ENTHALPY ? Concept Undestanding help

[kirsten_2009]

ENTHALPY ? Concept Undestanding help

Homework Statement

Hello, I am studying about enthalpy and I don't need any homework help but what I do desperately need is someone to explain to me how it works. I don't understand what enthalpy is and what the relationship is between heats of reactions at constant pressure, volume and temperature and work. As far as I know enthalpy is heat of a reaction under constant pressure and that no work is done under constant volume...but then I see practice questions in the textbook such as:

1. A 1.00g sample of Ne (g) at 1 atm pressure and 27 degrees Celsius is allowed to expand into an evacuated vessel of 2.50 L volume. Does the gas do work? Explain.

2. In each of the following processes, is any work done when the reaction is carried out at constant pressure in a vessel open to the atmosphere? If so, is work done by the reacting system or on it?

a.) Neutralization of Ba(OH)2 by HCl
b.) Conversion of gaseous nitrogen dioxide to gaseous dinitrogen tetroxide
c.) Decomposition of calcium carbonate to calcium oxide and carbon dioxode gas.

3. The internal energy of a fixed quantity of an ideal gas depends only on it's temperature. A sample of an ideal gas is allowed to expand at a constant temperature...

a.) Does the gas do work?
b.) What happens to the temperature of the gas?
c.) Does the gas exchange heat with it's surroundings?
d.) What is the change in internal energy (DeltaU) for the gas?

Homework Equations

DeltaH = DeltaU + P x DeltaV

DeltaH = q_p

DeltaU = DeltaH - P x DeltaV

The Attempt at a Solution

Like I said though, I don't need the answers, I have the correct answers I just need the conceptual reasoning behind them? Any help is truly appreciated! :)

 

[chemisttree]

Homework Statement

Hello, I am studying about enthalpy and I don't need any homework help but what I do desperately need is someone to explain to me how it works. I don't understand what enthalpy is and what the relationship is between heats of reactions at constant pressure, volume and temperature and work. As far as I know enthalpy is heat of a reaction under constant pressure and that no work is done under constant volume...but then I see practice questions in the textbook such as:

1. A 1.00g sample of Ne (g) at 1 atm pressure and 27 degrees Celsius is allowed to expand into an evacuated vessel of 2.50 L volume. Does the gas do work? Explain.

2. In each of the following processes, is any work done when the reaction is carried out at constant pressure in a vessel open to the atmosphere? If so, is work done by the reacting system or on it?

a.) Neutralization of Ba(OH)2 by HCl
b.) Conversion of gaseous nitrogen dioxide to gaseous dinitrogen tetroxide
c.) Decomposition of calcium carbonate to calcium oxide and carbon dioxode gas.

3. The internal energy of a fixed quantity of an ideal gas depends only on it's temperature. A sample of an ideal gas is allowed to expand at a constant temperature...

a.) Does the gas do work?
b.) What happens to the temperature of the gas?
c.) Does the gas exchange heat with it's surroundings?
d.) What is the change in internal energy (DeltaU) for the gas?

Homework Equations

DeltaH = DeltaU + P x DeltaV

DeltaH = q_p

DeltaU = DeltaH - P x DeltaV

The Attempt at a Solution

Like I said though, I don't need the answers, I have the correct answers I just need the conceptual reasoning behind them? Any help is truly appreciated! :)

Have you seen a relationships like:

W = PV ln(Vf/Vo) for dT = 0 and

W = P dV for dP = 0 ?

 

[kirsten_2009]

Hello,

No I have not.

 

[chemisttree]

Tell us what relationships that involve work you are familiar with. You need to start there to answer questions like, "Does the gas do work?"

 

[DeeNos]

Enthalpy is a thermodynamic property that represents the total heat content of a system. It is often denoted as H and is defined as the sum of the internal energy (U) of the system and the product of the pressure (P) and volume (V) of the system. In other words, enthalpy takes into account both the internal energy of a system and the work done by or on the system.

In a reaction at constant pressure, the change in enthalpy (DeltaH) is equal to the heat exchanged (q) at constant pressure. This is because in a closed system, the only way for energy to be transferred is through heat or work. Therefore, at constant pressure, the change in enthalpy is equal to the heat exchanged because no work is done.

In the first practice question, the gas is expanding into an evacuated vessel, meaning there is no external pressure acting on the gas. In this case, the gas is not doing any work because there is no opposing force to push against. Therefore, the change in enthalpy is equal to the heat exchanged.

In the second practice question, all three reactions are carried out at constant pressure in a vessel open to the atmosphere. In these cases, the change in enthalpy is equal to the heat exchanged and work is also done by or on the system. In the case of neutralization of Ba(OH)2 by HCl, the reaction is exothermic and the system releases heat to the surroundings, meaning work is done on the system by the surroundings. In the conversion of gaseous nitrogen dioxide to gaseous dinitrogen tetroxide, the reaction is endothermic and the system absorbs heat from the surroundings, meaning work is done by the system on the surroundings. In the decomposition of calcium carbonate, the change in enthalpy is equal to the heat exchanged, but no work is done because the volume of the system remains constant.

In the third practice question, the gas is allowed to expand at a constant temperature. In this case, the change in enthalpy is equal to the heat exchanged, but no work is done because the temperature remains constant. This is because the internal energy of an ideal gas depends only on its temperature, and since the temperature is constant, there is no change in internal energy.

I hope this helps to clarify the concept of enthalpy and its relationship to heat, work, and internal energy.