calorimetry

[Jennifer]

what would be a good way to create a colorimeter? i need to use a candle to heat up water and calculate the total enthalpy change. But meanwhile i cannot have any heat escape from the container. What would be a great wat to create my own calorimeter in order to do this, with only using day to day items?
Duct tape is not allowed.... we already asked. It makes the containers too sticky. We must insure that nothing will catch on fire.... if our professor must expell the fire extinguisher we will recieve a failing grade. Please help me with some ideas. I have tried and thought of nothing. It would be greatly appreciatd thank you.

[barretttomlinson]

Hi Jennifer:

It sounds like you have a challenging assignment. Here is a Sciences Buddies experimental writeup that includes making a calorimeter. It does not meet your criterion for not allowing any heat to escape however, so you might be able to do better.

http://www.sciencebuddies.org/science-f ... ?from=Home

The only calorimeter type I know that would meet your criteria of perfect heat capture is the so-called bomb calorimeter, which uses a pressurized oxygen sealed container to burn the sample. I doubt you can use that for both safety and cost considerations.

Have fun!

Barrett Tomlinson

[Craig_Bridge]

Your problem is similar to many exahust gas heat recovery heat exchanger design problems. You should look these up along with heat transfer by conduction and convection. You can't get anywhere close to 100% transfer; however, you can take steps to minimize your losses using a counter flow heat exchanger like design. Some things to consider:
1) Evaporation of liquid causes both a loss of mass and heat of vaporization times that mass. If you are using distilled water, the heat of vaporization is known so you can weigh before and after and determine what you lost via this path as long as your instructor allows you to do this adjustment.
2) By minimizing the amount of time the candle burns you will decrease the losses; however, it will also decrease the temperature difference making it harder to make accurate temperature measurements.
3) A narrow chimney can be used to keep the combustion gas contained and usable as a convection current heat exchange to the container with water being heated.

You can build a heat exchanger out of two different sized cans, turn the larger one upside down and cut a hole about half the diameter of the smaller can in the middle of the top to let out the combustion products and cut slots near the bottom to let in combustion air. The total area of these slots should equal the area of the top hole. Punch a couple of holes on opposite sides just under the top lip of the smaller can and attach a small gauge wire to hang the can from a nail laid across the vent hole in the larger can so that there is about 1/4 inch between the the top lip of the inner can and the vent end of the larger can. Note: you may have to adjust the height of one or both cans so that the candle will fit under the inner can. If you use a thin wall aluminum beverage can that has a domed botom for this inner water container, the hottest portion of the exaust gas will get trapped on the bottom surface of the can and have to cool down to escape making the exaust gas convection path start with a reverse flow. If you use a steel can for the outer chimney, it will take longer for its outer wall to heat up so less energy will be lost to the room air. If you can get the difference in diameters of the two cans to be between no more than 1 inch and closer to 1/2 inch, you will maximize the heat exchange of the chimney.

Don't assume that your instructor will let you submit something like what I describe without your understanding it and being able to explain why and how it works. I've given you quite a few tuning hints at some dimensions and materials that are close to optimal. You need to learn and think about the thermodynamics involved to understand them.

Have fun, you can learn a lot of thermodynamics from this experiment and various aparatuses that different people try.

[davidkallman]

Hi Jennifer,

Some possible bad news, in:
http://answers.yahoo.com/question/index ... 649AAyfA6E
(How can you make a homemade calorimeter?),
it's stated that "measuring the calories of a lemon would be very difficult with any type of homemade calorimeter. This page gives two issues (difficulty in burning the sample and accuracy) with the calorimeter. These issues would apply not just to lemons but to any food item.

I’m not sure if the first issue is valid, but the second issue appears valid.

The question above was generated as a result of the previously referenced sciencebuddies query:
http://www.sciencebuddies.org/mentoring ... p017.shtml
(Burning Calories: How Much Energy is Stored in Different Types of Food?).

I hate to be posting a message that raises a problem with a sciencebuddies write-up. So, please do your experiment and let us know how it works out. Note: everything you read on the internet could be suspect. And, I welcome other thoughts.

[Louise]

Hi Jennifer,

Some possible bad news, in:
http://answers.yahoo.com/question/index ... 649AAyfA6E
(How can you make a homemade calorimeter?),
it's stated that "measuring the calories of a lemon would be very difficult with any type of homemade calorimeter. This page gives two issues (difficulty in burning the sample and accuracy) with the calorimeter. These issues would apply not just to lemons but to any food item.

I’m not sure if the first issue is valid, but the second issue appears valid.

