VCE Chemistry Question Thread

[lzxnl]

1. 1-butanol can also be written as butan-1-ol. The 1 shows where the -OH is located, in this case the first carbon.
2. With combustion, usually the water produced is gaseous, as the temperature is high. Molar heat of combustion = energy released through the reaction (∆H), balanced chemical equation = the equation for that reaction.

Does this answer your questions?

'More' correct to call it butan-1-ol. The 1 should be next to the corresponding functional group.

[sweetcheeks]

Sorry forgot to mention its for a SAC. In the experiment, we are not boiling water just heating it by 30 degrees. So should I put the state as liquid?
Also the aim of the experiment is :"measuring energy released in combustion of alcohols and evalute their fuel efficiency"
In the experiment we are measuring the change in mass and temperature (although its meant to be kept constant at 30degrees) of the alcohols after burning. Is the dependent variable the energy released or the change in mass ?

The water used in the equation is produced as a result of the combustion of butan-1-ol. It isn't the water that is in the beaker. I suggest writing it as a gas.

The dependent variable* will be the energy released, as you are using that to compare the different energy densities of fuels. The change in mass varies between the fuels but you want to measure the same amount of energy released to determine the energy densities of the different fuels.

*It isn't really a variable, it is a constant as the value stays the same (energy used to heat water 30C).

[Guideme]

State two reasons why zinc does not fit into the usual pattern for transition elements.

I am very confused on what this question is asking so any answers to this question will be appreciated . Thank you!

[lzxnl]

State two reasons why zinc does not fit into the usual pattern for transition elements.

I am very confused on what this question is asking so any answers to this question will be appreciated . Thank you!

Not sure if this is what you're looking for, but here's a few of what I can think of.

1. Zinc has a full 3d subshell; it's the only one in that row that does.
2. Its solutions are generally colourless (actually related to the first point)
3. Rather dull redox chemistry, in that it doesn't exist in multiple oxidation states (for instance Fe(III) and Fe(II), Co(II) and Co(III), Cu(II) and Cu(I), Mn(II) to Mn(VII) etc)

[plsbegentle]

Can someone help me with this question? Thanks in advance.

Data tables give the heat output from the complete
combustion of ethane and ethene as 51.9 kJ g–1
and 50.3 kJ g–1 respectively. Write thermochemical
equations for the complete combustion of these
fuels, showing ΔH values in units of kJ mol–1.
 
i already found out both the balanced equations

[Syndicate]

Can someone help me with this question? Thanks in advance.

Data tables give the heat output from the complete
combustion of ethane and ethene as 51.9 kJ g–1
and 50.3 kJ g–1 respectively. Write thermochemical
equations for the complete combustion of these
fuels, showing ΔH values in units of kJ mol–1.
 
i already found out both the balanced equations

To get your kJ g^-1 value to kJ mol^-1 value, you will need to divide kJ g^-1 by molar mass of the substance.



Then multiply the kJ mol^-1 value by the coefficient of your fuel in the equation, and write ΔH next to the equation (Energy=mol x heat of combustion).

[Shadowxo]

To get your kJ g^-1 value to kJ mol^-1 value, you will need to divide kJ g^-1 by molar mass of the substance.



Then multiply the kJ mol^-1 value by the coefficient of your fuel in the equation, and write ΔH next to the equation (Energy=mol x heat of combustion).

Just with syndicate's answer, it's multiplying by the molar mass. Molar mass = grams per mol = gmol^-1
, then multiplying by the coefficient of the fuel in the equation.

[Butterflygirl]

I need help with this question please:

If 54.0 kJ of energy is required to convert 1.00 mol of liquid water to steam at 100 degrees celsius, the amount of heat energy, in kilojoule, required to convert 100g of water at 20 degrees celsius to steam at 100 degrees celsius is...

(Sorry about the repetition of 'degrees celsius', don't know how to include the symbol)

I don't understand why there's energy required to heat water from 20 degrees AND energy required to boil the water?
My answer was 33.4 kJ from using specific heat capacity (It's wrong though)

Can someone explain the reason for adding the two amounts of energy and what does the 33.4 kJ actually mean? Is that the energy released when heating from 20 degrees to 100?

Thankyou so muchh

[Shadowxo]

I need help with this question please:

If 54.0 kJ of energy is required to convert 1.00 mol of liquid water to steam at 100 degrees celsius, the amount of heat energy, in kilojoule, required to convert 100g of water at 20 degrees celsius to steam at 100 degrees celsius is...

(Sorry about the repetition of 'degrees celsius', don't know how to include the symbol)

I don't understand why there's energy required to heat water from 20 degrees AND energy required to boil the water?
My answer was 33.4 kJ from using specific heat capacity (It's wrong though)

Can someone explain the reason for adding the two amounts of energy and what does the 33.4 kJ actually mean? Is that the energy released when heating from 20 degrees to 100?

