A thermodynamic state function describing the total energy content of a system.

learn more… | top users | synonyms

0
votes
0answers
20 views

Determining Enthalpy of reactions using different reactions

So I've been stuck on this question for a while regarding enthalpy changes. I can't seem to figure it out as I've been trying for hours. Here's the question: Instructions: In this activity, calculate ...
2
votes
1answer
120 views

Stability of beryllium carbonate

Beryllium carbonate is reported to be unstable under normal conditions and should only be kept in a atmosphere of $\ce{CO2}$. Why and how this stability is maintained? Why does this compound decompose ...
0
votes
1answer
18 views

Molar enthalpy vs Enthalpy of formation?

(Note I've refocused a previous unanswered question into a more general question. You can find the original much more specific question here) There seems to be two main ways of looking at the enthalpy ...
-1
votes
0answers
24 views

The enthalpy of water molecule (distilled water)

Is the enthalpy of water less than that for individual hydrogen and oxygen molecules? And is the bond energy of distilled water sufficient enough to be used as a fuel in vehicles if the bonds are ...
0
votes
0answers
25 views

Calculate ΔH∘rxn for this reaction using standard enthalpies of formation

Propane ($\ce{C3H8}$) burns according to the following balanced equation: $\ce{C3H8(g) + 5O2(g) -> 3CO2(g) + 4H2O(g)}$ The standard enthalpy of formation of gaseous propane is ...
6
votes
1answer
108 views

What is the proper term for lines of constant volume and constant enthalpy on a thermodynamic diagram?

I know an isotherm is a line of constant temperature. Isobar is a constant pressure line. Isentrope is a line of constant entropy. But what do you call a line of constant volume? An Isochore? And ...
3
votes
1answer
32 views

Is it possible to calculate specific heat capacity of a compound based on its elements' heat capacities?

What the question says. For example, if I knew the specific heat capacities of lithium and oxygen, could I work out the specific heat capacity of lithium oxide with no further information?
1
vote
1answer
23 views

Is enthalpy of a reaction meaningful at variable pressure?

Why should the enthalpy of a reaction be defined at constant pressure. Why can't it be defined at variable pressure?
3
votes
0answers
55 views

Negative and Zero Enthalpy?

I've been investigating chemical kinetics and stumbled across the CHEMKIN program manuals and the JANAF tables. In the original CHEMKIN database they list 'standard enthalpy'. I've checked and this is ...
0
votes
1answer
26 views

Temperature Dependent Specific Heat of Nitrogen

Calculate the change in enthalpy as 1kg of nitrogen is heated from 1000K to 1500K, assuming the nitrogen is an ideal gas at a constant pressure. The tempreature dependent specific heat of nitrogen is ...
5
votes
1answer
101 views

Specific heat capacity question - mass of iron needed to melt ice and raise temperature of water?

My answer was that you needed 25041.67J to melt the ice and 16108.4J to raise the temperature to 22 degrees. Therefore $mc \Delta T(iron)$ = -(25041.67 + 16108.4) and dividing this by the mass of ...
5
votes
2answers
74 views

Can negative (abnormally) lattice enthalpies during the formation of ionic compounds exist?

Why do compounds form bonds? I've got three words for you: Less potential energy. I don't remember a case of positive lattice enthalpies $\ldots$ - My comment This makes me wonder. I remember ...
6
votes
2answers
225 views

How can I figure out whether a reaction is endothermic or exothermic without its enthalpy value?

I already know the way to determine whether if the given reaction is exothermic or endothermic by the enthalpy values. But is it possible without enthalpy values, and just by looking at the reaction? ...
3
votes
0answers
28 views

Reaction-enthalpy for synthesis of 40 g hydrazine

Problem: Calculate the reaction-enthalpy for the synthesis of 40 g hydrazine (rocketfuel): $$\ce{4 NH_3 (g) + Cl_2 (g) -> N_2 H_4 (l) + 2 NH_4 Cl (s)}$$ Attempt at solution: First we have to ...
7
votes
1answer
74 views

How is it possible for a substance to have a high heat of vaporization but a low boiling point?