The question above was generated as a result of the previously referenced sciencebuddies query:
http://www.sciencebuddies.org/mentoring ... p017.shtml
(Burning Calories: How Much Energy is Stored in Different Types of Food?).

I hate to be posting a message that raises a problem with a sciencebuddies write-up. So, please do your experiment and let us know how it works out. Note: everything you read on the internet could be suspect. And, I welcome other thoughts.

These are both valid concerns, which is why the sciencebuddies project recommends burning dry, high calorie foods. (In practice, the incomplete combustion issues is the one I've seen raised most often on these forums, particularly when students try other foods.) Just like any measurement, knowing the limits of your instrument is an important consideration.

People who do calorimetry for research often use something called a bomb calorimeter (as Barrett mentioned):
http://en.wikipedia.org/wiki/Calorimeter

(other calorimeters are also discussed in that link, including the coffee cup calorimeter. ) Bomb calorimeters are designed to both assure complete combustion (pressurized with ~20 atm of pure oxygen- hence the 'bomb'.) and to be extremely well insulated.

In this case, better engineering of the insulation will help with the second issue Dave raises (preventing heat loss should improve the accuracy for a given sample (assuming complete combustion)), and clever sample preparation will help with the first (incomplete combustion). In other threads on this project, several recommendations have been made for sample prep- particularly drying the sample and grinding it up to assure more complete combustion. Validating your instrument over many trials with a known quantity of food (with a specific calorie content) is important to determine the accuracy and precision of the instrument.

In this case, if I understand Jennifer correctly, she is just measuring the heat change of water, and not any tricky food sample. Furthermore, she isn't actually combusting anything, just heating water with a candle (so I am not sure how the teacher can evaluate that no heat is lost). So, focusing purely on the insulation is a good strategy, if you want to start with something like the coffee cup calorimeter. Craig gave you some great suggestions, and his apparatus sounds extremely non-flammable- so you wouldn't risk the 'fire-extinguisher'!

Louise

[davidkallman]

Louise, excerpting the last paragraphs of your post, which summarizes the rest of the post (hope you don't mind),

In this case, better engineering of the insulation will help with the second issue Dave raises (preventing heat loss should improve the accuracy for a given sample (assuming complete combustion)), and clever sample preparation will help with the first (incomplete combustion). In other threads on this project, several recommendations have been made for sample prep- particularly drying the sample and grinding it up to assure more complete combustion. Validating your instrument over many trials with a known quantity of food (with a specific calorie content) is important to determine the accuracy and precision of the instrument.

In this case, if I understand Jennifer correctly, she is just measuring the heat change of water, and not any tricky food sample. Furthermore, she isn't actually combusting anything, just heating water with a candle (so I am not sure how the teacher can evaluate that no heat is lost). So, focusing purely on the insulation is a good strategy, if you want to start with something like the coffee cup calorimeter. Craig gave you some great suggestions, and his apparatus sounds extremely non-flammable- so you wouldn't risk the 'fire-extinguisher'!

Louise, thanks for pointing out that the project here related to boiling water, not measuring the caloric value of food.

The second issue, accuracy, however, was not related to the design of the calorimeter. Instead, it related to the measured quantities. Quoting from the referenced page re:accuracy:

"The second problem is that lemons contain very few carlories so that your potential error will be on the order of the number of calories in your sample."

As you point out, Louise, accuracy can be improved by measuring dry, high calorie foods and the use of a bomb calorimeter. But, will the measuring of heating water have the result of a low calorie count?

I don't know. But, it's something to look at when, Jennifer, when performing the experiment.

[Louise]

The second issue, accuracy, however, was not related to the design of the calorimeter. Instead, it related to the measured quantities. Quoting from the referenced page re:accuracy:

"The second problem is that lemons contain very few carlories so that your potential error will be on the order of the number of calories in your sample."

Thanks for bring this up. I think this thread will be a useful reference for many students doing the calorimetry-type projects.

It seems to me that error in measured quantities is directly related to the device doing the measuring. Improve your device, and you can measure things 'better'.

The answer cited above is pretty ambigious about what they think is the source of potential error. I assume most of the potential error arises from the intrument itself and guessed insulation would be the main concern. I suppose error due to massing the water could also be on the order of the calories of the lemon and could be argued as being not related to instrment design. Dave, is this what you were thinking they meant? (Though, again, you could probably get around that problem with a redesign to change the amount of water to a quantity you could measure sufficiently well) Anyway, as I said, I believe improvements in the design of the instrument would reduce the error so that you could measure the calories in a lemon. My analysis, based purely on thinking about this, and not any test calculations or experiments, is as follows:

So the potential error referenced above comes from two main sources - incomplete combustion and the design of the apparatus. (There are many other sources of error that occur in this experiment, but I'd estimate these two are the largest.) If your calorimeter does not retain all heat, this is a source of error. If you improve the insulation, you decrease this error. Can you reduce it to the level needed to measure a single lemon? I don't know. You'd also have to make sure that this heat caused to a large enough change in the temperature of the water to detect on your thermometer, which is also a design consideration, but not one I mentioned in my previous answer. Note- the consideration would lead you to decrease the amount of water, so the temperature change would be larger. However, this could decrease the accuracy of you massing of water (see above).

