VCE Chemistry Question Thread

[sweetcheeks]

Thanks a lot! Seems so easy now that you explained it. If it's not too much trouble, could you please draw the diagram? I wanna learn how to do that myself.

Here is what I normally do. It may look like a lot of work when you start, but it gets much quicker when you do them a lot. They definitely help in SAC's and the exam as you are able to visualise what is going on and outline the steps you need to take, reducing your chance of making an error or spending time trying to determine what you need to do next.

[tasmia]

2. If 3.0 g of NaOH is added to 500 mL of 0.10 M hydrochloric acid, will the resulting solution be acidic or basic? What will the pH be

Could you please help me with this too :p

[Shadowxo]

Could you please help me with this too :p

n(NaOH) = m/M = 3.0/40.0 = 0.075 mol
n(HCl) = cV = 0.10*.500 = 0.050 mol
n(H+) = n(HCl) = 0.050 mol
n(OH-) = n(NaOH) = 0.075 mol
OH- (aq) + H+ (aq) -> H2O(l)
Therefore OH- is in excess by 0.025 mol
So solution is basic
pOH = -log10[OH-]
[OH-] = n/V = 0.025/0.500 = 0.050
pOH = 1.3
pH = 14 - pOH = 12.7

Hope this helps

[tasmia]

Hope this helps

Thanks! Helped me HEAPS more than when my teacher explained it

[Shadowxo]

No worries, glad to help 

[deStudent]

http://m.imgur.com/a/2xn4B

Need a little help!

Q4a) is the calibration factor of the calorimeter and water the same? Is this always true? I thought this was a trick question because the prior questions just said to "calculate the CF of the calorimeter", so I thought they'd be different. I ended up with 2 different results though.

b) am I missing something obvious? the correct answer is 4.14kJ/g but I can't think of any other way to do this?

Thanks

[Shadowxo]

http://m.imgur.com/a/2xn4B

Need a little help!

Q4a) is the calibration factor of the calorimeter and water the same? Is this always true? I thought this was a trick question because the prior questions just said to "calculate the CF of the calorimeter", so I thought they'd be different. I ended up with 2 different results though.

b) am I missing something obvious? the correct answer is 4.14kJ/g but I can't think of any other way to do this?

Thanks

a) It's asking for the calibration factor of the calorimeter plus water, so calculate the Joules per ºC (E = VIT then E/∆T)
b) Calibration factor is J per ºC, so multiply the calibration factor by by the change in temperature then divide by the mass for J per gram
CF = VIT/∆T = 3.50*3.00*240/1.80 = 1.40kJ/ºC
Energy content = CF * ∆T = 1.40*14.8 = 20.7kJ
Energy content per gram = 20.7/5.00 = 4.14kJ per gram
Hope this helps

[deStudent]

a) It's asking for the calibration factor of the calorimeter plus water, so calculate the Joules per ºC (E = VIT then E/∆T)
b) Calibration factor is J per ºC, so multiply the calibration factor by by the change in temperature then divide by the mass for J per gram
CF = VIT/∆T = 3.50*3.00*240/1.80 = 1.40kJ/ºC
Energy content = CF * ∆T = 1.40*14.8 = 20.7kJ
Energy content per gram = 20.7/5.00 = 4.14kJ per gram
Hope this helps

Edit:

For part a) if I just use E/deltaT, wouldn't this just be the CF of the calorimeter? This was correct though but I didn't add the water's CF.

[sweetcheeks]

Edit:

For part a) if I just use E/deltaT, wouldn't this just be the CF of the calorimeter? This was correct though but I didn't add the water's CF.

You don't need to add the waters CF, the CF of the calorimeter includes the water. Water has a specific heat capacity that can in theory be used to calculate energy. However in practical tests, energy/heat will be lost to the environment, not all the energy/heat will be retained in the water. We calibrate a calorimeter that has a specific amount of water in it. Electrical energy of a known amount is used to heat the calorimeter. Using the energy and change in temperature we can work out the CF/heat capacity of the calorimeter to account for the loss of energy.

