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Thermal changes accompanying physical and chemical changes
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Thermal changes accompanying physical and chemical changesThermal changes accompanying physical changes
- melting temperature
It is the temperature change when one mole of solute is dissolved in a given amount of solvent
(The amount of heat energy absorbed or released when one mole of solute is dissolved in a given amount of solvent).
During the dissolution process: two processes occur:
A- Dissociation: the breaking of bonds (endothermic reaction)
Sodium and chloride ions dissociate from the crystal lattice as a result of the ionic bond breaking.
The energy absorbed to break the bond is called the heat of the crystal lattice (bond energy).
- Crystal lattice temperature: It is the energy needed to break the bond between ions in the crystal lattice.
B- Hydration: The dissociated ions bind with water molecules to form hydration ions (exothermic process).
Rehydration temperature:
It is the amount of heat released when hydrating ions bind to water molecules.
If the crystal lattice temperature > heat of hydration
Dilution temperature:
It is the heat change per mole of solute when diluted from a concentrated solution to a dilute solution.
(It is the heat change of one mole of solute when diluted in a large amount of water (about 1000 moles of solvent)).
Sedimentation temperature:
The amount of heat released when one mole of an insoluble substance is completely precipitated.
- heat of combustion
It is the amount of heat released when one mole of a substance is completely burned in an abundance of oxygen. (The reaction is always exothermic).
Formation temperature:
It is the amount of heat released or absorbed when one mole of a compound is formed from its elemental elements so that the elements are in their standard state. (at 25 °C, a pressure of 76 x g).
Note:
Heat of formation of the elements = zero
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