38
votes
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What happens to a molecule while it is reacting?
Well, a lot of things happen to the reactants. Some bonds stretch (and maybe eventually break), the others shrink, and your molecules morph into different molecules, which are the products.
(source)
...
- 30.9k
23
votes
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Is activation energy temperature-independent?
There is a simple (some might say simplistic) way to get an intuition about this and it involves thinking at the molecular or atomic level rather than about the bulk properties of the reaction (the ...
- 35.1k
23
votes
Where does the energy from combustion come from?
Better is to say "Energy released by forming bonds of combustion products is bigger than the energy needed to break bonds of combustion reactants.".
Particularly breaking $\ce{C-C}$, $\ce{C-...
- 37.9k
22
votes
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Can carbon dioxide be reduced to carbon monoxide and oxygen to produce energy?
Unfortunately, the question as stated is thermodynamically impossible. Let's look at the proposed reaction:
$$\ce{CO2(g) -> CO(g) + O(g)}$$
This reaction is simply a bond dissociation (specifically,...
19
votes
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If aliens lived in a hydrogen (or any flammable gas) based atmosphere, would they perceive oxygen to be flammable?
Absolutely yes.
Lighting a torch in such an environment would simply be the reverse physical process (and same chemical process) of what is done in our oxygen-containing atmosphere. In the chamber ...
- 17.3k
18
votes
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Is iron the most stable element in the periodic table?
Yes, $^{56}\ce{Fe}$ has the most stable nucleus, and $\ce{He}$ is the most chemically inert element. These are different and unrelated qualities, pretty much like physical fitness and intelligence in ...
- 30.9k
17
votes
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Where does the energy in coal bonds come from?
Indeed it's in some way counterintuitive to say that the sun's energy is 'stored' in the coal bonds. Energy never comes from breaking bonds, it is released when forming bonds.
In the case at hand, the ...
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16
votes
Is the Gibbs standard free energy always constant?
In short, no, the standard Gibbs free energy change is not constant; it is a function of temperature. The same is true for practically all other standard-state quantities.
This gets a little ...
- 830
16
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Why do electrons jump back after absorbing energy and moving to a higher energy level?
This is a very fundamental question and for really understanding the "why" some advanced physics is involved. I will describe the process rather superficially.
As you might know, the level ...
- 2,386
16
votes
Why is it that protons and electrons undergo the same amount of deflection in an electric field if they have the same energy?
Now that's a mildly non-trivial observation. Why would they be equal, really?
Let's say a particle with mass $m$, charge $q$, and initial velocity $v$ enters an area of length $L$ where an electric ...
- 30.9k
14
votes
Accepted
Which has higher lattice energy: MgO or LiF?
$\ce {MgO}$ ($\approx 3800$ $\pu{kJ mol^{-1}}$) has higher lattice energy than $\ce {LiF}$ ($\approx 1045$ $\pu{kJ mol^{-1}}$) mainly because of the greater charge on $\ce{Mg^2^{+}}$ ion and $\ce {O^{...
- 5,345
14
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Can Rydberg constant be in joules?
Authors may be sloppy about notation in this matter. I recommend considering $R_\ce{H} \approx \pu{10973 cm-1}$ and $Ry \approx \pu{2.18e-18 J}$, noting $Ry = hc \cdot R_\ce{H}$. Units of wavenumbers $...
- 2,194
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Where does the energy in coal bonds come from?
As has been pointed out the energy comes in the form of visible light from the sun. The reaction written in summary or overview is
$$\displaystyle \ce{2H2O \overset{light}\to O2 + 4H^+ + 4e^-}$$
$$\...
- 29.8k
14
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Is there any process that converts water into hydrogen and oxygen by using light and a catalyst?
Probably several of them. The one I found at the top with a search for "photocatalytic water decomposition" is Ref. [1]:
The photocatalytic activity for water decomposition of alkaline ...
- 51.5k
13
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Wouldn't radiolabelled phosphorus in DNA break it apart as it disintegrates?
Of course it would break, just like you said; also, a high-energy $\beta$ particle would kill quite a lot of bystander molecules. Also, if not for other reason, the resulting molecule would no longer ...
