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

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25 views

Determining covalent bond strengths without hybridisation

Assuming pure $\ce{2s}$ and $\ce{2p}$ orbitals of carbon are used in forming $\ce{CH4}$ molecule, which of the following statements is false? (single choice question) Three $\ce{C-H}$ bonds ...
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1answer
33 views

Chemical bonding - lattice energy

Question: Why is ionic lattice energy inversely proportional to the radius of the atom? Most heterogeneous covalent molecules are polar to some extent. The degree of polarity, or the dipole moment, ...
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1answer
25 views

Is there a possible effect of low frequency mechanical vibration on the stability of organic molecules

Is there a possible theoretical effect of mechanical vibration on the stability of organic molecules. E.g. some medicines degrade if they are not stored in a fridge, is there a theoretical ...
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2answers
123 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 ...
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2answers
70 views

Does any element have nutritional (caloric) value?

Is there any single element (e.g. possibly carbon) that can be eaten, which has nutritional value in that it provides calories? I found on Wikipedia that http://en.wikipedia.org/wiki/Dietary_element ...
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1answer
108 views

Why is the equilibrium constant wrong for this Gibbs free energy problem?

Solid $\ce{NaHCO3}$ is heated to 90°C. At equilibrium the total pressure of the gases produced is 0.545 atm. Calculate $\ce{ΔG°}$ at 90°C for the reaction $\ce{2 NaHCO3(s) ⇌ Na2CO3(s) + H2O(g) + ...
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1answer
41 views

How much heat is gained/released based off of equation?

Using this equation, $\ce{CaCl2(s) -> Ca^{2+}(aq) + 2Cl^{-}(aq)}$ and $\Delta H = - 325.0 \; kJ $ If you have 15.0 grams of the solid, how much heat is released or gained in kilojoules?
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261 views

What are some good research topics for a high school student?

I absolutely love chemistry and I really want to take part in science fairs. I am also taking an AP chemistry class. Cleaner fuel and energy sources and air pollution (especially due to smog and ...
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2answers
104 views

Does covalent character increase or decrease bond strength?

I am a bit confused 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 Al2O3 to a value ...
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1answer
37 views

What is the energy change of ethene reacting with hydrogen gas?

$\ce{CH2CH2(g) + H2(g) -> CH3CH3(g)}$ Using this table, source: http://www.kentchemistry.com/images/links/Kinetics/TB08_003.GIF ...
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1answer
56 views

How much activation energy is required to combust propane?

According to WolframAlpha, the change in enthalphy in the combustion of propane is -2220 kJ/mol. How do I calculate the activation energy required start the reaction in the first place? Do you use the ...
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1answer
45 views

What exactly happens to mass during deuterium fusion?

I know that the mass of Helium-4 is slightly less than the mass of 2x Deuterium it may be made of and that it is the reason fusion gives a lot of energy. But where exactly is this missing mass taken ...
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1answer
82 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 ...
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1answer
60 views

Is there a relation between the ionization energy and the electron affinity?

I am aware of the definition and equation of IE and EA. I was wondering if there is a direct relation between this two quantities with the energy of the neutral specie.
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1answer
62 views

Endothermic and exothermic reactions

Today, in class, I learned about the two reactions. I know connecting bond releases energy, and breaking it requires and aborb/ consumes energy. But on my reference table I, 2C+H2 yielding C2H2 ...
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1answer
66 views

When a thin lead sheet is hammered, what happens to internal energy

When a thin lead sheet is hammered, what happens to internal energy, does it increase, decrease or remains the same or first increases and then decreases?
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1answer
40 views

Could this be a feasible fuel system?

First off, please forgive me, I am a chemistry novice! Overall Process Fuel for the system is distilled water Split water into Hydrogen and Oxygen (electrolysis or similar process) Exhaust Oxygen ...
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1answer
121 views

Activated Complex Products/Reactants

We know that an activation complex is the temporary unstable compound formed when activation energy is reached. But what dictates whether the products are indeed going to be products or if they become ...
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1answer
42 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 ...
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1answer
84 views

Thermodynamic stability of benzene ring

In learned that when benzene undergoes hydrogenation, $208KJ$ of energy is given off and thus $152KJ$ less energy than what would be given off if Kekule's structure was correct. My confusion is that ...
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1answer
118 views

Difficult calorimetry problems?

I figured out how to do all of the calorimetry problems except for the last few, which are the hardest of the bunch. Assume there is no heat exchanged with the environment. A. 1250 kJ of energy are ...
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1answer
68 views

Intuitive understanding of standard electrode potential

I have read many books on standard electrode potential and they define the term in various ways but I just can't seem to get an intuitive idea of it... $Cu^2+ 2e^- \longrightarrow Cu$ Standard ...
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2answers
109 views

How do I distinguish internal energy and enthalpy?

I am finding it hard to distinguish between the two. Can someone explain the two terms and the difference between them? I tried learning from Wikipedia, but it said for both the above terms that it ...
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1answer
1k views

Enthalpy change in an adiabatic system

This is a Chemistry GRE Question. Which of the following must be true for adiabatic processes? $C_v = C_p$ $\Delta H = 0$ $\Delta U = 0$ $\Delta S = 0$ $q = 0$ The answer is q = 0. From what I ...
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1answer
48 views

Where do the electrons not in the S1 orbital get this “extra energy” from?

