Enthalpy of Vaporization Enthalpy of Vaporization The enthalpy of vaporization, (symbol ), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure (often atmospheric pressure). It is often measured at the normal boiling point of a substance; although tabulated values are usually corrected to 298 K, the correction is often smaller than the uncertainty in the measured value. The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges and for Tr<<1.0. The heat of vaporization diminishes with increasing temperature and it vanishes completely at the critical temperature (Tr=1) because above the critical temperature the liquid and vapor phases no longer co-exist. Physical model for vaporization :- A simple physical model for the liquid-gas phase transformation has been proposed recently. It is suggested that the energy required to free an atom from the liquid is equivalent to the energy needed to overcome the surface resistance of the liquid.
Know More About :- Types of Pollution
Math.Tutorvista.com
Page No. :- 1/4
The model allows calculating the latent heat by multiplying the maximum surface area covering an atom with the surface tension and the number of atoms in the liquid. The calculated latent heat of vaporization values for the investigated 45 elements agrees well with experiments. Enthalpy of Formation :When a Compound is formed by its constituent elements then the reaction or process is accompanied by an enthalpy change. This change is called the enthalpy of formation. The conditions which can alter this enthalpy change or on which the change is dependent are: 1. Pressure of the reaction 2. Temperature of the reaction as well as surroundings 3. The state in which the reactants are present 4. The final state of products 5. The states can be gas, liquid or solid. We can also define the standard state of any substance as the form which remains the most stable at standard room temperature which is 25â&#x2C6;&#x2DC; C and 1atm. For eg : Let us discuss the enthalpy change in the reaction of formation of ethanol The reaction is: -2 C or graphite + 3 hydrogen (gas) + 12 oxygen (gas) -> ethanol (liquid) Here Î&#x201D; Hf or enthalpy change is -277 K J. Enthalpy Change Consider any chemical reaction, we can see the change in enthalpies in the reactants and the end products. This difference in the enthalpies of the products and the reactants can be called as Enthalpy Change.
Learn More :- Solubility Product Constant Math.Tutorvista.com
Page No. :- 2/4
In general we can defined it as: Note :- If the System changes from one state to another during a process, the change in enthalpies of the constituents is called as Enthalpy Change. It is expressed in KJ / mole. If H1 and H2 denote Initial and final state of enthalpy respectively, then change in enthalpy is given by Î&#x201D; H = H2 -H1. Where H1 = enthalpy of Initial state H2 = enthalpy of Final state. For example: The enthalpy of formation can also be called as one of the enthalpy change because it follows the same concept.
Math.Tutorvista.com
Page No. :- 4/4
Thank You For Watching
Presentation