Thankyou so muchh

There's the energy required to heat the the water up (from 20ºC to 100ºC) PLUS the energy required to make the water change state (from H₂O(l) to H₂O(g))
To heat the water up to 100ºC (staying liquid) it's E = mc∆T = 100*4.18*80 = 33kJ
1 mol of water = 18.0g
So energy required to convert water to steam - 54.0kJ per 1.00 mol or 54.0kJ per 18.0g = 3.00kJ per gram
Energy required to convert 100g of water at 100ºC to steam = 100*3.00 = 300kJ
Total energy required = 300 + 33 kJ = 333kJ

Hope this helps

[Butterflygirl]

There's the energy required to heat the the water up (from 20ºC to 100ºC) PLUS the energy required to make the water change state (from H₂O(l) to H₂O(g))
To heat the water up to 100ºC (staying liquid) it's E = mc∆T = 100*4.18*80 = 33kJ
1 mol of water = 18.0g
So energy required to convert water to steam - 54.0kJ per 1.00 mol or 54.0kJ per 18.0g = 3.00kJ per gram
Energy required to convert 100g of water at 100ºC to steam = 100*3.00 = 300kJ
Total energy required = 300 + 33 kJ = 333kJ

Hope this helps

Makes sense, thanks!!

[exit]

Hey, I'm getting a calculator with constants such as gravity and avogrado's constant (6.02*10^23) built in it but I'm not sure whether it is useful since its in more decimal places in the data sheet. Any advice?

[Sine]

Hey, I'm getting a calculator with constants such as gravity and avogrado's constant (6.02*10^23) built in it but I'm not sure whether it is useful since its in more decimal places in the data sheet. Any advice?

for chem doubt you'd need gravity

Yeah I would stick to the values given in the data booklet, could change your answer slightly i.e 0.33 vs 0.32 which would lose a mark.

[Gogo14]

Hi, can someone please tell me what from chapter 1 of the heinemann textbook (fuels) is actually relevant? The study design only has:
The definition of a fuel, including the distinction between fossil fuels and biofuels with reference to origin and renewability (ability of a resource to be replaced by natural processes within a relatively short period of time)
and
The comparison of fossil fuels (coal, crude oil, petroleum gas, coal seam gas) and biofuels (biogas, bioethanol, biodiesel) with reference to energy content, renewability and environmental impacts related to sourcing and combustion
and
The comparison of the suitability of petrodiesel and biodiesel as transport fuels with reference to sources, chemical structures, combustion products, flow along fuel lines (implications of hygroscopic properties and impact of outside temperature on viscosity) and the environmental impacts associated with their extraction and production.

but there is soooo much information in the textbook (and in my teacher's notes) and that is required to answer the chapter questions, that I don't know what info I actually need to know.

Can someone please summarise exactly the information I need to put in my own summary notes?

THanks
Emma

tbh its probs 99% junk. I suggest just looking at the end of exercise summaries in the book as they contain the relevant info.

I have a few more questions, sorry!
1.  When writing equations for incomplete combustion, how do you know when the products are CO and H2O, and when the products are C(s) and H2O?

2. Am I right in saying that when you use delta H to work out anything, you NEVER use signs? Eg: If you are given that the delta H of an exothermic equation is -56kJ/mol, and want to work out how much energy there is, you do E = 56 x n, not -56? And similarly, if you use delta H to work out anything else, you just use the magnitude?

3. Is heat of combustion (ΔH_C = E/n) the same as the amount of energy released from a combustion reaction? If a question asks you for the amount of energy released from a combustion reaction, do you find ΔH_C through ΔH_C = E/n, or do you find E = m c ΔT? Is the E you use in E = m c ΔT (i.e. mass x SHC x change in temperature) the energy released from a reaction, or is it just the energy that is present at the beginning? If the latter is not true, I have no idea what the difference between 'E' and ΔH is. Someone please explain cos I have a sac this week haha, thanks

Emma

1. question will usually say
2. yep
3. Heat of combustion is the amount of energy released per mole of reaction, and may not equal to the total amount of energy release in the reaction which is E. depends on teh question. If the question is "burning a fuel to heat water", then you can work out the energy required to heat the water, thus find the energy released or delta H value (if you know amount of mol)

[deStudent]

For this http://m.imgur.com/a/uwc5t

Is my working correct? The book has my answer but with an increase of factor of 1000 for both answer.

Also, for the combustion of butanol, is it in a liquid state? The answer wrote a gaseous state..

Thanks

[Shadowxo]

For this http://m.imgur.com/a/uwc5t

Is my working correct? The book has my answer but with an increase of factor of 1000 for both answer.

Also, for the combustion of butanol, is it in a liquid state? The answer wrote a gaseous state..

Thanks

I got the same answers as you,
Also with combustion, they'll often tell you the state but if I remember correctly C1-4 was gas, 5-10 liquid and above solid (someone may want to confirm this though). Usually it'll be given to you though.