HSC Chemistry Question Thread

[Kekemato_BAP]

Hey! I have a question:
 A student predicts 100mL of 10mol/L NaOH mixed with 100mL H₂SO₄ of 10mol/L with release about 114kJ. Is this prediction true?
Thank you

Does it give you the enthalpy heat value?

[Bubbly_bluey]

Does it give you the enthalpy heat value?

Is enthalpy value same as heat of neutralisation? because the previous question has a reaction between a strong acid and base to have 57kJ/mol of heat neutralisation.

[Bubbly_bluey]

Can someone explain the uses of Ka, Kb, Kw, etc? What are they used for? Thanks

Hey!
I think Ka is used to show the strength and weakness of acids. So stronger acids have a higher value of Ka because they completely dissociate.
The formula: Ka= [H+][A-] / [HA] shows the dissociation. When the [H+] is bigger = strong acid
Kb is for bases and Kw is for water = 1x10^-14.
The formula: ka x kb = kw shows that if we know the value Ka of an acid we can work out  its conjugate base's Kb.
hope this makes sense i could be wrong but i haven't seen a lot of questions in HSC relating to these   

[Shadowxo]

Hey! I have a question:
 A student predicts 100mL of 10mol/L NaOH mixed with 100mL H₂SO₄ of 10mol/L with release about 114kJ. Is this prediction true?
Thank you

The heat of neutralisation shows how much heat/energy is released during this reaction, so is therefore equal to the enthalpy / ∆H.
First find the number of each (n=cV), write out the equation, then figure out how many mols of the equation occur (should be equal to the number of mols of NaOH/H2SO4). E=∆H*n = 57*n kJ.

Would you need to know how to calculate pH of a non-neutral salt?
Eg. 20mL of 1M citric acid + 25mL of 0.5M sodium hydroxide? How do you calculate H+ concentration of a weak acid when it doesn't ionise fully? Thanks

A weak acid and a weak base (and weak acid and strong base) react completely with each other, even though they don't with water. So figure out the number of each , figure out which is in excess then figure out the concentration of the excess reactant, and therefore the concentration of H3O+ / OH-, taking into account how much it ionises.

[parthie]

Hey If anyone can help with the explanation in part b) of the question attached that would be great

Im just confused what i am meant to write like how to phrase it

Thankyou

[f_tan]

How do I work out the answer for this question?

Thanks!

[jakesilove]

Hey If anyone can help with the explanation in part b) of the question attached that would be great

Im just confused what i am meant to write like how to phrase it

Thankyou

Hey! Definitely a confusing statement; I would just figure out some equations for the reaction between carbonate and water, and see what we get!



Easy! Turns out, when the salt is added to a solution it produces Hydroxide ions. Hydroxide ions are what makes a solution basic, and thus the pH will be greater than 7. In this particular case, the pH is 8, but that isn't something we can really comment on except to say that it is a very weak base

[jakesilove]

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How do I work out the answer for this question?

Thanks!

I've always really liked this question; at first, it seems really difficult, but once you figure out the approach it isn't too bad!

Let's start off by calculating the number of moles of Charcoal.



Okay, great. We know that equal amounts of charcoal are used in each reaction. So, 16.65 moles of Charcoal must be consumed in each reaction! The first reaction 1:1 for C:CO. Thus, 16.65 moles of CO are produced as well. We can quickly find the mass of gas.



Okay. The second reaction is 1:1 C:CO2. Thus, 16.65 moles of CO2 are produced as well. We can quickly find the mass of the gas.



In total, this gives us a mass of 1.2kg, so the answer is B

[f_tan]

In total, this gives us a mass of 1.2kg, so the answer is B

Thank you!

Also, could you explain why the graph should have two lines of best fit?

[Butterflygirl]

I really need help with these questions because my SAC is tomorrow

Why is it better to use a highly conductive aqueous gel containing KNO3 for a salt bridge compared to a strip of filter paper dipped in KNO3 solution?

What effect would it have on measurements taken from a galvanic cell?

I don't understand how conductivity works and how it relates to the salt bridge.


Also,
If copper wire is used as a salt bridge, the voltage generated is higher but how does this happen? because doesn't the copper only contain cations (Cu2+)? How does it balance the charges in the half-cells and increase the voltage compared to using KNO3 solution or something?


Thankyou so much

[jakesilove]

Thank you!

Also, could you explain why the graph should have two lines of best fit?

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Honestly, I have absolutely no idea! I'm sure you can get full marks using one line of best fit. Potentially, they used two different scales on their graph (to take into account the quite large range over which Pressure was calculated). Still, there is no way I would have done that, and you would definitely get full marks by drawing one line of best fit.

[jakesilove]

I really need help with these questions because my SAC is tomorrow

Why is it better to use a highly conductive aqueous gel containing KNO3 for a salt bridge compared to a strip of filter paper dipped in KNO3 solution?

What effect would it have on measurements taken from a galvanic cell?

I don't understand how conductivity works and how it relates to the salt bridge.


Also,
If copper wire is used as a salt bridge, the voltage generated is higher but how does this happen? because doesn't the copper only contain cations (Cu2+)? How does it balance the charges in the half-cells and increase the voltage compared to using KNO3 solution or something?


Thankyou so much

Hey! I'm in NSW, so I didn't do the same curriculum, and can only really guess at answers. Using a 'highly conductive' strip will be better for simply that reason; it is highly conductive. When ions are transferred along the salt bridge, energy will not be lost as easily because the ions are free to flow. Thus, current will not be impeded as much (reducing total resistance), increasing the efficiency of the circuit. The measurement will likely read a higher voltage.

Think of a galvanic cell as a regular circuit. The only important thing is the conduction of ELECTRONS; in a salt bridge, that comes in the form of ions, however the electrons can be transmitted in any way. Thus, using a copper wire works well because it is very, very conductive! I can't imagine you need to be able to answer these questions in any more depth than that

Jake

[Aaron12038488]

So I'm having trouble with the validity of thermal decomposition of Copper Carbonate. I wrote that as the experiment was qualitatively instead of quantitatively, that validity was needed for the exp. 

[jakesilove]

So I'm having trouble with the validity of thermal decomposition of Copper Carbonate. I wrote that as the experiment was qualitatively instead of quantitatively, that validity was needed for the exp.

Hey! Is this for one of the options? Not sure if I've heard of the decomposition of Copper carbonate as part of the course?

[Kekemato_BAP]

Hi, how do i calculate the percentage by mass of a mixture? And also finding the empirical formulas? I'm having trouble understanding this concept