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Chapter 4

CHMB20 Chapter 4

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Jamie Donaldson

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Chapter 4: Physical Transformations Of Pure Substances Phase Diagrams • Phase: form of matter that is uniform throughout in chemical composition and physical state. • Phase Transition: the spontaneous conversion of one phase into another and may be studied by techniques that include thermal analysis. o Transition Temperature (Ttrs): the temperature at which the two phases are in equilibrium and Gibbs energy of the system is minimized at the prevailing pressure. o Thermal Analysis: the transition is detected by noting that the temperature does not change even though heat is being supplied or removed from the sample. o Metastable Phases: thermodynamically unstable phases that persist because the transition is kinetically hindered. • The thermodynamic analysis of phases is based on the fact that at equilibrium, the chemical potential of a substance is the same throughout the sample, regardless of how many phases are present. o Chemical Potential (μ ): measure of the potential that a substance has for undergoing change in a system and is synonyms with molar Gibbs energy (μ = Gm). o At equilibrium, μ 1 = μ 2 and then, there is no change in G. • A susbtance characterized by a variety of parameters that can be identified on its phase diagram. o Phase Diagram: shows the regions of pressure and temperature at which its various phases are thermodynamically stable. o Phase Boundaries: the lines separating the regions that show the values of p and T at which two phases coexist in equilibrium and their chemical potentials are equal. o Vapor Pressure: the pressure of a vapor in equilibrium with the liquid. o Sublimation Vapor Pressure: the vapor pressure of the solid phase. o The vapor pressure of a substance increases with temperature because at higher temperatures, more molecules have enough energy to escape from their neighbors. o Boiling: the condition of free vaporization throughout the liquid. o Boiling Temperature: the temperature at which the vapor pressure of a liquid is equal to the external pressure.  At normal boiling point, temperature where external pressure is 1 atm.  At standard boiling point, temperature where vapor pressure is 1 bar. o Critical Temperature (Tc): temperature at which the surface disappears. o Critical Pressure (pc): vapor pressure at the critical temperature. o Supercritical Fluid: a single uniform phase at and above the critical temperature. o Melting Temperature: the temperature at which the liquid and solid phases of a substance coexist in equilibrium. o Freezing Temperature: because a substance melts at exactly the same temperature as it freezes, melting temperature is the same as its freezing temperature.  Normal Freezing Point (Tf): freezing temperature when pressure is 1 atm.  Standard Freezing Point (Ts): freezing point when pressure is 1 bar. o Triple Point: a point at which the three phase boundaries meet; it occurs at a single definite pressure and temperature characteristic of the substance. o If the slope of the solid-liquid phase boundary is shown, then the triple point also marks the lowest temperature at which the liquid can exist; the critical temperature is the upper limit. • The phase rule relates the number of variables that may be changed while the phases of a system remain in mutual equilibrium. o Phase Rule: F = C – P + 2, where F is the variance, C is the number of components and P is the number of phases at equilibrium. o Component: chemically independent constituent of a system; the minimum number of types of independent species necessary to define the composition of all the phases present in the system. o Constituent: a chemical species that is present.  A mixture of ethanol and water has two constituents.  A solution of sodium chloride has three constituents; Na+, Cl- and H2O. o Variance: the number of intensive variables that can b
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