PHASE CHANGES EVAPORATION EVAPORATION PHYSICAL PROPERTIES OF LIQUID PHYSICAL PROPERTIES OF LIQUID SOME PROPERTIES OF LIQUIDS 2014/08/08

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1 PHYSICAL PROPERTIES OF LIQUID PHYSICAL PROPERTIES OF LIQUID A physical property is a property that can be changed without changing the fundamental components of a substance. SOME PROPERTIES OF LIQUIDS The following properties of liquids are going to be considered: Evaporation (Vaporization) and Condensation Vapor Pressure Boiling point Critical Temperature and Critical Pressure Surface Tension Capillary Action Viscosity PHASE CHANGES This is a change from solid to liquid, from liquid to gas or from solid directly to gas. But here, because we are dealing with liquid, we are going to look at evaporation or vaporization and condensation only. EVAPORATION This is a process in which the liquid changes from the liquid state to the gaseous state. The energy is required to overcome the intermolecular force/s between/among the liquid particles and make it/them as weak as the intermolecular force/s between/among the gaseous particles Evaporation is therefore an endothermic reaction. EVAPORATION This energy is called the molar enthalpy of vaporization, vap H o Liquid vaporization energy absorbed by the liquid vap H o = molar enthalpy of vaporization 1

2 This is a process in which the gas changes from the gaseous state to the liquid state. The energy must be liberated to strengthen the intermolecular force/s between/among the gas particles and make it/them as strong as the intermolecular force/s between/among the liquid particles. Condensation is therefore an exothermic reaction. Vapor condensation energy released by the vapor - vap H o = molar enthalpy of condensation The molar enthalpy of evaporation is equal to but opposite in sign to the molar enthalpy of condensation. EVAPORATION AND For an example, the energy required to evaporate 1.00 mol of water at 100 o C is 40.7kJ. The energy required to condense 1.00 mol of water vapor at 100 o C is -40.7kJ. VAPOUR This is called an equilibrium vapor pressure. It is a pressure exerted by a substance at equilibrium with the liquid. The higher the vapor pressure at a particular temperature, the higher the volatility of that substance. Different substances has different vapor pressure at the same temperatures temperature. See this on the figure below: VAPOUR VAPOUR On the figure above, it is shown that the vapor pressure is directly proportional to temperature. All the points along the vapor pressure versus temperature represent a condition of pressure and temperature at which the liquid and the vapor are at equilibrium. 2

3 VAPOUR,ENTHALPY OF The enthalpy of vaporization of a substance can be calculated by using the equation that relates the: Vapor pressure P The gas constant The temperature at which equilibrium is attained. The constant for a particular liquid c The equation used to calculate this is called Clausius-Clapeyron equation. Excluding the constants one may say, this shows the relationship between the vapor pressure, enthalpy and temperature. It is given as: Where C is a constant characteristic of a particular liquid, This shows a linear relationship between the natural look of P (lnp) and the inverse of the temperature 1/T with a negative slope. This is the same as: y = -mx + c For the partial pressures of the same liquid at different temperatures, this can be written as: The boiling point of a liquid is a temperature at which the vapor pressure of a liquid is equal to the atmospheric pressure. For example: if you have water in an open container, the atmospheric pressure presses the molecules of this water down and they cannot evaporate. 3

4 When increasing the temperature of this water to a certain temperature, these molecules get enough energy to overcome the atmospheric pressure, form some bubbles and escape from the surface of water. That is the water boils. This temperature is called a boiling point. The normal boiling point of a substance is a boiling point of that substance at 760 mmhg. The boiling points of different substances differ and depends on different altitudes. For an example the boiling point of water is 100 o C in most countries. CRITICALTEMPERATURE AND A critical temperature (T c ) is a temperature at which one cannot differentiate between the liquid and the gaseous state of a substance. The same thing applies to a critical pressure (P c ) is a pressure at which one cannot differentiate between the liquid and the gaseous state of a substance. CRITICALTEMPERATURE AND A triple point is the temperature and pressure at which a substance can exist in equilibrium in the liquid, solid, and gaseous states. The triple point of pure water is at o C. An example of a critical temperature, critical pressure and triple point is shown on the figure below: A critical temperature, critical pressure and triple point of H 2 O: 4

5 A critical temperature, critical pressure and triple point of CO 2 : SURFACE TENSION This is a force of attraction between the molecules on the surface of a liquid. The molecules on the surface of liquid has a force of attraction between each other and between each molecules and the underlying molecules. This form a strong tension on the surface of the water to resist an easy entrance to or exit from the liquid. SURFACE TENSION This is the cause of the formation of bubbles in the liquid. Unlike the molecules inside, which experience a force of attraction from all sides. CAPPILLARY ACTION This is the force that pulls the liquid up in the capillary tube or any polar substances. This is caused by the cohesion or adhesion force. Cohesion is a force of attraction between the same molecules. For an example: A force of attraction between water molecules. Adhesion is a force of attraction between the liquid molecules and the walls of the container (between different molecules). CAPPILLARY ACTION Adhesion is a force of attraction between the between different molecules. For an example: A force of attraction between the liquid molecules and the walls of the container. It makes the surface of some liquids, (e.g. water) to form a concave. It cause the movement of the mobile phase in chromatography. It again cause the surface of some liquids (e.g. mercury) to be convex. VISCOSITY This is the easiness or difficulty of the liquid to flow. For an examples: when one have two containers of equal amount of oil and water and try to decant: Water will be easily be decanted while oil will be decanted with difficulty. This is because oil has high viscosity while water is less viscous. 5