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27 views

Two methods for calculating ΔH yield different results [on hold]

A problem in my book says: $\pu{140 g}$ of $\ce{N2}$ (perfect gas) changes from $\pu{200 K}$ to $\pu{400 K}$ at constant volume. Calculate the change in internal energy $\Delta E$ and the change in ...
0
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1answer
43 views

Calculating bond energy given the enthalpy value of burning reaction

The burning enthalpy (exothermic) of $\ce{NO +O2 ->NO2}$ is $\pu{57 kJ mol^-1}$ and $\ce{O=O}$ bond energy is $\ce{494 kJ mol^{-1}}$, then what is the bond energy of $\ce{N-O}$ ? So I balanced the ...
-1
votes
1answer
46 views

Reaction of sulphur with calcium carbonate

Can sulphur react with calcium carbonate to form sulphur trioxide and calcium carbide, as the energy of 3(S=O) is greater than ((C=O) +2(C-O))? EDIT: i forgot to say that this is in an oxygen-free ...
1
vote
1answer
70 views

Why enthalpy changes in phase transition when there is no change of temperature?

Why there is a change in enthalpy in phase transition? I'm asking because enthalpy depends only on temperature: $$\mathrm dH = C_p\,\mathrm dT,$$ and the change in temperature is $\mathrm dT = 0.$
0
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1answer
32 views

Calculating bond dissociation enthalpy of F-F bond

$$\ce{N2 + 3 F2 -> 2 NF3} \quad ΔH = \pu{-263 kJ mol-1}$$ The bond dissociation enthalpies of the nitrogen triple bond and nitrogen–flourine bond are $\pu{946 kJ mol-1}$ and $\pu{272 kJ mol-1}....
4
votes
2answers
80 views

Exothermic: enthalpy or heat?

I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat". Since change in ...
1
vote
2answers
64 views

Relation between ΔH and ΔE for the reaction in gaseous phase at constant temperature and pressure

Which of the following statements is correct for the reaction at constant temperature and pressure: $$\ce{CO(g) + \frac{1}{2}O_2(g) -> CO2(g)}$$ $\Delta H = \Delta E$ $\Delta H > \...
0
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1answer
82 views

Comparing strength of the C-H, N-H, and O-H bonds

$$ \begin{align} \ce{CH4(g) + Cl(g) &-> CH3(g) + HCl(g)} &\quad ΔH^\circ &= \pu{-14 kJ mol-1}_\mathrm{rxn}\\ \ce{NH4(g) + Cl(g) &-> NH2(g) + HCl(g)} &\quad ΔH^\circ &= \...
2
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2answers
66 views

Which substances have the highest/lowest enthalpy of formation per gram?

Which substances have the most extreme enthalpies of formation per gram at standard temperature and pressure?
1
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1answer
73 views

Heat of formation of aqueous H⁺

In my book it is given that by convention heat of formation of aqueous $\ce{H+}$ is taken to be zero. Heat of formation is energy released or absorbed when 1 mole of a compound is formed from its ...
1
vote
1answer
23 views

Overall enthalpy balance on a mixing process

Let's say I have a mixing process involving $\ce{A}$ and $\ce{B}$ at temperatures $T_\ce{A}$ and $T_\ce{B}$, respectively. The $\ce{AB}$ mix is at temperature $T_p$. Our $ΔH_\mathrm{mix}$ is defined ...
8
votes
2answers
239 views

Calculating the strength of an ionic bond that contains poly-atomic ions

So the bond association enthalpy for ionic compounds like $\ce{NaCl}$ and $\ce{NaBr}$ can be easily calculated from a Born-Haber cycle. But the way a Born-Haber cycle is constructed it uses info that ...
0
votes
1answer
67 views

What is the difference between ΔfH and H-H° in NIST–JANAF tables?

Can someone explain the difference between $Δ_\mathrm{f}H$ and $H-H^\circ$ in NIST–JANAF tables?
1
vote
2answers
101 views

Why enthalpy is defined at a constant pressure?

How absolute enthalpy and enthalpy change are defined? What is the clear difference between enthalpy and heat? Is it the constancy of pressure what makes the enthalpy a state function? My teacher ...
5
votes
1answer
129 views

Using thermodynamics to predict the acid-base character of fluoride ion in water

I was working through the end-of-chapter exercises of the acid-base chapter in Shriver's Inorganic Chemistry when I came across the following problem: 4.7. The effective proton affinity $\ce{A^{'}_{...
-2
votes
3answers
385 views

Why is the enthalpy of a reaction equal to the difference between the enthalpies of combustion of the reactants and the products?

My textbook gives me the following formula for calculating the enthalpy change of any reaction: $$\Delta H_\mathrm{r}^\circ=\sum(\Delta H_\mathrm{c}^\circ)_\text{reactants}-\sum(\Delta H_\mathrm{c}^\...
-2
votes
2answers
549 views

Difference between Enthalpy and Heat transferred in a reaction?

