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An Introduction to Thermodynamics phần 7

Gần gũi hơn với tình hình thực nghiệm phương pháp tiếp cận cân bằng đúng, chúng ta có thể có sự đảm bảo rằng câu trả lời đầy đủ. Ví dụ, trong các khoản trên 6 phương trình, trong đó năng lượng nhiệt được chuyển giao cho hệ thống (đá) từ môi trường xung quanh (nhiệt) ở nhiệt độ cao hơn một chút, không có lý do để nghi ngờ kết luận. | f p c - 1 2 - c p - 1 c - p 2 36 Figure 5. Each area corresponds to a single phase. The lines are the common borders and thus represent two phases in equilibrium or three phases at a point . which is Gibbs phase rule for two external degrees of freedom. You will recall from experience with drawing graphs that a point or any finite number of points corresponds to no degrees of freedom. A line or a finite number of lines represents one degree of freedom setting one variable determines the other. Two degrees of freedom yield an area or more than one area . ONE-COMPONENT PHASE DIAGRAMS. Water is a single component c 1 so the degrees of freedom may vary from zero to two. c 1 f c-p 2 3 -p 37 A single phase vapor liquid or solid is therefore represented by an area. Two phases solidliquid solid-vapor or vapor-liquid are restricted to a line. Three phases yield a single point called the triple point solid liquid vapor . The phase diagram with T on the vertical axis to match following diagrams is shown in Figure 5. You can calculate the slopes of the three lines from the Clapeyron and Clausius-Clapeyron equations knowing the heats of fusion and of vaporization or sublimation and the densities of the phases. The upper end of the liquid-vapor line simply stops above the critical point the distinction between liquid and vapor disappears. TWO-COMPONENT PHASE DIAGRAMS AT CONSTANT PRESSURE. We could not represent a single phase with two components on a two-dimensional page if we kept both temperature and pressure as external variables. We consider here the restricted example where pressure is maintained constant e.g. at atmospheric pressure . Then Gibbs phase rule tells us the degrees of freedom are again with c 2 c 2 constP f c -p 1 3 -p 38 It follows that wherever there is a single phase present that phase must be represented by an area two degrees of freedom . If there are two phases present the compositions are restricted to a line or sometimes more than one line but still one