To Measure The Specific Latent Heat of Vaporisation of Water. Background Information: 1. When a substance warms or cools, heat energy is transferred from or to the substance, and the substance changes temperature (up or down). Heat energy used depends on: The mass- M Specific Heat Capacity- c Change in temperature -∆θ 2. When a substance is changing state, it can take in or give out heat Example: A thermometer will show that water will remain at 100C while it is boiling. Melting ice will stay at 0c, even though heat energy is being added to it. 3. The heat energy needed to change state without change in temperature is called The Latent Heat. The latent heat needed to change from a solid to a liquid is called The Latent Heat of Fusion 4. Definition: The Specific Latent Heat Of Fusion of a substance is the amount of heat energy needed to change 1Kg of that substance from a solid to a liquid without the change in temperature. 5. Unit Of Specific Latent Heat: joule per kilogram (Jkgˉ1) 6. Formula to find the latent heat of fusion: (Heat gained by water)+ (Heat gained by calorimeter) = (Heat given out by steam in turning to water at 100C) + (Heat lost by condensed steam) Or (M of water × C of water ×∆θ) + (M of copper calorimeter × C of copper × ∆θ) = (M of steam × latent heat of fusion) + (M of steam ×C of water × ∆θ) The values of all the variables in this formula are known, except for l (latent heat of fusion) , hence l can be calculated. Equipment Needed: 1. 2. 3. 4. 5. 6. 7. Stand Round Bottom Flask Bunsen Burner Two Thermometers Steam Trap or Tube Wrapped in Cotton Wool Calorimeter Lid and Insulation for Calorimeter. Precautions: To avoid melting the crushed ice, transfer it with a plastic spatula. Use a stirrer to ensure the heat energy is evenly distributed Ensure the calorimeter is insulated so that heat does not escape into the environment Use a lid to prevent the loss of heat to the environment Use a low specific heat capacity thermometer to ensure minimal energy is removed from the water in order to raise the length of the column of liquid. Use a digital thermometer correct to 2 decimal places.. Polish the inside of the calorimeter before the experiment as the shiny surface will reflect heat inwards, thus preventing heat loss to the environment. Ensure the temperature of the water begins 7˚C below room temperature and finishes 7˚C above room temperature. By starting below room temperature some heat will be taken in from the environment and when the temperature rises above room temperature heat will be given off. By starting and finishing with the same degree of difference to room temperature heat lost will balance heat gained, thus negating the effect on our results and making them more accurate. Also, because we are using cold water the steam will cool more quickly; thus the experiment will happen faster and hence less heat will be transferred in or out. Lastly, using cool water allows us to use a greater mass of steam which will reduce percentage error in reading the mass of steam. Method: 1. 2. 3. 4. 5. 6. 7. 8. Set up apparatus as seen in diagram. Put the dry, empty calorimeter on a balance scale to find mass of calorimeter. mcal. Fill calorimeter about ½ full of cool water, about 70C under room temperature*. Find the mass of the calorimeter and water. m1** The mass of the cool water can now be calculated by mw is m1 – mcal. Record the temperature of the calorimeter plus water θ1. Boil water in round bottom flask. Place calorimeter in insulation and lid to prevent heat loss and gain to the surrounding environment 9. To ensure the steam used is dry***, we use a steam trap. (in the absence of a steam trap, we can use a rubber tubing insulated with cotton wool, BUT the first steam will condense and we must wait until only steam is coming out of the delivery tube before inserted it UNDER the cool water in the calorimeter). 10. Immerse the end of the steam tube into the water. 11. Heat the water until about 70C above room temperature. 12. Record the final temperature θ2 of the calorimeter plus water plus condensed steam. The fall in temperature of the steam θ1 is 100 °C – θ2. 13. The rise in the temperature of the calorimeter plus water θ 2 is θ2 – θ1. 14. Once experiment is completed immediately find the mass of the calorimeter, water and steam, m2, so to determine mass of steam. (The mass of the condensed steam ms is m2 – m1). Data Collected: Room TemperatureMass of water- Mass of CalorimeterFinal Temperature- Initial TemperatureMass of Steam- Apply Information to the following equation to find latent heat of vaporisation: (M of water × C of water ×∆θ) + (M of copper calorimeter × C of copper × ∆θ) = (M of steam × latent heat of fusion) + (M of steam ×C of water × ∆θ) * The temperature must begin about 70C below room temperature and finish 70C above room temperature because this ensures that any heat lost or gained to the surrounding atmosphere will balance. ** To determine the mass of the water, subtract the mass of calorimeter from the mass of the calorimeter and water. Also to determine the mass of the steam subtract the mass of the calorimeter and water from the mass of the steam, water and calorimeter. ***The steam must be dry so the result found for latent heat of fusion is purely steam, not additional water Results: Mass of the calorimeter Mass of the water plus calorimeter Mass of the cooled water Temperature of the calorimeter plus water Final temperature of the calorimeter plus water plus condensed steam Fall in temperature of the steam Rise in the temperature of the calorimeter plus water Mass of the calorimeter plus water plus condensed steam Mass of the condensed steam mcal m1 mw θ1 θ2 = = = m1 – mcal = = = θ1 = 100 °C – θ2 = = θ2 – θ1 = θ 2 m2 = ms = m2 – m1 = Energy lost by steam = energy gained by calorimeter + energy gained by the water msl + mscw θ1 = mcalcc θ 2 + mwcw θ 2 . Additional Information: If you are asked to transfer hot copper into a calorimeter: Energy lost by copper rivets = energy gained by copper calorimeter + the energy gained by the water mcocc θ1 = mcalcc θ 2 + mwcw θ 2 . Ensure that the rivets are small and that you have a thermometer in the middle of them. Leave for 15 minutes after the water comes to the boil to ensure the copper is at 1000C. If you are asked the room temperature: Answer: It is half way between the initial temp and the finishing temp.