Chemistry Questions (1 Viewer)

[golgo13]

Re: Percentage yield question

Just checking is ur provided answer correct, because no matter how many times i do it i still get 0.3588t

 

[Aysce]

Re: Percentage yield question

Just checking is ur provided answer correct, because no matter how many times i do it i still get 0.3588t

That's the textbook answer, I also always got the answer of 0.36. And would the significant figures here for the answer be 2 SF since the % can be expressed as 0.92?

 

[golgo13]

Re: Percentage yield question

i like 4 but u would prolly go for 2, umm is it from the conquering chem book cause its been known to have mistakes here and there. But i would throw my back behind 0.36 because logically speaking 1t with 92% efficiency translate to 0.92 tonnes anyways and when you work yield both Na and Cl and add them up u get the 0.92t

 

[Aysce]

Re: Percentage yield question

i like 4 but u would prolly go for 2, umm is it from the conquering chem book cause its been known to have mistakes here and there. But i would throw my back behind 0.36 because logically speaking 1t with 92% efficiency translate to 0.92 tonnes anyways and when you work yield both Na and Cl and add them up u get the 0.92t

It is conquering chem so yeah I think it may be a mistake

 

[Aysce]

Enthalpy changes

There is this part of the textbook I'm failing to understand clearly.

Here is the quote:

Side note: DH = Changes in enthalpy or more specifically, Enthalpy of products - enthalpy of reactants.

"The definition of DH above includes the term per mole of specified reactant or product, because it is often possible to write the reaction in more than one way. For example, the statement that DH for the formation of water from hydrogen and oxygen is -572kJ is ambiguous because it does not make clear whether the reaction concerned is:

2H2 (g) + O2 (g) -----------> 2H2O (l)

or H2 (g) + (1/2)O2 (g) -------> H2O

Ambiguity is removed by saying:

DH = -572kJ per mole of Oxygen

or DH = -286kJ per mole of water (or of hydrogen)"

I simply have very little idea of what it's trying to tell me.

How do the two different values of DH remove "ambiguity"? For further reference, it is in Conquering Chemistry preliminary, page 277.

Also, why is the first value of DH only for a mole of O2 and not for hydrogen or water? Same thing applies for the second value.

Thanks.

 

[nightweaver066]

Re: Enthalpy changes

It is ambiguous if you simply say that the DH of the formation of water from hydrogen and oxygen is -572kJ because you can write the chemical equation in different ways (like in the two equations you typed up, in one, 2 moles of water is formed and in the other, 1 mole of water is formed). So it is unclear which equation it is referring to.

However, if you say DH = -572kJ per mole of oxygen, now we know how to set up the equation such that the DH is relevant & applicable, i.e., using the 1st equation.

Hopefully that made sense and it didn't seem like I was just restated what was said :S

 

[Aysce]

Re: Enthalpy changes

It is ambiguous if you simply say that the DH of the formation of water from hydrogen and oxygen is -572kJ because you can write the chemical equation in different ways (like in the two equations you typed up, in one, 2 moles of water is formed and in the other, 1 mole of water is formed). So it is unclear which equation it is referring to.

However, if you say DH = -572kJ per mole of oxygen, now we know how to set up the equation such that the DH is relevant & applicable, i.e., using the 1st equation.

Hopefully that made sense and it didn't seem like I was just restated what was said :S

Do you mind clarifying this bit? It's the only part I don't really get

 

[nightweaver066]

Re: Enthalpy changes

Do you mind clarifying this bit? It's the only part I don't really get

So if we're looking at the DH per mole of oxygen in the formation of water from H & O, we set up the equation such that its in terms of 1 mole of oxygen, i.e.

2H2 + O2 -> 2H2O

But if we were looking at DH per mole of hydrogen (or water), divide everything by 2 to get:

H2 + (1/2)O2 -> H2O

 

[Aysce]

Re: Enthalpy changes

So if we're looking at the DH per mole of oxygen in the formation of water from H & O, we set up the equation such that its in terms of 1 mole of oxygen, i.e.

2H2 + O2 -> 2H2O

But if we were looking at DH per mole of hydrogen (or water), divide everything by 2 to get:

H2 + (1/2)O2 -> H2O

Ohhhhh alright, I get it. Thanks man

 

[Aysce]

Q. Write equations for the reaction of butane with chlorine in the presence of u.v. light. How many monosubstituted chloro-compounds (that is, compounds with only one Cl atom per molecule) can be formed? Draw their structures.

I've got the equation:

C4H10 + Cl2 + (u.v) -----> C4H9Cl + HCl

There answer is 2 monosubstituted chloro-compounds and I don't understand where the other one comes from, apart from the one already in the chemical equation. Thank you.

