Assumptions made when doing calorimetry / errors that occur

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Homework Statement:
What are the assumptions that we make when doing a calorimetry experiment? (when the calorimeter is a foam cup)?

What are the sources of error when doing a calorimetry experiment? (when the calorimeter is a foam cup)?
Relevant Equations:
this question does not really need any equations to be answered, but Q=mc(delta T)
I just know that one assumption is that the specific heat capacity of water (as the surrounding) stays the same (or the change in its heat capacity is negligible). Please Help me with the rest.
 
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  • #2
Borek
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I just know that one assumption is that the specific heat capacity of water (as the surrounding) stays the same
That's a bit off. We often do assume that's the case, but nothing stops you from using exact specific heat of the solution if you know it.

Which brings us to another problem with your statement: you say "water as surroundings". That's not entirely correct either. It is not water that is heat up/cooled down by the reaction heat, it is whole solution - water plus whatever the reaction mixture contained and whatever its total mass is.

Does the temperature of the calorimeter itself change?

Is the reaction mixture perfectly isolated?
 
  • #3
That's a bit off. We often do assume that's the case, but nothing stops you from using exact specific heat of the solution if you know it.

Which brings us to another problem with your statement: you say "water as surroundings". That's not entirely correct either. It is not water that is heat up/cooled down by the reaction heat, it is whole solution - water plus whatever the reaction mixture contained and whatever its total mass is.

Does the temperature of the calorimeter itself change?

Is the reaction mixture perfectly isolated?
This is part of a lab procedure question.
We are told that the specific heat capacity of the water remain roughly the same because the solution is dilute.
Reaction 1- solid NaOH + water (aq) temperature change= 5.8
Reaction 2- Solid NaOH + HCl (aq) temperature change =12.6
Reaction 3- NaOH (aq) + HCl (aq) temperature change =6.7

Yes the temperature changes.
It is not isolated perfectly; A foam up is used as the calorimeter and it has no led on it.
Sorry I didn't clarify it at first. Can you help me with it now?
 
  • #4
Borek
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We are told that the specific heat capacity of the water remain roughly the same because the solution is dilute.

And that's an assumption you are making: that the specific heat capacity doesn't change by much.

It is not isolated perfectly; A foam up is used as the calorimeter and it has no led on it.

Think what it does mean for the heat balance - is all heat used for heating the solution?

Basically questions I asked where not for you to give simple answers, but to make you understand what is going on in the experiment and realize what the assumptions are. That's the way we help here.
 
  • #5
And that's an assumption you are making: that the specific heat capacity doesn't change by much.



Think what it does mean for the heat balance - is all heat used for heating the solution?
So I know one assumption could be that calorimetry and the outside environment do not exchange energy. And one source of error could be the fact that foam cup is not isolated. but the thing is the assumption I made and the error kind of contradict each other, that's why I need help
 
  • #6
Borek
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the thing is the assumption I made and the error kind of contradict each other

Not sure what you mean by that, please elaborate.

There are still more assumptions and more error sources.
 
  • #7
Not sure what you mean by that, please elaborate.

There are still more assumptions and more error sources.
I mean when I say one assumption is that no heat is transferred between outside environment and the calorimetry then I cant say one error that can occur is that heat can be transferred between outside environment and calorimetry.
I am really struggling with this can you tell me what are other assumptions ad errors?
 
  • #8
Borek
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I mean when I say one assumption is that no heat is transferred between outside environment and the calorimetry then I cant say one error that can occur is that heat can be transferred between outside environment and calorimetry.

Sure you can say that. These assumptions are reasonably correct, but not perfect - that makes it possible to do the calculations at all. But it also means that - as the assumptions are only approximate - they can be the source of error.

We don't have better way to do calculations, so we have to assume some factors can be neglected. It doesn't mean they don't exist at all, it only means the error they result in is reasonably small.

Try to look at every question I asked you from this point of view.
 

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