Questions tagged [enthalpy]

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

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Why is my ethanol–water mixture showing negative change in enthalpy of mixing?

I am conducting an experiment where I mix two solutions $\text{A}$ and $\text{B}$ and then I am measuring the temperature change (for real solutions, i.e., $\Delta_\mathrm{mix}H\neq 0).$ I took the ...
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What is an example of an exothermic reaction with negative ∆S?

In high school we learn that when a reaction has both negative $\Delta{}S$ (entropy change) and negative $\Delta{}H$ (enthalpy change) it occurs spontaneously at lower temperatures, but becomes non-...
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Homemade Flash Paper [closed]

I am making homemade flash paper (nitrocellulose) and had a question regarding the safety and stability of the mixture. I have purchased a respirator, beakers, stirring rods ect so i stay safe. After ...
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Physical meaning of enthalpy change when internal pressure Is not constant [closed]

My chemistry textbook states the following "general enthalpy" formula for the change in enthalpy when the internal pressure is not constant: $∆H= ∆U + ∆(PV)$ How is the internal pressure ...
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How strongly affected is the buffering capacity of sodium carbonate - sodium bicarbonate by temperature changes?

I would like to do a lab regarding the affects of temperature on the buffering capacity of sodium carbonate - sodium bicarbonate in particular for sulfuric acid. Currently I have two competing ...
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How to calculate enthalpy of mixing for non-ideal solutions?

I have conducted a series of experiments where I basically mixed two solutions to form a non-ideal mixture and then I measured the temperature rise/fall (since both positive and negative deviation are ...
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Enthalpy/Temperature change in a chemical reaction

I was wondering how exactly absolute temperature change in a chemical reaction could be calculated. For example, consider this reaction: 3H₂+N₂→2NH₃, ΔH=-46kJ/mol (298.15K) Assuming the given reaction ...
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Confusion on when to use mass of entire solution when performing q=mc∆T

I hope you are having a good day. I am studying for the AP Chemistry exam in May 2020. While studying, I encountered a doubt. In Problem 1 (the question on the top), the educator saw that the ...
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Why does the trend of the enthalpy of reaction per atom of carbon in hydrocarbons approach a near-constant value?

I was looking at trends for enthalpies of hydrocarbons for a high school project and found an interesting paper regarding it, stating that the enthalpy per carbon atom approaches a constant. I'm ...
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Endothermic reaction with dV < 0 [closed]

I am looking for an endothermic reaction dH > 0 for which the net volume change is negative, e.g. gases reacting to form a liquid. Does anybody have an illustrative example?
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Does entropy contribute work?

I've seen that Gibbs free energy is defined as the maximum amount of work that a system can do. From this I gather, this is how much energy it will release - right? But, I've also learnt that the heat ...
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1answer
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How to compute change in Gibbs energy given change in entropy and enthalpy found over a temperature range?

I was given the following question by my tutor: The variation of enthalpy ($\Delta H$) and entropy ($\Delta S$) in a reaction carried out at constant pressure in the temperature range between $\pu{...
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At equilibrium, which one of the following is always Zero? [duplicate]

At equilibrium, which one of the following is always zero? (A)∆H (B)∆S(Total) (C)∆S(System) (D)∆G° I know that at equilibrium ∆G=0, and was facing difficulty in connecting the above mentioned ...
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Databases for Enthalpy of Solution and Ionic Radii for Ionic Salts

I am looking to investigate the relation between experimental enthalpies of solution and theoretical enthalpies of solution calculated using theoretical equations for the lattice enthalpy and ...
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Justifying assumptions about method to find equilibrium pressure for co-existence of graphite and diamond

The standard state Gibbs free energies of formation of $\ce{C(graphite)}$ and $\ce{C(diamond)}$ at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively. The conversion of ...
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Why is it sometimes kJ only, and in other times kJ/mol? What's the difference? [closed]

Here is an example in my book. Computing the enthalpy of formation $\Delta{}H_\mathrm{f}$ via multiple enthalpies of reaction $\Delta{}H_\mathrm{r}$ I notice some of the equations run by $\pu{kJ/mol}$...
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Why is the enthalpy change of an ideal gas a function only of temperature?

