Colligtatives Properties
Páginas: 5 (1245 palabras)
Publicado: 14 de enero de 2013
Lab: I Scream, We All Scream for …Colligative Properties!?
Introduction:
When a solute is added to water the physical properties of freezing point and boiling point change. Water normally freezes at 0oC and boils at 100oC. As more solute is added, the freezing point drops (“freezing point depression”) and the boiling point increases (“boiling point elevation”). This property is useful in ourlives. Antifreeze is added to car radiators to keep the water used as a coolant from freezing in winter and boiling in the summer. Antifreeze is added to a solvent to “de-ice” planes before take-off to prevent ice forming on the wings. When roads ice over in the winter, salt is added along with sand. The sand is used to give cars traction and the salt is used to lower the freezing point of thewater so the ice melts. In the kitchen we can use this same property to make ice cream. Water freezes at 0oC. But milk, although it contains mostly water, normally freezes at about –4oC. In order to get the milk to freeze, we add salt to the ice. This changes the melting point of ice to –6oC. This is low enough to freeze the milk and make ice cream. The purpose of this lab is to use the property oflowering freezing points by adding salt to water to produce ice cream.
The traditional method of ice cream making, using ice and rock salt to freeze a milk and sugar mixture, involves the application of several chemistry principles:
Colligative Properties: Physical properties determined by the concentration of dissolved particles in a mixture, and not affected by the type of dissolvedparticles. Osmotic pressure (tendency of water to flow from high water concentration to low water concentration as seen when applying salt to a slug), boiling point elevation and, most importantly in this activity, freezing point depression are examples of colligative properties.
When 1 mole of the ionic compound NaCl dissolves in a liter of water it dissociates into 1 mole of sodium ions and 1 moleof chloride ions, or a total of 2 moles of particles per liter of mixture (this is called a 2M or 2-Molar concentration). When 1 mole of the covalent compound sugar, C11 H22 O11 , dissolves in a liter of water there is just 1 mole of sugar molecules per liter (onlya 1-Molar concentration) because sugar molecules remain intact when dissolved in water and do not dissociate like ionic compoundsdo. The greater the particle concentration (molarity M): the stronger the colligative effect.
Ice cream, a mixture of milk, sugar and vanilla, is really an aqueous mixture with many particles in each liter. Because of this, the freezing point of ice cream is lower that of water. To freeze ice cream, or to keep it frozen, you must keep its temperature significantly below 00 Celsius. Adding rocksalt to ice produces a melting ice and saltwater mixture, with a depressed freezing point in which the ice cream can be frozen.
Heat of Fusion: Melting a solid to form a liquid is an endothermic process. The heat of fusion of a substance is the amount of energy needed to change 1 gram of solid to liquid. The heat of fusion of water is 334 joules/gram. Since the room temperature and the milkmixture are both warmer than the freezing point of the ice mixture, energy will be transferred into the ice mixture on all sides. This energy will be used in melting the ice (334 joules per gram melted) and the temperature of the ice mixture will stay at its freezing point as long as there is still ice to melt. This process will effectively draw energy out of the milk mixture, lowering itstemperature and, eventually, freezing it!
Hess' Law and Enthalpy Change: The energy absorbed by the ice mixture is transferred from the room and the milk mixture so ΔH ice = ΔH milk mix + ΔH room. This is an application of Hess' Law.
Pre-Lab. Use your textbook (pp. 471-475) to answer the pre-lab questions below.
Define COLLIGATIVE PROPERTIES:
Give four examples of COLLIGATIVE...
Introduction:
When a solute is added to water the physical properties of freezing point and boiling point change. Water normally freezes at 0oC and boils at 100oC. As more solute is added, the freezing point drops (“freezing point depression”) and the boiling point increases (“boiling point elevation”). This property is useful in ourlives. Antifreeze is added to car radiators to keep the water used as a coolant from freezing in winter and boiling in the summer. Antifreeze is added to a solvent to “de-ice” planes before take-off to prevent ice forming on the wings. When roads ice over in the winter, salt is added along with sand. The sand is used to give cars traction and the salt is used to lower the freezing point of thewater so the ice melts. In the kitchen we can use this same property to make ice cream. Water freezes at 0oC. But milk, although it contains mostly water, normally freezes at about –4oC. In order to get the milk to freeze, we add salt to the ice. This changes the melting point of ice to –6oC. This is low enough to freeze the milk and make ice cream. The purpose of this lab is to use the property oflowering freezing points by adding salt to water to produce ice cream.
The traditional method of ice cream making, using ice and rock salt to freeze a milk and sugar mixture, involves the application of several chemistry principles:
Colligative Properties: Physical properties determined by the concentration of dissolved particles in a mixture, and not affected by the type of dissolvedparticles. Osmotic pressure (tendency of water to flow from high water concentration to low water concentration as seen when applying salt to a slug), boiling point elevation and, most importantly in this activity, freezing point depression are examples of colligative properties.
When 1 mole of the ionic compound NaCl dissolves in a liter of water it dissociates into 1 mole of sodium ions and 1 moleof chloride ions, or a total of 2 moles of particles per liter of mixture (this is called a 2M or 2-Molar concentration). When 1 mole of the covalent compound sugar, C11 H22 O11 , dissolves in a liter of water there is just 1 mole of sugar molecules per liter (onlya 1-Molar concentration) because sugar molecules remain intact when dissolved in water and do not dissociate like ionic compoundsdo. The greater the particle concentration (molarity M): the stronger the colligative effect.
Ice cream, a mixture of milk, sugar and vanilla, is really an aqueous mixture with many particles in each liter. Because of this, the freezing point of ice cream is lower that of water. To freeze ice cream, or to keep it frozen, you must keep its temperature significantly below 00 Celsius. Adding rocksalt to ice produces a melting ice and saltwater mixture, with a depressed freezing point in which the ice cream can be frozen.
Heat of Fusion: Melting a solid to form a liquid is an endothermic process. The heat of fusion of a substance is the amount of energy needed to change 1 gram of solid to liquid. The heat of fusion of water is 334 joules/gram. Since the room temperature and the milkmixture are both warmer than the freezing point of the ice mixture, energy will be transferred into the ice mixture on all sides. This energy will be used in melting the ice (334 joules per gram melted) and the temperature of the ice mixture will stay at its freezing point as long as there is still ice to melt. This process will effectively draw energy out of the milk mixture, lowering itstemperature and, eventually, freezing it!
Hess' Law and Enthalpy Change: The energy absorbed by the ice mixture is transferred from the room and the milk mixture so ΔH ice = ΔH milk mix + ΔH room. This is an application of Hess' Law.
Pre-Lab. Use your textbook (pp. 471-475) to answer the pre-lab questions below.
Define COLLIGATIVE PROPERTIES:
Give four examples of COLLIGATIVE...
Leer documento completo
Regístrate para leer el documento completo.