Heat Of Solution (1959)

[Music] The heat absorbed or produced when one mole of a solute is dissolved to make a solution of a particular concentration and at a specific temperature is known as the heat of solution. One or more of a number of complex changes of a chemical nature such as the formation of hydrates usually accompany the ordinary physical change of solution itself. Each of these changes is associated with an absorption or evolution of heat. Also, the summation of all the heat effects of the several changes that occur when a substance is dissolved is evidently what we have called the heat of solution. It thus becomes possible to understand why the solution of a solid in a liquid is sometimes actually coupled with an evolution of heat. In these cases, the heat of fusion of the solid, which is always negative, that is heat is absorbed, is merely one of several factors in a sum and is overshadowed by positive heat factors from one or more chemical changes taking place during the solution process. The solute which we will use in this experiment is zinc sulfate which has a positive heat of solution that is it dissolves in water with the evolution of heat. Assembled on the table are the components of a crude calerimeter. The large beaker is one liter in capacity and the small beaker 400 ml. The weight of the small beaker has been determined before the experiment starts and has been found to be 137 g. A stirer has been prepared by bending one end of a glass rod into a triangle. The small beaker is now placed in the large beaker. A cardboard disc has been cut to fit into the larger beaker and over the smaller beaker and will serve as a cover. The stirer will now be inserted through the disc and placed in the small beaker. A second larger piece of cardboard serves as a cover of the large beaker and will now be inserted on the stir. A thermometer is now inserted through the two cardboard covers. This thermometer is graduated in fifths of a degree so that accurate temperature readings may be obtained. The inner beaker is supported on three corks to insulate it from the bottom of the larger beaker. The larger beaker serves merely as a container and screens the smaller beaker off from outside conditions. The apparatus will now be dissembled and prepared for the experiment. First, we must determine the dimensions of the empty small beaker. We will use a small metric ruler and first measure the diameter of the beaker. The diameter is found to be 7.5 cm. Next, the height of the beaker will be measured and is seen to be 10.5 cm. We will now place exactly 250 ml of distilled water in the beaker. And next measure the height to which the water stands in the beak. This height is 6.3 cm. Knowing the weight of the empty beaker and the dimensions we have just obtained, it is now an easy matter to calculate the weight of the glass actually in contact with the water. This will now be done at the board. You will remember that the diameter of the beaker was measured and found to be 7.5 cm. We also determined the height of the beaker to be 10.5 cm. Now applying a little elementary geometry, we can calculate the total area of the beaker and we find it to be 291.6 square cm. The water stood to a height of 6.3 cm in the beaker. And again, we can calculate the area of the beaker in contact with the water to be 192.6 square cm. The total weight of the beaker was 137 g. And it's then a simple matter uh to set up a ratio and to calculate the weight of the beaker actually in contact with the water. That weight we will put equal to x. Well, the weight of the beaker in contact with the water is to the total weight of the beaker approximately at least as the area of the beaker in contact with the water is to the total area of the beaker. And solving this equation for X, we find that the weight of the beaker in contact with the water is about 90.5 g. The stirer has previously been weighed and its weight determined to be 31.6 g. In order to determine the weight of the stirer in contact with the water in the beaker, we must first determine the overall length of the stirer. The length of the handle from the top to the triangular base has been measured and is 32 cm. The triangular base will now be measured and is found to be 4 cm on each side, making a total of 12 cm in the base and an overall length of glass in the stir of 46 cm. The stirer will now be placed in the beaker. This was done previously and a red mark has been made on the stirer at the surface of the water. The stir will now be withdrawn from the beaker and the length of the handle from the red mark to the base of the stir will be measured. This length is 6.1 cm. And since 12 cm of glass occur in the triangular base, we know that 18.1 cm of glass are in contact with the water. From this data, it is easy to calculate the weight of glass in contact with the water. The approximate weight of the stir in contact with the water uh may be calculated in a simple and entirely similar fashion. Uh we've measured the total weight of the stir, its total length and the length of the stir in the water and can then set up a simple ratio. The weight of the stir in the water is to the total weight of the stir as the length of the stir in the water is to the total length of the stir. And solving this