While teaching us thermodynamics our chemistry teacher told us that enthalpy change of a real gas is a function of temperature, pressure etc. but the enthalpy change of an ideal gas is only a function ...
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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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1answer
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Why does the heat of formation of organic radicals and positive ions decrease with their size and degree of branching at the radical or ionic site?

I am currently studying the textbook Mass Spectrometry, third edition, by Jürgen H. Gross. Chapter 2.4.3 Bond Dissociation Energies and Heats of Formation says the following: The heat of formation ...
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Where is my non-expansive work in combustion?

Consider if I burn some octane. The enthalpy released is $5430 \;\text{kJ/mol}$. The Gibbs free energy released is $5537 \;\text{kJ/mol}$. From my understanding, enthalpy is the heat change in the ...
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What's the equilibrium pressure of CO2 gas over sodium hydroxide?

Many (too many) decades ago in physical chemistry lessons, I seem to recall that given the enthalpy changes of reactions, and maybe phase diagrams, one could work out what equilibrium a reaction would ...
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Calculating ΔH from bomb calorimetry

Suppose we carry out a reaction in a bomb calorimeter whose starting temperature is $298.15\ \mathrm K$. Here we assume $\Delta V$ is close enough to zero that we consider the process to be at ...
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Why is the enthalpy of formation of 2,2-dimethylpentane larger than that of 2,2,3-trimethylbutane?

Firstly, it is commonly known and taught that branched alkanes are more stable than linear ones, and so have larger enthalpies of formation. But why is that the case? Are there some steric ...
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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?
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How to calculate the enthalpy of formation of hydrazine from tabulated values?

Estimate the standard enthalpy of formation of hydrazine, $\ce{N2H4(g)}$, from the following data. \begin{align} \Delta{}H_\mathrm{B}(\ce{H2}) &= 436~\mathrm{kJ/mol}\\ \Delta{}H_\mathrm{B}(\ce{N2})...
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857 views

Why is the zero of standard enthalpy of formation a convention?

The standard enthalpy of formation $\Delta H_f^°$ of pure elements is zero by definition. Why is that a convention? It is true that enthalpy is defined unless a constant (like energy and entropy), but ...
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Are spontaneous exothermic reactions spontaneous due to more stable products or to an increase in the entropy of the surroundings? [closed]

The lattice formation of an ionic compound from its infinitely separated gaseous ions is spontaneous under standard conditions, but I cannot figure out the 'driving force' behind this. One way to look ...
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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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Why are branched alkanes used as fuels instead of straight chain alkanes?

My textbook states that branched alkanes are thermodynamically more stable than straight chain alkanes. Of course, iso-octane has an enthalpy of formation of $\Delta H_\mathrm f$= $\pu{-259.34 kJ mol-...
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How change in potential energy corresponds to change in enthalpy?

So while reading about activation energy , I came across this graph : Here, change in potential energy of reactant and product is depicted as ${ΔH}$, but at constant pressure and temperature , ...
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How to calculate a reaction's DeltaH using a MM forcefield calculator software?

I want to calculate $\Delta H$ for hydrogenation of benzene and producing cyclohexane using MM+ Force Field Energy. I am using Hyperchem for energy calculation and optimization. I calculated the ...
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Calculating enthalpy of dissolution

In a polystyrene cup calorimeter, $4.3\ \mathrm g$ of ammonium nitrate, $\ce{NH4NO}$, was added to $60.0\ \mathrm g$ of water and stirred to dissolve the solid completely. The initial temperature ...
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Ideal gasses reaction under const temp and pressure

Conceptual problem: Let $A + 2B \rightarrow C + 2D$ be a reaction under constant pressure ($P$) and temperature ($T$), that goes all the way towards the products. $\ce{A,B,C,D}$ are ideal gases. Let $...
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Enthalpy Change

If I mixed $25.0\ \mathrm{cm^3}$ of $0.350\ \mathrm{mol\space dm^{-3}}$ sodium hydroxide solution with $25.0\ \mathrm{cm^3}$ of $0.350\ \mathrm{mol \space dm^{-3}}$ hydrochloric acid. The temperature ...
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Is the standard enthalpy of formation always non-positive?

