Entalpy Change: Pressure & Temperature Effects

  • Thread starter yifan
  • Start date
  • Tags
    Change
In summary, enthalpy is a state property of a substance that varies with temperature and pressure. It is calculated using the equation H=U+PV, where U is the internal energy and P and V are pressure and volume. This means that at different temperatures and pressures, a substance will have different enthalpies, which will affect the deltaH in a reaction between two substances. However, enthalpy does not directly pertain to the energy of a chemical reaction, as the PV term relates to a separate work function. The energy of a specific chemical reaction is constant, and in most cases PV is considered negligible. Thus, the enthalpy of the reaction itself does not change, but the amount of energy released may vary at
  • #1
yifan
8
0
Does anybody know why the entalpy change for a reaction differs in different pressure and temperature?
 
Chemistry news on Phys.org
  • #2
enthalpy is a state property of a substance, like temperature and pressure
it also varies with temperature and pressure via H=U+PV
U is the internal energy, and you know what P and V are

so at different temps and pressures, a substance will have different enthalpies
this will change the deltaH in a reaction between two substances
 
  • #3
Enthalpy pertains to reactions in open systems. To be truthful, enthalpy does not directly pertain to the energy of a chemical reaction, the PV which shrumeo mentioned relates to a separate work function...that is when a exothermic chemical reaction occurs, the energy can be used to change the internal energy U as well as expand against the atmosphere PV.

The energy of a specific chemical reaction is constant, and in most cases PV is considered negligible. Think bond energies.

Thus the enthalpy of the reaction does not change, rather different amounts of energy is released at different temperature and pressure since these two factors influence how many of such individual reactions take place.

----------
http://groups.msn.com/GeneralChemistryHomework
 
Last edited by a moderator:

1. What is enthalpy change?

Enthalpy change is a measure of the amount of energy that is absorbed or released during a chemical or physical process. It is typically represented by the symbol ΔH and is measured in units of Joules (J).

2. How does pressure affect enthalpy change?

According to the ideal gas law, pressure and volume are inversely related. When pressure increases, volume decreases and vice versa. This means that when pressure is increased, the particles in a system are forced closer together, resulting in stronger intermolecular forces and a decrease in enthalpy (ΔH). Therefore, an increase in pressure generally leads to a decrease in enthalpy.

3. How does temperature affect enthalpy change?

Temperature and enthalpy are directly related. An increase in temperature leads to an increase in the kinetic energy of particles in a system, causing them to move faster and with more force. This results in weaker intermolecular forces and an increase in enthalpy (ΔH). On the other hand, a decrease in temperature leads to a decrease in enthalpy.

4. What is the difference between endothermic and exothermic reactions in terms of enthalpy change?

An endothermic reaction is a process in which energy is absorbed from the surroundings, resulting in a positive enthalpy change (ΔH). This means that the products have a greater enthalpy than the reactants. On the other hand, an exothermic reaction is a process in which energy is released to the surroundings, resulting in a negative enthalpy change (ΔH). This means that the reactants have a greater enthalpy than the products.

5. How is enthalpy change calculated?

Enthalpy change is calculated by taking the difference between the enthalpy of the products and the enthalpy of the reactants. This can be represented by the equation ΔH = H(products) - H(reactants). The enthalpy values can be obtained from tables or by using calorimetry, which measures the heat flow of a reaction.

Similar threads

What does "limit of zero pressure" mean for ideal gas temperature?
    • Chemistry
Replies
14
Views
2K
Does the initial temperature affect the change in temp?
    • Chemistry
Replies
13
Views
5K
Vapor pressure -- How does water still boil at 100°C in an open pot?
    • Chemistry
2
Replies
35
Views
3K
Andrew's experiment on liquefaction of gases
    • Chemistry
Replies
3
Views
2K
What is the difference between internal energy and enthalpy?
    • Chemistry
Replies
12
Views
2K
Why would a reaction be nonspontaneous at higher temperatures?
    • Chemistry
Replies
3
Views
1K
Does pressure affect equilibrium vapor pressure (or RH)?
    • Chemistry
Replies
8
Views
2K
Questions about the common-ion effect
    • Chemistry
Replies
4
Views
806
Calculating Energy Released by Sodium Reacting with Water
    • Chemistry
Replies
3
Views
2K
Is the Triple Point of Water Affected by Atmospheric Pressure?
    • Chemistry
Replies
6
Views
1K

Hot Threads

Recent Insights

Back
Top