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

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1answer
25 views

Lowering energy in molecular orbital mixing

When drawing molecular orbital diagrams for mixing two orbitals we always end up with one orbital which is lower in energy than the initial orbitals and another which is higher in energy. The ...
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0answers
19 views

Trying to work out the Mean Bond Energy for C-H Bond

The question is asking me to find a value for the mean bond energy of the C-H bonds in Ethene and Ethane. I am given the mean bond energies for the C=C, the H-H and the C-C bonds which are 612kj/mol, ...
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0answers
25 views

potential profile in kinetics and spontaneity in reactions

We usually think of the criterion of spontaneity of a reaction as the change of Gibbs energy. By Clausius inequality, one of various forms of the 2nd law, we know that $ ...
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3answers
200 views

kJ per mole or kJ per 2 moles?

From Chemguide: Is the measurement unit $\ce{kJ*mol^{-1}}$ correct here? Shouldn't it be measured simply in kJ, because the $-572$ figure is for the whole reaction, i.e. for 2 moles of the product? ...
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0answers
21 views

How to calculate minimum temperature of a reaction?

For burning ethanol (its reaction with oxygen), we need a minimum activation energy (a minimum temperature) to start the reaction. Is it possible to calculate what are the minimum temperatures ...
2
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0answers
63 views

Is a substance's band gap related to it's flame test colour?

This is really not my field at all, but I am intruiged by the cause of different metals emitting different wavelengths of light. To my knowledge, the more energy needed to "excite" an electron, the ...
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0answers
25 views

Confusion with degrees of freedom, some say CO2 has 9, others say 6 [duplicate]

My textbook doesn't consider vibration modes when determining the degree of freedom. Some other sources I have found don't either. But wikipedia, other sources, and even other users here apparently ...
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0answers
19 views

Can you derive a formula for any internal energy of a gas as long as you know the degree of freedom?

I have the following formula that I derived from studying my textbook that generalizes to all ideal gases: Monoatomic molecular gases ($\ce{He, Ne}$ etc.) Degree of freedom: 3 $\ce \frac{3}{2}nRT = ...
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1answer
71 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 ...
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1answer
31 views

“Exactly Equal” and “At Least” in electron excitation

When we examine IR spectra, we see troughs corresponding to absorption at exactly a specific frequency that corresponds to the energy needed to stretch certain bonds (although translational motions ...
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0answers
20 views

Difference between bulk free energy and cohesive enrgy

I am writing a lammp's code to calculate size dependent surface energy of FCC copper nano particles. First of all I need to calculate total bulk energy of each atom in the crystal. my question is that ...
4
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2answers
360 views

How does an energy input actually change a molecule?

I'm reading an online biochemistry textbook for fun, and a question that has bothered me on chemistry before popped up again. The question here is not specific for the chemical process described, but ...
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1answer
43 views

How can you explain the energy change in this phenomenon?

Tagged is the diagram of the apparatus used in this thought experiment. Pure water is taken in one side of the beaker and a solution of common salt in water in the other. After a very long time, the ...
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0answers
17 views

Temperature when greater work leaving than heat entering

A reaction takes place in an open system where q is entering the system, work is leaving the system and the magnitude of q is less than w. The question (among others) is would the temperature of the ...
0
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2answers
38 views

Are electron orbitals in a helium atom degenerate?

In a hydrogen atom, the presence of only one electron allows various orbitals' energy states to be dependent only on the principal quantum number and not on angular momentum. Orbital degeneracy and ...
0
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1answer
16 views

Finding the energy given out when 2 moles of reactants are mixed and ignited from a balanced equation

The question I am trying to answer is this: I only know about Hess' law, bond enthalpies and enthalpies of combustion and have tried to use all concepts in order to somehow get an answer but none of ...
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2answers
22 views

Lattice Energy (enthalpy)

I know that when making a bond the reaction is exothermic (positive) and breaking a bond the reaction is endothermic (negative). Lattice enthalpy or lattice formation enthalpy is defined as the ...
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1answer
72 views

Internal energy calculation

The temperature of 1 mole of a liquid is raised by heating it with 750 joules of energy. It expands and does 200 joules of work, calculate the change in internal energy of the liquid. I want to ...
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2answers
70 views

derivation of enthalpy internal energy relationship [closed]

A question asks us to derive a relationship between the enthalpy change and the change in internal energy for a reaction. Now, I know the relationship is: $\Delta H = \Delta U + V \Delta p + p \Delta ...
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0answers
41 views

Difference between enthalpy and internal energy for a reaction

Zinc metal reacts with dilute nitric acid according to the equation: Zn + 2(HNO3) = H2 + Zn(NO3)2 calculate the difference between the delta H and delta U for this reaction at a temperature of 298 ...
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2answers
75 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, ...
0
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1answer
47 views

Energetics of Bond formation

Back to chemistry after 2 months and got stuck in the beginning! That's why I don't like it. Anyways, come to the question please. Following are the words from my textbook (Don't ask the name ;-) ...
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2answers
96 views

How much energy is stored when H+ is converted to H2?

I'm doing a write-up for this for my students: Source: Amirav Research Group (Facebook) The energy of formation of isopropyl alcohol is $-318.2~\mathrm{kJ/mol}$ (kilojoules per mole), that of ...
7
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1answer
93 views

Ballistic behavior of molecules on potential energy surfaces

I came across the following sentence in a publication$^1$: "At this point we have to remember that the computed MEP path [sic] represents the reaction in the absence of any kinetic energy, while in ...
3
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2answers
111 views

How does a catalyst accelerate both forward/reverse reactions simultaneously?

