VCE Chemistry Question Thread

[vcestressed]

They look correct to me.

The last one is a bit difficult. I believe that it is referring to covalent network, something like graphite. It follows the same style as the example, except that this one has a high conductivity.

Thank you so much. Yes, its a weird one because if its a covalent network, it would still have the same electroconductivity as the first one, but in this case it is high in solid. :/

[sweetcheeks]

Thank you so much. Yes, its a weird one because if its a covalent network, it would still have the same electroconductivity as the first one, but in this case it is high in solid. :/

If you look at the structure of graphite, it is a covalent network with delocalised electrons, allowing it to readily conduct electricity. If you look at another form of carbon, diamond, it is also a covalent network but it does not have any delocalised electrons and cannot conduct electricity as a solid.

[vcestressed]

Hi, can I please get help with b,d and e? thank you.
1.32g of propane (c3h8) was combusted to heat 250.0ml of water from 20degrees to 87.5degrees.
A) density of water is 1.00g mL^-1. Calculate the mass of water present in 250.0mL. (I got 250g)
b) Calculate the quantity of energy absorbed by the water from this reaction.
c) calculate the number of moles of propane combusted. (I got 0.299mol)
d) calculate the enthralpy change of propane via combustion.
e) write a thermochemical equation of propane complete combustion.

[Vaike]

Hi, can I please get help with b,d and e? thank you.

Heya. For b), you'll need to know the specific heat capacity (SHC) of water, and an equation involving the SHC, mass and temperature change. Have a go and see if you can get it. For d), you'll need to get the heat of combustion value for propane from the data booklet, and for e), its just asking for a standard complete combustion equation. If you can't get any of them, have a go anyway so we can see what specific bit you are getting stuck on  

[nicholas9027]

Hi, can i please get help with b)? thank you
Many transport vehicles are fuelled by compressed natural gas (CNG) rather than petrol. CNG is largely
methane. The thermochemical equation for the
complete combustion of methane is:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
ΔH = –802 kJ mol–1
a) How much energy, in MJ, is released by the complete combustion of 1.00 kg of methane? (i got 50.1MJ)
b) What volume of carbon dioxide is formed, at standard laboratory conditions (SLC), for each MJ of energy produced?

[A TART]

Hi, can i please get help with b)? thank you
Many transport vehicles are fuelled by compressed natural gas (CNG) rather than petrol. CNG is largely
methane. The thermochemical equation for the
complete combustion of methane is:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
ΔH = –802 kJ mol–1
a) How much energy, in MJ, is released by the complete combustion of 1.00 kg of methane? (i got 50.1MJ)
b) What volume of carbon dioxide is formed, at standard laboratory conditions (SLC), for each MJ of energy produced?

a)I got that too

b) So we need to find the volume. We can use this equation given it's at SLC: n=V/Vm where Vm is the constant 24.8

We are now missing n (mol) of CO2.

To find this, we need to find how much CO2 is released per 1MJ of energy produced.

To do this, we first convert 1MJ into 1000KJ. Then we divide it by the heat of combustion (802). This will give us around 1.25

Now if we multiply the equation by that number, you will see this:

1.25CH4(g) + 2.5O2(g) → 1.25CO2(g) + 2.5H2O(g)
ΔH = –802 x 1.25 kJ mol–1
      =1000KJ mol-1
       = 1MJ mol-1 (approx)

This tells us that 1.25mol of CO2 will be released per 1MJ

Now we have n, we can substitute this into the gas equation

1.25=V/24.8

V=1.25 x 24.8
 =31.0L (3SF I think)

EDIT: The key is to find the factor/coefficient thing (I dunno what to call it)

[Bri MT]

Hi, can i please get help with b)? thank you
Many transport vehicles are fuelled by compressed natural gas (CNG) rather than petrol. CNG is largely
methane. The thermochemical equation for the
complete combustion of methane is:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
ΔH = –802 kJ mol–1
a) How much energy, in MJ, is released by the complete combustion of 1.00 kg of methane? (i got 50.1MJ)
b) What volume of carbon dioxide is formed, at standard laboratory conditions (SLC), for each MJ of energy produced?

