Latent Heat Introduction The amount of heat transfer per unit mass required to produce a phase change (with no temperature change) in a substance is called the latent heat. If the phase change is from solid to liquid (or vice versa), then it is called the latent heat of fusion. If the phase change is from liquid to gas (or vice versa), then it is called the latent heat of vaporization. This relation is given by the following equation:  Q = ±mL Q = heat (J) m = mass of the substance (kg) L = latent heat (J/kg) The sign in the equation must be put in manually to indicate the direction of the phase change. The purpose of this lab is to test the theoretical value of heat of fusion for water. In this experiment, we will throw ice cubes into hot water. We can use the principle of conservation of energy along with the accepted value of the specific heat capacity of water and experimental values of temperature and mass to calculate the experimental value of heat of fusion for water. A conservation of energy statement for this experiment can be written as follows:  ΣQ = 0 In this experiment, the ice will melt and then change its temperature to the equilibrium value. The hot water will simply change its temperature to the equilibrium value. These three processes need to be taken into account in equation . Also, recall that the relationship between heat and temperature change is Q = mcΔT. Equation  can be written more specifically as follows:  miLf + micw(Teq – Ti) + mhcw(Teq – Th) = 0 mi = mass of ice Lf = heat of fusion for water cw = specific heat capacity of water mh = mass of hot water Teq = equilibrium temperature Ti = initial temperature of ice Th = initial temperature of hot water Equation  can be solved for Lf in terms of known or measurable quantities. This is left as an exercise for the student. Experimental Procedures 1) Measure and record the mass of a Styrofoam cup and lid. Fill the cup about half full of hot tap water and measure the total mass with the lid placed next to the cup. Calculate the difference to obtain the mass of the water. 2) Measure and record the initial temperature of the water once the temperature is fairly steady. 3) Using a towel, dry off cubes of ice and put them in the hot water. Add 3 to 6 cubes and cover. Occasionally gently shake the cup until the temperature is steady. Record the final equilibrium temperature. 4) Measure the final mass of the cup and water. Calculate the difference between this value and your previous measurement to obtain the mass of the ice. 5) Use equation  to calculate the latent heat of fusion for water. Compare your experimental result to the theoretical value of (3.34 ± 0.01) E5 J/kg. The specific heat of liquid H2O is (4190 ± 15) J/(C° kg). 6) Complete this experiment two more times with a different initial temperature of water, different amount of hot water, or different amount of ice.
Pages to are hidden for
"Latent Heat - DOC"Please download to view full document