to measure the specific latent heat of vaporisation of water (1)

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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.
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