In this video, Sal Khan says that an electron reaches a higher energy state if the electron possesses more energy. So I assume that, say in the case of carbon, the electrons not in the S1 orbital have ...
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1answer
85 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 Na+ ion and energy is given off (hydration enthalpy) Likewise, ...
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1answer
81 views

When temperature is decreased, why do reactions occur at all?

I admit that my knowledge of collision theory may be lacking, but, as I understand it, when particles collide, a reaction will not occur without overcoming the activation energy. That being said, as ...
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1answer
60 views

Does minimizing ANY type of energy ALWAYS predict a state of equilibrium?

My intuition says no, but I am having trouble coming up with concrete examples. I know that minimizing Gibbs free energy predicts a state of equilibrium, while minimizing kinetic or internal energy ...
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2answers
93 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 : ...
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1answer
346 views

Volume of gas needed to heat air in room

How much methane gas ($\text{cm}^3$) is required heat $150~\text{m}^3$ of air by $10~^\circ\text{C}$? Please help! I thought this wouldn't be too difficult but I've been stuck for at least an hour ...
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0answers
49 views

X-ray fluorescence: where do expelled electrons go?

I'm studying the XRF method cause I'm about to use a WDXRF spectrometer at college so I'm getting all the information about the physics and chemistry behind it. I'm just curious about one thing, as ...
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0answers
38 views

Reversibility, and ratio of products and reactants

I'm reading a book on molecular biology but the chemistry part is getting me confused. If more reactants are added to a reversible reaction, the reaction will proceed toward products until an ...
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2answers
101 views

Can a battery powered by iron and air really provide a feasible power source for automobiles?

I'm referring to the Shuttle Battery recently announced (read more on it here) that suggests that: The energy density of the device is up to 7,700 Wh/L. It is more than 5 times higher than ...
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1answer
132 views

thermochemistry - energy and enthalpy of different substances in water

I conducted an experiment in which I added Sodium Nitrate, Potassium Nitrate, Potassium Chloride, and Sodium Hydroxide - all separately into water - then measured the temperature change of each. The ...
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1answer
195 views

Is it customary to express heat loss as a negative number?

In my chemistry class, we had a problem akin to the following: If an object with mass bla and specific heat bla is placed into a bla g calorimeter with an initial temperature of bla and a specific ...
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1answer
267 views

How should I calculate the heat capacity of a calorimeter in this problem?

I've looked at the other similarly worded questions but I can't seem to make heads or tails out of them. So here's my case. I need to find the heat capacity of the calorimeter (in Joule/degree ...
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0answers
23 views

Characteristics of a methane combustion chamber? (direct injection)

While I've found several combustion chamber descriptions, it's been impossible to find anything about a methane one (direct injection). Could anyone please point me to any article or resource where I ...
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1answer
121 views

Activation energy clarification

This is probably a very small clarification I seek, but it has been bothering me from quite a while. In the Arrhenius rate equation $$k=Ae^{-\frac{E_a}{RT}}$$ (where $E_a$ is the Activation energy), ...
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26 views

Why do s orbitals have lowest energy? [duplicate]

My chem textbook says s orbitals have lowest orbital energy, and then p orbitals and so forth. But since electrons on s orbitals have the most Zeff, shouldn’t s orbitals have highest orbital energy ...
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1answer
86 views

Number of $\pi$-electrons in an energy state

I am learning quantum chemistry and I am trying to understand the statement in a .pdf I'm reading which states "The energy transition associated with the spectral transitions observed is: $$\Delta E = ...
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1answer
148 views

Storing kinetic energy in bonds

Let's assume a setup with a static linear molecule with three identical atoms connected by bonds and a single atom, identical to the other three, being shot at the molecule. Let's also assume that ...
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2answers
5k views

EZ-Water - Fraud or breakthrough?

Recently the quack medicine folks online have been promoting the research of a certain Dr. Gerald Pollack who claims to have discovered a "forth phase of water", and who has recently published a book ...
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1answer
1k 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.
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1answer
134 views

How can I calculate the energy of a solution by knowing all of it's elements?

How can I calculate the energy of a solution by knowing all of it's elements? For example: If I have 2 liters of salting water $\mathbf{NaCl_{(aq)}+H_2O_{(l)}}$ contains 1.5 g of salt, how can I ...
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73 views

energy efficiency, transformation from chemical to thermal energy, for district heating plants

If I have a system in which I introduce 100e.u.F (energy units) derived from chemical energy (i.e. fuel) and generate thermal energy to heat water what are e.u. absorbed by water? I.e. the efficiency ...
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0answers
66 views

bond energy calories

I am attempting to optimize interactions for a series of random and test molecules to a protein using docking software. The software uses QM/MMGBSA to calculate free energy in joules (yes it is not a ...
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3answers
110 views

Can natural gas appliances run on Hydrogen?

Can natural gas appliances run on Hydrogen? If so, does the appliance have to be modified and how significant of a modification would it need? Thanks
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3answers
6k views

How can melting point equal freezing point?

I don't understand how melting point = freezing point. For example, if liquid water freezes at 0*C how can ice also melt at 0*C?
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2answers
175 views

Create my own solar cells?

How would I grow my own and build functional solar cells? I've read and researched that they're mostly a crystal structure of something like silicon. I would assume I'd be able to grow my own crystals ...
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225 views

Software Bond Dissociation Energies, Bond Energy and Hybridization with Polarity

I want to calculate BDE to measure the cost of hybridization of two molecules. There are many examples of protein ligand docking on the internet but there are no examples of hybridization of ligand to ...