[-273.15]

HI,
I'm a bit confused about the difference between crude oil, petroleum and petroleum gas.

The study design says "..the comparison of fossil fuels (coal, crude oil, petroleum gas, coal seam gas)..."
What is the difference between petroleum and petroleum gas? And how are crude oil, petroleum and petroleum gas linked?

Thanks so much

[sweetcheeks]

HI,
I'm a bit confused about the difference between crude oil, petroleum and petroleum gas.

The study design says "..the comparison of fossil fuels (coal, crude oil, petroleum gas, coal seam gas)..."
What is the difference between petroleum and petroleum gas? And how are crude oil, petroleum and petroleum gas linked?

Thanks so much

This is quite a broad field so I will give you the basic information you will need.

Crude oil is what is extracted from the ground. It contains many different types of molecules, mainly hydrocarbons of varying sizes. Crude oil isn't very useful in its current state, instead it is processed at refineries to seperate the different groups of molecules, as can be seen in the attached image.

Petroleum is what unleaded petrol is, which is mainly composed of octane. Petroleum gas is what LPG (liquified petroleum gas) is, composed of small, volatile molecules like propane and butane.

Petroleum and petroleum gas are constituents of crude oil, with petroleum gas being composed of smaller molecules than petroleum.

[deStudent]

For electrolysis to occur, does the top right, bottom left rule still have to hold for electrolytic cells? (so the oxidant's E-value must be above the reductant's E-value)? Our notes says yes but the book's examples/questions suggests that it doesn't have to be http://m.imgur.com/a/PgXDn (a question I did, oxidant was below the reductant?)

Since we're applying a voltage because elctrolytic cells aren't spontaneous, wouldn't the position of the oxidant relative to the reductant on the electrochemical series be irrelevant? Since this voltage provided is going to force the reaction to occur regardless?

Thanks

[Butterflygirl]

Hey guyss,

For fuels (theory part of chem), do I just need to memorise the different components of each fuel? Because some of the MC questions in checkpoints are really based on facts...

Or is there another way?


Thanks!

[Shadowxo]

For electrolysis to occur, does the top right, bottom left rule still have to hold for electrolytic cells? (so the oxidant's E-value must be above the reductant's E-value)? Our notes says yes but the book's examples/questions suggests that it doesn't have to be http://m.imgur.com/a/PgXDn (a question I did, oxidant was below the reductant?)

Since we're applying a voltage because elctrolytic cells aren't spontaneous, wouldn't the position of the oxidant relative to the reductant on the electrochemical series be irrelevant? Since this voltage provided is going to force the reaction to occur regardless?

Thanks

Forgive me if I'm a bit rusty, but I'll try to help you out
For a regular galvanic cell, there's the top left, bottom right rule. Electrolysis is the opposite, so top right, bottom left, like if the initial reaction has finished leaving the two products that wouldn't spontaneously react. Electrolysis is a bit like one of those wind up cars that you drag backwards across the floor (the electrolysis part) and then you let go and it goes forwards until it stops (the spontaneous reaction). I think of electrolysis as forcing the electrons out of the top right and then pushing them into the bottom left, which then gives them energy that they would later use to react spontaneously.
So the top right is undergoing oxidation aka is the reductant, and the bottom left is undergoing reduction, aka is the oxidant, so, to answer your question, the oxidant and the reductant is 'flipped' in this scenario as the species that would normally under reduction undergoes oxidation and vice versa, so the oxidant in this case is below the reductant's E-value. If, after electrolysis, they were allowed to react however, the oxidant would be above the reactant.

The position of the reductant to the oxidant is only important as if the oxidant is above the reductant, the reaction would occur spontaneously, meaning no need for electrolysis to happen.

Hope this helps a little bit, and I hope I didn't confuse you

[Butterflygirl]

I have another question:

Is this definition for a fuel okay? or is there anything that should be added, such as "it stores chemical energy which can be easily released"

A fuel is a substance that produces energy as a result of a chemical or nuclear reaction.


Thanks