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Questions tagged [enthalpy]

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

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1answer
2k views

Calculating enthalpy change for a real gas

Calculate $∆H$ for $1$ mole of real gas undergoing change of state from $\pu{10^5 Pa}$, $\pu{300 K}$ to $\pu{2e5 Pa}$, $\pu{600 K}$. The values of real gas constants are $a = 0$ and $b = \pu{50 mL/mol}...
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364 views

How do you calculate the delta H for multiple reactions?

I am having difficulty in a question: Given that 1. $\ce{N_2(g) \rightarrow 2N(g), \Delta H = 941 kJ/mol}$ 2. $\ce{N_2(g) \rightarrow N_2^+ + e^-, \Delta H = 1501 kJ/mol}$ 3. $\ce{N(g) \rightarrow N^+...
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When to use internal energy and enthalpy in balances

I am confused as to when you use ΔU and when to use ΔH, I don’t really understand the difference. Is it that ΔU is used for closed systems whilst ΔH is used for open systems?
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Experimental measurement on the ring-strain

for my thesis i calculated the ring-strain of a molecule by MP2 on the computer. I was wondering if there is an easy possibility to get the experimental value of the ring strain. I guess I have to ...
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1answer
360 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^{'}_{p}...
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Sign of enthalpy for exothermic and endothermic reactions

In Cambridge Chemistry Coursebook [1, p. 94] it’s written that A rise in temperature is given a positive sign. So the value of $\Delta H$ is negative for an exothermic reaction. A fall in temperature ...
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1answer
351 views

How can there be an enthalpy of formation for gaseous water?

Under standard conditions( 1 bar and 298.15 K), water will be in the liquid state. So how can you find the enthalpy of formation for gaseous water at standard conditions(as found in the back of all ...
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1answer
958 views

Why do we use pV term independent of U in the equation H = U + pV? [duplicate]

I have read the following definition of enthalpy in my textbook: A substance has to occupy some space in its surroundings depending upon its volume ($V$). It does against the compressing influence ...
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1answer
107 views

thermochemistry problem confusion

I can understand upto where the ideal gas equation is used to calculate volume of steam. But on trying to find out $ \Delta V$ , i saw that there are no gases on the reactant side so then $P\Delta V=(\...
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1answer
1k views

Insoluble salts and Gibbs free energy

Is it true that according to gibbs free energy equation, insoluble salts such as AgCl don't dissolve because the process would be too endothermic? The right side of the equation, TΔS, would have to be ...
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1answer
907 views

How to calculate the enthalpy of reaction using Gaussian 09?

I'm having a hard time trying to understand what's the correct way to calculate the enthalpy of reaction given that I've already calculated the frequencies of my compounds in Gaussian. Below I show ...
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Why is creating water exothermic?

I have to calculate the bond dissociation energy of steam. I'm a total noob, so don't go too hard on me. (I'm translating the exercise by myself, sorry if there is a mistake.) Calculate the bond ...
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1answer
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Enthalpy of neutralization of strong acid and strong base differing from enthalpy of formation of water from ions

If the neutralization between strong acid and bases has enthalpy of neutralization of around $\pu{-57.1 kJ mol^-1}$, why is it that when I try to calculate the enthalpy of formation of water from $\ce{...
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What factors contribute in making a solution more endothermic?

For example, a solution between potassium nitrate and water is more endothermic than sodium nitrate in water. Can someone explain?
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113 views

Oxidation of glucose

By oxidation of 9 g of glucose it is released 150 kJ of heat. Calculate the standard enthalpy of derivation of glucose (in kJ/mol), if the given enthalpies of the reactants are ΔfH (H2O(l)) = -285,8 ...
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Why are negative enthalpies becoming more positive described as decreasing?

This website and my lecturer have describe negative enthalpies that are becoming less exothermic as decreasing. A relevant diagram from the linked to website to illustrate what I mean: I see the ...
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1answer
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Where can I find the total enthalpy and heat of dissolution in water of a given components?

I don't really have a background in chemistry but I'm actually studying in aerospace engineering, sorry if this question may seem trivial. I am interested in the decomposition process of the hydrogen ...
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2answers
219 views

How can change in Gibbs free energy ever be positive?