The final paragraph of Dissenter's question here is worthy of standing alone: [H]ow does one square a high heat of vaporization with a low boiling point? If it takes a lot of energy to vaporize ...
3
votes
1answer
54 views

Temperature dependence of equilibrium constant

On looking over the usefulness of quantity enthalpy change, I found that it can also be used to calculate temperature dependence of equilibrium constant, and I was wondering how could it be done, it ...
0
votes
1answer
36 views

How to calculate the enthalpy change of water?

How do I calculate the $\Delta H$ of the reaction where $2.6\:\mathrm{g}$ of water, $C_s= 4.184 \mathrm{\frac{J}{g\:K}}$ is heated, raising the temperature increases from $298\:\mathrm{K}$ to ...
10
votes
0answers
90 views

How different concentration of HCl on magnesium affect the enthalpy change

I had conducted an experiment where I reacted a $0.3\ \mathrm{g}$ strip of magnesium with $100\ \mathrm{mL}$ of varying concentrations of $\ce{HCl}$: $1\ \mathrm{mol/L}$, $2\ \mathrm{mol/L}$ and $3\ ...
5
votes
1answer
54 views

Enthalpy and Bond Dissociation Energies

I'm not sure if I understand the reasoning behind this particular question. The sigma and pi bonds of $\ce{C=C}$ have a combined bond dissociation energy of 632 kJ/mol. Using this information, ...
2
votes
1answer
58 views

What is the enthalpy of formation of 1 mol of sulfuric acid?

In the contact process, the commercial process of producing $\ce{H2SO4}$, several reactions occur before the final product is produced: $\ce{S8_{(s)} +8O2_{(g)} -> 8SO2_{(g)}}~~~~~~~~~~\Delta ...
1
vote
1answer
25 views

What is heat of vaporization? How can it be used at temperature as low as 25 °C?

When I first learned this term. I was introduced to this graph: The heat in this case is at 100 °C, which I had ever since believed to be the only temperature of vaporization, assuming vaporization ...
2
votes
1answer
70 views

Is there more than one equation for Delta T(reaction)?

So, I have to find the $\Delta T_{\mathrm{reaction}}$ of a solution for my lab report. Earlier in the lab, it gave the equation as $\Delta ...
0
votes
2answers
87 views

If the temperature of a solution decreases, what is the sign of the enthalpy change?

Here is what seems to be a straightforward question: you dissolve some ammonium chloride in water at $25\ \mathrm{^\circ C}$, lowering the temperature of the solution. Is the change in enthalpy for ...
2
votes
1answer
57 views

Plotting ΔG vs Temp for Electrochemical Reaction

I’m doing an electrochemistry lab that requires me to construct various galvanic cells in an attempt to solve for $\Delta H$, $\Delta G$, and $\Delta S$ values of an unknown. During one part of the ...
5
votes
2answers
80 views

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 $353~\mathrm{K}$ and the enthalpy of ...
0
votes
0answers
63 views

How do I convert J to kJ/mol?

I'd like to convert from J to kJ/mol. I understand how to convert between J to kJ, however, I don't know which amount of moles I'm supposed to use from the reaction. Products? What if I have two ...
1
vote
1answer
45 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\ ...
1
vote
1answer
37 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}\ ...
4
votes
1answer
95 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 ...
2
votes
1answer
165 views

What is the enthalpy of combustion for ethene given the values of enthalpy of formation?

I have answered this question and got a value of $\mathrm {1409 \space kJ\ mol^{-1}}$ however, the answer sheet says $\mathrm{-1009}$. Using the equation: $$\ce{C2H4 + 3O2 -> 2CO2 + 2H2O}$$ I ...
0
votes
1answer
82 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 ...
2
votes
1answer
85 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 ...
-2
votes
1answer
46 views

What is the enthalpy change for the following equation? [closed]

The equation we need it to change to \[\ce{FeO + CO ->Fe + CO2}\] The three equations and delta heat: \[ \begin{align*} \ce{3Fe2O3 + CO &-> 2Fe3O4 + CO2} &\Delta H=-47.0\ \mathrm{kJ}\\ ...
2
votes
2answers
92 views

Are all naturally occuring reactions exothermic?

