Table of Contents
Do elements have standard enthalpy of formation of zero?
All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.
What does it mean if the enthalpy is zero?
Enthalpy is zero for elements because they are in their natural, ground state. For most thermodynamic calculations, we consider anything in its ground state to have a value of 0. It’s not that an element doesn’t contain energy, it just doesn’t contain energy usable in a typical chemical reaction.
Is enthalpy zero in isothermal process?
As the temperature is constant in an isothermal process, change in enthalpy of the process is zero. . For ideal gases, enthalpy is a function of the only temperature. Thus, in an isothermal process involving only ideal gases, the change in enthalpy is zero.
Why is standard enthalpy of formation of diamond not zero?
The standard enthalpy of formation of diamond is not zero because it is slightly less stable form of carbon. Its ∆fHΘ is 1.89 kJ mol−1 .
Why is standard enthalpy of formation of diamond not zero although it is an element?
Out of the various allotropic forms of an element, only the standard heat of formation of the most stable allotropic form is taken as zero. Since graphite and not diamond is the most stable form of carbon therefore, the enthalpy of formation of diamond is not zero.
What is the standard heat of formation of diamond is?
Standard Heat of Formation
| Substance | ΔHfº (kJ/mol) |
|---|---|
| Br2(g) | 30.91 |
| C (s, graphite) | 0 |
| C (s, diamond) | 1.90 |
| CH4(g) | -74.85 |
Why is the enthalpy of a substance zero?
Answer Wiki. The enthalpy is zero because of the conventions we use in our definitions. A standard enthalpy is, by definition, the amount of energy released or gained when 1 mole of a substance is made from the elements under standard conditions of temperature and pressure.
What is the standard enthalpy of formation of an element?
Using this as our definition we can see that it takes no energy to produce an element from an element. For example, it takes no energy to make O 2 from O 2, or H 2 from H 2, or carbon from carbon. As a consequence, the standard enthalpy of formation of an element in its standard state is always zero.
What is the enthalpy of an element in kJ mol?
Regarding elements, by definition the standard enthalpy is zero for the most stable modification and phase. For example, bromine gas has a SEF of 112 kJ/mol; ozone (contains only one element, oxygen) has a SEF of 143 kJ/mol. Monocline sulfur 0.3 kJ/mol.
What is the change of enthalpy of oxygen gas?
This is the change of enthalpy when one mole of a substance in its standard state is formed from its elements under standard state conditions of 1 atmosphere pressure and 298K temperature. Oxygen gas consists of its elements already in the standard state, so there isn’t any change here.