I have a question regarding this formula for Gibbs free energy of a system: $$\Delta G=\Delta H-T\,\Delta S$$ According to the second law of thermodynamics: $T\,\Delta S\ge\Delta Q$ and to my ...
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1answer
55 views

Predicting the temperature of an object using the heat of its surroundings after a certain amount of time?

how would you predict what the temperature of something would be if it spends a certain amount of time exposed to heat from its surroundings? is this possible? ex. what temperature would a hot ...
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Why is ΔH (enthalpy)=Σreactants-Σproducts and not Σproducts-Σreactants? [duplicate]

I was told that ΔH=Σreactants-Σproducts in regards to finding the enthalpy, given average bond enthalpies. Why is the case? It seems counterintuitive, because of the delta, which would imply ΔH=...
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1answer
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Finding Gibbs energy at different temperatures

How can I find $∆G$ at a higher/lower temperature if I am given $∆G°$ and $∆H°$ $\pu{298 K}$, and a reaction equation (solubility)?
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1answer
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Calculating the enthalpy of polymerisation of ethylene given the bond strengths

Given the average bond dissociation enthalpies of a $\ce{C-C}$ bond (say $x$) and a $\ce{C=C}$ bond (say $y$), find the enthalpy of the following polymerisation reaction (per mole of ethylene): $$\ce{...
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239 views

Anomalous trend in formation enthalpy of alkali metal halides

I read that in alkali metal halides, the formation enthalpy for fluorides become less negative as we go down the group, whereas the reverse is true for chlorides, bromides and iodides. Why is it so? ...
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1answer
360 views

The sign of enthalpy of formation of magnesium oxide

I'm currently doing a lab to calculate the enthalpy of formation for $\ce{MgO}$. However at the moment me and my lab partner are having a disagreement. We've both calculated and agreed upon the same ...
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1answer
491 views

Would ionic compounds dissolve in methanol?

Question: Determine whether liquid hexane ($\ce{C6H14}$) or liquid methanol ($\ce{CH3OH}$) is the more appropriate solvent for the substances grease ($\ce{C20H42}$) and potassium iodide ($\ce{KI}$)....
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What really are the units of enthalpy change? [duplicate]

A reasonable definition of standard enthalpy change of formation would be: 'The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions' ...
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2answers
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Is Kirchoffs law valid at all pressures?

$$\Large H_{T_\mathrm{f}, p}=H_{T_\mathrm{i},p}+\int_{T_\mathrm{f}}^{T_\mathrm{i}}c_p(T)dT$$ This can be derived by integrating: $$C_v =\left(\frac{\mathrm{d}U}{\mathrm{d}T}\right)_V$$ Applying this ...
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1answer
989 views

Why are there two contradicting formulas for Enthalpy (Delta H): [duplicate]

1) Delta H = Bonds broken - Bonds Formed 2) Delta H = $H_{products} - H_{reactants}$ In the first formula, bonds break in the reactants and form in the products, so its basically saying "bonds ...
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How do the pressure acting on the reactants and the initial temperature of the reactants affect the enthalpy of reaction?

In an exothermic reaction that occurs under constant pressure, does the pressure acting on the reactants affect the enthalpy of reaction? Does the initial temperature of the reactants affect the ...
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1answer
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Is the bond dissociation enthalpy of Cl-F greater than that of Cl-Cl?

In my book, it is mentioned that $\ce{Cl-F}$ has greater bond dissociation enthalpy than $\ce{Cl-Cl}$. Is it true? I know that interhalogen molecules usually have weaker bonds than dihalogen molecules ...
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2answers
184 views

Why don't we consider VΔP when we define Q?

We only define $Q = \Delta U + W_\text{exp}$ (expansion work = $-P\Delta V$). If heat can cause $\Delta U$ and work, why work is defined only as expansion work in the first place where there are other ...
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923 views

What is V times dP?

Work is defined when there is a change of volume. Mathematically, $W=-\int P\, dV$. When pressure is constant $W= P\, \Delta V$. For an ideal gas expansion we have $W= nRT\ln\frac{V_2}{V_1}$. If its ...
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3answers
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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}^\...
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1answer
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What is the difference between enthalpy of reaction and standard enthalpy?