We know at const pressure change in enthalpy is change in internal energy plus work done. Here by taking enthalpy as heat i can easily think the situation in mind that heat absorbed is due to the two ...
4
votes
1answer
357 views

Enthalpy calculation

What is the value of enthalpy change for the following reaction $$\ce{X + 2Y -> 2Z}$$ given \begin{align} \ce{W + X &-> 2Y} &\qquad &\Delta_\mathrm{r}H_1 = \pu{-200 ...
0
votes
1answer
473 views

Entropy and enthalpy at the melting point of benzene

For the melting of Benzene, since it is melting, it feels intuitive to think that the change in entropy will be positive (more disorder) and enthalpy will be positive (exothermic). However, when I ...
6
votes
1answer
112 views

Is dH(S,P) = dH(T,P) true (always)?

I came across an interesting question with some physical chemistry students today. Based on the following steps, we're uncertain whether the statement in the title is/could be true. Assuming $dN = 0$, ...
8
votes
1answer
2k views

Why is $\Delta G = -T \Delta S_{\mathrm{total}}$ valid only at constant pressure?

According to my book(Elements of Physical Chemistry by Atkins and de Paula, 5th ed.), $\Delta G = - T \Delta S_{\mathrm{total}}$ is valid only for constant pressure and temperature. Enthalpy is ...
-1
votes
1answer
210 views

Why must NaOH be in excess if we wish to measure the effect acid strength has on the enthalpy of neutralisation?

A few different acids are given i.e $\ce{HCl}$ , ethanoic acid, $\ce{H2SO4}$, ethanedioic acid. We want to conduct an experiment in which we measure the temperature rise for neutralisation as a ...
0
votes
1answer
202 views

Why does the standard enthalpy of formation diverge so far from the standard Gibbs free energy of formation for some substances?

If you look at a Table of Thermodynamic Values for chemical substances, most substances have very close values for their standard enthalpy of formation ($\Delta H_{\mathrm{f}}^\circ$) and their ...
-1
votes
1answer
72 views

Enthalpy Change

In calculating the change of enthalpy in converting Fulminic to Isofulminic acid, would one simply take the sum of the negative enthalpies of the bonds of Isofulminic acid minus the sum of the ...
-1
votes
1answer
127 views

Can the enthalpy change of allotrope transformation can be done with Hess cycle?

My A- level textbook has some points about the uses of Hess cycle. For example: indirectly determining the lattice enthalpy, vaporisation enthalpy etc. But, my book also says it is possible to ...
1
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0answers
22 views

Finding Enthalpy [closed]

Given Information $\ce{CH_4 + 2 O_2 -> CO_2 + 2 H_2O}$ "The Heat of reaction is $-802000\, \frac{J}{mol}$ of $\ce{CH_4}$ at $25\, ^\circ C$. Rate of Moles of $\ce{CH_4}$ used is $0.625\, \frac{...
1
vote
1answer
100 views

How do I evaluate the PV term when calculating a change in enthalpy?

As we know enthalpy is defined by the equation $H = E+PV$. Now to calculate this $PV$ factor for $\Delta H$, Sometime we take $P \Delta V$ and sometime $P_2V_2-P_1V_1$. Why are there so many ...
3
votes
2answers
5k views

Difference between lattice energy and thermal stability

For an ionic compound, is it always true that the greater the thermal stability, the greater the lattice energy. E.g. for 2 ionic compounds MX and MY, if MX has a higher thermal stability than MY, ...
1
vote
3answers
448 views

Calculating standard combustion enthalpy of dodecane from that of heptane & octane

These are the problem from UKChO-2015. Given the standard enthalpy changes of combustion ($\Delta_\mathrm{c}H$) for heptane ($n = 7$) and octane ($n = 8$) are $-4816$ and $-5470~\mathrm{kJ~mol^{-1}}...
1
vote
2answers
2k views

How to find the temperature change of a solution?

Right now I'm doing a calorimetry lab and for the pre-lab we have been asked to determine the temperature change (in Celsius) of ammonium chloride in water. The question asks for the expected ...
1
vote
1answer
417 views

How do you find the enthalpy change for a given reaction?

$0.500\,\mathrm g$ of magnesium chips are placed in a coffee-cup calorimeter and $100.0\,\mathrm{mL}$ of $1.00\,\mathrm M$ $\ce{HCl}$ is added to it. The reaction is: $$\ce{Mg(s) + 2HCl(aq) -> ...
0
votes
1answer
184 views

Validating Phase Diagram of a mixture

In lab I did several different runs to create a phase diagram for the mixture between biphenyl and naphthalene. My phase diagram is attached, just for visual purposes. My question is I'm supposed to ...
3
votes
0answers
387 views

Example of an endothermic reaction that can sustain itself for at least an hour?

