54
votes
Why does wood burn but not sugar?
Combustion is a gas phase reaction. The heat of the flame vapourises the substrate and it's the vapour that reacts with the air. That's why heat is needed to get combustion started.
Anyhow, wood ...
- 1,983
13
votes
Fluorine detection in organic compounds
Unfortunately, your analytical question is not an easy one, and no test can be done safely at home or by using a handheld gadget. The problem is fluorine, and tiny amounts of it in the sample. ...
- 38.9k
12
votes
Why does wood burn but not sugar?
With hydrocarbons a certain amount of oxygen (n) and a certain amount of heat energy (Q) are required for complete combustion. In complete combustion the byproducts are carbon dioxide and water in ...
- 2,264
8
votes
Aspirin Hydrolysis at pH 2
To understand the pH rate profile of the Aspirin hydrolysis, we have to look at all the possible mechanisms through which the hydrolysis may take place.
A good rule of thumb for interpreting a pH rate ...
- 5,637
8
votes
Accepted
How do we obtain the π molecular orbitals for allene via Hückel theory?
For p-orbitals $i$ and $j$ which are orthogonal to each other (i.e. different colour in your diagram), the off-diagonal matrix elements are simply zero:
$$\langle i | H | j \rangle = 0$$
If we ignore ...
- 71.1k
8
votes
Difference between intermediates and transition states
Throughout, I will be using definitions from a Journal of Chemical Education article$^1$ by Keith Laidler.
I'll start with describing an intermediate, as I think it is an easier concept to grasp. An ...
- 11.6k
7
votes
How can isotope labelling be used to determine the mechanism of ether cleavage with hydrogen iodide?
If your products are simply an alcohol plus an alkyl iodide, then you really don't need the isotopic labelling. If you have an $\mathrm{S_N1}$ mechanism operating, then iodide ion attacks the tert-...
- 71.1k
6
votes
Accepted
Why doesn't tetrachloromethane react with aqueous sodium hydroxide under reflux?
In chloroform, there are three electron withdrawing groups ($\ce{Cl}$) which are able to stabilize a negative charge. Thus, the first step is the deprotonation of chloroform, typically with ...
- 28.4k
6
votes
What will be the product of 1 eqv Benzene reacts with 1eqv H2?
The answer makes sense (and doesn't break the law of chemical equivalence) when you consider the reactivity and stability of the intermediates in the reaction
Hydrogenation of benzene proceeds in ...
- 35.4k
5
votes
How can isotope labelling be used to determine the mechanism of ether cleavage with hydrogen iodide?
The function of the oxygen isotope is to determine what mechanism the reaction undergo. The mechanism of ether cleavage is SN1 or SN2 depending on the nature of the carbons bonded to oxygen. If both ...
- 463
5
votes
Why does the dipole moment of molecules go from positive to negative?
You have assessed the problem correctly. Physicists and chemists use just the opposite directions. Keep in mind that both are arbitrary choices, however the physicist's convention is consistent with ...
- 38.9k
5
votes
Why does wood burn but not sugar?
It's a matter of physics more than of chemistry. If you apply a lit match to a log, it won't catch fire. Doing the same to a (headless) matchstick will set it alight immediately. This is because the ...
- 219
5
votes
Out of tropolone and tropone, which is more soluble in water?
The presence of internal hydrogen bonding does not automatically mean lower water solubility. The water solubility difference between fumaric and maleic acid provides a good example.
Maleic acid (the ...
- 42.6k
4
votes
Accepted
Why is the boiling point of sugarcane juice lower than the boiling point of water?
A more plausible explanation is that it is not water that is boiling. The "organic substances" and maybe the "nitrogenous substances" in the juice could include some more ...
- 53.1k
4
votes
Accepted
How does addition of side chains affect solubility and processibility of polyphenylene vinylene?
Conjugated chains, especially if delocalisation does indeed take place, tend to be flat or at least stiff.
