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Bomb Calorimeters - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

A bomb calorimeter is used for measuring energy released in a combustion reaction. This reaction takes place in a water bath, so that the water absorbs the energy released and we can measure how much its temperature rises accordingly.

Alright so let's talk about a bomb calorimeter. A bomb calorimeters are used, they're instruments used in Chemistry and they're used for measuring energy released in a combustion reaction. So we know that combustion reactions are extremely exothermic meaning they release a lot of heat when they go forward. So how do we know how much heat they actually do release? Well we have this thing a bomb calorimeter and this is a very simple drawing of what one looks like and essentially what happens is we have inside the bomb calorimeter we have a reaction that's contained within a water bath. And so the reaction is going to take place, the combustion reaction is going to release a lot of energy notice the red around it saying energy, energy, energy and what that energy is going to do according to the law of conservation of energy is going to, the same amount of energy that's released is going to be absorbed by the water surrounding it. So we can actually determine the amount of energy that this reaction is actually releasing by looking at the temperature change of the water.

Alright so let's put it in a practice, so we have a glucose molcule and it's reacting with oxygen it's a simple combustion reaction and we're producing carbon dioxide and water and we found that, when this reaction happens, for every 1 mol of glucose that combust it produces negative or produces 2,808 kilo joules of energy. Okay so what should I expect if I have 50 grams of water in the calorimeter, so we have 50 grams in here of water and then I have 54 grams of glucose. How much energy should I expect or how, or how much of this water heat up if this is the true reaction and the true change of energy of change [IB]. So let's actually do this together, okay the first thing I want to know is, I know for 1 mol of glucose it releases 2,808 kilo joules of heat but I don't have 1 mol and I have 54 grams, well how many mols is that? Let's figure that out, so I'm going to say 54 grams of glucose I'm going to convert that to mols by looking at the molar mass and I know that's 180. So for every 1 mol, it's 180 gram and so I know that I have 0.3 mols of glucose not 1 mol I have 0.3 mols of glucose. So how much energy should be released, so I'm going to multiply it by the unit for 1 mol which is 2,808 kilo joules and that should be 842.4 kilo joules.

Okay but again notice that we, I like things in joules so I'm going into, I'm going to convert this to joules and I'll show you the reason why for a second. So my, if I want to convert this to joules I have, moving the decimal point over 3 places so 1, 2, 3 so 842400 joules. Okay so this is how much energy is released from this reaction if I have 54 of glucose. But now I want to know how much heat is going to be released from this water, so this is the energy that's released from the reaction which is also going to be equal, is also going to be the same as the energy absorbed by the water. Because the law of conservation of energy states that the energy released is going to be the amount of energy. We can't lose any energy anywhere, absorbed. Okay so if we know [IB] the temperature change I'm going to have to go to my specific heat equation.

My specific heat equation remember is q equals mc delta t okay. So this is the q for the water that is absorbed, the water is going to absorb this much. The mass of the water is 50 grams, the specific heat of water if you don't, to remember is 4.184 joules per gram degrees Celsius and this is the reason why I had to convert to joules because my specific heat is in measurement of joules. So I want to make sure that my energy is also in joules, so it's 4.184 joules per gram degrees Celsius units are really important in this case. And I want to figure out the temperature change, so I'm just going to say delta t. Okay so I multiply these 2 together and divide it by 842,400 and if get 4.03 degrees Celsius. That's why temperature change, now I want to figure out the final temperature, so my initial temperature was 25 degrees Celsius is it going to get increase in temperature or decrease in temperature? Well the reaction that is going on inside is releasing energy, that means it's going to increase in temperature so the water is going to go up. So I initially started 25 degrees Celsius and went up by 4 degrees Celsius, so 25+4 is going to equal 29.03 degrees Celsius which is my final temperature of the water.

And if that were the case, then we can agree that this is the true reaction, that if for every mol of glucose that decomposes or combust they're going to give off 2,808 kilo joules and this will prove that. Now we know that if we were to put this in practice, this actually might not be, you might not get this you're going to get some error and why is the error? Well if you look back at the chamber at the bomb calorimeter the reaction is going to release some energy but it's going to be absorbed by several things within the bomb calorimeter probably the container that the reaction is in. The water also and then also the calorimeter itself, so there're, it might not go increase the temperature of the water exactly that but it's because water, the energy is being absorbed by other thing. But just remember that energy cannot be created or destroyed according to law of conservation of energy and this should prove that.