Chemistry 3/4 2013 Thread

[Will Sparks]

Are we required to be able to balance redox reactions in basic conditions?

Yeah, unless it specifically says you don't need to balance the equation, ALWAYS balance the equation (whether it's redox or anything else!)

[thushan]

From the study design:
So alkanes, alkenes, alkanols, carboxylic acids, haloalkanes, amines, amides and esters. Shouldn't need to know anything else I don't think - though benzene and cycloalkanes seem to come up even if they're not mentioned here. Plus carbohydrates, proteins and DNA stuff too I guess.

You shouldn't need to know how to name benzene and cyclo compounds.

[Alwin]

You shouldn't need to know how to name benzene and cyclo compounds.

True, naming of aromatic compounds is not in the 3/4 course, but you still need to know some basic reactions (especially if you made aspirin as part of sac 2) and be able to count the number of hydrogen atoms in a molecule such as salicylic acid. Counting hydrogen atoms may sound dumb, but a surprisingly large number in my class got it wrong.

[lzxnl]

Hi guys,

Does anyone know what actually determines whether a substance is a strong acid or base? I mean, considering oxygen is more electronegative than chlorine, why is HCl a stronger acid than CH3COOH? A considering an OH bond is highly polarised, why are water and alcohols such weak acids?

Thanks 

Bond polarity does not necessarily mean acid strength. In fact, bond polarity means that there are electrostatic attractions between the atoms, which would strengthen the bond and hence reduce dissocation.

Let's look at the hydrogen halides. HF is a weak acid because the H-F bond is very polar, so the bond is harder to break. Also, fluorine is a small atom; in fact, the smallest atomic anion you can have aside from the hydride ion (which is a massively powerful base). Therefore, its bond length with hydrogen will be quite small and so the bond will be even stronger. Take into account HF's ability to hydrogen bond with water and other HF molecules, and you can see how it may be unfavourable for HF to dissociate.
HCl, however, is a strong acid because the bond is not quite as polar. Also, chlorine is larger than fluorine, so the bond length will be larger, leading to a weaker H-Cl bond. This makes the abstraction of the hydrogen in HCl a lot easier.
Similarly, HBr and HI have larger halogen atomic radii, which lead to decreasing bond strengths. In fact, HI, which isn't really polar at all, is an incredibly powerful acid, at least 100 times as strong as HCl and around 10^7 times as strong as sulfuric acid.
Also note how all of the halide ions fulfil the octet rule, which confers additional stability.

I'll look at methanoic acid for simplicity. Let's take a look at the methanoate ion. We do have resonance forms in which a Lewis structure can be drawn with the double bond on either oxygen. It averages out for each oxygen to have a -1/2 charge. This does mean that carboxylic acids are much stronger acids than alcohols due to this resonance stabilization.
However, O-H bonds are relatively strong. Also, inductive effects aren't really present here with only two oxygens in total on a carbon atom bonded to nothing else. Carbon isn't very electronegative, so the oxygens retain most of the negative charge which is quite attractive for hydrogen ions to attack and reform the acid.
These inductive effects are important if we consider the acidity of HClOn, where n is an integer from 1 to 4. HOCl, hypochlorous acid, isn't a very strong acid at all (pKa around 7.5), but acid strength increases with increasing n. In HClO3, for instance, we have two oxygens doubly bonded to a chlorine which is then singly bonded to an OH group. The oxygens with double bonds sap electron density from the chlorine, leaving it positively charged. This positive charge from the chlorine then saps electron density from the OH oxygen, which in turn drags even more electron density off the H. This means that OH bond is weakened considerably. HClO3 is a strong acid with a pKa of around -1, and HClO4 is a ridiculously powerful acid of pKa -8, and this extra acidity is accounted for by the extra induction by the extra oxygen which increases the positive charge on the chlorine.
This is all helped by chlorine's electronegativity. It has an ability to draw some electron density from oxygen atoms, especially if there are other oxygens forming double bonds to it. Another atom with a similar electronegativity, nitrogen, exhibits similar behaviour. Nitric acid, HNO3, is of similar strength to HClO3. In contrast, phosphoric acid, H3PO4, may have four oxygens bonded to the phosphorus, but the phosphorus isn't strong enough at attracting electron density. Also, H3PO4 consists of three OH groups bonded to P and then a double bond to O. The inductive effects also aren't as strong.

I hope this gives you an idea of what determines acid strength (:

[Limista]

Suppose we conduct an experiment on aspirin that tells us to use 1mL sulphuric acid as a catalyst.

What happens if we use more than 1mL & what happens if we use less than 1mL?

Who determines what the prescribed amount of sulphuric acid to use should be?

Thanks.

[drake]

A sample contains two compounds with the same molecular formula, but different structural formulas. If they are introduced into a Gas Chromatography Instrument with a Flame Ionisation Detector and the sample has the same concentration of BOTH compounds, would the peak area of the two peaks be the same on the chromatogram??

I am a little confused, because peak area is proportional to amount. TSFX says that the peak area will be the same, however Heinemann Textbook suggests that the peak area is dependent on the sensitivity of the Flame Ionisation Detector, and because the compounds have different structural formulas, they will produce different ions in the flame and hence different peak areas.

WHICH ONE IS RIGHT?!

[saba.ay]

A sample contains two compounds with the same molecular formula, but different structural formulas. If they are introduced into a Gas Chromatography Instrument with a Flame Ionisation Detector and the sample has the same concentration of BOTH compounds, would the peak area of the two peaks be the same on the chromatogram??