What is the difference between enthalpy of reaction ($\Delta H$) and standard enthalpy ($\Delta H^\circ$)? I was told that the standard enthalpy of reaction is the change in heat when one mole of ...
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1answer
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Estimation of change in enthalpy of a gas phase reaction using bond enthalpies [closed]

Estimate ∆H for the following gas phase reaction: $$\ce{C2H4 + 3 O2 ⟶ 2 CO2 + 2H2O}$$ I have been having trouble on this problem for a long time, and I was hoping someone would help me with it. I ...
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1answer
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Enthalpy of formaton [closed]

today I was doing a set of HW problems regarding enthalpy and stumble upon one problem that I found confusing.It was the standard enthalpy of formation for CaCO3 is -1207.6 kJ mol when 30.7 g of ...
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2answers
786 views

Expansion under isobaric adiabatic condition

A 3 mole sample of a triatomic gas at $\pu{300 K}$ is allowed to expand under isobaric adiabatic condition from $\pu{5L}$ to $\pu{40L}$. The value of change in enthalpy is: $\pu{12.46 KJ}$ ...
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Why 'Enthalpy change' (Delta H) is equal to 'Heat transfer at constant pressure' (Qp)? [closed]

Why 'Enthalpy change' (∆H) is equal to 'Heat transfer at constant pressure' (Qp)? ∆H = ∆U + ∆pV, here only expansion work done by the sustem is added. If non expansion work is done on the system then ...
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4answers
287 views

How do energy changes in reactions avoid violating the conservation of momentum?

As mentioned in this question, the idea of chemical potential energy explains how the thermal/kinetic energy of particles can change in a reaction; energy is taken in / released by the bonds broken / ...
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Shouldn't enthalpy be constant measured at constant volume?

Shouldn't enthalpy be measured at constant volume? Then the work done on and by the gas is 0 meaning that the temperature change is caused solely by heat released or absorbed by the system. You'd be ...
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1answer
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Acetone Enthalpy & Observations

I was looking at the enthalpy of various liquids using WolframAlpha and noticed that at 30 °C, while water and ethanol have a positive enthalpy, acetone has a negative enthalpy. If I'm understanding ...
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4answers
362 views

Is it possible to freeze water by dissolving a salt?

Theoretically, by dissolving a salt in water the melting point lowers, approximately 1.86 K kg/mol, making it more difficult to freeze water. However, the process of dissolution of certain salts is ...
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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 ...
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2answers
374 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 ...
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1answer
863 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 ...
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0answers
161 views

Change in enthalpy and entropy for the formation of nickel tetracarbonyl

I was working on an enthalpy/entropy problem and I had a question on predicting the signs of some reactions. For the reaction $$\ce{Ni (s) + 4 CO (g) -> Ni(CO)4 (g)}$$ my textbooks says that ...
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0answers
786 views

What is wrong with my enthalpy calculations?

I need to calculate the enthalpy of combustion for butane using bond formation energies. Thus I first balance the equation: $$\ce{C4H10 + 6.5 O2 -> 4 CO2 + 5 H2O}$$ Then, I calculate the total ...
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1answer
26k views

Calculating the heat of formation of propane given its enthalpy of combustion

Given the equation $\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}$ and that enthalpies of formation for $\ce{H2O (l)}$ is $\pu{-285.3 kJ/mol}$ and $\ce{CO2 (g)}$ is $\pu{-393.5 kJ/mol}$, and the enthalpy of ...
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1answer
2k views

Why can we use standard entropy when determining temperature at which a reaction becomes spontaneous?

In this chemistry problem, $\Delta H$ and $\Delta S$ are given at $298\text{ K}$, and it asks for the temperature at which the reaction becomes spontaneous: For the reaction $2\text{SO}_3 \...
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1answer
782 views

Can absolute enthalpy values be used to determine reaction enthalpy?

Our teacher added this question to an exam, and there was a huge disagreement with my classmates and teacher about the correct answer. One side favoured answer 2, the other side favoured answer 4 ...

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