HSC Chemistry Question Thread

[jakesilove]

how would i go about this question. Describe the cause and effect relationship between the appropriate properties of water to earthquakes?

I'm honestly unsure! I imagine that the answer goes something like this. Earthquakes are caused by seismic shifts in tectonic plates. However, usually, these shifts occur in the middle of the ocean. The energy released by earthquakes are able to travel to land by propagating through water, as water is a medium that easily allows the transfer of energy. The ocean can also cause Tsunamis to form, increasing the effect of the earthquake.

[Jyrgal]

hey! i've been practicing in titrating using phenolphthalein with sodium carbonate as standard and nitric acid. I've noticed that whenever i get some colour change, after some few seconds of swirling the colour disappears, and I know that this happens because CO2 dissolves to make the pH slightly lower so colour goes away. I was just wondering how do you exactly determine which colour change to stop since 'permanent' colour change doesnt rly work for calculating concentration if co2 reacts with the sodium carbonate as well

[jakesilove]

hey! i've been practicing in titrating using phenolphthalein with sodium carbonate as standard and nitric acid. I've noticed that whenever i get some colour change, after some few seconds of swirling the colour disappears, and I know that this happens because CO2 dissolves to make the pH slightly lower so colour goes away. I was just wondering how do you exactly determine which colour change to stop since 'permanent' colour change doesnt rly work for calculating concentration if co2 reacts with the sodium carbonate as well

Hey! The general rule I always used was 'if the colour stays the same for 10 seconds, you've reached the end point'. If you then let it rest, and noticed a few minutes later that the colour had reverted back, you don't need to go and change your results! However, I don't know if this is necessarily a 'cover-all' rule, it's just what I use! As long as you have a solid 'method', so that you get reliable results (by using the same criteria each time), you'll be fine

[Jyrgal]

hello again another Q cause my exams r soon 

For esterification, is there an explanation to why the -OH group comes from the alkanoic acid rather than the alkanol? (in other words: why is the -OH group in alkanoic acid weaker than the -OH group in alkanol?)

Cheers!

[jakesilove]

hello again another Q cause my exams r soon 

For esterification, is there an explanation to why the -OH group comes from the alkanoic acid rather than the alkanol? (in other words: why is the -OH group in alkanoic acid weaker than the -OH group in alkanol?)

Cheers!

Nope, there absolutely isn't. It's a nice thing to know, but also not entirely necessary. Basically, we figured that out by carefully looking at the molecules when physically doing the experiment.

[Piza]

I need some guidance in answering these types of questions, can someone do their working out for A so that I can do B and C, thank you!

[jakesilove]

I need some guidance in answering these types of questions, can someone do their working out for A so that I can do B and C, thank you!

Definitely a weird question! We can use the hint to realise that ppm=mg/L. So,

We know that absorbance is proportional to concentration. We need to 'convert' our known standard of 0.85 to 0.8, and then perform the same operation on the concentration.



Great! Hopefully you followed the above; we know have the concentration of Iron in the 250mL volumetric flask. To find the total mass of Iron we divide by four (as there are 4.71mg per L, therefore 0.471/4 mg per 0.25L).

This results in a total mass of 1.178mg of Iron in the original tablet. You can divide this by 100mg to find the w/w concentration of Iron in the tablet.

Hope that all made sense!

[Piza]

Definitely a weird question! We can use the hint to realise that ppm=mg/L. So,

We know that absorbance is proportional to concentration. We need to 'convert' our known standard of 0.85 to 0.8, and then perform the same operation on the concentration.



Great! Hopefully you followed the above; we know have the concentration of Iron in the 250mL volumetric flask. To find the total mass of Iron we divide by four (as there are 4.71mg per L, therefore 0.471/4 mg per 0.25L).

This results in a total mass of 1.178mg of Iron in the original tablet. You can divide this by 100mg to find the w/w concentration of Iron in the tablet.

Hope that all made sense!

Thank you so much, this helped me a lot!!

[beau77bro]

Hey guys so I have a prac assessment followed shortly by a processing skills assessment. But I need to worry more about the practical because they have gotten me in strife a lot before. Are there any (and all) prac tips you guys could give me? Even basic ones to make sure I do as much right as possible

[jakesilove]

Hey guys so I have a prac assessment followed shortly by a processing skills assessment. But I need to worry more about the practical because they have gotten me in strife a lot before. Are there any (and all) prac tips you guys could give me? Even basic ones to make sure I do as much right as possible

Hey! Bit of a tough question to answer, but I'll try my best.

First and foremost, make sure you totally understand the theory behind whatever practical task you could be asked to do. Presumably, you don't know what the prac is? Perhaps go through the curriculum, picking out pracs and making sure you understand them. However, don't limit yourself there; make sure there aren't areas of the syllabus that you're particularly weak with, in case the particular talk assesses those points.

Now, the practical task itself will usually require a couple of things.

Firstly, you will probably be asked to write a method/procedure/experimental plan. Get used to the structure; something along the lines of aim, hypothesis, equipment/material, method, results, discussion, conclusion. You probably won't need to do this entire process, however having the general shape of an experimental design in mind is very helpful when completing a practical task. Make sure you understand each of the above mentioned sections; if not, let me know, and I'll elaborate!