fraction for the weight of the stir in the water uh we find this value to be 12.5 g. We must next calculate the volume of thermometer in contact with the water in the beaker. We have previously immersed the thermometer in the water and placed a small red mark at the water level. In the graduated cylinder, we have placed exactly 6 ml of water. We will now insert the thermometer into the cylinder and lower it just to the red mark. The level of the water in the cylinder has risen from 6 to 8 ml, an increase of 2 millilit. We know now therefore that the volume of thermometer in contact with the water in the beaker will be about 2 milliliters. We now need to calculate the so-called calerimeter constant uh which uh is the figure which tells us how many calories uh per degree centigrade are absorbed by the calerimeter itself. We can calculate the weight of the glass in contact with the water because we know that it will be the sum of the weight of the beaker plus the weight of the stir and those two values have been found to be 90.5 and 12.5 respectively. So the weight of glass in contact with the water is 103 g. uh by looking up in our lab book we find that the specific heat of glass is.19 calorie per gram per deg centigrade change in temperature and then simply multiplying the weight of the glass by the specific heat of glass we find that the beaker and stirer will together absorb 19.6 6 calories for each degree centigrade change in temperature. We have found the volume of the thermometer in the water to be about 2 milliliters. Now the specific heat of the thermometer bulb is a difficult figure to calculate because both glass and mercury are involved. But again reference to the lab book tells us that the thermometer constant is about49 calories per milliliter per deg centigrade. And since our thermometer occupies 2 milliliters of volume, we have to multiply that by 049 and obtain about uh one calorie uh per deg centigrade absorbed by the thermometer. The calerimeter constant then is equal to the sum of the calories per deg centigrade absorbed by the beaker and by the thermometer and in this case is equal to about 20.6 calories per degree centigrade. This figure will be used in future calculations as the calerimeter constant. Approximately 12 g of anhydrris zinc sulfate has been added to this test tube and the tube and it contents and the stopper carefully weighed. The weight of the tube and content is 38.12 gram. We will now place the tube and its contents in the water in the small beaker in the calerimeter. This is done in order to bring the zinc sulfate to the same temperature as the water. We will now allow this setup to stand for about 15 minutes. The initial temperature is determined by the thermometer and is found to be 28.25°. We will now remove the test tube. Dry it quickly with a towel and rapidly pour most of the powder into the water and the calerimeter. We replace the cover and stir vigorously. The highest temperature recorded is 33.2°. We will now weigh the tube and the remaining zinc sulfate. And we find that the tube and its contents now weigh 25.55 g. We can now proceed to the calculation of the heat of solution of zinc sulfate. The initial temperature of the calerimeter and the water we found to be 28.25°. The highest temperature which the system reached was as you saw 33.20°. Therefore, the temperature change was 4.95°. Now the weight of the tube containing the zinc sulfate was 38.12 g and then this tube after the zinc sulfate had been removed was 25.55 g. So this rise in temperature uh was caused by 12.57 g of zinc sulfate added to 250 ml of water in the calerimeter. Now we know that the specific heat of water is one. So that the number of calories gained by the water equals the weight of the water multiplied by the degrees of temperature change or 1237.5 calories. The calories gained by the water. The calories gained by the calerimeter can be calculated if we know the calerimeter constant which we previously found to be 20.6 and multiply that by the temperature change of the calorimeter. This comes out to be 102 calories. Adding these two figures together, we then have for the total calories gained by the calerimeter and the water 1339.5 calories. Now this value also equals the number of calories evolved when the zinc sulfate dissolved. And we need then to proceed to calculate the heat of solution of one mole of zinc sulfate knowing that 12.57 g evolved 1339 12 calories. This involves a simple ratio. the calories evolved over the weight of zinc sulfate used compared to x calories the heat of solution per mole divided by the molecular weight of zinc sulfate 161.4 and solving this expression for x we find x to equal 17,199 calories. This is our experimental value for the heat of solution of zinc sulfate. Our reference to the lab book shows us that the accepted value for this heat of solution is 1800 18,430 calories. Our value was 17,199. The difference is,231 calories. Our percent error therefore is the difference between the two values divided by the accepted value multiplied by 100 or 6.7% error. And considering the uh simplicity and almost crudeness of the apparatus, the lack of insulation uh of the small beaker from the large beaker and so forth. Uh this value certainly is not an excessive error. [Music]