you read a pressure-enthalpy diagram…
Enthalpy diagram for an ..
(Change in Enthalpy) for photosynthesis
To evaporate (i.e. vaporize) water, energy is needed to breakthe hydrogen bonds between water molecules. This energy can come from radiationor sensible heat from the air, water, soil, etc. If the energy comes fromsensible heat, kinetic energy is removed from the air and changed to latentheat. This causes the temperature to decrease. When water condenses, hydrogenbonds form and latent heat is released back to sensible heat causing thetemperature to rise. The total heat content (i.e. enthalpy) of the air is thesum of the sensible and latent heat content.
Notice also that the equation and amount of heat evolved in the hydrogen case is exactly the same as you have already come across further up the page. At that time, it was illustrating the enthalpy of formation of water. That can happen in some simple cases. Talking about the enthalpy change of formation of water is exactly the same as talking about the enthalpy change of combustion of hydrogen.
Enthalpy Changes | Enthalpy | Chemical Reactions
For instance, the diagram in , panel b shows that molecules with C–C, H–H, and C–H bonds have rather similar enthalpies and thus readily explains why reactions between “fuels of high energy density” such as C and H2 are in fact not strongly exothermic without O2.
Calculated values for a range ofand are given in Table 3.1 and for arange of and in Table 3.2. Values forand depend only on the water vapourcontent of the air and hence the latent heat content of the air. Whenor is high, then is often considerably higher than the air temperature, which implies highertotal heat content (i.e. higher enthalpy). Therefore, when is close to the air is dry, there is less heat in the airand there is more chance of frost damage.
or only happens inside a complex system such as photosynthesis.
Chemical Energy Notes | Chemical Reactions | Enthalpy
09/09/1970 · Chemistry 30: Chemical Energy Notes
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Enthalpy Changes Definitions
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