Questions tagged [energy]

For questions relating to the energy required for or produced by reactions, including questions of endothermicity/exothermicity, bond enthalpy, etc.

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78
votes
7answers
114k views

Why is the 2s orbital lower in energy than the 2p orbital when the electrons in 2s are usually farther from the nucleus?

My chemistry book explains that even though electrons in the $\mathrm{2p}$ orbital are closer to the nucleus on average, electrons from the $\mathrm{2s}$ orbital spend a very short time very close to ...
8
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5answers
5k views

Is Bond Formation “Strictly” Exothermic?

Is bond formation "strictly" exothermic? The IUPAC definition of exothermic doesn't make any reference to bond formation. However, I have seen the aforementioned statement before - that bond formation ...
19
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4answers
10k views

Is not possible to find absolute value of internal energy?

Today in thermodynamics lecture my teacher told that it is not possible to find absolute value of internal energy so we have to calculate change in internal energy. So my question is why is it not ...
13
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4answers
4k views

How is chemical energy stored in carbon

I'm fairly new to chemistry, and I have a hard time understanding how chemical energy is stored in carbon (I'm aware that the question can be generalised to 'how is chemical energy stored', but I'm ...
5
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2answers
35k views

Strength of intramolecular vs intermolecular hydrogen bonds

Why are intramolecular hydrogen bonds weaker than intermolecular hydrogen bonds?
7
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1answer
1k views

Difference between sodium ion and a transition metal ion dissolving in water?

In class, I learned that when $\ce{NaCl}$ is dissolved in water, the partially negative oxygen part of the water molecules surround the $\ce{Na+}$ ion and energy is given off (hydration enthalpy) ...
11
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2answers
10k views

Rotational degrees of freedom (3N-5 and 3N-6)

In spectroscopy we described the electric energy with the approximative separability of internal motions as: \begin{equation} E=E_e+E_v+E_r+E_{ns} \end{equation} (energies: electronic, vibratory, ...
3
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2answers
11k views

What is the correct definition of the Gibbs free energy of activation?

Is the following correct? Gibbs free energy of activation is used in energy profiles where the stabilities of the species are expressed as changes in Gibbs energy, while the activation energy Ea is ...
1
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2answers
704 views

Do catalysts shift equilibrium constant towards 1?

I want to be able to understand shifts in equilibrium from the maxwell boltzmann distribution. One thing I cannot get my head around is the effect of catalysts on the equilibrium position - supposedly ...
15
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5answers
1k views

Define, (actually define) the “stability” and “energy” of a compound

When we say a conformer or compound is of "higher energy" than another, are we quantifying energy in terms of the bond strength? Or are we going off of how much energy it takes to break the bond? I ...
11
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3answers
2k views

Is the transformation of pi bonds to sigma bonds always energetically favorable?

Is the move from pi bonds to sigma bonds always favorable? My professor claims so, except in the case of conjugated pi bonds. I can see how pi bonds going to sigma bonds might be favorable; I know ...
21
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3answers
5k views

How to relate a reaction barrier to the time the reaction needs to proceed?

As I am writing this I am at a conference and one of the participants just asked a question where he linked reaction barriers to durations for the reaction to complete. To paraphrase: From our ...
11
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2answers
4k views

Is the Gibbs standard free energy always constant?

I am a biochemistry student and we are learning about thermodynamics. Is the Gibbs standard free energy for a reaction always constant? The equation below suggests that it changes with temperature: $$...
14
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4answers
14k views

Is activation energy temperature-independent?

I know that activation energy for a reaction is the extra energy given to the reactants to reach the threshold energy so that they can collide and react. But then, why is it said that the activation ...
10
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1answer
6k views

Why do tetrahedral complexes have approximately 4/9 the field split of octahedral complexes?

I am trying to calculate the relationship between the octahedral field splitting parameter ($\Delta_\mathrm{o}$) and the square planar field splitting parameter ($\Delta_\mathrm{sp}$) and thought a ...
8
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1answer
2k views

Understanding and usage of Arrhenius equation

A widely used rule-of-thumb for the temperature dependence of a reaction rate is that a ten degree rise in the temperature approximately doubles the rate. This is not generally true, especially when a ...
8
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3answers
37k views

ATP break down and energy release?

I have a very basic question. Energy is absorbed to break bonds. Bond-breaking is an endothermic process. Energy is released when new bonds form. Bond-making is an exothermic process. But ATP breaks ...
5
votes
1answer
20k views

How does Carbon Dioxide have a degree of freedom of 6?