Recall that in physical chemistry, the spontaneity of a reaction at constant pressure and a given temperature $T$ can be quantified by the Gibbs free energy $$\Delta G = \Delta H - T \Delta S$$ A ...
0
votes
3answers
433 views

Enthalpy of a reaction changes with temperature, does this mean bond strengths change with temperature?

During chemical reactions, the bonds between atoms break or form to either absorb or release energy. The result is a change to the potential energy of the system. The heat absorbed or released from a ...
1
vote
3answers
3k views

What is the correct definition of hydration enthalpy and why is it always negative?

The first definition of hydration enthalpy I found on the net is: The hydration enthalpy is the enthalpy change when $1$ mole of gaseous ions dissolve in sufficient water to give an infinitely ...
3
votes
1answer
341 views

Discrepancies between specific heat of vaporization of water and enthalpy of formation of water and steam

According to Wikipedia, the enthalpy of formation of water is $-285.8~\mathrm{kJ/mol}$ while the enthalpy of formation of steam is $-241.818~\mathrm{kJ/mol}$, implying the following: $$\ce{H2O(l) -&...
1
vote
2answers
713 views

Internal Energy vs Enthalpy

I struggle to see the difference between internal energy and enthalpy, or at least the need for differentiation between the two. I understand that enthalpy, supposedly unlike internal energy, accounts ...
2
votes
1answer
3k views

Is enthalpy of activation the same as activation energy? Is there a purely mathematical way to find that value?

I asked about the effect of pressure on a reaction, and since then I learned a lot more. One of the things I learned was the Eyring equation: $$\kappa = \frac{\kappa_B T}{h} e^\frac{\Delta^\ddagger S°...
0
votes
1answer
93 views

For what is reaction enthalpy more relevant than reaction free energy?

I know enthalpy is a component of the Gibb's Free Energy (along with entropy). For practical purposes, though, isn't Gibb's Free Energy what a chemist is actually interested in, as that determines the ...
1
vote
0answers
528 views

Why is the specific heat of ethanol not representing experiments

I am in my last year of high school completing an assignment hot and cold packs, and particular the enthalpy behind them. My major lack of understanding is why my calculations are working for heat of ...
1
vote
2answers
503 views

Understanding Gibbs free energy and enthalpy

Let's say that a reaction releases -50kJ/mol of Gibbs free energy (delta G), and -75kJ/mol of enthalpy (we know that -25kJ/mol went to make sure that the total entropy of universe increased). Now let'...
0
votes
2answers
775 views

Enthalpy and Internal Energy in Constant Volume Calorimeter

It is my understanding that enthalpy differs from internal energy only in that the change in enthalpy takes into account any pressure-volume work done on the system (or by the system) in a reaction, ...
6
votes
5answers
1k 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 ...
1
vote
2answers
7k views

Relationship between enthalpy change and endo/exothermic reactions

Something that confused me, is there a relationship between enthalpy change and endo/exothermic? Does region "x" represent the enthalpy change? What represents enthalpy change, kinectic energy, ...
13
votes
1answer
783 views

How is enthalpy change equal to heat supplied at constant volume?

We know that: $$\begin{align} H &= U + pV \\ \Delta H &= \Delta U + \Delta (pV) \end{align}$$ Now, If $p$ is constant, $$\begin{align} \Delta H &= \Delta U + p\Delta V \\ &= q_p + ...
10
votes
3answers
1k views

How to calculate an enthalpy change for this reaction

I've got an assignment to solve problem number 4 in this worksheet, that is to calculate change in enthalpy for reaction: $$\ce{CHCl3 + O -> COCl2 + HCl}$$ How do I solve it? Why is oxygen $\ce{O}$...
1
vote
2answers
699 views

(P1,T1) -> to (P2,T2) then what will be the change in enthalpy for ideal gas?

I need to prove that it is equal to the integral of $C_pdT$ from $T_1$ to $T_2$. But should not be the case only when the pressure is constant?
1
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0answers
682 views

How to calculate enthalpy change of neutralisation - what mass?

How to calculate enthalpy change of neutralisation - what mass? I made an experiment where I reacted 100ml HCL 2M with 100ml NaOH 2M in 1300ml Water, and got a temperature change dT. My question is ...
1
vote
0answers
128 views

Energy required to break salt bridges in peptides and proteins

Is there a way to calculate the energy required to break a salt bridge in a peptide or on the exposed surface of a protein in aqueous solution? Is there any useful literature on this topic? I'm ...
2
votes
2answers
2k views

Enthalpy change and equilibrium

I don't understand why enthalpy change won't change at an equilibrium? The reason I ask this is because of this question: At first I thought the answer is C, but I realised that the amount depends ...