What is saturated vapor pressure

Sexl Physik 5 RG, textbook

Vapor Liquid Thermometer Manometer p 102.1 Experiment to measure the vapor pressure Pressure in kPa p Temperature in ° C 0 20 40 60 80 100 120 100 80 60 40 20 0  102.2 The saturation vapor pressure of water between 0 ° C and 100 ° C rises close the boiling point steeply. 102.3 Both the density and the thermal movement of the molecules in the vapor increase with temperature. 102.4 If the volume is increased, more molecules pass into the vapor space until the original density is restored. b The vapor pressure In order to study the liquid-gas transition of a substance (e.g. water), we have to prevent the loss of liquid through evaporation. To do this, we lock the substance in a pressure vessel. High-energy molecules leave the surface of the liquid and move in the space above the liquid, while molecules return to the liquid at the same time. At constant temperature, a dynamic equilibrium is created between the liquid and the gaseous state. As a result, the gaseous component reaches a constant density and pressure, which only depend on the temperature. (102.2) The vapor in equilibrium with the liquid is called saturated vapor. Its pressure on the surface of the liquid and the vessel wall is called the vapor pressure of the liquid.  There is usually additional air in the space above the liquid, and the pressure above the liquid is made up of the vapor pressure and the pressure of the enclosed air. To study the vapor pressure only, we should first remove the air. We then have a single substance (e.g. water) in two states (liquid and gaseous) in the vessel. How does vapor pressure change with temperature? Experiment: Steam pressure of water 102.1 E 1 You need: a pressure vessel with thermometer and manometer, heating plate, water Fill the pressure vessel with water and heat it until the pressure of the steam exceeds the external air pressure. Let any residual air and some water vapor out through a valve in the lid so that there is no more air in the vessel. The vessel then contains the liquid and above it its saturated vapor (102.1). As the vessel cools down, create a diagram (vapor pressure curve) that shows the pressure of the steam as a function of temperature (102.2). In diagram (102.2) the vapor pressure curve separates the liquid state from the gaseous state. The points on the curve describe water in equilibrium between the liquid and the gaseous state. In the range from 0 ° C to 100 ° C, the vapor pressure increases from 6 mbar to 1 013 mbar. In the area to the left of the curve there is only liquid water, to the right only gaseous water. The vapor pressure curve (102.2, 105.1) shows that, in contrast to the behavior of an ideal gas, the vapor pressure does not rise proportionally to the temperature, but rather more. In the ideal gas, the number of particles remains the same, the pressure increases proportionally to the temperature. In the liquid-gas system, when the temperature rises, on the one hand the mean kinetic energy of the molecules increases and on the other hand, more molecules leave the liquid. A new dynamic equilibrium is established with a higher vapor pressure. Does the steam pressure change when we change the steam volume? If the volume of the steam is increased while the temperature remains the same, the density and pressure initially decrease. Until the original density of the vapor is reached again, more molecules can leave the liquid than return into it, (102.2). As long as there is still liquid, the vapor density and the vapor pressure above the liquid do not depend on the volume. The pressure of a saturated steam increases with increasing temperature, but is independent of the volume. The vapor pressure curve ends abruptly at the critical point, a pressure and temperature value that is characteristic of the liquid. In the case of water, the critical temperature T k = 374.15 ° C. and the critical pressure p k = 221.15 bar (102.1). At the critical point, the difference between liquid and gas disappears. The thermal movement is now so strong that the particles do not remain bound to their closest neighbors, even if the pressure is even greater. Above the critical point there is only gas (105.1). 102 WÄRMELEHRE For testing purposes only - property of the publisher öbv

Made with FlippingBook