Thermodynamic Properties of Water PSC 151 Laboratory Activity 7 Thermodynamic Properties of Water Heat of Fusion of Ice

What causes the temperatures of two objects placed in thermal contact to change? Something must move from the high temperature object to the low temperature object. Is it matter or energy? If it is matter the mass of the high temperature object would decrease while the mass of the low temperature object would increase. This is not observed. It must be energy flowing between the two objects that causes a change in their temperatures.

The energy flowing between the two objects must be on the microscopic level because we can not see it. Energy Review Kinetic energy, KE-the macroscopic energy an object has due to its motion, measured in Joules, J. Gravitational potential energy energy, GPE-the macroscopic energy an object has due to its position, measured in Joules, J. Total mechanical energy, E-the sum of an object’s kinetic and potential energies, measured in Joules, J. Work, W-the process by which the total mechanical energy can be changed, measured in Joules, J.

Work and the Related Changes in Macroscopic Energy

All macroscopic objects are composed of microscopic objects: atoms and molecules These atoms and molecules are moving so they have a microscopic kinetic energy. These atoms and molecules are subject to conservative forces (gravitational and electrical) so they have a microscopic potential energy. The sum of these microscopic kinetic and potential energies is called Thermal Energy, U.

When two objects with different temperatures are placed in thermal contact, thermal energy flows from the higher temperature object to the lower temperature object until thermal equilibrium is reached. The thermal energy that flows between two objects because of a difference in temperature is called heat, Q. Since heat is a form of energy it is measured in Joules, J. For historical reasons another unit of thermal energy or heat is sometimes used: calorie, cal or kilocalorie, kcal. Conversion Factor 1kcal = 4186J

When work is done on or by an object there is a change in the object’s kinetic energy or its gravitational potential energy or both. The change in kinetic energy is perceived as a change in object’s velocity. The change in gravitational potential energy is perceived as a change in object’s position (height above the reference level). When heat flows into or out of an object there is a change in the object’s thermal energy. How is the change in the thermal energy of an object perceived? A change in thermal energy is perceived as a change in the object’s temperature or phase (solid, liquid, or gas). It has been observed that the change in temperature and the change in phase never occur at the same time.

When the temperature is changing the phase remains constant and when the phase is changing the temperature remains constant. or

What variables determine the magnitude of the change in temperature? More heat results in a larger change in temperature: change in temperature is directly proportional to the amount of heat.

When the same quantity of heat flows into (or out of) a larger mass the change in temperature is less.The change in temperature is inversely proportional to the mass.

When the same quantity of heat flows into (or out of) equal masses of different substances the change in temperature is different. The change in temperature depends on the specific heat, c of the substance. The specific heat of a substance is the amount of heat required to change the temperature of 1kg of the substance by 1C°.

The substance with the higher specific heat experiences a smaller change in temperature. The change in temperature is inversely proportional to the specific heat.

Combing All of the Proportions Heat flow that results in a phase change can not be described by the equation above since during a phase change the temperature remains constant…  T=0. It has been observed that phase changes can only occur at certain temperatures which depend on the particular substance.

Temperature remains constant Ice begins melting 0°C 0°C is the melting (or freezing) point of water. As more heat is added the temperature will remain constant as more ice melts. Once all of the ice has melted the addition of more heat will result in an increase in the temperature of the water. Temperature increases

Temperature remains constant Water begins boiling 100°C is the boiling point of water. As more heat is added the temperature will remain constant as more water converts to steam. Once all of the water has converted to steam the addition of more heat will result in an increase in the temperature of the steam. Temperature increases

What determines how much ice melts or water converts to steam? More heat results in a larger mass of ice melting. The mass of ice melted is directly proportional to the quantity of heat.

When the same quantity of heat flows into (or out of) equal masses of the solid phase of different substances, each at its melting point, different masses will melt. The mass that melts depends on the latent heat of fusion, Q f of the substance.

The latent heat of fusion of a substance is the quantity of heat required to change 1kg of the solid phase of the substance, at its melting point, to 1kg of liquid at the same temperature. The substance with the greater heat of fusion experiences less melting. The mass melted is inversely proportional to the heat of fusion

Combing All of the Proportions There is a similar equation describing the phase change between liquid and gas. Q v is the latent heat of vaporization, the quantity of heat required to change 1kg of the liquid phase of the substance, at its boiling point, to 1kg of gas at the same temperature.

Summary of Thermodynamic Properties and Relationships I) Phase Change Temperatures- °C, K, °F A) Melting / Freezing Point, T f B) Boiling Point, T b A) Solid Phase, c solid B) Liquid Phase, c liquid C) Gas (vapor) Phase, c gas 1. c p, constant pressure 2. c v, constant volume

A) Latent Heat of Fusion, Q f B) Latent Heat of Vaporization, Q v IV) Thermodynamic Relationships

Example: Water T f = 0°C, 273K, 32°F T b = 100°C, 373K, 212°F

Graphical Representation of Heat Flow, Temperature Change, and Phase Change TfTf TbTb T initial T final Step #1 Raise the temperature to the melting point Step #2 Melt all of the solid Step #3 Raise temperature of liquid to boiling point Step #4 Convert the liquid to gas Step #5 Raise the temperature of the gas to the final temperature

Calorimetry

When heat transfer between two systems occurs inside of an insulated environment, no heat is lost to the external environment and no heat enters the systems from the outside. In this case we can say that the heat lost by the high temperature system equals the heat gained by the low temperature system. Further, we can recognize when the two systems have reached thermal equilibrium by noting when the temperature of the combined system remains constant. Suppose we place a known mass of tap water (m w ) into an insulated calorimeter cup and measuring its initial temperature (T i ). Then a small amount of ice (assumed to be at 0°C) is added to the cup, and when thermal equilibrium is reached, the final temperature (T f ) is measured. Heat will flow from the tap water into the ice.

The temperature of the tap water will decrease while the ice will first melt and then the temperature of the ice water will increase until a final common temperature is reached. The mass of ice (m i ) added can be determined by measuring the increase in the amount of water in the cup.

Procedure Step 1. Measure the mass of the empty calorimeter cup…Record in Data Table 1. Step 2. Place about 150ml of tap water in the calorimeter cup and measure the mass…Record in Data Table 1. Step 3. Insert the temperature probe, and measure the initial temperature (T i ) of the tap water …Record in Data Table 1. Step 4. Remove the temperature probe. Place 3-4 pieces of ice in the calorimeter cup and quickly replace the lid. Step 5. Place the Styrofoam cover on the cup and insert the temperature probe. Wait until thermal equilibrium is reached (temperature reading remains constant). This will ensure that all of the ice has melted and all of the water has reached the common final temperature. Record the final equilibrium temperature in Data Table 1.

Step 6. Remove the temperature probe and lid. Measure the final mass and record in Data Table 1.

Measure mass of cup and water, subtract the mass of the cup to determine the mass of tap water - m w, kg. Measure initial temperature of tap water -T i, °C. Measure mass of cup. Place about 150ml of water in the cup. Insert the CBL temperature probe.

Measure final mass of water, subtract initial mass to determine mass of ice added - m i, kg Measure final equilibrium temperature -T f, °C Add 3-4 pieces of ice to the water in the cup. Quickly place the top on the calorimeter cup and reinsert the temperature probe. Wait for thermal equilibrium to be reached. Temperature Constant Place the inner cup inside the outer cup.

Step 7. Calculate the heat of fusion of ice.