Heat Transfer and Change of Phase

Heat Transfer and Change of Phase

Important Terms to Remember:

Heat transfer / Phase change / Boiling Point / Controls
Variables / Melting point / Evaporation / Condensation

* Lab adapted from the work of Dr. Christopher Sirola, Marquette University (2003)

Prior to lab: describe how water changes phases from solid to liquid to gas. What is required for this to take place?

Introduction

Matter exists in four common states or phases: solid, liquid, gas and plasma (which is the illuminating gas found in fluorescent and other vapor lamps). Whenever matter changes phases, a transfer of thermal energy is involved. The amount of energy needed to change a substance from solid to liquid is called the “heat of fusion” and from liquid to gas the “heat of vaporization”. Evaporation is the change of phase at a liquid’s surface whereas boiling is a rapid state of evaporation within a liquid as well as at its surface (which is why we see “bubbling” during boiling). Sublimation is that change of phase directly to a gas from a solid (as in “dry ice”). In each of these cases, the kinetic energy of molecules increases. Energy can be transferred to these molecules from a variety of sources. Various substances require different levels of energy in order to changes phases. We will investigate these phase changes and some of the factors that influence these changes.

Transitions between solid, liquid and gaseous phases typically involve large amounts of energy. If heat were added to ice (solid water) at a constant rate, phase changes from solid to liquid to gas (stream) would lead to plateaus in a time vs. temperature graph (see Figure 1 below).

T (ºC)

125

Gas phase

100Boiling point

Liquid phase

0Melting point

Solid phase

- 25

Time

Figure 1. Temperature vs. Time for a Specimen of Water.

Energy must be added to a substance such as water in order to raise its temperature. Part of that energy increases the kinetic energy of the molecules, causing individual molecules to move more quickly. During a phase change, the energy goes into weakening or breaking chemical bonds between the molecules. The amount of energy to accomplish this depends on the strength of the original bonds.

Thermal energy is the energy of random molecular motion. Heat is thermal energy transferred. If heat Q is added or removed from an object, then the temperature of the object changes. The heat added or removed Q and the change in temperature T are related by:

Q = mcT (J)(1)

where m and c are the mass and specific heat of the substance. Heat Q is transferred by conduction, convection, or radiation.

In the first experiment, electrical energy is converted into heat energy in a heating coil. This heat is transferred to water, by conduction. Heat losses from the water are miniimized by insulating the apparatus. When the heater is turned off, the water cools at a rate determined by the heat loss process.

Objectives:

To investigate the concept of heat transfer and phase changes of water
To distinguish between boiling point and time taken for water to boil
To investigate the impact of adding various substances to water on the boiling point of water
To set controls and variables in scientific experiments
To contrast the meanings of melting and boiling points
To contrast evaporation and condensation

Part 1: Phase Change

The apparatus you will use is shown below.

Power Thermometer

Supply Stirrer

Heater

Note: As you set your experimental process, consider your controls and variables – some are set for you, but not labeled as such. Consider what else you might add. Consult with your team and your instructor.

Data Collection

Record the mass of the inner canister, and then fill the canister about 1/3 to ½ full of crushed ice. Measure the mass of the canister and ice together and record the results in your own data section.

Place the small canister inside the empty larger one. Position the end of the thermometer in the middle of the small canister.

Allow the canister to sit for a few minutes until some slush (water) has formed. Gently mix the ice and slush. Using both the alcohol thermometer and the temperature probe, monitor the temperature of the slush forming in the beaker (do not add any water!). Record the temperature of the icy slush in your own data section.

Turn on the power supply and set the voltage to 10.0 V. Record the voltage (V) and current (I) from the power supply. You may need to make a slight adjustment of the power supply from time to time to hold the voltage constant at 10.0 V.

As soon as the power supply is set, switch on the timer. Record the temperature at one minute intervals; stir gently for about 15 seconds before reading the thermometer. Also record the phase of the water during each interval

(a)Note when and at what temperature ice begins to melt and when it has completely melted. What is the melting point of ice water? How does the temperature of the water change while it is melting?

Melting Point of Ice Water = ______

(b)Note the times at which the water is making the transition from solid to liquid in your own data section.

The ice will eventually melt completely. Continue to record the time and temperature until the water boils (i.e. bubbles appear). Take temperature readings for a few minutes while the water bubbles. Record this temperature as the “final” temperature in your own data section.

Note: Be careful not to burn yourself with the coil, the liquid water or the gaseous water (steam). Also, do not attempt to take the temperature of the steam itself as the thermometer might burst.

(a)Note when and at what temperature the liquid water begins to evaporate. What is the boiling point of liquid water? How does the temperature of the water change while it is boiling? Do your temperature measurement devices differ? How much? Why?

Boiling Point of Liquid Water = ______

Allow the water to cool to room temperature. Record the final mass of the water in your own data section.

Repeat this experiment with the same mass of water but now start at room temperature. Record the time and temperature (probe and thermometer) in your own data section until the water is boiling again.

Question 1:Is there any difference between the two experimentsin the time needed for water to boil? Did the boiling point change? Are there differences in the readings? Why or why not?

Graph your results of temperature vs. time for both situations.

Question 2: Reconsider the controls and variable in this procedure. Was anything added to the list? Was anything taken off the list?

Data Analysis

In this experiment electrical energy is converted to thermal energy. You will measure the energy dissipated by a metal coil in a canister of water and compare it to the change in the water’s thermal energy.

The electrical power is the current I in amperes (A) times the voltage in volts (V). The electrical energy dissipated during time t(in seconds) is:

E = IVt(Joules)(2)

Calculate how much energy was produced by the power supply while you were heating the water from its liquid to boiling point.

The heat input to the water during that time is:

Q = mw cwT + mc ccT(Joules)(3)

where the specific heats are cw = 4,186 J/kgC and cc = 900 J/kgC. This takes into account the energy needed to heat the water plus the energy needed to heat the canister. With your data calculate how much energy was used to heat the water and canister by T=100C.

Question 1. Was the heat Qtransferred to the water greater or less than the energy E dissipated by the resistive wire? Calculate the % discrepency between the heat input to the water and the electrical energy dissipated.

percent discrepancy = [E – Q/E]  100% (4)

Part 2: Newton’s Law of Cooling

As soon as the temperature starts to fall, re-start the clock and record the temperature at 3 minute intervals for about 30 minutes. Stir before making a temperature measurement.

Graph your results of temperature vs. time.

Newton’s law of cooling states that the rate at which an object cools in proportional to the difference between its temperature and the surrounding temperature. This implies that the temperature T will decreases exponentially with time t.

Question 3. Take a look at your cooling graph, when the system was cooling did it cool faster at the beginning of the experiment or toward the end of the experiment? Is this what you would expect from Newton’s Law of cooling? Explain.