Anyway, Jennifer, the point you should take away from all this, is that if you are building an apparatus, you need to decide what order of magnitude number you need to measure, and what your tolerance for error is. If you are measuring the calories in a lemon or in a block on TNT, your design parameters are going to be very different. Your problem seems a little easier than either of these extremes. Thinking through all the sources of error, and trying to estimate what each one contributes to your over all error is really useful. Talking to many people is a great idea, because people see different problems. Lastly, it is really excellent if you can do either some simple calculations to test these hypothesis abouot error and orders of magnitude or run some simple experiments to directly measure these values. Then, once your device is built- test it! Make sure it performs to your (or your teacher's) specifications.

This is a really fun project, so I hope you have a great time thinking about these things, and building it. Good luck!

Louise

[Louise]

Just to add a clarification- my first post on this topic containes a mistake. This technique (calorimetry) is used to measure the energies contained in _anything_. Before I said you aren't combusting anything (the mistake!), which is of course wrong. You are buring the candle. As I understand your project, you are measuring the calories/joules contained in a candle (provided by your teacher?... might be good to get the dimensions/burn time of the candle), using a standard technique of transfering the heat of combustion to water and then measuring the temperature change. (As I mentioned before, this sample is advantageous, since sample prep is easy. Candles burn! Lemons don't!) You need to make sure that all heat from combustion is transfered fully to the water, and that the water does not release heat to the environment. Everything else is okay- I was just sloppy in pointing out that trying to burn food wasn't the issue for this particular application.

Sorry for any confusion.

Louise

[davidkallman]

Hi Louise,

You got it almost correct; my interpretation of the referenced webpage was related to one paragraph in your post:

"The point you should take away from all this, is that if you are building an apparatus, you need to decide what order of magnitude number you need to measure, and what your tolerance for error is. If you are measuring the calories in a lemon or in a block on TNT, your design parameters are going to be very different. Your problem seems a little easier than either of these extremes."

It's my understanding of the referenced webpage is that is difficult to measure numbers at the lower extreme, and it becomes easier to measure a quantity as the measured number becomes bigger. So, the easiest number to measure is at the high end of the measurement interval.

[Louise]

Hi Louise,

You got it almost correct; my interpretation of the referenced webpage was related to one paragraph in your post:

"The point you should take away from all this, is that if you are building an apparatus, you need to decide what order of magnitude number you need to measure, and what your tolerance for error is. If you are measuring the calories in a lemon or in a block on TNT, your design parameters are going to be very different. Your problem seems a little easier than either of these extremes."

It's my understanding of the referenced webpage is that is difficult to measure numbers at the lower extreme, and it becomes easier to measure a quantity as the measured number becomes bigger. So, the easiest number to measure is at the high end of the measurement interval.

It is difficult to measure outside the operational range of the instrument. (Note, that is just as true for the high end as the low end, though the nature of the limit may be different). For the coffee cup calorimeter, the yahoo expert postulated that the value of the lemon was equal to the error of the instrument. To measure a value in this range (where magnitude of the error is equal to magnitude of the signal), you must improve your instrument, by reducing the error. This extends the operational range to lower values. This is the point of my paragraph that you quote- knowing what range your instrument is and what value you need to measure. The specific examples I've discussed has been a) the lemon, which may be outside the limit of the coffee cup calorimeter, and requires a redesign and b) 'high calorie' food, which is solidly in the range of the coffee cup calorimeter, and probably does not require a redesign.

Anyway , I believe we are moving dramatically outside the scope of the original problem. I don't think there is a problem with the sciencebuddies procedure, which I believe was a concern, as long as the student understands the range of the instrument and sources of error. I've outlined several possible sources of error for the calorimetry project, and suggested that sample calculations or trials be run for a given object of interest. Unless Jennifer has a specific question about our conversation, or another student wishes to ask about the calorimetry of a specific food item, I think there is limitted utility in continuing with this topic.

Louise

[davidkallman]

Hi Louise,

Re: lack of utility in continuing with this topic unless a student has a question or comment, agreed.