Chemistry

Alice10

thanks. and do you have to balance that before you start you know them things with Delta H ?! sorry i know that doesnt really make sense

H2student

eh? Are you talking about thermochemistry.

Not sure if you're asking this but if the question says:

The heat of combustion for Methane is (say) 5 KJ mol^-1, then that is the value for the balanced equation. Also, I think you need to learn how to balance combustion reactions yourself, since they can ask it for almost any hydrocarbons and it's relatively straightforward to do.

Alice10

the Q is : write a balanced equation for the combustion of methane. if the heat of combustion of methane is -890kJ mol-1 , etc
i jus need to know the balanced equation for the combustion of methane, i know i to work the rest of it out

jumpguy

Methane is CH4.

A hydrocarbon burns to give water and CO2. You need oxygen to burn things.

CH4 + 2O2 = 2H20 + CO2

Alice10

thanks

do you know what thing has an octane number of 100 ?

Making It Bad

If you're struggling make sure you have the basics down and understand it all: electronegativity, chemical bonding, periodic table, mole concept and so on (in other words the first few chapters of your book). For example, you have no hope of learning how to name the alcohols if can't name the corresponding alkanes, or you have no hope in understanding why they mix with water if you don't understand electronegativity. The subject really builds on it self, which is why if you didn't work in 5th year of chemistry you're struggle with the 6th year stuff even though it isn't actually harder.

seanaor

Alice10 wrote: »

thanks

do you know what thing has an octane number of 100 ?

2,2,4 trimethylpentane!

Making It Bad

Alice10 wrote: »

thanks

do you know what thing has an octane number of 100 ?

trimethyl pentane I believe

Edit: seanaor beat me to it

Alice10

Thanks
are you supposed to learn those off or is there a way to make them out?! :S

Do you know what a LPG is?
it says name 2 components of LPG

Making It Bad

Alice10 wrote: »

Thanks
are you supposed to learn those off or is there a way to make them out?! :S

Do you know what a LPG is?
it says name 2 components of LPG

Use google, "Liquefied petroleum gas, propane and butane". I don't think those kinda questions would come up on the L.C

felic

LPG - liquid petrolium gas - are u sure the question is not asking what are the 2 main components of LPG? Butane and isobutene and then mercaptan is added to give it a strong odour for fast detection of leakages. Is this the option stuff of the course?

CNG - compressed natural gas - methane.

seanaor

Alice10 wrote: »

are you supposed to learn those off or is there a way to make them out?! :S

You just need to know what has an octane number of 0 (heptane) and 100 (trimethylpentane). Theres no method on the syllabus to work them out...

Itd also be wise to learn the four methods of increasing the octane number of a fuel!

Fad

Alice10 wrote: »

does anyone know the equation for the combustion of methane???

An alkane burns in air to form carbon dioxide and water.

CH4 + 2O2 -> CO2 + 2H20

Seriously though, if you cant figure that out at this stage, I'd worry.

Never looked at the 3rd page of the thread *facepalm*

rainbowtrout

Alice10 wrote: »

Thanks
are you supposed to learn those off or is there a way to make them out?! :S

Do you know what a LPG is?
it says name 2 components of LPG

No offence Alice, but it sounds like you're getting other people (who have been very helpful here) to do your homework. You're far more likely to remember it if you get off the computer and open your chemistry book and look it up. you're going to find it pretty hard in June if you are struggling with the basics at this point of the year.

@everyone else, i think that series of questions was a leaving cert organic question from a couple of years ago, possibly the 02 paper, the first year it was examined- they did ask what does LPG stand for!:eek:

EDIT: yep, it was

http://www.examinations.ie/archive/exampapers/2002/LC022ALPO00EV.pdf

seanaor

I was thinkin that series of q's was familiar...that was part of my xmas test. Ha!

Alice10

rainbowtrout wrote: »

No offence Alice, but it sounds like you're getting other people (who have been very helpful here) to do your homework. You're far more likely to remember it if you get off the computer and open your chemistry book and look it up. you're going to find it pretty hard in June if you are struggling with the basics at this point of the year.

@everyone else, i think that series of questions was a leaving cert organic question from a couple of years ago, possibly the 02 paper, the first year it was examined- they did ask what does LPG stand for!:eek:

EDIT: yep, it was

http://www.examinations.ie/archive/exampapers/2002/LC022ALPO00EV.pdf

Excuse me for your information I have my Chemistry book open in front of me an have been studying all nigth. The only reason I asked the questions on this is because I cannot find the answers in the book! and no, it is not not homework nor would i get people here to do my homework for me. I am revising a topic we covered last year - nothing whatsoever to do with what we are doing in school at the moment!
And no where in my chemistry book does it Explain what LPG is hence me asking the question

Making It Bad

rainbowtrout wrote: »

@everyone else, i think that series of questions was a leaving cert organic question from a couple of years ago, possibly the 02 paper, the first year it was examined- they did ask what does LPG stand for!:eek:

EDIT: yep, it was

http://www.examinations.ie/archive/exampapers/2002/LC022ALPO00EV.pdf

Eugh, that sucks. I've never seen that in my textbook. Hate all those stupid little weird memory questions.

felic

So.. proof of my points earlier that they DID ask stuff that wasnt on the syllabus! I saw a question on one of the recent papers as well asking for the octane number of some compound. Thats not on the LC course either!

jumpguy

Alice10 wrote: »

the Q is : write a balanced equation for the combustion of methane. if the heat of combustion of methane is -890kJ mol-1 , etc
i jus need to know the balanced equation for the combustion of methane, i know i to work the rest of it out

Alice10 wrote: »

thanks

do you know what thing has an octane number of 100 ?

