Questions relating to heat and its relation to other forms of energy in chemistry.

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How can I work out the entropy change of benzene when it changes from vapour into liquid?

I'm stuck on a particular problem which I can't work my head around: Information has been given that :The boiling point of benzene at atmospheric pressure is 353 K and the enthalpy of vaporization of ...
2
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2answers
36 views

Does the third law of thermodynamics apply to all compounds and elements?

According to wikipedia, the third law of thermodynamics is "The entropy of a perfect crystal, at absolute zero (zero kelvins), is exactly equal to zero.". Then, theoretically, could all substances, ...
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Is ln(K2/K1) = (∆h°rxn/R)(1/T1 - 1/T2) even when ∆h°rxn varies strongly with temperature?

In all of my physical chemistry books I find the same expression for estimating the equilibrium constant of a reaction at a non-standard temperature. The derivation starts with $R \ln K_\text{eq} = ...
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2answers
30 views

Given a forward reaction with positive enthalpy, is its reverse reaction be endothermic or exothermic?

I'm pretty sure the sign changes. But, I want to be sure if it becomes endothermic or exothermic.
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1answer
30 views

Why do exothermic dissolution reactions occur?

I've always been taught that an exothermic dissolution means that the forming of solute-solvent bonds releases more energy than the energy consumed to break solute-solute bonds and solvent-solvent ...
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0answers
13 views

How much ice will reduce the chicken carcass temperature from 32 to 8 degree C

I am interested to learn through heat claculations the quantity of ice that would achieve a chicken carcass core temperature of 8 degree C. My chilling tank has a volume of 15,000 liters of flowing ...
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0answers
15 views

Approximation with Gibbs Free Energy (Kp or Kc?) [on hold]

I have a question about the following relation : $\Delta G^{\circ} = -RT\ln K$. I am aware the $K$ (equilibrium constant) is given in activity, however approximations using either concentration $Kc$ ...
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1answer
32 views

Predict Entropy based on the Balanced Equation

You cannot use values but must determine from looking The equation I have trouble with is as follows: $$\ce{2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2}$$ According to my teacher the entropy is ...
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2answers
64 views

why everything goes in the direction of positive entropy change? [duplicate]

I read about entropy from different sources. Still don't get why (universal) entropy change has to be greater than zero for irreversible process. Is it a result of pure observation or is there any ...
4
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2answers
52 views

What is the difference between ΔG and ΔG “with an o on top”?

I want to know the difference between $\Delta G$ and $\Delta G$ "with an o on top"? I couldn't figure it out. Ex: $\Delta G^\circ = -RT\ln K$ $\Delta G = \Delta G^\circ + RT \ln K$ What is the ...
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40 views

Why is it that increasing the molar mass generally increases the Entropy?

I've just found read that entropy increases through these reasons: Changing the phase of the elements to a gas. Increasing the molar mass of that specific element increases the entropy. I ...
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1answer
21 views

Hess's Law - enthalpy of combustion

The enthalpy of combustion of a solid carbon to form carbon dioxide is $-393.7\ \mathrm{kJ/mol}$ carbon, and the enthalpy of combustion of carbon monoxide to form carbon dioxide is $-283.3\ ...
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1answer
152 views

Exo- or endothermic? Clarification required regarding a few cases

Are the following processes exothermic or endothermic? a) When solid $KBr$ is dissolved in water, the solution gets colder. b) Natural gas ($CH_4$) is burned in a surface. c) When concentrated ...
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0answers
11 views

Why do we include the mass of a sample in the total mass of the solution in thermodynamics? [closed]

Why,in thermodynamics problems, we include the mass of a sample such as NH_3 in the total mass despite we use the specific heat of water in our calculations of the solution heat. We usually deal with ...
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1answer
42 views

How to determine whether the enthalpy of solution is positive or negative by calorimetry?

How can we determine if the enthalpy of solution of a compound is positive or negative? In order to make a clear idea about what I mean, take the following problem: A 3.5 g sample of $\ce{NH4NO3}$ ...
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0answers
56 views

Is time a measurement of Entropy?

Is time a measurement of entropy? Is it because time is increasing forever? Well after reading about the Second Law of Thermodynamics and working with Entropy, I began to realize that if time can be ...
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1answer
22 views

Gibbs Free Energy of Solutes and Solutions

Using the equation $\ce{CaSO4 (s) <=> Ca^2+ (aq) + SO4^2- (aq)}$, and $\Delta H/\Delta S$ values, calculate $\Delta G$ at 50 degrees Celsius when the solution is saturated with $\ce{Ca^2+}$ and ...
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2answers
56 views

What causes spray cans to get cooler when shaken?