A catalyst is known to speed up both forward/backward reactions of a reversible reaction. But how does this work, because the mechanism for the forward and backward reactions could be different right? ...
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0answers
49 views

If chemical reactions occur because everything tends to a lower energy state, why is there such a thing as equilibrium? [duplicate]

It just doesn't make any sense, if a reaction occurs because the reactants go into a lower energy state, then there should be no such thing as equilibrium. right? Because that would mean that the ...
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0answers
20 views

How to calculate the energy of each electron transiction level , bohr

What does the Z on -2,18x10^18xZ^2 / n^2 means? I want to calculate the energy of each level on the transiction of the electron using that formula .
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0answers
29 views

Born Haber Cycles - Calcium Chloride [closed]

I'm really confused about this aspect of Born Haber cycles. Why is the lattice enthalpy of CaCl3 more exothermic than CaCl2 when CaCl2 is actually more stable? I thought that the more exothermic a ...
1
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1answer
44 views

Changes of Internal Energy un open system

My professor says the internal energy of a system can be calculated as $U=n T c_v$. Now I always thought that we could only calculate a change in internal energy $\Delta U = n c_v \Delta T$. ...
2
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0answers
26 views

What is the correct order of subshell energies? [duplicate]

I was taught in high school that the order is given by the Aufbau Principle: 1s>2s>2p>3s>3p>4s>3d>4p>5s>4d>5p>6s>4f>5d>6p... Why is the 3d subshell higher in energy than the 4s subshell if it is in a ...
0
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0answers
38 views

Physical basis for activation energy

So I'm doing a lab where we are calculating the k constant and graphing it as a first order reaction. Meaning the y-axis will be $\ln k$ and the x-axis will be $\frac 1T$. We got our k constant and ...
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1answer
107 views

Does humidity increase oxygen levels in the air we breathe? [closed]

I'm testing a new form of meditation and it requires certain breathing exercises in order to generate a specific effect on the nervous system; I have noted significant differences in the results being ...
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0answers
41 views

Particle in an Infinite Well and Particle in a finite Well

a) What conditions give rise to quantisation of energy in the the particle in a finite well model? b) State the symmetry condition or conditions on the resulting stationary wavefunctions ...
5
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1answer
204 views

Energy profile of cyclohexene conformations

The energy profile for cyclohexane flipping looks like this: How does the corresponding diagram for cyclohexene look like?
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36 views

Entropy as “extent of energy dispersal” vs. entropy as defined by the Clausius equation

Currently, the concept of entropy as "energy dispersal" has been proved to be more easy to understand than the concept of "disorder". While the first can be applied successfully to explain the ...
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0answers
49 views

Does internal potential energy affects temperature or only kinetic energy?

If I take an extreme case, where a body has only an internal potential energy with zero internal kinetic energy, does this body have a temperature? Another question related to it: if two objects A and ...
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0answers
20 views

Find dE dissolution for salt

Part of my lab is to find the dEdissolution for a series of salts. I am told that dE ~ dH and a table which looks like this: Unfortunately in class we have never been given a lattice energy or ...
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2answers
97 views

How much energy is there in 1 mol of diamond and how can you get it?

Diamond is made of carbon which is during maybe 3 billion years under pressure is formed in the earth. Imagine you found a big source of diamonds such as coal...how much is the energy of the diamond ...
5
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1answer
72 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 ...
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0answers
39 views

Why are hydrogenation reactions exothermic?

I learnt that all reactions that yield hydrogen are endothermic (such has reforming) and reactions, that use up hydrogen, are exothermic (FCC cracking, hydrogenation, etc.). But why is this so? I know ...
3
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2answers
292 views

Why 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 ...
2
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0answers
39 views

Using electron affinity to calculate lattice energy

I am trying to calculate the lattice energy of $\ce{CaBr2}$, I am uncertain of how to incorporate the electron affinity value provided for $\ce{Br2}$ into my calculation. Since electron affinity is ...
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0answers
19 views

Chemical energy + energy of electrons

The amount of energy released in a chemical reaction is equal to variation of the enthalpy of this system (In general). The energy released is usually is in form of light and heat. When light is ...
3
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2answers
101 views

What is the qualitative definition of enthalpy in beginner terms?

So in my coursebook it says enthalpy is a property that accounts for the increase in internal energy and the accompanying work done by the system and that its equal to the sum of the internal energy ...
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1answer
61 views

Positive vs. negative heat of solution confusion

Given that I have absolutely zero formal (or informal) education in chemistry (sorry about that), I'm struggling to get my head around what appears to be repeated confusions of the term "heat of ...
5
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1answer
163 views

How does ice melt?

I know that when water freezes, it forms a crystalline structure created by the hydrogen bonds between polar water molecules. Also, I know that during a phase change, the potential energy of the ...
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0answers
57 views

Calculating energy change for calcium carbonate decomposition

The $\Delta H^0$ for the decomposition of $\ce{CaCO3(s)}$ is $178.3~\mathrm{kJ/mol}$ at $25~\mathrm{^\circ C}$ and $1.000~\mathrm{atm}$. What is the $\Delta E$ for the decomposition process? ...
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1answer
174 views

What factors effect the energy change in a reaction?

The reaction of zinc metal and copper sulfate solution produces copper metal and zinc sulfate solution. What factors affect how much energy is given out for this reaction? So far I have:The mass of ...
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1answer
41 views

Calculate the heat of a reaction

Say I would like to calculate the heat produced by the following reaction: $$\ce{2SO2 + O2\longrightarrow2SO3}$$ I have calculated the moles of the reagents. I can't seem to figure out how to ...
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0answers
105 views

How to prove that the forward and reverse reactions have the same rate at equilibrium?

I was trying to answer a question from Zumdahl and Zumdahl's Chemistry textbook which asks me to show that the rate constant K is related to the forward and reverse reaction constants: ...