This results in a tendency to adhere to each others. In addition, their stiffness also means ...
- 3,596
3
votes
What's the least common element in the universe?
(edited based on @JonCuster's comment) The rarest known elements are most probably the man-made heavy elements. These isotopes (elements) are very unstable. They most probably also formed in ...
- 5,363
3
votes
Accepted
What can we say about the dipole moment of terephthalic acid?
You are indeed correct, a study by Karthikeyan et. al.[1] suggests that the Dipole moment of terephthalic acid is indeed around ~2.6D (in air), the same as your latter source.
From the paper,
The ...
- 146
3
votes
Ratio of uncharged to charged side chains
I think you are simply misunderstanding the "uncharged to charged" part of the question. Imidazole is a base, so it gains a proton to become charged when the pH < pKa. That's why you got the right ...
- 700
3
votes
Accepted
Why do we need Equilibrium Constant as well as Acid Disocciation Constant?
For any system at equilibrium we can define an equilibrium constant as follows:
$$\ce{A(aq) + B(aq) <=> C(aq) + D(aq) }$$
$$K_\text{eq} = \frac{[\ce{C}][\ce{D}]} {[\ce{A}][\ce{B}]}$$
When the ...
- 1,158
3
votes
What will be the product of 1 eqv Benzene reacts with 1eqv H2?
Hydrogenation of benzene requires high temperature and high pressure of hydrogen. An equimolar mix of benzene and hydrogen (the term "equivalent" is unfortunate here) with catalyst and ...
- 1,811
3
votes
Find percentage of carbon monoxide reacted using ideal gas equation
Using the ideal gas state equation as the starting point:
$pV=nRT \implies \frac{p_1V}{p_2V}=\frac{n_1RT}{n_2RT} \\
\implies \frac{p_1}{p_2}=\frac{n_1}{n_2}$
leading to the consequence that the gas ...
- 39.1k
3
votes
Accepted
Can species with diffrent number of electrons be isoelectronic?
The wikipedia page for isoelectronicity shows an example image of Serine, Cysteine, and Selenocysteine.
These three molecules can all be described by chemical formula "HOOC−CH(−NH2)−CH2−XH" ...
2
votes
Difference between intermediates and transition states
A transition state cannot be isolated while an intermediate can be isolated. A transition state is a chemical species which has only fleeting existence and represents an energy ...
- 29
2
votes
Accepted
Why do weak acids like ethanol not act as a strong base in water?
There seems to be a misunderstanding: The conjugate of a strong base like EtONa can't be a weak acid, because weak acids, like acetic acid for example, are still quite acidic.
Strong acid (HCl) - ...
- 12k
2
votes
Zinc and copper
I believe you have some conceptual flaws:
I like to think of potential energy as the energy held in a system but more formally it is the energy possessed by a body by virtue of its position relative ...
- 247
2
votes
which one is the correct statement of "Law of definite proportion"?
The Law of Definite Proportions was defined before we knew about isotopes. Sure, you can give a modern version of it, but that's not the original version.
If you have no notion of isotopes, then the ...
- 17.3k
2
votes
Why has nature evolved to put a porphyrin (i.e. chlorophyll) as the chromophore in a leaf?
Chlorophyll (chl) is involved in two ways in photosynthetic organisms firstly as a light gathering pigment and second in electron transfer once the light is captured. It is unique in this role.
In ...
- 30k
2
votes
Enzyme Kinetics
Application of the Michaelis-Menten equation generally assumes steady state concentration of the enzymatic complex intermediate. Then $K_M$ allows two interpretations$^1$:
an apparent dissociation ...
- 20.8k
2
votes
Enzyme Kinetics
The Michaelis–Menten kinetics model formula is:
$$ V= \frac{(d¦P¦)}{dt} = \frac{(Vmax)¦S¦}{Km + ¦S¦}$$
Reaction rate V is the rate of formation of product, ¦P¦ is the concentration of product and the ¦...
- 1,821
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