Questions tagged [enthalpy]

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

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How do you determine whether is it [products] - [reactants] or [reactants] - [products] when calculating the enthalpy change of a reaction?

In the reaction above, knowing the enthalpy of formation of each individual compound, how do I determine whether it is: [products] - [reactants] or [reactants] - [products] To calculate the standard ...
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Which of the following reaction will have greater enthalpy and why? [closed]

Which of the following reaction will have greater enthalpy and why? $\ce{C (g) + 4 H (g) -> CH4(g)}$ (reaction enthalpy = $x\ \mathrm{kJ/mol}$) $\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ ...
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Need help understanding Enthalpy

My chemistry textbook says that: Enthalpy (Internal Energy) = chemical potential energy + thermal energy Now, I know that Enthalpy and Internal Energy are different so I really don't understand ...
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how to compute enthalpy of combustion for methanoic acid [closed]

My data source gives the following values for various enthalpies of formation and combustion: formation of CO2 = -393.5 formation of HCOOH = -425 combustion of HCOOH = -254.3 Wikipedia gives ...
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Enthalpy of reaction between ammonia and oxygen

I have to ask this here because I asked literally everybody I know and I still disagree with them. Using the information in the table below, calculate the heat of released from the following ...
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Enthalpy of formation for Lithium ion in ethylene carbonate

The following link has the enthalpy of formation for aqueous lithium http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf I am wondering how to get the enthalpy of ...
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How do you find the bond enthalpy from an equation?

Which equation represents the $\ce{N-H}$ bond enthalpy in $\ce{NH3}?$ $$ \begin{align} &\textbf{A.} &\ce{NH3(g) &-> N(g) + 3 H(g)}\\ &\textbf{B.} &\ce{1/3 NH3(g) &-> ...
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1answer
38 views

Ground state elevation for phase transition

I have a question about the illustration from chem.libretexts.org: the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, ...
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1answer
32 views

Is the heat for constant pressure the same as for constant volume in the formation of nitric oxide?

In the reaction $\ce{N2 (g) + O2(g) -> 2NO (g)}$ my textbook[1] says $q_v$ and $q_p$ are not the same, without any explanation. I know that \begin{align} \Delta U &= q_p + w\\ \Delta U &=...
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How to calculate the temperature change when CaCl2, NaCl, and water added together at an initial temperature of 298 K?

My question: How to calculate the temperature change when adding $\pu{0.1 kg}$ of $\ce{CaCl2}$ and $\pu{0.1 kg}$ of $\ce{NaCl}$ to $\pu{1 kg}$ water with an initial temperature of $\pu{298 K}$? I ...
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Calculating ΔH with non-integer coefficients in the balanced equation

I am working a practice problem out Ebbing's General Chemistry and for part of the problem I needed to calculate $ΔH.$ The problem considers the burning of ethane in oxygen to yield carbon dioxide and ...
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Enthalpy Changes [closed]

0.50 g of Mg is added to a 50 mL HCl solution. The temperature increased by 5.2 degree Celsius. Calculate the enthalpy change for the reaction per mole of Mg that reacted.
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What does higher electron gain enthalpy mean here?

While talking about ionic bonds my book states : Higher the value of electron gain enthalpy of the atom, greater the ease of formation of the anion from it, i.e., other atom should have high value ...
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How to get the enthalpy of a reaction from the enthalpy of formations with Hess' law?

Let's consider a generic reaction: $$\sum_{i=0}^{i=I}n_{i}A_{i} \rightarrow \sum_{j=0}^{j=J}m_{j}B_{j} $$ where $n$ and $m$ are the stoichiometric coefficients, while $A$ and $B$ are reagents and ...
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Thermochemical data for thiocyanatoiron(III) ion

I am currently writing a lab report about the formation constant of $\ce{FeSCN^2+}.$ All I could find online was a formation constant at $\pu{25 °C}$ $(890),$ which was 70% higher than the ...
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Why when deriving the Clapeyron equation can you assume that dH = dq, as surely the pressure is not constant?

Part of the Clapeyron equation involves replacing $\Delta S$ with $\Delta H/T$. I can see the step from $\Delta S$ to $\Delta q/T$ but I'm not sure why you can replace $\Delta q$ with $\Delta H$. From ...
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Is it possible to use the laws of termodynamics for non-ideal systems?

For the reaction $$\ce{2CO(g) + O2(g) -> 2CO2(g)}\quad\Delta H = \pu{−560 kJ}$$ it is given that the reaction occurs at constant volume of $\pu{1L}$ and the initial and final pressures are $\pu{...
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Heat involved in isenthalpic process

Question 28 from [1, p. 4.4]: An ideal gas is expanded irreversibly from $\pu{5 L}$ to $\pu{10 L}$ against a constant external pressure of $\pu{1 bar}.$ The value of heat involved $(q)$ in this ...
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Is enthalpy of combustion the 'heat released' or 'enthalpy change'?

In a constant volume calorimeter, $\pu{3.5 g}$ of a gas with molar weight $\pu{28 g}$ was burnt in excess $\ce{O2}$ at $\pu{298 K}.$ Temperature of calorimeter increased from $298$ to $\pu{298.45 K}$ ...
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Change in enthalpy equation [closed]

If the formula for change in enthalpy is ΔH=ΔU+pΔV, and ΔU = q - pΔV then shouldn't the formula be simplified to ΔH = q regardless of whether the volume or pressure is constant? Is this only true when ...
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Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?