- 30.9k
13
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How does an electrons's wave function change when it moves between energy levels?
You need to go back to the very start. Here, you're kind of asking: I have a solution $\psi_0$, how do I get the next solution $\psi_1$? The answer is to look at how $\psi_0$ was obtained, and it ...
- 70.9k
12
votes
Is iron the most stable element in the periodic table?
No, nickel-62 is the most stable on a binding energy per nucleon basis. Fe-58 is second and Fe-56 is third.
See Fewell, M. P., "The Atomic Nuclide with the Highest Mean Binding Energy", Am. J. Phys....
- 40.1k
12
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Why is the Gibbs free energy for phase changes zero at constant temperature and pressure?
The initial and final thermodynamic equilibrium states of your system are as follows:
State 1: $\pu{1kg}$ liquid water at $\pu{0 ^\circ C}$ and $\pu{1 atm}$
State 2: $\pu{1 kg}$ water ice at $\pu{...
- 12.8k
12
votes
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Does the potential energy increase when temperature is raised?
Yes, potential energy increases with increasing temperature for at least the following three reasons:
At a higher temperature, more atoms/molecules are in excited electronic states. Higher ...
- 40.1k
12
votes
Accepted
kJ per mole or kJ per 2 moles?
the whole reaction, i.e. for 2 moles of the product?
This is your mistake.
The balanced equation $\ce{2H2 + O2 -> 2H2O}$ does not mean that 2 moles of the product are being formed.
The numbers, ...
- 70.9k
11
votes
Why exactly are standard potentials additive?
Following on Derek's great answer, it is very important to remind that the conventional way we use to add half-cell potentials is a consequence of the conservation of energy. Therefore, we should look ...
- 301
11
votes
Why is oxygen so special?
Dioxygen, $\ce{O2}$ is a very special molecule. A good majority of organisms on earth use dioxygen to breathe and survive. Oxygen is also the second most abundant element in the sky, as well as the ...
11
votes
Where does the energy required to initially begin the reaction of iron and oxygen come from?
Do all reactions initially require a input of energy?
Yes, it is called activation energy. When you say "initially required", it does not mean just when the overall reaction starts. Each ...
- 37.9k
10
votes
Accepted
Why is oxygen so special?
Your question is quite broad, so I will tackle it in parts.
Element abundance
there is nothing as familiar to mankind (in terms of the frequency in
their occurrence) as the reactions involving ...
- 9,049
10
votes
Accepted
Why does increasing the temperature of a *solid* increase its entropy?
All right, someone bearing the standard of thermodynamics will give you the equations shortly... From a layman to another, here goes my attempt at a simpler explanation.
Entropy may be seen as the "...
- 2,737
10
votes
Define, (actually define) the "stability" and "energy" of a compound
As a chemist, I would agree that textbooks are not clear about the term stability and energy. It is not your fault and undergraduate organic chemistry books make the situation worse. I cannot recall ...
- 38.6k
10
votes
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Why is the principal energy of an electron lower for excited electrons in a higher energy state?
Notice that when $n=1$, we have,
$$
E=-E_0=-13.6~\mathrm{eV}
$$
which is the negative of the energy required to remove an electron from the ground state of a hydrogen atom.
If we increase $n$ to say $...
- 12.9k
10
votes
Can Rydberg constant be in joules?
Rydberg constant $R_∞$ is usually given in reciprocal length units historically and because it's determined from hydrogen and deuterium transition frequencies [1].
Current value (in $\pu{m-1}$) is ...
- 37.3k
10
votes
Does energy come from fuel or oxygen? (surprise?)
[OP:] It is usually said that fuel contains energy and that oxygen only enables the release of energy in the sense like enzymes enable reactions.
An enzyme is a catalyst, so it does not change the ...
- 37.9k
10
votes
Accepted
Pressure exerted by a particle of ideal gas
Here's your confusion:
You need to consider two different things:
The momentum transfer per particle per collision. There, since we assume an instantaneous collision, it doesn't make sense to try to ...
- 12k
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