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Quick question: Grade 12 stuff (Heat capacity)

  1. May 3, 2006 #1
    Chemistry question: Grade 12 stuff (Heat energy)

    So we're going to have a lab soon, and its goal is to find the heat energy value (Q). We're going to take a beaker filled with water, record its mass and initial temprature, and then bring a really hot iron washer inside and record its final temprature.

    The first step is to calculate the heat energy the water gained, which is Q (The heat energy of the surroundings). Q = mcT, we have all the values. We're good on that part.
    The second step is to calculate the energy the iron washer lost, that's just the negative of the first answer.
    The third step is to find the change in temprature of the iron washer.. we have the heat capacity (c) of iron, mass, and heat energy (Q) lost, so it's easy to find the T value (re-arranging the equation).

    However, the last question asks us to find the temprature of the bunsen burner flame. How do we do that? Any hints? Is it the same as the temprature change of the iron washer?
    Last edited: May 3, 2006
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  3. May 3, 2006 #2


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    You heated the iron washer inside the hottest part of the bunsen flame for "a long time" right ? You have the change in temperature experienced by the washer after equilibriation with the water bath, and you've calculated the change in temperature whilst in the water bath. So what's the initial temp of the iron washer (just after withdrawing from the flame) ?

    What does the zeroth law of thermodynamics state ?
  4. May 3, 2006 #3


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    One other subtle point is that radiant loss of heat from the iron washer is being disregarded. While it is in the flame, it will be negligible (since the environment is hotter than the washer), but once removed from the flame (in mid-air) and all the time in the water bath, it's losing a significant quantity of heat by radiation. I don't think the insulation used in this experiment can stop the radiant loss (did you silver the exterior of the beaker in addition to wrapping it up with air filled materials) ?
  5. May 3, 2006 #4

    So what you're saying is, the bunsen flame's temprature is the change of temprature of the water + the rate of temprature of the washer?
    I can calculate both, so I can just add them up to get the total change (AKA the temprature gained by the flame or in other words the temprature OF the flame?)

    Zeroth's law: Meaning #1: the law that if two bodies are in thermal equilibrium with a third body then the first two bodies are in thermal equilibrium with each other

    We didn't study that yet, but I think I got it right. My mind is just really mixed up now, help me clarify this?

    Thanks for the help!
  6. May 4, 2006 #5


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    No, I don't get where you're bring "rate" into it. I'm simply saying that the temperature of the washer after heating in the flame would be equal or close to the temperature of the flame because thermal equilibriation would've occured between washer and flame. Since you can measure the final temperature of the washer (which is the same as the final temperature of the water bath (since thermal equilibriation has occured between washer and water) and you know the change of temperature the washer has undergone since being introduced into the water, you can compute the temperature of the washer just after the flaming has been done. That's the temperature of the flame.

    It's easier to put it mathematically. Here Tfinal refers to the final common temperature of the water bath and washer, Tinitial, water refers to the starting temp of the water and Tinitial, washer refers to the hottest temp of the washer (which is the temp of the flame).

    Then by Conservation of Energy, [tex]m_{water}c_{water}(T_{final} - T_{initial,water}) = m_{washer}c_{washer}(T_{initial,washer} - T_{final})[/tex]

    most of which you've already figured out. Every quantity there is either given or measurable except for the [tex]T_{initial,washer}[/tex] which you have to find by algebra and this is the temp of the flame.

    I've already mentioned some of the limitations of this simple approach - the inability to account for radiant losses, etc.

    I was bringing up the zeroth law because it formalises the concept of temperature and its relation to thermal equilibriation. It's a deceptively simple concept which many people forget about.
  7. May 4, 2006 #6


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    assuming that iron had reached a temperature equilibrium with the flame and ignoring the cooling effects, the initial temperature of the iron would have been the temperature of the flame.
  8. May 4, 2006 #7
    Why is it T(initial, washer) - T(final)?
    Shouldn't it be T(final) - T(initial, washer) - in the conservation of energy equation that you posted.
  9. May 4, 2006 #8
    Well, if there is a heat transfer, don't you think one of the objects (the washer) would have a higher initial temperature than the water?
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