If a reaction is endothermic, then the reaction must take energy from the environment to occur (as far as I know), and if this is the case, then to me the reaction seems energetically unfavorable, as ...
0
votes
0answers
15 views

Why is it that reactions that cause an increase of temperature are exothermic? [duplicate]

I've been taught that if you have two aqueous reactants that react together and produce heat it is exothermic. Shouldn't that be endothermic? Did my professor misspeak? If it's gaining heat, that ...
4
votes
2answers
189 views

Where to find data for gibbs energy, enthalpy, and entropy digitally

Where can you find the enthalpy and gibbs free energy of formation and entropy of compounds either as a data file or web api (I technically only need two since I can figure out the other one from two ...
2
votes
1answer
123 views

Determining the Average Bond Enthalpy for the C-F bond

Given the following data, how would you work out the average bond enthalpy for C-F bond. I've tried setting up the chemical equations and applying Hess's Law, but that's not getting me anywhere. ...
0
votes
1answer
25 views

How do you figure out the dissociation energy for ions?

Specifically $\ce{KF}$ into the $\ce{K+}$ and $\ce{F-}$ ions. I can find tables on the DE into atoms, but where would I go to find the DE between ions? I am also given that the bond length is ...
3
votes
2answers
82 views

How to calculate the enthalpy change for the formation of ethylene epoxide from ethylene?

Question 4, iii.) in here: (iii) The equation for an important reaction of ethene is given below. I get everything up until the very end. We need to identify the bond enthalpies first: ...
2
votes
2answers
53 views

How to determine the sign of the enthalpy of formation?

During the formation of $\ce{Al2O3}$ from 5.4 grams of $\ce{Al}$ and enough oxygen, the temperature of 2 kg of water climbs by 20 degrees. What’s the enthalpy of formation of $\ce{Al2O3}$ (per mole) ? ...
3
votes
1answer
41 views

Trying to figure out heat of reaction

A question on a textbook is to find the "standard heat of reaction" of this reaction: $\ce{C8H16N2O3(aq) + H2O(l) = C6H13NO2(aq) + C2H5NO2(aq)}$ As far as I understand it, it should be a rather ...
4
votes
3answers
115 views

Relationship between thermodynamic reversibility and reactions

Often we're concerned with changes in thermodynamic properties during reactions, like $\Delta H_p = Q_{reaction}$ (change in enthalpy for an isobaric process). What seems odd to me is that in the ...
1
vote
1answer
19 views

Ideal gasses reaction under const temp and pressure

Conceptual problem: let $A + 2B \rightarrow C + 2D$ be a reaction under constant pressure (P) and temp (T), that goes all the way towards the products. A,B,C,D are ideal gases. let $\Delta H$ be the ...
5
votes
1answer
149 views

Why is enthalpy of atomization of Mn exceptionally low?

Transition elements usually have high values, especially so for greater number of unpaired electrons. Why the exception here?
3
votes
1answer
92 views

Can a reaction with positive Gibbs free energy change ever occur?

If you have an endothermic reaction with a negative entropy change, is it still possible to induce the reaction in some way despite the fact that the Gibbs free energy change is positive or all ...
0
votes
0answers
88 views

How to measure the enthalpy change of hydration?

I am learning about lattice enthalpies, and my text book talks about determining the lattice enthalpy from the standard enthalpy change of solution and the enthalpy change of hydration of the ions, ...
-1
votes
1answer
50 views

Helium gas expand isobaric isothermal

If 4 mol helium gas expand against constant pressure in isothermal process. How can calculate w? Is it w=-nrtLn(v2:v1) as define by isothermal or is it w=-p(v2-v1)as define from isobaric? Also, is ...
4
votes
1answer
60 views

How to calculate bond enthalpies from first principles?

Is it possible to calculate bond enthalpies using the periodic table and the molecule's geometric structure? I am interested in a way of calculating this without getting the answer from measurements ...
1
vote
2answers
81 views

Gibbs free energy or reaction enthalphy

If I conduct a reaction say, $$\ce{CaO(s) + H2O(l) \rightarrow Ca(OH)2 (aq)}$$ let us say , I want to measure the energy generated from the reaction.Will this energy I calculate equal the change in ...
7
votes
2answers
129 views

What is an example of a very low temperature endothermic reaction?

Shakashiri describes the reaction of solid Ammonium Thiocyanate and solid Barium Hydroxide Octahydrate resulting in an endothermic reaction leading to subzero temperatures. I've performed this ...