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Calorimetry Question (1 Viewer)
- Thread starter lisarh
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Jase
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Re: 回复: Calorimetry Question
Don't quote me on this but i think its because the calorimeter is insulated from the surroundings. Any measurements on heat wont be accurate if heat energy is escaping into the surroundings.
Don't quote me on this but i think its because the calorimeter is insulated from the surroundings. Any measurements on heat wont be accurate if heat energy is escaping into the surroundings.
The heat released and absorbed from chemical reactions can be readily measured using the specific heat capacity. If it is an exothermic reaction, heat is released, causing the temperature of water to increase. On the other hand, if it is endothermic, heat is absorbed from the water, causing its temperature to fall.lisarh said:
As many chemical reactions occur in solution, we are able to measure the molar heat of solution through the use of a calorimeter. A calorimeter provides a 'closed' environment, in the form of an insulated container, hence preventing energy loss or gain from the surroundings. The heat released or absorbed by a reaction changes the temperature of the surrounding body of water. From here, we are able to measure the temperature change through the use of a thermometer and in turn, calculate the molar heat of solution.
It is important to note whether the calorimeter is able to absorb heat or not as it may affect your final calculations. In most schools styrofoam/polystyrene cups are often used as they are assumed to have minimal heat capacity.
Moreover, care must be taken to mix the solution well to ensure that there is no accumulation of hot/cold water in one area of the calorimeter.
johndownes
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So that not much heat is lost/gained to effect the reading on the thermometer.
Aerath
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To improve the accuracy of calorimetry measurements, various factors must be taken into account:Twickel said:
Insulation
The calorimeter vessel must be insulated to avoid heat losses to the environment so that the measurement of temperature change is as accurate as possible. The usual approach is to use an insulator that absorbs little heat: one with a high specific heat capacity. Common materials used, along with their specific heat capacities, in J/K/g include polystyrene (1.3), polyethylene (2.2), cork (2.0), paper (1.9), wool (1.4) and air (1.0). In comparison, glass has a specific heat capacity of 0.6 J/K/g.
Thermal capacity
When heat is liberated from a reaction, it is absorbed not only by the products, but also by the reaction vessel. Each of these has a different mass and specific heat capacity and therefore, different thermal capacities. Accurate experimentation requires calculating the heat that each of these materials absorbs. For example, consider 50g calorimeters made of glass or copper. If a reaction in these vessels causes a temperature rise in the water and the vessel of 1oC, then the heat gained by these vessels is 30J and 20J, respectively.
Thermal conductivity
The previous example assumed that the reaction solution and vessel both reached the same temperature during the experiment. This is probably true for the copper calorimeter, with its high thermal conductivity of 401 J/s/K/m. However, other materials used, along with their low thermal conductivities in J/s/K/m, include glass (1), polystyrene (0.08), polyethylene (0.3), water (0.6), air (0.02) and cork (0.05). Thus, a polystyrene foam calorimeter would absorb a negligible amount of heat during an experiment, making it an ideal material for a high-school calorimetry experiment.
Amongst other things....
Um. You could calculate how much energy has been absorbed by both, then comment on why both results differ. In turn, you could then suggest ways to improve your experiment =x.Twickel said:
uh. Just for the record, i would have thought that the beaker would have absorbed less heat energy than the copper one =/, but I'm not really sure.. so don't quote me on it
So err, copper has a heat capacity of 0.387 so thats good right more accurate then beaker? btw the copper calorimeter im using isnt really a calorimeter its just copper shaped in a cup arent calorimters meant to be closed?
Wait thats worse the higher the heat capicyt the better?
Wait thats worse the higher the heat capicyt the better?
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Hold on specifi heat capaciy of glass is 0.6
so if I do 50ml of water with a heat change of 15
0.387x50x15 Less amount of energy
0.60x50x15 Higher amount of energy
Isnt that saying that the lower the specific heat capacity the better?
so if I do 50ml of water with a heat change of 15
0.387x50x15 Less amount of energy
0.60x50x15 Higher amount of energy
Isnt that saying that the lower the specific heat capacity the better?
In the formula Δ [FONT="]h= mc[/FONT]Δ tTwickel said:
c stands for the specific heat capacity of the solution in which the reaction is taking place. So in both reactions it would be 4.18 has they both occur in water.
oh and btw im putting a spirit burner underneath the beaker and the copper cup. as i said they are not really calorimeters, more like cups. im trying to see which fuels would be better by heating the water in a copper cup and beaker both to the same temperature. incase that changes anything.
So still its better to use a beaker then copper because it has a higher heat capacity
my chem teacher is saying do this
for copper mass of copper cupx heat capicity of waterx cahnge in temp of water + mass of water x 4.18 x change in temp isnt that wrong?
So still its better to use a beaker then copper because it has a higher heat capacity
my chem teacher is saying do this
for copper mass of copper cupx heat capicity of waterx cahnge in temp of water + mass of water x 4.18 x change in temp isnt that wrong?
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Aerath
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Yeah, that's what I would've thought. Need to ask Deeming, cause I swear my source (Jacaranda) said that.12o9 said:uh. Just for the record, i would have thought that the beaker would have absorbed less heat energy than the copper one =/, but I'm not really sure.. so don't quote me on it
Why can't we use a copper sheet to cover the top of the polystyrene cup?Twickel said:
In what context? Water is 'better' as a habitat where temperature extremes are less than nearby terrestrial habitats because it moderates the temperature due to its high specific heat capacity. But that means that it's a bitch to boil water (relative to other things).Twickel said:
And for anything else I didn't reply to, I don't really understand what you mean.
According to Jacaranda, the copper cup is, but I'll go and ask my teacher today.Twickel said:
Ok today I did the prac, I was heating 50ml of water from 18 degrees to 30 degrees using ethanol and methanol as fuels.
When the water was heated in the copper cup only 0.43g of methanol was used and only 0.28g of ethanol was used. However once the 50ml of water was heated in the beaker 1g of methanol was used and 0.45g of ethanol was used.
How can I explain these results?
Obviously the reason why a little amount of fuel was used when water was in copper was because it has a lower specific heat capacity hence the water heated up quicker, however its thermal conductivity is also high so more enery is used because we can assume that the copper cup temp increased by the same amount the water did.
When the water was heated in a beaker almost double the amount of each fuel was used, but its thermal conductivity is low so we can assum that all energy released by the fuels went to the water and not beaker.
Now how can I explain the differnce in fuel consumption?
When the water was heated in the copper cup only 0.43g of methanol was used and only 0.28g of ethanol was used. However once the 50ml of water was heated in the beaker 1g of methanol was used and 0.45g of ethanol was used.
How can I explain these results?
Obviously the reason why a little amount of fuel was used when water was in copper was because it has a lower specific heat capacity hence the water heated up quicker, however its thermal conductivity is also high so more enery is used because we can assume that the copper cup temp increased by the same amount the water did.
When the water was heated in a beaker almost double the amount of each fuel was used, but its thermal conductivity is low so we can assum that all energy released by the fuels went to the water and not beaker.
Now how can I explain the differnce in fuel consumption?