Questions tagged [physical-chemistry]

The study of chemical systems using the laws and concepts of physics. This usually requires the techniques of thermodynamics, statistical mechanics and quantum mechanics.

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21 views

Help to kick out the anomaly in proving $Q$ = $K_\text{eq}$

Consider a hypothetical bidirectional reaction , $\ce{aA + bB <=> cC + dD}$ (A , B are reactant(s) and C , D are reaction product(s)) , As we know the following expressions $Q=\frac{P_C^c.P_D^d}...
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0answers
22 views

Why does the voltage of a cell depend on its chemical elements?

I know that the standard reduction potential of $\ce{Zn^{2+}}$ to $\ce{Zn} $ is $\pu{-0.76 V}$ and the one of $\ce{Ca^{2+}}$ to $\ce{Ca}$ is $\pu{-2.87 V}.$ But on the atomic level, why does this ...
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4answers
114 views

Formula for rate constant for the first order reaction

My lecturer mentioned that the formula for the rate constant $k$ for the first order reaction is $$k = \frac{2.0303}{t_{1/2}}\,\log\frac{[\ce{A}]_0}{[\ce{A}]_{t_{1/2}} - [\ce{A}]_0},\tag{1}$$ where $...
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22 views

Why aren't all the fluorescence transitions the same wavelength?

If all the excited molecules at higher S1 vibrational states rapidly relax to the lowest S1 vibrational state, why do the fluorescence transitions from v' = 1, v' = 2, v' = 3, and other states to v&...
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23 views

Why is there a mirror image effect in the absorbance and fluorescence spectra?

I've read some answers online but I still can't seem to understand the mirror image effect. Why is the highest energy absorption (v" = 0 to v' = 4) the lowest energy fluorescence (v' = 4 to v&...
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0answers
34 views

Finding rate constant from a change in pressure experiment

I measured the change in pressure for the decomposition of hydrogen peroxide in the presence of a manganese(IV) oxide catalyst at 5 different temperatures: $$\ce{2 H2O2 (l) ->[MnO2 (s)] O2 (g) + 2 ...
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1answer
32 views

Decline in current during electrolysis

I observed a sharp decrease in the current when I carried out the electrolysis of water. Just to set the context, I used a voltage of $\pu{12V}$ and the concentration of my electrolytes were $\pu{0.15 ...
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0answers
22 views

Effect of electrolytes on the rate of evolution of hydrogen [closed]

Why does the concentration of electrolytes like $\ce{Ca^{2+}}$ and $\ce{Sr^{2+}}$ only affect the current density rather than cation charge or mobility of the ion?
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2answers
59 views

Concept of Equivalent Weight [duplicate]

I'm having a hard time understanding what equivalent weight means physically. I've looked at many articles on Google but the definition always seems nebulous to me. For example, if the equivalent of $...
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0answers
9 views

Estimating enthalpy change of vaporization using bond strengths

If we were to consider the vaporization of liquid water to gaseous water, my instructor stated that it would be "possible to estimate the enthalpy of vaporization based on differences in bond ...
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0answers
15 views

How are potential energy curves obtained for diatomic molecules using Hartree-Fock? [closed]

I am looking for a brief explaination on this, or a source that explains this more.
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42 views

Confusion in solution for this Question regarding mole concept

$\pu{48g}$ of graphite and $\pu{16g}$ of $\ce{O2}$ is given. The question is to count the no of molecules of $\ce{CO2}$ produced? Now, the solution in book is this No of $\ce{C}$ atoms = $\frac{\pu{...
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25 views

Equilibrium Expression [closed]

Just wanna check my answers last month. Please anybody write the equilibrium expressions of the given reactions.
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1answer
36 views

How does precipitate affect cell potential?

A cell is set up with copper and lead electrodes in contact with $\ce{CuSO4(aq)}$ and $\ce{Pb(NO3)2(aq)},$ respectively, at $\pu{25 °C}.$ The standard reduction potentials are: $$ \begin{align} \ce{Pb^...
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0answers
37 views

Rate constant at temperature

So, I don't know where I have went wrong. The question is: The initial method was used to investigate the reaction: $$\ce{2H2 + 2NO -> 2H2O + N2}$$ $$ \begin{array}{clrr} \hline & \ce{H2} &...
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37 views

What happens when solution containing two or more cations are electrolysed at an excess potential?