For any question in this style you can follow these steps:
1. Find the number of times the equation occurs for 1 MJ of energy
2. Find how many mols of CO2 are produced
3. Use n * Vm = V

Edit: didn't see previous reply - this is the same process as A TART used

[TheAspiringDoc]

Can someone please clarify the difference between renewable and sustainable?
Is wind considered renewable, sustainable, or both? What about biofuels?
I'm not sure whether to include non-finite energy sources such as wind into the definition of renewability.

Thanks

[sweetcheeks]

Can someone please clarify the difference between renewable and sustainable?
Is wind considered renewable, sustainable, or both? What about biofuels?
I'm not sure whether to include non-finite energy sources such as wind into the definition of renewability.

Thanks

This concept is a little bit difficult to explain. In terms of energy sources we often say something is renewable if it can be replenished within a human timescale (like sunlight, wind, but not fossil fuels). Sustainable means that it is replenished at a faster rate than it is being used up (and can continually be done so). If biofuel can be replenished within a short amount of time, it is renewable and if it can be produced at a rate faster than it is being used then it is also sustainable.

[TheAspiringDoc]

Thanks sweetcheecks.

Apparently coal emissions can be reduced by "removing water from brown coal by heating; the volatile hydrocarbons that are driven off are burnt to produce electricity".
What does "volatile" refer to in this context and how are the volatile hydrocarbons driven off with the water as compared to a normal method of removing water from brown coal?

 

[Vaike]

Apparently coal emissions can be reduced by "removing water from brown coal by heating; the volatile hydrocarbons that are driven off are burnt to produce electricity".
What does "volatile" refer to in this context and how are the volatile hydrocarbons driven off with the water as compared to a normal method of removing water from brown coal?

Howdy! Don't worry too much about this; I don't think you'd ever be examined on something this specific.

However, I believe all that paragraph is attempting to convey is that the energy conversions in the process of burning brown coal can be increasing if the brown coal is heated first. Brown coal has a relatively high percentage of water (30-70% iirc). This high percentage of water results in inefficient energy conversions in a coal fired power plant, not only because it reduces the amount of carbon being combusted per amount (e.g per kilogram) of coal, but also because some of the heat upon combustion goes into heating and evaporating the water in the coal, instead of the water in the boiler. 

In this context, 'volatile' refers to its definition: (of a substance) easily evaporated at normal temperatures. When you heat the water, which has a boiling point of 100 degrees C, other smaller (gaseous) hydrocarbons trapped with the coal will also evaporate out (mainly coal seam gas, a.k.a methane). These extracted hydrocarbons can then be combusted to produce energy. This results in an increased energy output per volume, ensuring these fuels do not go to waste.

In regards to the 'normal method' of removing water from brown coal, as far as I know heating is the usual method. However, it is also possible to just combust the brown coal without heating, which as previously mentioned, is less efficient due to high water content than black coal or dried brown coal, containing a lower amount of moisture.

Like I said, you definitely don't need to know all of this stuff, but it can be helpful to have a little bit of background knowledge. Hope this helps clear up a few things

[Yertle the Turtle]

Since I find Chemistry one of the harder subjects to understand, as I find it less intuitive than Physics or Maths, should I focus more on it, or should I focus less on it and try the best I can in the other subjects?

Also is it wise to study 1/2 Chemistry again this year to look at the base concepts of Chemistry, so that what is built on that in 3/4 will make more sense? Is this going to be study-efficient?

[Bri MT]

Since I find Chemistry one of the harder subjects to understand, as I find it less intuitive than Physics or Maths, should I focus more on it, or should I focus less on it and try the best I can in the other subjects?

Also is it wise to study 1/2 Chemistry again this year to look at the base concepts of Chemistry, so that what is built on that in 3/4 will make more sense? Is this going to be study-efficient?

It's very hard to predict what will actually be your best and worse subjects - I would focus more on chemistry.

Not all of units 1 & 2 is relevant, so certainly don't study the whole course. Ensure that you are able to do basics such as converting to and from mols, and after that look at things like solubility etc.

[Yertle the Turtle]

It's very hard to predict what will actually be your best and worse subjects - I would focus more on chemistry.

Not all of units 1 & 2 is relevant, so certainly don't study the whole course. Ensure that you are able to do basics such as converting to and from mols, and after that look at things like solubility etc.

Thanks for your help miniturtle, will do!

[TheAspiringDoc]

Would it be necessary or useful to memorise the specific order of different bio/fossil fuels in order of highest to lowest energy content?