If it helps, here's the condensed structural formula of the second one:

 

[nightweaver066]

Re: Reaction of alkanes question

The chlorine atom can substitute any of the H atoms.

The one that you posted up is one of them, 2-chlorobutane, and there's another possible one, 1-chlorobutane, when the chlorine atom substitutes a H from either ends of the molecule.

Notice how if you substitute a H from the 2nd atom from the left with a Cl, it's exactly the same as the one you posted up (if you look at the molecule from behind) so it's not considered different.

So only 2 different monosubstitued chloro-compounds can be formed.

 

[someth1ng]

Re: Reaction of alkanes question

That is absolutely correct.

When looking at isomers and things like that, don't forget that if flipped, it's the same molecule.

 

[Aysce]

Re: Reaction of alkanes question

Thanks for the help Nightweaver and Someth1ng. I haven't learnt this yet but curiosity stops me from moving ahead with the content:

How does one interpret 1,2-dibromoethane? The structural formula is given in the textbook (Page 9 Conquering Chem HSC) but I'm not sure about how to name it? (It's still too early)

 

[nightweaver066]

Re: Reaction of alkanes question

Thanks for the help Nightweaver and Someth1ng. I haven't learnt this yet but curiosity stops me from moving ahead with the content:

How does one interpret 1,2-dibromoethane? The structural formula is given in the textbook (Page 9 Conquering Chem HSC) but I'm not sure about how to name it? (It's still too early)

Let's break down the name first: 1,2-dibromoethane

Bolded: Tells us the carbon structure of the compound. Since it's ethane, we can draw 2 C's.

Italics: Tells us any added atoms. "Bromo" tells us there is bromine, and "di" tells us there are two of them.

Underlined: Tells us the positioning of the atoms. Since it is "1,2", there is one bromine atom on each C atom. We always count from ascending to descending.

Now we just fill in the rest of the empty spaces (make sure each C atom has 4 bonds) with H's.

I'm not too sure what you mean by what you said after the question.

 

[Aysce]

Re: Reaction of alkanes question

Let's break down the name first: 1,2-dibromoethane

Italics: Tells us any added atoms. "Bromo" tells us there is bromine, and "di" tells us there are two of them.

Underlined: Tells us the positioning of the atoms. Since it is "1,2", there is one bromine atom on each C atom. We always count from ascending to descending.

Oh, I just meant that I started the course yesterday and I'm assuming you learn this later?

And from ascending to descending you just mean left to right? So for alkanes, do we always go from ascending to descending? Because for alkenes I recall in prelim you just number them based on the direction that minimises the number for the double bond :S

 

[nightweaver066]

Re: Reaction of alkanes question

Oh, I just meant that I started the course yesterday and I'm assuming you learn this later?

And from ascending to descending you just mean left to right? So for alkanes, do we always go from ascending to descending? Because for alkenes I recall in prelim you just number them based on the direction that minimises the number for the double bond :S

IIRC, i learnt how to do naming in Energy in the prelim course.

Its not always necessarily from left to right, but from the side such that you start with the lowest number.

lol i just realised what i said made no sense at all.. "ascending to descending". What i meant was what i just said above :L Sorry about that

 

[Aysce]

Re: Reaction of alkanes question

IIRC, i learnt how to do naming in Energy in the prelim course.

Its not always necessarily from left to right, but from the side such that you start with the lowest number.

lol i just realised what i said made no sense at all.. "ascending to descending". What i meant was what i just said above :L Sorry about that

I've never seen things like "1,2" and I always thought this was only applied to alkenes in general lol

And for this structural formula:



Is it possible to name it 3-bromo-4-hexanol or is there some certain order we have to confine to? I'm going from right to left.

 

[nightweaver066]

Re: Reaction of alkanes question

I've never seen things like "1,2" and I always thought this was only applied to alkenes in general lol

And for this structural formula:



Is it possible to name it 3-bromo-4-hexanol or is there some certain order we have to confine to? I'm going from right to left.

The OH takes precedence over the bromo so we have to count such that the OH has the lowest number.

Read over this site:
http://www.chem.ucalgary.ca/courses/351/orgnom/main/rules.html

 

[theind1996]

Re: Reaction of alkanes question

I've never seen things like "1,2" and I always thought this was only applied to alkenes in general lol

And for this structural formula:



Is it possible to name it 3-bromo-4-hexanol or is there some certain order we have to confine to? I'm going from right to left.

Naming goes alphabetical order Iirc - so B before H etc.

And you give the smaller number to the OH afaik. You start naming from the end closest to the OH group.

 

[theind1996]

Re: Reaction of alkanes question

The OH takes precedence over the bromo so we have to count such that the OH has the lowest number.

Read over this site:
http://www.chem.ucalgary.ca/courses/351/orgnom/main/rules.html

Fakn nightrider posted before me.