I've been searching the web for this phenomenon and surprisingly I found only one post about it. And the reply to this question about canned air begins with words: I don't know about the shaking ...
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3answers
1k views

Why does water evaporate spontaneously at room temperature despite dG > 0?

Standard Gibbs free energy of formation of liquid water at 298 K is −237.17 kJ/mol and that of water vapour is −228.57 kJ/mol. Therefore, $$\ce{H2O(l)->H2O(g)}~~\Delta G=8.43~\mathrm{kJ/mol}$$ ...
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1answer
31 views

How to determine the bond enthalpy of the phosphine chlorine bond? [duplicate]

How to determine the bond enthalpy of P-Cl in the equation: $$\ce{PH3 + 3HCl -> PCl3 + 3H2}$$
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1answer
23 views

Ways to change the spontaneity of a reaction

I can think of any other way to influence the value of $\Delta G$ than (1) change the temperature (2) providing an external source to overcome the negative cell potential The relationship between ...
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0answers
25 views

How do tie-line slopes affect raffinate/extract envelope in ternary phase diagrams?

I am struggling to understand ternary phase diagrams for reactor engineering. The course slides offer little to no help so I'm hoping to find an answer to this question How does the slope of the ...
2
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1answer
24 views

Relating Delta E (U) to Delta H question

The question is For which exothermic reaction is $\Delta\,E$ more negative than $\Delta\,H$? (A) $\ce{Br2(l) <=> Br2(g)}$ (B) 2C(s) + O2(g) r 2CO(g) (C) $\ce{H2(g) + F2(g) ...
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0answers
9 views

Calculating activity using a different standard state

Here's a question from a thermodynamics textbook: Consider crystalline A to be in equilibrium with water saturated with A at a pressure of 1234.0 bars and a temperature of 567.0◦C. A is very ...
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1answer
54 views

Is there a way to efficiently estimate the enthalpy of formation for an organic molecule in computational chemistry?

I'm exploring the platonic hydrocarbons and one of the methods I'd like to use is to compare their enthalpies of formation. The issue is that carbon's standard form is a covalent lattice and I can't ...
2
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0answers
29 views

heat capacity and thermal equilibrium

A $52\ \mathrm{g}$ piece of iron at $250\ \mathrm{^\circ C}$ is added to a mixture of $100\ \mathrm{g}$ of ice and $100\ \mathrm{g}$ of liquid water at thermal equilibrium $(0\ \mathrm{^\circ C})$. ...
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1answer
29 views

Why does the mixture's temperature change while phase transition?

Consider a mixture made up of alcohol and water at room temperature. If I freeze this mixture, why does it change its temperature during the phase change? I don't understand the difference between ...
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1answer
23 views

Why heat of reaction under neither constant pressure or constant volume is considered (in my textbook, or probably somewhere else)?

So my textbook gives me this random equation: $\Delta U = q + w$. Then consider two conditions, under constant pressure and under constant volume. Because $w = -P\Delta V$, constant volume will make ...
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1answer
63 views

When and why do hot material take heat from cold material?

This is what I read on Chemwiki: Rudolf Clausius was a German physicist, and he developed the Clausius statement, which says "Heat generally cannot flow spontaneously from a material at a lower ...
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0answers
25 views

The concentration is unity! Why? [duplicate]

I want to know why concentration of solids and liquids are taken unity in expression of equilibrium constant( heterogenous equilibrium). It is usually mentioned that the density of solids and liquids( ...
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1answer
970 views

Why is NaCl3 possible?

There. And there. Almost a year ago, a group of scientists claimed to have reached compounds of $\ce{Na}$ and $\ce{Cl}$ with weird stochiometries ($\ce{NaCl3, Na3Cl, NaCl7, Na3Cl2}$ and $\ce{Na2Cl}$). ...
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1answer
23 views

Decomposition of hexaurea-chromic-chloride-trihydrate

I am sitting over a TG diagram of the decomposition steps of hexaurea-chromic-chloride-trihydrate ($\ce{[Cr(CO(NH2)2)6]Cl3 \cdot 3 H2O}$) and cannot really make sense of the products it might ...
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0answers
22 views

Stirring the reaction between hydrochloric acid and a powdered substance

For an experiment, I'm adding several substances (as powders) to hydrochloric acid, and measuring the temperature changes to calculate enthalpy. However when adding the powders, the mixture requires ...
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1answer
44 views

Exercising to use up energy consumed

Suppose I eat a whole $450~ \rm{g}$ chocolate Easter rabbit. The enthalpy of combustion of chocolate is $−22.4~\rm{ kJ/g}$. One kilogram of body fat contains $39000 ~\rm{kJ}$ of energy. Walking for an ...
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1answer
28 views

How to lose as little heat as possible to the surroundings from an endothermic reaction?