Carbon Dioxide has a degree of freedom of 6, yet from what I understand, it is a linear molecular shape. Therefore, the number of axis of linear movement is 3 (x,y,z) but there are only two axis of ...
12
votes
1answer
542 views

A theoretical molecule that could match uranium mole-for-mole

XKCD #1162 piqued my curiosity. Obviously, there's nothing we generally use as a chemical fuel that can match the energy density of uranium fission. But I wondered what it would take, hypothetically, ...
7
votes
1answer
944 views

What is the nature of chemical energy? [closed]

Is chemical energy basically stored energy? I'm not sure how to define "chemical energy" while I know many physical formulas for energy e.g. potential, movement and other. Is chemical energy an ...
5
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1answer
5k views

Why is it possible to form bromoalkanes but not iodoalkanes from free radical substitution?

For a free radical substitution (FRS) reaction between iodine $\ce{I2}$ with ethane and bromine $\ce{Br2}$ with ethane $\ce{CH3CH3}$: $E_\mathrm d(\ce{H-I})=299\ \mathrm{kJ\ mol^{-1}}$ $E_\mathrm d(\...
5
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2answers
1k views

Counting Nodal Planes in cyclopropane

The energy of molecule orbitals increases with more nodal planes. W1 (in the attached picture) has no nodal plane. I'd like to know how to draw the nodal planes in cyclopropane molecule orbitals but ...
5
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1answer
2k views

Molecular orbital diagram for nitrogen monoxide, the nitrosyl cation and the nitrosyl anion

As I understand it, all of them should have same order of energy of molecular orbitals as a model for $\ce{NO}$ should withstand ionization and adding an electron. JD Lee Pg 109 shows the same ...
3
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2answers
856 views

Is the E in Arrhenius equation the change of enthalpy?

I'd like to know whether the activation energy in Arrhenius equation is the enthalpy difference between the transition state and the reactant. If it's not enthalpy, is it Gibbs energy or something ...
2
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3answers
4k views

In nuclear chemistry, how does a neutron split to form a proton and an electron?

I'm studying radioisotopes at the moment and balancing nuclear reactions isn't making sense in that more matter is coming out of the equation in negative β decay equations : $^\textrm{14}_\textrm{6}\...
2
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1answer
10k views

What is the exact definition of state functions? How to determine state functions?

How do you know if something is a state function? I know Internal Energy (U) is a state function since it is the sum of energies, which is constant at a given time. However I don't know for sure if $...
2
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1answer
2k views

Why is ionization energy measured in gaseous phase?

When I looked up the definition of ionization energy, it says:- it is the energy required to remove one electron from the shell to infinity in an atom in gaseous state. Why is it measured in ...
16
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2answers
28k views

In Crystal Field Theory what does Δo=10Dq mean?

While studying the Crystal Field Theory I was told $\mathrm{Dq}$ is a unit, related to the unit $\Delta_{\mathrm{O}}$ by the relation $\Delta_{\mathrm{O}} = 10\ \mathrm{Dq}$. But aren't $\Delta_{\...
3
votes
1answer
2k views

What tools can be used to estimate binding energies?

I want to estimate the binding energies of various molecules. What are the analytical and numerical tools I can use for this purpose? In particular I'm interested in bonding energies of dyes to ...
2
votes
1answer
839 views

Understanding Gibbs free energy

I want to verify a thought I had about Gibbs free energy. In some ways, I'm actually wondering why we are using enthalpy instead of the internal energy $U$. Right now, I'm thinking that enthalpy is ...
16
votes
3answers
806 views

What really happens atomically in an explosion?

Let's say a room is filled with butane, I then throw a cigarette into the room. What happens to the atoms/molecules of the butane when they are in contact with the heat from the cigarette?
9
votes
3answers
173k views

How can melting point equal freezing point?

I don't understand how the freezing point of a substance is the same temperature as the melting point of the same substance. For example, if liquid water freezes at 0 °C how can ice also melts at 0 °...
7
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1answer
368 views

Periodically oscillating chemical reactions?

I've seen before chemistry demonstrations where solutions are mixed with one another and subsequently where the resulting product goes through periodic color changes: for example blue to orange and ...
6
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1answer
3k views

What's the energy density of liquid nitrogen?

