VCE Chemistry Question Thread

[dankfrank420]

Quick question on thermochemical equations.

In the formation of water, the delta H is negative. Shouldn't it be positive?

Isn't an input of energy required to form the bonds between the O and H atoms that constitute the water molecule?

[grannysmith]

Quick question on thermochemical equations.

In the formation of water, the delta H is negative. Shouldn't it be positive?

Isn't an input of energy required to form the bonds between the O and H atoms that constitute the water molecule?

Formation of new bonds generally releases energy, whereas breaking them requires energy.

[cosine]

Formation of new bonds generally releases energy, whereas breaking them requires energy.

But in saying that, the presence of an enzyme is still required in both scenarios, right?

[grannysmith]

But in saying that, the presence of an enzyme is still required in both scenarios, right?

In an enzyme-catalysed reaction, yes. Enzymes only lower the activation energy, but don't affect enthalpy change.

[Dkontro]

An organic compound that weighs 3.411 grams is made up of carbon,  nitrogen and hyrogen.  When burned,  7.2 grams of CO2 are produced and 2.5 grams of NO2 are produced

1. What is the percentage composition of carbon in the compound
2. What is the empirical formula of the compound

[knightrider]

What is the number of subshells that make up the third shell

[knightrider]

a hydrocarbon has the empirical formula   and a molecular mass of 54 .
The molecular formula is?

[Dkontro]

C4H6
\\\

a hydrocarbon has the empirical formula   and a molecular mass of 54 .
The molecular formula is?

[knightrider]

C4H6
\\\

How did you work that out?

[IndefatigableLover]

How did you work that out?

Trial and error using the fact that the molar mass of Carbon is 12 and Hydrogen is 1. Subbing n=1, n=2 etc. until n=4 whilst you multiply it all out will yield C4H6.

[cosine]

How many marks can you drop on the exam to obtain a B+ on it? If it's hard to predict this year, what was last year's cut off?

Thanks

[keltingmeith]

Trial and error using the fact that the molar mass of Carbon is 12 and Hydrogen is 1. Subbing n=1, n=2 etc. until n=4 whilst you multiply it all out will yield C4H6.

You can also do it slightly more generally.

We know that CnH(2n-2) has a molar mass of 12*n+1*(2n-2)=14n-2, and the molecule we want has a molar mass of 54. So, we get:

14n-2=54
14n=56
n=4

So, the molecule must be C4H(2*4-2)=C4H6

[IndefatigableLover]

You can also do it slightly more generally.

We know that CnH(2n-2) has a molar mass of 12*n+1*(2n-2)=14n-2, and the molecule we want has a molar mass of 54. So, we get:

14n-2=54
14n=56
n=4

So, the molecule must be C4H(2*4-2)=C4H6

Oh LOLOL the general method is definitely the better way to go (too much memorising molar masses >.<)

How many marks can you drop on the exam to obtain a B+ on it? If it's hard to predict this year, what was last year's cut off?

Thanks

Using this, you could lose 41.5 marks and get a low B+ (from a range of 157-178/240).

[ekay]

hey guys,

just had a quick question about naming amines.

in vce, is it more preferred to name a molecule as 2-aminopentane or pentan-2-amine?

just wondering because if we're naming something like a carboxylic acid molecule we would mention the amino group as a prefix and say which number carbon it's attached to (e.g. 2-aminopentanoic acid), so do we do the same if it's just a basic alkane?

thanks!

[grannysmith]

Because pentan-2-amine has no other functional group that takes precedence over the amino group (e.g. carboxyl, hydroxyl), this is the preferred name. In your second example, that would indeed be correct because now you've got a carboxyl which takes precedence, hence 2-aminopentanoic acid (and NOT pentan-2-aminoic())