VCE Chemistry Question Thread

[HasibA]

what is the relationship between heat of content, and heat of combustion, and heat of reaction? are they the same? thanks
extra question- is delta t found by final temp minus intial, or is it intial temp minus final? internet is confusing me, thanks

[sweetiepi]

Hey HasibA!
For your extra question- delta T is always calculated by final minus initial.
(I'll let the chem squad answer the other question )

[HasibA]

Hey HasibA!
For your extra question- delta T is always calculated by final minus initial.
(I'll let the chem squad answer the other question )

sweet-ty! just another quick one. suppose i was doing a question like those ones that are E=SHC x M x DELTA T, and my temp goes from 100 to 20, my delta t would be 20-100=-80 , correct? i'd use that negative 'number' in my calculations, right? such a simple question xD

[sweetiepi]

In my experience, I haven't seen a question that has a higher initial temperature than final (unless my life has been a lie and I've only ever seen questions that do have bigger final or a positive delta T)
However, that'd be correct, if that would be the situation I didn't read that correctly, jyce is correct.

[jyce]

sweet-ty! just another quick one. suppose i was doing a question like those ones that are E=SHC x M x DELTA T, and my temp goes from 100 to 20, my delta t would be 20-100=-80 , correct? i'd use that negative 'number' in my calculations, right? such a simple question xD

No you wouldn't use a negative sign. In that equation, you're calculating energy in Joules. This is a scalar value with no specified direction.

You DO use a negative/positive sign for changes in enthalpy, though.

[HasibA]

No you wouldn't use a negative sign. In that equation, you're calculating energy in Joules. This is a scalar value with no specific direction.

You DO use a negative/positive sign for changes in enthalpy, though.

so I would use 80 in my working out not negative 80 ? Correct?
Also, by changes in enthalpy you mean like deltaH products minus reactants etc. Or does it extend to some thermochemical things and calorimeter things too. Right ? Thanks

[jyce]

so I would use 80 in my working out not negative 80 ? Correct?
Also, by changes in enthalpy you mean like deltaH products minus reactants etc. Or does it extend to some thermochemical things and calorimeter things too. Right ? Thanks

Yes to everything.

[blacksanta62]

Hey, could anyone help me with these questions please? I'm not that confident with heat of combustion so some explanation of key steps would be appreciated
Thank you

[sweetcheeks]

Hey, could anyone help me with these questions please? I'm not that confident with heat of combustion so some explanation of key steps would be appreciated
Thank you

Q12.
a) Heat of combustion for a substance is how much energy is released when one mole of it combusts. The equation shows the heat of combustion if two moles are burnt. Divide -2599 by 2 to get the heat of combustion this is -1300kJ. To get kJ/g you must work out how many grams of ethyne there is in 1 mole (22.0g/mol). 22 grams of ethyne releases 1300kJ of energy 1 mole is 1300kJ/22grams = -59.1kJ/g

b) This question is asking how much ethyne is being burnt in a minute and how much energy does this release. The question says that 500ml of ehtyne (or 0.500L) is being combusted at STP. One mole of a gas at STP (101.3 kPA and 273K) occupies of volume of 22.4L (this can be found in the data booklet). The formula is V/22.4L=moles. Sub in 0.500L/22.4L=0.0223 moles of ethyne per minute

1 mole of ethyne gives us 1300kJ of energy 0.0223 moles gives us 29 kJ of energy. 29 kJ of energy is being released in 1 minute so the answer should be 29kJ/minute. I am not sure if there should be a negative sign.

[sweetcheeks]

Hey, could anyone help me with these questions please? I'm not that confident with heat of combustion so some explanation of key steps would be appreciated
Thank you

Q22.

a) This reaction has a + delta h. This means that it will be drawing energy from the environment.
The first thing to do is set out our enthalpy change equation. CxmxT=aJoules
Where C= specific heat capacity of water m=mass of water T=change in temperature and a= energy in joules.
We have all the information required but it is hidden.
To find a, we must first work out how many moles of ammonium nitrate we have and how much energy it will intake. To work out moles we have the mass, 25.0g, and molar mass, 80.0g, n=m/Mw =25.0/80.0 =0.313 moles of ammonium nitrate.

According to the equation one mole of ammonium nitrate takes in 25kJ of energy. We can then multiple 25kJ by the number of moles of ammonium nitrate to get the energy in kJ. 25kJx0.313moles = 7.81kJ. But if you notice, the enthalpy equation requires joules not kilojoules, so multiply by 1000 and we get 7810Joules of energy

Next we need to find the mass of water. It says a 2.00L container is used. 2.00L is the same as 2000mL. It says mass of water, good thing that water has an approximate density of 1g/ml so 2000mL will be 2000grams.

C, the specific heat capacity of the water, can be found in the data booklet and is 4.18j/g/c. It is the amount of energy, in joules, required to heat one gram of water one degree.

We are looking to find what the change in temperature is going to be.

Using the equation cxmxT=joules we can sub in what we know. 4.18j/g/c x 2000grams x T =7810 joules. Rearrange so that we can get the change in temperature 7810joules/(2000grams * 4.18j/g/c) and the change in temperature should be around 0.935C.

To find the lowest temperature possible; 19.00C (initial temperature) -0.935C (as energy is being absorbed the change in temperature will be negative)= 18.07C if my calculations are correct.

B) What are some possible reasons? Impurities in the ammonium nitrate? heat being absorbed from the environment?

[blacksanta62]

Thank you for the help sweetcheeks, cleared everything up for me

[Sine]

How much do we need to know about fuel cells? My Sac is on tomorrow and I feel that there isn't really much to know.

[jyce]

How much do we need to know about fuel cells? My Sac is on tomorrow and I feel that there isn't really much to know.

You should know the difference between fuel cells and other galvanic cells. You should also be able to give advantages and disadvantages of fuel cells for generating electricity compared to other galvanic cells and compared to power stations. You should be able to interpret diagrams of fuel cells. It'd also be good to know the overall reaction common to many fuel cells such as the hydrogen fuel cell and phosphoric acid fuel cell (2H₂ + O₂ -> 2H₂O), as well as to be able to locate the relevant half-equations in the electrochemical series, for either acidic or alkaline conditions.

[Jakeybaby]

I've been given an Issues Investigation task, we need to determine a topic and research question within an area with is relevant to society. For example: Water Treatment, Fuel Cells.

I'm having trouble designing a question, but I am interested in the areas of Water Treatment or Pharmaceutical Drugs. Does anyone have any ideas for research questions with regards to either of these topics?

[Sine]

You should know the difference between fuel cells and other galvanic cells. You should also be able to give advantages and disadvantages of fuel cells for generating electricity compared to other galvanic cells and compared to power stations. You should be able to interpret diagrams of fuel cells. It'd also be good to know the overall reaction common to many fuel cells such as the hydrogen fuel cell and phosphoric acid fuel cell (2H₂ + O₂ -> 2H₂O), as well as to be able to locate the relevant half-equations in the electrochemical series, for either acidic or alkaline conditions.

Thanks I have covered pretty much all of that kinda just expected more for a sac. haha