11.12
加热和冷却曲线
当与环境隔离的物质发生热变化时，会观察到该物质的温度和相位的相应变化；这通过加热和冷却曲线以图形方式表示。
例如，加热会升高固体的温度；吸收的热量取决于固体的热容量 (q = mcsolidΔT)。根据热化学，物质吸收或释放的热量q及其伴随的温度变化ΔT之间的关系是：
其中 m 是物质的质量， c…
加热或冷却物质会导致温度变化，然后发生相变。加热某物质会增加其分子的热能，这反映为温度的升高，直到达到转变点为止。当物质吸收了足够的热量时，其分子之间的吸引力便被克服，导致在恒定温度下 发生相变。一旦转变完成，加热 将导致温度再次升高。当热量从物质中去除时，其分子的热能下降，对应温度会下降，直到达到转变点为止。然后，在相变过程中会重新建立 更强的分子间力，同时温度保持恒定。可以使用加热曲线或冷却曲线对物质响应温度变化的行为 进行建模，其中对温度变化与所添加热量或所去除热量的函数关系 进行绘图。来看一下一个装满冰块的烧杯，初始温度为 20°C。随着热量的流入，冰的温度 稳步上升。在使冰变暖过程中吸收的热量 取决于冰的比热容。一旦达到冰的熔点，尽管热量不断流动，温度 会停止升高。吸收的热量会减弱分子间作用力，直到冰完全 融化成液态水。有一个固液平衡平台是 恒温下相变的特征。平台的开始点和结束点之间的焓变 指示融化过程所需的热量，或者换句话说，是水的熔化焓。融化过程结束后，吸收的热量会导致温度 相应地线性上升。水的比热容 决定了所吸收的热的量。达到沸点时，温度停止上升。吸收的热量反而 有助于克服水分子之间的吸引力，直到水完全蒸发为止。液-气平衡平台代表 恒定温度下的相变。平台开始点和结束点之间 的焓变是 水的蒸发焓。在所有液体转化为蒸气之后，额外的热量会导致温度再次升高。
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Q1: Why does temperature remain constant during a phase transition?
During a phase transition, absorbed heat overcomes intermolecular forces rather than increasing molecular kinetic energy. The temperature stays constant until the transition completes because all heat energy goes toward breaking attractive forces between molecules. Once the phase change finishes, additional heat resumes raising temperature.
Q2: What does the plateau on a heating curve represent?
A plateau represents a phase transition occurring at constant temperature. During melting or boiling, the temperature does not rise despite continuous heat input. The plateau's length indicates the duration of the phase change, while its vertical position shows the melting or boiling point temperature.
Q3: How does specific heat capacity affect temperature change in a heating curve?
Specific heat capacity determines how much heat a substance absorbs to change temperature by one degree. Materials with higher specific heat capacities require more heat for the same temperature increase, resulting in steeper slopes on heating curves. The relationship is expressed as q equals mass times specific heat times temperature change.
Q4: What is the difference between enthalpy of fusion and enthalpy of vaporization?
Enthalpy of fusion measures heat required to melt a solid into liquid at constant temperature. Enthalpy of vaporization measures heat required to vaporize a liquid into gas at constant temperature. Vaporization typically requires significantly more energy than fusion because gas molecules are farther apart and intermolecular forces are completely overcome.
Q5: How does a cooling curve differ from a heating curve?
A cooling curve plots temperature decrease as heat is removed from a substance, creating a mirror image of a heating curve. Like heating curves, cooling curves show plateaus at phase transitions where temperature remains constant as intermolecular forces re-establish. The slopes reflect specific heat capacities of each phase during cooling.
Q6: Why does boiling become more vigorous when heat is applied faster?
When heat is supplied at a greater rate during boiling, the temperature remains constant because the substance is already at its boiling point. Instead of raising temperature, excess heat accelerates the vaporization process, causing more vigorous bubble formation. The temperature plateau persists until all liquid has transformed into gas.
Q7: What determines the slope of a heating curve between phase transitions?
The slope between phase transitions depends on the specific heat capacity of the phase being heated. Solids, liquids, and gases have different specific heat capacities, producing different slopes. A steeper slope indicates lower specific heat capacity, meaning less heat is needed to raise temperature by one degree.