If the standard enthalpy of formation is defined as the energy change when 1 mole of a substance is formed from its elements in their standard states and forming chemical bonds is an exothermic ...
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How to calculate what temperature change in entropy for the reaction equal to the change in entropy for the surroundings given Enthalpy and entropy

A reaction has $\Delta H_\textrm{rxn} = \pu{-107 kJ}$ and $\Delta S_\textrm{rxn} = \pu{285 J/K}$. At what temperature is the change in entropy for the reaction equal to the change in entropy for the ...
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Joules to kJ/mol conversion

In my AP Chem class we are working on testing Hess's Law and conducted three reactions. Then enthalpy changes of the 1) $\ce{NaOH + HCl}$ and 2) $\ce{NaOH + NH4Cl}$ to predict the enthalpy change of 3)...
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Why is enthalpy change = heat change only for constant pressure?

Please help me, a mathematician doing his elective course on Physical Chemistry, out with this very simple question: why does the relation between enthalpy and heat $$\Delta H = \Delta U + p\,\Delta V ...
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Is the change in enthalpy (ΔH) for dissolution of urea in water positive or negative?

To test the properties of a fertilizer, $\pu{15.0g}$ of urea, $\ce{NH2CONH2_{(s)}}$, is dissolved in $\pu{150 mL}$ of water in a simple calorimeter. A temperature change from $\pu{20.6^\circ C}$ to $\...
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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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When is the Assumption that Enthalpy is Constant over Temperature Change Invalid?

In a question, I was asked to integrate the Gibbs-Helmholtz equation to derive some formula. Then the next question asked: Identify one assumption that you have made in deriving the above equation ...
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What is the difference between ΔH = ΔU +Δ(PV) and ΔU = q + w? [closed]

Here, ΔH = Enthalpy change, ΔU = Change in internal energy, P = pressure and V = volume. I know that ΔH(Enthalpy) is heat given/taken to/from system at constant pressure. But I have been practicing ...
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Why does the C−H bond dissociation energy vary in a homologous series of primary alcohols?

Specifically, for the primary carbon atom in the alcohol. Here is bond dissociation energy (BDE) data from chapter three of Luo's Comprehensive handbook of chemical bond energies [1] (boldface refers ...
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Enthalpy in the van't Hoff equation

In the van't Hoff equation, why do we say that the change in enthalpy at standard pressure is constant? My book derives the equation for a mixture of ideal gases reacting with each other. The proof ...
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Relation between enthalpy and internal energy for chemical reaction at constant volume

Question The combustion of benzene(l) gives $\ce{CO2(g)}$ and $\ce{H2O(l)}.$ Given that the heat of combustion of benzene at constant volume is $\pu{-3263.9 kJ mol-1}$ at $\pu{25 ^\circ C}$; heat of ...
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Finding the enthalpy of formation of magnesium oxide using Hess' Law

I have the following reactions: $$\begin{align} \ce{Mg(s) + 2HCl(aq) &-> MgCl2(aq) + H2(g)}\\ \ce{MgO(s) + 2HCl(aq) &-> MgCl2(aq) + H2O(aq)}\\ \ce{H2(g) + 1/2O2(g) &-> H2O(l)}\\ \...
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Why won't water freeze if you put ice in it, despite ice being frozen?

Just now I got some room temperature water and added some ice to cool it, and it just came to me, like the scientist I wish I was ... why doesn't frozen (freezing point) ice added to higher ...
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How to find enthalpy of solvation for gas phase cation using Hess's law?

Find enthalpy of solvation $\Delta_\mathrm{solv}H$ using Hess's law, and show the expression that allows to calculate the enthalpy of solvation. $$\ce{Ca^2+(g) -> Ca2+(aq)}$$ I am not told which ...
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How can change in Enthalpy for a reaction be written like this?

If a gas expands in a container with constant external pressure then: $W=-P_{ext}\Delta V$ Instead of $P_{ext}$ if we use $P_{int}$ then we will not get the same value because $P_{int}$ is constantly ...
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Is it the solution energy or the reduction potentials (or both) that drives a galvanic cell?

I have received conflicting explanations for why the redox reaction in a galvanic cell occurs. Explanation 1: Some say that it occurs because the anodic metal dissolves more easily than the cathodic ...

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