I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow. Specifically, here are IUPAC'...
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Enthalpy and heat relationship

They say that enthalpy of reaction is the heat when at constant temperature and pressure. So if enthalpy is $$\mathrm dH = \mathrm dU + p\,\mathrm dV = T\,\mathrm dS - p\,\mathrm dV + p\,\mathrm dV + ...
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Enthalpy change of water [closed]

A given amount of (liquid) water in an open vessel is heated from 20 degree celsius to 50 degree celsius. Suppose it absorbs x joule in the process. What is the enthalpy change in the process? Is it = ...
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Why do alkyl substituents stabilize alkenes?

My textbook states that because $sp^2-sp^3$ bonds are stronger than $sp^3-sp^3$ bonds, alkenes are more stable than alkanes. The rule given is that bonds with more $s$ character are stronger. However,...
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Chemistry 12: Calculating changes in enthalpy [closed]

I was wondering if I could please get some help with this: In a coffee-cup calorimeter $\pu{100.0 mL}$ of $\pu{1.0 M}\ \ce{NaOH}$ and $\pu{100.0 mL}$ of $\pu{1.0 M}\ \ce{HCl}$ are mixed. Both ...
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How to find the Cp, difference of Cp and heat of fusion from the dependence of vapor pressure on temperature?

The vapor pressure of zinc varies with temperature as $$\log P(\pu{mm Hg}) = -6850/T - 0.755\log T + 11.24 \label{eqn:1}\tag{1}$$ and that of liquid zinc as $$\log P(\pu{mm Hg}) = -6620/...
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Spontaneity of an exothermic chemical reaction

When the process is exothermic (ΔHsystem​<0), and the entropy of the system increases (ΔSsystem​>0), the sign of ΔGsystem​ is negative at all temperatures. Thus, the process is always spontaneous. ...
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Why can't calorimetry be used to measure fast reactions?

As my previous post was a bit unclear and unspecific, I am reformulating it. First of all this is a question that I had while reading some concepts my book introduces, so this isn't a homework ...
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I was told that the more exothermic the solution, the more soluble the salt. Is this statement wrong?

I was also told that $$\Delta H(\text{solution}) = \Delta H(\text{hydration}) - \Delta H(\text{lattice E})$$ However, for $\Delta H(\text{solution})$ to be exothermic, it has to have a negative value ...
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Computing enthalpy of formation of dissolved gas from its enthalpy of dissolution

Provided that I know the enthalpy of formation $\Delta H_f^0(X_{g})$ of a substance $X$ in gas phase and its enthalpy of dissolution $\Delta H^0_{sol}(X)$ in the same conditions, considering that the ...
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What is total enthalpy?

I've seen that it's defined as $h_0 = h + \frac{v^2}{2}$, where $h_0$ is the total enthalpy per unit mass. I can see that the $\frac{v^2}{2}$ term probably has something to do with kinetic energy, but ...
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How do you find the heat capacity ratio of combustion products?

For example, if gaseous oxygen $(O_2)$ and paraffin wax $(C_{32}H_{66})$ were to stoichiometrically combust at STP, how would one go about finding the heat capacity ratio ($\gamma={c_p \over c_v}$) of ...
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Infinite Increase in Entropy when Energy added to Absolute Zero

My textbook states the following: If a system were at absolute zero, an additional small amount of heat energy would lead to an infinite increase in entropy. Such a state is impossible. Absolute ...
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Facing a problem in enthalpy calculation

If 1 mole of gaseous carbon atoms are converted into diamonds, calculate enthalpy change of process given that bond enthalpy of $\ce{C-C}$ bond is $\pu{400 kJ mol-1}.$ According to me, the answer ...
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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 ...
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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 ...
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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 $\mathrm dT = 0.$
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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}....
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I can't find reaction enthalpy ∆H for water formation reaction

$$\ce{1/2 O2(g) + H2(g) → H2O (l)}$$ The tabulated value is $\Delta H = \pu{−285.8 kJ/mol}$ I'm using the bond energies values for each molecule and for some reason, I am getting a different result, ...
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Question about calculating enthalpy change

$$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds made}\end{array}\right)$$ $$\Delta ...
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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 ...
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What makes determining entropy changes theoretically so hard compared to enthalpy?

It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
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Enthalpy of combustion of 1-butanol

1-butanol $\ce{C4H10O}$ So I have absolutely no idea how to find the enthalpy and I did some looking on the internet with no use since I don't understand. I came here hoping someone generous could ...
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Reaction enthalpy using bond energy in resonating molecules

To find out and enthalpy of a reaction using bond energy we use the formula $$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$ since we use all ...
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Why are Born-Haber enthalpy calculations inaccurate for transition metal compounds?

A senior high/secondary school level exam question is about discrepancies between calculated and empirical enthalpies of formation. It asks why a discrepancy “usually” exists for d-block compounds ...
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Iron(III) nitrate and hydrogen peroxide reaction

When iron(III) nitrate is added to hydrogen peroxide as a catalyst, the temperature initially decreases and begins to increase after 30 seconds. Why is this the case? (is there any intermediate ...
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Energy absorbed by a system in isothermic process in which the phase changes from liquid to gas

A system contains $x$ mole of material, with surface area $S$ is pressed by a mass $M_1$ (no fraction and no any other outer forces involves). Let $\Delta H_\mathrm{l\rightarrow g}$ be the transition ...
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Relation between equilibrium constant and entropy change

Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant? Consider the ...
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Need to normalize lattice enthalpy by no. of nearest-neighbor (NN) interactions to equate to NN interaction strength?

I've seen electrostatic interaction energy (from $E\sim \frac{q_1 q_2}{r}$) used to explain differences in lattice enthalpies between different crystalline solids. For example, I've seen it explained ...
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Thermodynamic data related to diazotization reaction

Is there any thermodynamic data available for diazotization of aniline and also for the reduction of the diazonium salt using sodium sulfite? I also want to know the heat of reaction for both the ...