When it comes to methods particularly, make sure to write them in past tense, and number them. Some people say you must being with a verb; you are more than welcome to do this, although I didn't really focus on that.

With results etc. make you sure you are comfortable with drawing tables to summarise data. Make sure you put units in the heading of each row/column, and you box in your table.

With your discussion, be very familiar with the terms accuracy, reliability and validity. Know how to assess each of these, and how they inter relate. If you have question on any of this, feel free to ask, I just don't want to go over anything you already know really well!

Be knowledgeable of the names of various glass ware, and the way by which you can write a risk assessment (ie risk, precaution, response table).

Uhhhh I'm trying to think of anything else...

Will keep thinking. I'm sure others will contribute. Please let me know if I can elaborate on any of the above!!!

[beau77bro]

Hey! Bit of a tough question to answer, but I'll try my best.

First and foremost, make sure you totally understand the theory behind whatever practical task you could be asked to do. Presumably, you don't know what the prac is? Perhaps go through the curriculum, picking out pracs and making sure you understand them. However, don't limit yourself there; make sure there aren't areas of the syllabus that you're particularly weak with, in case the particular talk assesses those points.

Now, the practical task itself will usually require a couple of things.

Firstly, you will probably be asked to write a method/procedure/experimental plan. Get used to the structure; something along the lines of aim, hypothesis, equipment/material, method, results, discussion, conclusion. You probably won't need to do this entire process, however having the general shape of an experimental design in mind is very helpful when completing a practical task. Make sure you understand each of the above mentioned sections; if not, let me know, and I'll elaborate!

When it comes to methods particularly, make sure to write them in past tense, and number them. Some people say you must being with a verb; you are more than welcome to do this, although I didn't really focus on that.

With results etc. make you sure you are comfortable with drawing tables to summarise data. Make sure you put units in the heading of each row/column, and you box in your table.

With your discussion, be very familiar with the terms accuracy, reliability and validity. Know how to assess each of these, and how they inter relate. If you have question on any of this, feel free to ask, I just don't want to go over anything you already know really well!

Be knowledgeable of the names of various glass ware, and the way by which you can write a risk assessment (ie risk, precaution, response table).

Uhhhh I'm trying to think of anything else...

Will keep thinking. I'm sure others will contribute. Please let me know if I can elaborate on any of the above!!!

hmmm yea im going through the pracs and picking out the ones to revise and go through, its only the ones we did as a class. but the problem is, there isnt actually a conclusion or discussion. there is a plan, results and the majority will consist on carrying out the practical well, and unfortunately that's where im worst at ahahahha. so like i need to know all the basic practical procedures and the tricks to them, like whenever you make a standardised solution you rinse the flask with water first, then put the substance to be dissolved in the flask and then fill. because filling and then adding the solid doesnt take into account volume of the solid. like that kinda stuff, i know im asking a weird and ambiguous/broad question.

to narrow it down the pracs are only the first two mods, minus esters.

thankyou and sorry ahahha

[jakesilove]

hmmm yea im going through the pracs and picking out the ones to revise and go through, its only the ones we did as a class. but the problem is, there isnt actually a conclusion or discussion. there is a plan, results and the majority will consist on carrying out the practical well, and unfortunately that's where im worst at ahahahha. so like i need to know all the basic practical procedures and the tricks to them, like whenever you make a standardised solution you rinse the flask with water first, then put the substance to be dissolved in the flask and then fill. because filling and then adding the solid doesnt take into account volume of the solid. like that kinda stuff, i know im asking a weird and ambiguous/broad question.

to narrow it down the pracs are only the first two mods, minus esters.

thankyou and sorry ahahha

Ah, okay, I see! Could you do me a favour and go through, figure out which pracs you might be assessed on, and throw up a list? Then, I can talk through each, giving you points/methods etc. Most of them are pretty straight forward; you'll probably be doing a Titration aha. Still, pop up a list, and I'll give you whatever I think you need for each

[beau77bro]

this looks like most of the ones we could actually be assessed on (theres also a fermentation prac but it would take to long and i doubt very much sir would do it)
Molar heat of combustion:
Galvanic Cells:
Potential difference of metals in an electrode:
Natural Indicators:
Decarbonation of a soft drink:
Titrations:

thanks so much jake your a life saver

[jakesilove]

this looks like most of the ones we could actually be assessed on (theres also a fermentation prac but it would take to long and i doubt very much sir would do it)
Molar heat of combustion:
Galvanic Cells:
Potential difference of metals in an electrode:
Natural Indicators:
Decarbonation of a soft drink:
Titrations:

thanks so much jake your a life saver

Let's do this

Molar heat of combustion:

Here, we are trying to extract the heat released by combusting one mole of a substance. For arguments sake, let's assume your prac is Ethanol, however it could foreseeably be any other alcohol.

We look at the formula, which is


This will tell us how much energy has been released, depending on the increased temperature of the water.

