However, you haven't directly answered the question.

[jakesilove]

is it possible if I post up a past preliminary paper with my answers, and receive marking?

Hey! Probably not an entire paper, but feel free to chuck up a few questions you're not sure about, and I'll take a look at your answers!

[Alalamc]

Draw up an electrochemical series consisting of metals A, B, C and D and their ions and H+ and H2 given the following information:

Metal A reacts with B2+ but does not react with C2+.

Metal D reacts with A2+.

Only B does not react with H+ to form H2 gas.
hey guys I know this may seem reasonable simply but I havent come across this type of question before

[RuiAce]

Draw up an electrochemical series consisting of metals A, B, C and D and their ions and H+ and H2 given the following information:

Metal A reacts with B2+ but does not react with C2+.

Metal D reacts with A2+.

Only B does not react with H+ to form H2 gas.
hey guys I know this may seem reasonable simply but I havent come across this type of question before

A reacts with B2+ so A is more reactive than B. (Because A will turn into A2+ and B2+ will go back to B.)
A does not react with C2+ so C is more reactive with A. (Conversely)

C > A > B

D reacts with A2+ so D is also more reactive than A. (D gets oxidised to become D2+ whereas A2+ is reduced.)

B does not react with H+ to become B2+, but A will react with H+ to become A2+
So the reactivity of H is between A and B

C > A > H > B

D > A > H > B
D is also more reactive than A but we aren't told any information to compare between D and C to properly rank them.

(I just hope I did not do this in reverse.)