Atoms and atomic theory
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English-for-Chemistry-1-After-lesson-1
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- Vapor pressure
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6.2 Reading: In our study of gases, we noted that at high pressures and low temperatures intermolecular forces cause gas behaviour to depart from ideality. When these forces are sufficiently strong, a gas condenses to a liquid. That is, the intermolecular forces keep the molecules in such close proximity that they are confined to a definite volume, as expected for the liquid state. Intermolecular forces are important in establishing the form and behaviour of matter. The origin of intermolecular forces, those interactions between molecules, arises from the permanent and momentary unequal distribution of electron density within molecules. The intermolecular forces that occur between molecules are collectively known as van der Waals forces. The most common intermolecular forces of attraction are those between instantaneous and induced dipoles (dispersion forces, or London forces). The magnitudes of dispersion forces depend on how easily electron displacements within molecules cause a temporary imbalance of electron charge distribution, that is, on the polarizability of the molecule. In polar substances, there are also dipole dipole forces. Some hydrogen- containing substances exhibit significant intermolecular attractions called hydrogen bonds, English in chemistry 1 – 2017 TS. Nguyễn Tuyết Phương – TS. Trần Thu Phương 19 in which H atoms bonded to highly electronegative atoms N, O, or F in a molecule are simultaneously attracted to other highly electronegative atoms in the same molecule or in different molecules. Hydrogen bonding has a profound effect on physical properties, such as boiling points and is a vital intermolecular force in living systems. Vapor pressure, the pressure exerted by a vapor in equilibrium with a liquid, is a measure of the volatility of a liquid and is related to the strength of intermolecular forces. The conversion of a liquid to a vapor is called vaporization or evaporation; the reverse process is called condensation. When the pressure exerted by the escaping molecules from the surface of the liquid equals the pressure exerted by the molecules in the atmosphere, boiling is said to occur. The normal boiling point is the temperature at which the vapor pressure of the liquid equals 1 atm. The critical point is the condition of temperature and pressure at which a liquid and its vapor become indistinguishable. When crystalline solids are heated, a temperature is reached where the solid state is converted to a liquid, melting occurs. The temperature at which this occurs is the melting tải về 0.52 Mb. Chia sẻ với bạn bè của bạn:
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