For an experiment, I'm adding several substances (as powders) to hydrogen chloride, and measuring the temperature changes to calculate enthalpy. What is a container I could perform it in (other than a ...
2
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1answer
42 views

Spontaneous Reaction and Gibbs Free Energy

I know how to derive the expression for the equilibrium constant starting from $dG=0$, $$\ln(K)=-\frac{\Delta G ^o}{RT}$$ However, if we are not at equilibrium, $dG$ is not necessarily zero. In this ...
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1answer
364 views

What's the source of “2.303” in Van't Hoff equation?

I've seen a "2.303" term multiplied to R in Van't Hoff equation several times. Here is one example: http://en.wikipedia.org/wiki/Van_%27t_Hoff_equation#Error_propagation ΔH0 = 2.303 R (logK1 - ...
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1answer
51 views

If Gibbs energy of activation is positive, how is the change to the transition state possible?

The Gibbs energy increases from the reactants to the transition state, but according to the Second Law of Thermodynamics, entropy of the universe can only increase. Why is the transition state ...
6
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1answer
164 views

What is non-expansion work?

I learnt that Gibbs free energy is the maximum amount of non-expansion work. But the phrase non expansion work confuses me. Work is defined as pressure times change in volume. If there is no ...
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15 views

What is the justification behind the principle of corresponding states?

Two fluids with same reduced volume and temperature have the same reduced pressure. Two fluids with same reduced pressure and temperature have the same compression factor. What justifies these two ...
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1answer
24 views

What is the significance of enthalpy definition not under constant pressure?

Under constant pressure (and only PV work), change in enthalpy can be shown to equal change in heat. If enthalpy is not meaningful outside of constant pressure, then why not use heat instead? So I ...
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1answer
29 views

The change in enthalpy of carbon in diamond form to graphite is negative .This suggests that graphite should be more stable than diamond. is it true?

What I thought was that graphite has van derr waal forces while diamond has covalent forces so diamond should be more stable. but the melting point of graphite is more than diamond which implies that ...
2
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1answer
29 views

Heat Capacity temperature dependence and Gibbs energy

I got this problem in school as a tool to get ready for International Chemistry Olympiad, but I have some problems with this challenge. So, the problem is to calculate as precise as possible the Gibbs ...
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0answers
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HHO devices for cars (hydrogen enchancement of fuel burning reaction) [duplicate]

Recently I got quite an interesting offer for increasing my fuel economy (which for my 4.0 liter gasoline engine is quite bad as is) which was an HHO device. What it basically is, as I had it ...
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1answer
39 views

Calculating Energy Change of Acetic acid [closed]

A 10.0 g sample of pure acetic acid (CH3COOH) is completely burned. The heat released from the combustion is used to warm 2.00 L of water from 22.3 ℃ to 39.6 ℃. Assuming no heat was lost to the ...
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1answer
86 views

Relative energies between reactants and products

I've been asked to draw a relative energy sketch of the reactants and the two products. The reaction being examined was adding copper acetate monohydrate to glycine in two different fashions to create ...
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1answer
73 views

How to calculate the bond energy of C-Cl?

$$\ce{C(s) + 2 Cl2 (g) -> CCl4 (g)}$$ \begin{align} \Delta{}H^\circ_\mathrm{f} (\ce{C(g)}) &= 121~\mathrm{kJ/mol}\\ \Delta{}H^\circ_\mathrm{f} (\ce{Cl (g)}) &= 716~\mathrm{kJ/mol}\\ ...
3
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35 views

Radial Distribution Function for a Hard Sphere Gas in the Percus-Yevick Approximation - Is There a Quick Way to Evaluate it Numerically?

The question is as in the title. I know that for a one-component hard-sphere gas, there is an analytic solution in the $\vec{k}$-space (PRA, 5, 939). I know that to obtain the correct solution in ...
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0answers
123 views

Calculating Vapor Pressure from Delta G/ Kp

For the process, CH3OH(l) --> CH3OH(g) ∆G° = 4.30 kJ/mol at 25 °C. What is the vapor pressure of CH3OH(l) at 25 °C in mm Hg? A) 0.176 mm Hg (B) 14.0 mm Hg (C) 134 mm Hg (D) 759 mm Hg This question is ...
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2answers
62 views

Thermochemistry: forms of energy & pressure

I've been in a few classes now that teach about energy, and I feel they're always so bad. There is always such a rush to start using equations to get problems solved, and I often feel I don't have an ...