$\pu{250 mL}$ of a waste solution obtained from the workshop of a goldsmith contains $\pu{0.1 M}$ $\ce{AgNO3}$ and $\pu{0.1 M}$ $\ce{AuCl}$. The solution was electrolyzed at $\pu{2 V}$ by passing a ...
2
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1answer
83 views

How is Gibbs free energy related to stability? [closed]

I have been learning thermodynamics for quite some time and I fail to relate Gibbs free energy and stability. For example, how do I compare stability of metal oxides using the following data? Also, do ...
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34 views

Is it possible to solve for the equilibrium value given only the initial amounts of reactants? [closed]

At $\pu{90 °C}$ following equilibrium is established: $$\ce{H2(g) + S(s) <=> H2S(g)}$$ If $\pu{0.20 mol}$ of hydrogen and $\pu{0.1 mol}$ of sulfur are heated to $\pu{90 °C}$ in a $\pu{1.0 L}$ ...
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Factors Affecting Rates of Reaction [closed]

1-Bromopropane reacts with sodium hydroxide solution to give propan-1-ol. The hydroxide ions replace the bromine in the organic molecule: $$\ce{CH3CH2CH2Br + OH- -> CH3CH2CH2OH + Br-}$$ This is a ...
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0answers
47 views

What is the effect on Kp on doubling the volume?

Following reaction takes place at $\pu{127 ^\circ C}$ and $\pu{5 L}$ vessel, $$\ce{SO2Cl2(g) <=> SO2(g) + Cl2(g)} \qquad K_p = \pu{1 atm}$$ (a) If initially only $\ce{SO2Cl2(g)}$ was present at $...
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1answer
31 views

Can endergonic reactions be driven by heat?

I am having trouble conceptualizing endergonic reactions and their difference from endothermic reactions. An endergonic reaction is one that has a positive Gibbs free energy change, such that energy ...
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0answers
28 views

How do we know orbitals of atoms and elements exist? [duplicate]

How would we know if we have never experimentally seen them?
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1answer
31 views

Removing Heavy Rust from Very Old Carbon Steel Carving Knife on the Cheap

I am trying to remove heavy rust from a carbon steel 10-inch carving knife. Mothers Polish did not get rid of much of the rust. I turned to vinegar soaking and scrubbing: At first it took off a good ...
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0answers
19 views

Does population of vibrational states depend on population of electronic states?

We know that molecules have different energy states (vibrational, rotational, electronic). Using Boltzmann distribution we can find the popoulation of each energy state. If we have only electronic ...
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0answers
37 views

What spectrum of light does penetrate fog?

I am wondering about what spectrum of light does penetrate fog the best. I would like to know if IR light penetrate fog better than UV light and why is that. Thank you for answers.
6
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1answer
62 views

Why does the specific conductivity against concentration curve for NaCl curve more than KCl?

I understand why KCl has a higher specific conductivity than NaCl.. but I dont understand why the curvature is more significant for NaCl (KCl is a straight line). Ive provided a picture of this here, ...
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35 views

What is a possible conventional interpretation of Wilfred Hagen's puzzling Electron Paramagnetic Resonance spectrum?

In searching for a conventional interpretation, Wilfred Hagen of Delft Technical University is stumped by an Electron Paramagnetic Resonance spectrum (aka Electron Spin Resonance spectrum) in "...
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1answer
46 views

Equilibrium for the synthesis of methanol

Liquid methanol is obtained with carbon monoxide gas and hydrogen gas in a reactor at $\pu{300 °C}$ and $\pu{250 atm}.$ $K_p = \pu{9.28E-3}.$ Find $K_c.$ With only these data, do I have to use the ...
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0answers
67 views

Why is U the symbol for lattice energy?

I often see $U$ or even $\Delta U$ being the symbol for lattice energy. Why is that? Does it refer to internal energy?
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17 views

KMnO4 and Na2C2O4 solutions were used in the reactions that took place in a back titration to determine the amount of H2O2 in a sample [closed]

KMnO4 and Na2C2O4 solutions were used in the reactions that took place in a back titration to determine the amount of H2O2 in a sample. Calculate the concentration of H2O2 in the sample (w / v) as% by ...
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0answers
21 views

How much should I scan an aromatic dihedral?

Suppose I have a napthalene-like molecule (attached), and I would like to scan (relaxed) its aromatic dihedral (highlighted atoms). I used CGENFF to obtain its dihedral parameters as follows, HGA4 ...
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1answer
65 views

Why a catalyst is not consumed in a reaction when all steps are reversible?