Liquid nitrogen seems to be attracting a bit of attention at the moment as a medium of energy storage, both for electricity grid applications and for transport. For example, Highview are doing round-...
6
votes
1answer
24k views

How much energy is needed to break a water molecule?

How do you calculate how much energy is needed to break a water molecule into its constituent atoms? note This is not homework. I just want to know what is required to get hydrogen out of water.
3
votes
2answers
349 views

Interaction between energy and entropy in highly elastic polymers?

Consider the interaction of energy and entropy in the highly elastic materials of an ideal polymeric network. Now Gibb's free energy cannot be used directly $\Delta H = \Delta G + T \Delta S$ where $\...
2
votes
1answer
375 views

Gibbs free energy=potential free energy? (Transition state theory)

Online I found the plot Gibbs free energy vs. reaction coordinate: https://en.wikipedia.org/wiki/Transition_state_theory and on many books I found the plot potential energy vs. reaction coordinate (...
9
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3answers
3k views

Can Rydberg constant be in joules?

In my textbook (Chemistry Part - I for Class XI published by NCERT), there is an equation for the energy of an electron in an energy state: $$E_n = -R_\mathrm H\left(\frac{1}{n^2}\right)$$ and there ...
7
votes
2answers
5k views

Significance of single point energy when calculating interaction energies

I am currently investigating about the interaction behavior of a few atoms in certain conditions. Is it possible to use the concept of single point energy to represent the atomic interaction energies ...
5
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1answer
2k views

How can I produce hydrogen gas efficiently and cheaply?

I have been intrigued by one of the principles of chemistry lately- electrolysis. In my knowledge, electrolysis is a quite dangerous operation as the decomposition of water produces hydrogen and ...
4
votes
2answers
168 views

In equilibrium, if bonds are constantly being created and broken, where does the energy that does so come from?

I'm a student who recently learned about equilibrium and how it is not a state where nothing is made, but rather, a state where the product is decomposing at the same rate as the reactants are ...
4
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1answer
158 views

Correlation energy in quantum chemistry

In density functional theory (DFT) there is classic Coulomb energy and non-classic correlation energy. But what is difference between them? In books these energies are explained very similarly. Both ...
3
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1answer
338 views

A reversible exothermic reaction between a molecule in an excited triplet state and a reducing agent

Say I promote a molecule to a triplet state via repeated electronic excitation events. Here, this is going to mean that an electron from the HOMO level, originally with a spin-UP and spin-DOWN ...
2
votes
1answer
587 views

Are the vibrational modes of CO2 active at room temperature?

I was doing an experiment whereby I had to measure the specific heat ratios of certain gases. Carbon dioxide came out to be around 1.3, and checking with the accepted values this is close. My question ...
2
votes
2answers
4k views

What does the “total energy” of a molecule mean for the Chem3D MM2 minimized energy calculation?

I was fiddling around with Chem3D and the MM2 calculation for finding a molecules minimized energy. Benzene was assigned a total energy of -0.5741 kcal/mol while biphenyl was assigned a total energy ...
1
vote
1answer
750 views

Using enthalpies of formation and combustion to find formation enthalpy of Acetylene (Hess' Law)

I understand this is a simple question (I’m in high school), but for some reason the numbers just are not working for me. Given: $$\Delta H^\circ_\text{c} = -1301\ \mathrm{kJ}\ \text{for}\ \ce{C2H2(...
1
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3answers
5k views

Are 1st Electron Affinities positive or negative?

According to this ChemistryLibretexts website and this StackExchange answer, 1st electron affinites are negative and 2nd electron affinities are positive. However, according to this Study.com ...
1
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3answers
1k views

Why are molecular reactions endo- or exo-thermic?

It seems strange to me that chemical reactions should be exothermic, meaning the molecules move faster after the reaction. Normally, in physics when two moving objects collide and stick together, ...
-1
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2answers
10k views

Does covalent character increase or decrease bond strength? [closed]

On one hand, covalent character increases the bond dissociation enthalpy (i.e., more energy is needed to be put in) and on the other it lowers the melting point of $\ce{Al2O3}$ to a value lower then ...
9
votes
1answer
284 views

Does the methyl rotation in propene have a symmetric or asymmetric potential energy surface?

Depending on how one imagines the methyl rotation of propene to occur, it seems like it could exhibit either a symmetric or an asymmetric potential energy surface. If the $\ce{H2C=CH\! -}$ portion ...