SPECIFIC HEAT CAPACITY: COOLING CURVES AIM To determine the specific heat capacity of a liquid using cooling curves of liquids. APPARATUS Calorimeter, thermometer, stop watch, measuring cylinder, paraffin, massmeter, steam bath, tongs INTRODUCTION According to Newton's cooling law, the rate of loss of heat by a body is directly proportional to the excess temperature of the body above that of its surroundings. This law holds for large temperature differences provided the body cools under the influence of a strong draught. This law may be written as dQ k0 (1) dt Where 0 represents the excess temperature. The rate of loss of heat by a body depends upon the nature and extent of the radiating surface. It also depends upon the temperature of the body and that of the surroundings. The particular form of the cooling law need not be known if the conditions of cooling are identical in the two cases. Then at any given temperature the rate of loss of heat by the calorimeter and contents must be the same. That is, if both liquids fall through the same temperature range, then dQ₁_dQ₂ dt₂ (2) dt, If m, m₁ and m2 are the masses of the calorimeter, water and paraffin respectively, c, C₁, C2 are their respective specific heat capacities, de de and dt dt The rates of fall of temperature of water and paraffin respectively at temperature 0, then equation (2) can be rewritten in the form de (m, c, + mc •{(do) 1 = (m₂c₂. 1 = (m₂c₂ + mc) (de) 2 dt dt 25 Practical Coordinator Ms M.N Mphelane = PROCEDURE Past test No me / Nowe 25. 1) Record the temperature of the room. 2) Measure the mass of the calorimeter 7. g 3) Fill the calorimeter with a specified volume of water 4) Measure the mass of the calorimeter and water 5) Heat the calorimeter in a water bath to at least 20°C above the room temperature. N.B: Do not heat the liquids too high to minimize evaporation 6) Remove the calorimeter from the water bath 7) Starting at a certain temperature, take temperature reading every 2 minutes as the water cools down N.B: The liquids must be stirred continuously as the readings are taken 8) Clean out the calorimeter and refill it with an equal volume of paraffin. 9) Repeat steps (4) to (7). 10) Fill table 1 and 2 Table 1: Masses Calorimeter Calorimeter + Water only Water Calorimeter + Paraffin only 7.9 27.0 paraffin тал 22.8 14.9. Table 2: Cooling rate t (min) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 °C Water 45 41 38 35 33 31 30 29 28 275 27 26.5 26 25.5 25 25 0 °C Paraffin 45 /40/37 30 29 28 /27 25. hs 23 11) On the same axes, plot graphs of temperature against time for water and for paraffin 2 grap 12) Draw a horizontal line on the graph at room temperature and label it. 13) Which liquid cools faster and why? Paraffin 14) Calculate specific heat capacity of paraffin. Q-MCAT Physica/hemmedo go - heat energy Qw=Qp say why. Bon't and Discussion ma -t Why paraffin cods juster & Concluse Practical Coordinator Ms M.N Mphelane 26 °C 2011 26