It is said that a catalyst speeds up the rate of a reaction but is not consumed (assuming no side reactions take place). Suppose we have the following reaction: $$\ce{A + B <=> C} $$ catalysed ...
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2answers
37 views

Planes of symmetry of a molecule

Are we limited to use only xy, yz and zx planes to check for planes of symmetry of a molecule? Because I see a plane of symmetry in the following molecule through the two shown H atoms or OH groups. ...
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2answers
98 views

Total entropy change for a spontaneous reaction

For the oxidation of iron, $$\ce{4 Fe(s) + 3 H2O(g) -> 2 Fe2O3(s) + 3 H2(g)}$$ the entropy change is $ \pu{–549.4 J K^{-1} mol^{-1}}$ at $\pu{298 K}.$ In spite of the negative entropy change of ...
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1answer
50 views

Basis of mole concept? [closed]

We all have read that 1 mole of any substance contains $6.022 \times10^{23}$ atoms included in the substance. And: $$Mole=\frac{weight}{molar\,\,mass}$$ Where molar mass is gram atomic mass such that ...
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0answers
41 views

Basis of equivalence principle? [closed]

I have read in physical chemistry that for any reaction like: $$A+B \rightarrow C+D$$ Where $$Eq(A)=Eq(B)=Eq(C)=Eq(D)$$ Where $eq$ refers to equiavlaent of reactant/product. But my doubt is that what ...
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0answers
27 views

General Method For Calculation of Enthalpy Change for a chemical reaction [duplicate]

1. Enthalpy of reaction defined as the enthalpy change observed in a constituent of a thermodynamic system when one mole of substance reacts completely. 2. Enthalpy of formation defined as the ...
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2answers
114 views

Origin of electrode potential and its connection with electric double layer

Recently, I was learning about electrode processes and I came across the topic of "electrode potentials". I understood that at equilibrium, a potential difference was generated at the ...
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2answers
116 views

How to relate the conductance of an aqueous electrolyte to its conductivity

In this answer on Physics Stack Exchange, the author states that conductance $G$ is related to conductivity $\sigma$ by $$G = \sigma\frac{A}{l}$$ where $A$ is the cross-sectional area perpendicular to ...
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3answers
243 views

How to assign overlapping multiplets in 1H NMR spectra?

In the above $\ce{^1H}$ NMR spectrum ($\pu{400 MHz}$, $\ce{CDCl3}$) the methyl, alkene, and $\ce{CCl3CH2}$ protons are fairly easy to assign however, the protons in the cyclohexene ring show overlap ...
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2answers
64 views

Is it valid to calculate concentrations given equilibrium constant and initial concentration?

Suppose we have the following reaction: $$\ce{2A <=> B + C}\tag{1}$$ which can be thought the sum of the following reactions: $$\ce{A <=> B}\tag{2}$$ $$\ce{A <=> C}\tag{3}$$ with ...
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1answer
36 views

Do I have to systematically show the protons in my reaction equation to find the Pourbaix diagram's line equations?

I was trying to get the line equation for the Pourbaix diagram for the couple $\ce{Ni(OH)2}/\ce{Ni}.$ I can write two chemical equations for basic and acidic conditions, respectively: $$ \begin{align} ...
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1answer
32 views

Entropy of Dissolution of Hydrocarbons

Here is what I think I know: The entropy of dissolution reactions increases as methylene groups are added (i.e. butanol has higher entropy of dissolution than propanol). Also, acyclic saturated ...
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0answers
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Why do we omit OH- ions from H2O and acid equations [closed]

And the converse for bases why do we omit $\ce{H3O+}$ ions? Is it correct to think that when an acid is added to water it shifts the equilibrium to the right for the dissociation of water? ...
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0answers
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Is standard change in activation Gibbs free energy indepedent of pressure?

For a chemical reaction the activation volume is defined as: $$Δ^{\ddagger}V={V^\ddagger}-V$$ where both $V^\ddagger$ and $V$ refer to standard states (I have omitted the symbol for simplicity). One ...
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0answers
18 views

adiabatic reversible process problem

A compressor recieved air at To= 300 K and at pressure Po= 1 atm. At the exit, P1 = 2 atm. The process inside the compressor is (A) adiabatic and reversible (B) gas is an ideal gas (3) gamma = Cp/Cv = ...
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1answer
74 views

What is the general phenomenon taking place here?

For many cases while calculating the activity of a particular species I find them calculating like this: $$a=\frac{p_{Mn}}{p^{\circ}_{Mn}}$$ What I really do not understand is what is evaporating into ...
6
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3answers
863 views

Why is the sum of two inexact differentials exact?

From the first law of thermodynamics, $\mathrm{d}U = đQ + đW$, where $\mathrm{d}$ represents an exact differential and $đ$ an inexact differential. Exact differentials correspond to state functions ...
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26 views

Relation between Entropy, Resonance and stability

Why is it so that the continuous Delocalisation of electrons in a molecule tends to lower the total internal energy of the molecule? I understand that there is a distribution of energy in multiple ...
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2answers
2k views

Why does a 57.15% ABV spirit (ethanol+water) have a density of 923 kg per cubic meter?

According to the UK proof standard, a 100 proof spirit was defined as a spirit with gravity of $\frac{12}{13}$ that of water or $\pu{923 kg m-3}$, which is equivalent to $\mathrm{57.15\% \ ABV}$. One ...

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