We set up the below prac



First, we weight the mass of the substance (eg. Ethanol). Then, we light it, and let it heat up the water. Finally, we extinguish the Ethanol, measuring A) the change in mass of the Ethanol and b) the change in temperature of the water.

Let's say the water heated up by 20 degrees, and one gram of Ethanol was combusted. First, we find the energy that must have been 'used' to heat up the water by this amount, using the formula above



Where m is the mass of the water, and C is the specific heat capacity. This will give us some value of H, which was released when one gram of Ethanol was burned. Then, we convert this kJ/g measurement into kJ/mol (my multiplying by the molar mass), and we have a total value!

Some notes on the 'reliability' etc part of this experiment. Firstly, we would perform it multiple times, and average the result. Second, validity is quite limited in this experiment for two reasons: 1) A lot of heat energy is lost to the environment and 2) incomplete combustion can occur (if there is a black smudge underneath the tripod. These are all things you would talk about in the written portion of the prac.

One down. Like 100 to go.

Galvanic Cells:

For a Galvanic Cell, you need to be really confident re how to draw one out/set one up/label it. The below is a bit of a blurry example, but you get the idea.


So, set it up by placing the liquids in two beakers, and the appropriate electrode in each beaker, connected to a voltmeter in series. Also, make sure to connect the two half cells with a salt bridge, to allow ions to travel between (essentially completing the circuit!).

For an experiment like this, there isn't much to it. You might be asked to change the concentration of the solution, and measure the affect of this on the voltage etc. You don't need to know anything about any possible variances in advance; you'll have to actually do it on the day.

Be comfortable with half-cell notation, redox equations, etc.

I've never heard of a practical task that was a Galvanic cell, so I think this is pretty unlikely

Potential difference of metals in an electrode:

This is the same as a Galvanic cell. Potentially (lol) you'll need to switch metals, compare the output voltage etc. This can all be predicted using the table of standard potentials on your formula sheet; you find the was oxidising, and switch the signs of the potential on the sheet. Then, you add it to the one reducing, and presto! You've got a voltage out. So, the highest voltage would be a cell between a substance very high on the list, and a substance very low on the list. Be comfortable with this table, with balancing equations, which creating net ionic equations and half equations. Happy to give some examples if you think that will help.

Natural Indicators:

Also a pretty straight forward practical task. Usually, you'll use something like red cabbage. Here's how I did the prac.

Cut up the cabbage into smallish pieces, and place it in a beaker full of water. Smoosh it around, so it releases its delicious delicious juices. Then, boil it for about 5 minutes, also to ensure that you get it's sick ass ability to detect changes in pH. Drain the remaining liquid, so you retain only the solution, and throw away the soggy Cabbage.

Now, you've got your indicator! We need to test how useful it is, and what range it will be able to recognise. So, prepare a range of test tubes (ideally, multiple at each pH scale for reliability) such that each test tube has a different pH. I would try to get a test tube with pH 1, a test tube with pH 2.... a test tube with pH 12 and a test tube with pH 13. If not possible, just ensure you have the full range. Drip a few drops of the indicator into each test tube, and record any colour change. It will be obvious what 'sort' of indicator yours is once you've done this! ie. if it's red for all substances up until a pH of 9, and then blue for pHs higher than 10, it would be useful in determining if something is very basic.

Ooft. Getting there.

Decarbonation of a soft drink:

This one is pretty chilled. However, there is a bit of nuance to it. First, you'll get a can of soft drink. Weigh it. Then, place it on a scale, and open the can. MAKE SURE NOT TO RIP THE TAGGY BIT OFF, OR YOU'LL BE CHANGING THE WEIGHT! Let the soft drink sit, and record changes in mass over whatever time period you have (if it's an assessment, probably not long). Plot this change in weight against time, and you'll have a nice curvey/straight lineish graph.

What's happening is that Carbon dioxide is being released. Based on the change in mass, you can calculate exactly how much Carbon dioxide has been released, based on its molar mass. I assume you can do this calculation.

Now, if you want to be really smart, consider the main issue with validity. So, you're watching Carbon dioxide escape, but what else? Water is evaporating. So, if you want to be clever, get a different can, empty it, and fill it with water. Simultaneously measure any change in mass in this can, which would be entirely due to water loss. Approximate that the evaporation in the second can is the same as the first, and 'add' the additional weight to your data. eg. if you notice that the water can loses 3g, but the soft drink can loses 5g, you can only actually attribute 2g to Carbon dioxide escaping.

Remember that this is all done in terms of Le Chatelier's principle, although the experiment itself doesn't seem to involve it. Just be comfortable with the principle and it's applications in order to answer any theoretical questions.

Titrations:

AAAAAAAND now we get to the tough one. Let me see if I have any helpful images...

I DO INDEED!


Hell yeah. This genuinely has everything you need. The LHS is about creating a standard, the RHS is about performing the titration.
I think it's actually fairly comprehensive; let me know if I can elaborate on this at all.

Phew.

[jakesilove]

this looks like most of the ones we could actually be assessed on (theres also a fermentation prac but it would take to long and i doubt very much sir would do it)

I love that you call him sir aha I do not miss High School