Alice10 wrote: »

Excuse me for your information I have my Chemistry book open in front of me an have been studying all nigth. The only reason I asked the questions on this is because I cannot find the answers in the book! and no, it is not not homework nor would i get people here to do my homework for me. I am revising a topic we covered last year - nothing whatsoever to do with what we are doing in school at the moment!
And no where in my chemistry book does it Explain what LPG is hence me asking the question

What chemistry book do you have? It must be fairly crap if it doesn't explain burning hydrocarbons in oxygen, or what is used in comparison octane tests (octane number 100)?

BTW Making It Bad, you need to have the 2, 2, 4 in trimethyl pentane (2, 2, 4 trimethyl pentane). Correct me if I'm wrong someone though!

dambarude

felic wrote: »

I saw a question on one of the recent papers as well asking for the octane number of some compound. Thats not on the LC course either!

Octane numbers are on the LC course though

felic

But can they ask say Give the Octane number of Propane?

dambarude

felic wrote: »

But can they ask say Give the Octane number of Propane?

They can ask particularly well known octane numbers like 2,2,4-trimethyl pentane and heptane. I don't ever remember coming across a question asking for the octane number of a compound they just plucked out of the air.

H2student

I'm pretty sure they only ask for the Octane number of Heptan and 2,2,4-trimethylpentane (yeah you need 2,2,4-) as they're used as a comparasion for other fuels. (0 and 100 being convenient numbers)

Also, I think LPG is on the syllabus but I don't think chemistry live explains the components, or it's pretty obscured. Thankfully, I think they asked all the awkward questions in the last 8 already so you can just check the answers in the marking schemes.

rainbowtrout

Alice10 wrote: »

Excuse me for your information I have my Chemistry book open in front of me an have been studying all nigth. The only reason I asked the questions on this is because I cannot find the answers in the book! and no, it is not not homework nor would i get people here to do my homework for me. I am revising a topic we covered last year - nothing whatsoever to do with what we are doing in school at the moment!
And no where in my chemistry book does it Explain what LPG is hence me asking the question

Calm down, and get off your high horse. I don't have the book to hand but if I remember rightly Chemistry Live (if that's the book you're using) has a section on hydrocarbons and fuels in Chapter 21. There is a diagram of a combustion engine on the page and it goes on to explain what LPG, mercaptans and octane numbers are. It has diagrams of some of the main compounds and their octane number is stated clearly below. From what I remember it shows benzene, propane, 2-2-4trimethylpentane (iso-octane), so to be fair, if that's the book , it's not that hard to find the information.

rainbowtrout

H2student wrote: »

I'm pretty sure they only ask for the Octane number of Heptan and 2,2,4-trimethylpentane (yeah you need 2,2,4-) as they're used as a comparasion for other fuels. (0 and 100 being convenient numbers)

Also, I think LPG is on the syllabus but I don't think chemistry live explains the components, or it's pretty obscured. Thankfully, I think they asked all the awkward questions in the last 8 already so you can just check the answers in the marking schemes.

It's not a concept as such, it's only asking what the letters stand for

OxfordComma

I did Chemistry for the LC last year and loved it, but found it quite tough. It's probably the hardest of the sciences in my opinion - it can't simply be learnt by heart like Biology, and I find that Physics is easier to understand than Chemistry almost all of the time.

Additionally, in Physics, if you find one particular section, like say Electricity or Light, very difficult, you are safe in the knowledge that your understanding of different areas like Mechanics or whatever isn't heavily reliant upon your grasp of these sections. There's no such comfort when it comes to Chemistry - the various sections of the course you study aren't discrete and independent like they are in Physics, and the same concepts keep reappearing all over the place. For example, you can't understand Organic Chem if you don't "get" bonding, oxidation and reduction, and so on. Chemistry is like a language in some ways, and I think that's part of the reason why people struggle more with it than with other subjects.

The main reason why people find Chemistry difficult is that they don't approach it in the right way. It can't simply be "learnt" like History or Geography, it must be understood. A lot of students don't put in the effort trying to get their heads around Chemistry, and things start to seem overwhelming quickly.

Making It Bad

jumpguy wrote: »

BTW Making It Bad, you need to have the 2, 2, 4 in trimethyl pentane (2, 2, 4 trimethyl pentane). Correct me if I'm wrong someone though!

Yeah, you're probably right there Jumpguy.