I am a little confused, because peak area is proportional to amount. TSFX says that the peak area will be the same, however Heinemann Textbook suggests that the peak area is dependent on the sensitivity of the Flame Ionisation Detector, and because the compounds have different structural formulas, they will produce different ions in the flame and hence different peak areas.

WHICH ONE IS RIGHT?!

Well, my teacher has said that the workings on the flame ionisation detector isn't on the course this year, so I'd say we use the logic that peak area is proportional to 'concetration/amount'.

[teletubbies_95]

Hi guys!
Just had a quick question regarding with one of the organic chem practicals. When we are reacting chloroalkane of some kind, why do we have to add both ethanol and KOH? Can someone explain to me how this works in terms of bonding?
My teacher said it was just necessary and didnt give me any further info.

oh another question... why is h2S04(l) used instead of H2S04 (aq) catalyst in esterfication reactions?

Thanks guys! Shall try to answer questions later. Wave of sac period atm.

[Patches]

A sample contains two compounds with the same molecular formula, but different structural formulas. If they are introduced into a Gas Chromatography Instrument with a Flame Ionisation Detector and the sample has the same concentration of BOTH compounds, would the peak area of the two peaks be the same on the chromatogram??

I am a little confused, because peak area is proportional to amount. TSFX says that the peak area will be the same, however Heinemann Textbook suggests that the peak area is dependent on the sensitivity of the Flame Ionisation Detector, and because the compounds have different structural formulas, they will produce different ions in the flame and hence different peak areas.

WHICH ONE IS RIGHT?!

Definitely the second. Different detectors are going to detect different substances to greater or lesser degrees. This is why, for instance, you can't use one calibration curve for every substance.

[jgoudie]

Not sure what you mean by adding Ethanol and KOH, what are your trying to make?  Normally in VCE you will be looking to create an alcohol, so you would want to react it with KOH (OH^-).

Second question H₂SO₄(l) is used as it is concentrated sulphuric acid.  The reason we add it in the first place for the dehydrating affect, if you were to add (aq) you would be adding water and thus driving the reaction more in reverse.

Hi guys!
Just had a quick question regarding with one of the organic chem practicals. When we are reacting chloroalkane of some kind, why do we have to add both ethanol and KOH? Can someone explain to me how this works in terms of bonding?
My teacher said it was just necessary and didnt give me any further info.

oh another question... why is h2S04(l) used instead of H2S04 (aq) catalyst in esterfication reactions?

Thanks guys! Shall try to answer questions later. Wave of sac period atm.

[teletubbies_95]

Not sure what you mean by adding Ethanol and KOH, what are your trying to make?  Normally in VCE you will be looking to create an alcohol, so you would want to react it with KOH (OH^-).

Second question H₂SO₄(l) is used as it is concentrated sulphuric acid.  The reason we add it in the first place for the dehydrating affect, if you were to add (aq) you would be adding water and thus driving the reaction more in reverse.

Umm. In the student workbook, one of the pracs is asking for the product of the reaction of KOH and chloroalkane? My question is , why do we have to add ethanol for the reaction to occur?

Thanks .

[Mr Keshy]

Umm. In the student workbook, one of the pracs is asking for the product of the reaction of KOH and chloroalkane? My question is , why do we have to add ethanol for the reaction to occur?

Thanks .

I'm not sure if we're doing the same prac But apparently, the ethanol helps dissolve the potassium hydroxide in the mixture. (page 96? haha)

[teletubbies_95]

Yep!!That's the prac. Lol.
But I don't know why it helps dissolve the KOH?

[Alwin]

Hi guys!
Just had a quick question regarding with one of the organic chem practicals. When we are reacting chloroalkane of some kind, why do we have to add both ethanol and KOH? Can someone explain to me how this works in terms of bonding?
My teacher said it was just necessary and didnt give me any further info.

The solvent is usually a 50/50 mixture of ethanol and water, since everything will dissolve in it. The chloroalkane is insoluble in water. Remember that chloroalkanes are non-polar. If you used just water alone, the chloroalkane and the sodium hydroxide solution wouldn't mix and the reaction could only happen where the two layers met.

oh another question... why is h2S04(l) used instead of H2S04 (aq) catalyst in esterfication reactions?

Again, more unit 4 chem but simply, if you use H₂SO₄ (aq) then there is water present. However, if you look at your equation, water is actually formed (condensation reaction). So, for the reaction to go as fast as possible, you don't want water there initially since it will 'inhibit' (for lack of a better word) the production of water which is one of the products. This will make the reaction want to go 'backwards' and slow it down.

Hope it makes sense!

[Daniel_Le]

Hello,

Thank u for answering my questions earlier.

I'm want to ask about the troughs in IR Spec, if it's more intense or sharp, does it mean there is more of that particular bond at the particular wavenumber? i.e. for alkyl groups (2850–3300cm^-1), it tells us there is many alkyl groups (which meaning larger size of the alkyl group?) in the particular molecule?

Then I'm want to ask how about the area of the trough? That's different than how intense or sharp it is right? If so, what does area correspond to in IR peak? My teacher say that compared to other spectroscopic methods, the area is not correspond to the concentration of the bond producing the trough in IR? Then may I ask what is it correspond or show us? This is important to know?

Thank you friends,