Chemistry

[Lois Graham]

We've collected data for "Burning Calories: How Much Energy is Stored in Different Types of Food." We're having trouble calculating Q water = mc^T. Can you explain where to plug in our ^T, ^w, mass of water information?

[Louise]

We've collected data for "Burning Calories: How Much Energy is Stored in Different Types of Food." We're having trouble calculating Q water = mc^T. Can you explain where to plug in our ^T, ^w, mass of water information?

I guess I don't understand what your question is, since you have the formula in your question and you measured all the variables.

Qwater = mcΔT

where:
m is the mass of the water, in grams (g); c is the specific heat capacity of water, which is 1 cal/g°C; and ΔT is the final temperature minus the initial temperature.

So, if you had 100 g water (m=100 g) and the temperature changes 5 degrees (ΔT=5 °C)


you get: Qwater = mcΔT= (100 g)*(1 cal/g °C)*(5 °C)

If you are asking something else, please post back here with clarification.


Louise

[Craig_Bridge]

In Louise's example, the units cancel to yield 500 calories.

[Lois Graham]

We've collected data for "Burning Calories: How Much Energy is Stored in Different Types of Food." We're having trouble calculating Q water = mc^T. Can you explain where to plug in our ^T, ^w, mass of water information?

I guess I don't understand what your question is, since you have the formula in your question and you measured all the variables.

Qwater = mcΔT

where:
m is the mass of the water, in grams (g); c is the specific heat capacity of water, which is 1 cal/g°C; and ΔT is the final temperature minus the initial temperature.

So, if you had 100 g water (m=100 g) and the temperature changes 5 degrees (ΔT=5 °C)


you get: Qwater = mcΔT= (100 g)*(1 cal/g °C)*(5 °C)

If you are asking something else, please post back here with clarification.


Louise

Don't the change in weight [ i.e., (w)i - (w)f ] of the foods get figured in?

[Louise]

We've collected data for "Burning Calories: How Much Energy is Stored in Different Types of Food." We're having trouble calculating Q water = mc^T. Can you explain where to plug in our ^T, ^w, mass of water information?

I guess I don't understand what your question is, since you have the formula in your question and you measured all the variables.

Qwater = mcΔT

where:
m is the mass of the water, in grams (g); c is the specific heat capacity of water, which is 1 cal/g°C; and ΔT is the final temperature minus the initial temperature.

So, if you had 100 g water (m=100 g) and the temperature changes 5 degrees (ΔT=5 °C)


you get: Qwater = mcΔT= (100 g)*(1 cal/g °C)*(5 °C)

If you are asking something else, please post back here with clarification.


Louise

Don't the change in weight [ i.e., (w)i - (w)f ] of the foods get figured in?

Talking about the project at :
https://www.sciencebuddies.org/mentorin ... ?from=Home

Don't the change in weight [ i.e., (w)i - (w)f ] of the foods get figured in?

No. The food is changed to burned food + gas (+energy, which is the quantity you care about) when you burn it, which is why it apparently changes weight. [that is, burned food + gas= the mass of the original food. When the gas leaves, you lose part of the mass] Ideally, in the "real" version of this experiment (called bomb calorimetry) you have the fire inside a every strong container (called a bomb) and there is not a mass change because no gas can leave. What you care about is the energy change, not if some of the food is as a charred solid or an escaped gas. What this measurement (change in food mass) can tell you is how complete the combustion was... ideally to get the total energy of a food item, you should have total combustion. See for example, http://en.wikipedia.org/wiki/Combustion
This type of calculation is probably beyond the scope of your project, but you can use this value qualitatively. For example, suppose one food item barely changed weight... maybe it didn't burn completely, and then you could hypothesize that the caloric value you measure is incorrect. If you think about it, you can even figure out if your number of calories is too big or too small.

Anyway...

First you calculate the heat that goes in to the water. That is what is in the equation above...

The heat that goes in to the water must have come from the food. For calculation, you can calculate the number of calories per gram. To do this, you would use the intial mass.

Hopefully this was clear. If not, ask again!

Louise

[Craig_Bridge]

Don't the change in weight [ i.e., (w)i - (w)f ] of the foods get figured in?

The total calories produced by burning the food isn't affected by the weight change.

Now if you want to calculate something like an energy density for a given food, say calories per gram of food burned, then you might want to divide the total calories produced by the change in food weight.

I say might because the unburned food may or may not have useful energy in it. The burn proceedure in this experiment is rather crude. Grinding up the food would probably increase the percentage burned; however, you would need a container to hold it. The container would absorb heat from the combustion so you would have to come up with a way of measuring that heat as well.

There are lots of scientific issues with figuring out how useful this experiment is in terms of what the data really means.

[Lois Graham]

We've collected data for "Burning Calories: How Much Energy is Stored in Different Types of Food." We're having trouble calculating Q water = mc^T. Can you explain where to plug in our ^T, ^w, mass of water information?

I guess I don't understand what your question is, since you have the formula in your question and you measured all the variables.

Qwater = mcΔT

where:
m is the mass of the water, in grams (g); c is the specific heat capacity of water, which is 1 cal/g°C; and ΔT is the final temperature minus the initial temperature.

So, if you had 100 g water (m=100 g) and the temperature changes 5 degrees (ΔT=5 °C)


you get: Qwater = mcΔT= (100 g)*(1 cal/g °C)*(5 °C)

If you are asking something else, please post back here with clarification.


Louise

Don't the change in weight [ i.e., (w)i - (w)f ] of the foods get figured in?

Talking about the project at :
https://www.sciencebuddies.org/mentorin ... ?from=Home

Don't the change in weight [ i.e., (w)i - (w)f ] of the foods get figured in?

No. The food is changed to burned food + gas (+energy, which is the quantity you care about) when you burn it, which is why it apparently changes weight. [that is, burned food + gas= the mass of the original food. When the gas leaves, you lose part of the mass] Ideally, in the "real" version of this experiment (called bomb calorimetry) you have the fire inside a every strong container (called a bomb) and there is not a mass change because no gas can leave. What you care about is the energy change, not if some of the food is as a charred solid or an escaped gas. What this measurement (change in food mass) can tell you is how complete the combustion was... ideally to get the total energy of a food item, you should have total combustion. See for example, http://en.wikipedia.org/wiki/Combustion
This type of calculation is probably beyond the scope of your project, but you can use this value qualitatively. For example, suppose one food item barely changed weight... maybe it didn't burn completely, and then you could hypothesize that the caloric value you measure is incorrect. If you think about it, you can even figure out if your number of calories is too big or too small.

Anyway...

First you calculate the heat that goes in to the water. That is what is in the equation above...

The heat that goes in to the water must have come from the food. For calculation, you can calculate the number of calories per gram. To do this, you would use the intial mass.

Hopefully this was clear. If not, ask again!

Louise

Thanks for the complete explanation - now we get it!!

[Louise]

[quote="Lois Graham]

Thanks for the complete explanation - now we get it!![/quote]


Glad to help! Good luck with the science fair.

Louise