Constant-Volume Calorimetry - Video Tutorials & Practice Problems
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Constant-Volume Calorimetry uses a bomb calorimeter and a combustion reaction to determine its enthalpy of reaction.
Constant-Volume Calorimetry
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concept
Constant-Volume Calorimetry
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Here we're going to say that the heat of combustion is the amount of heat released when a mole of a substance is burned or combusted. Now recall a combustion reaction normally involves a compound with carbon and hydrogen or carbon hydrogen and oxygen reacting with o 2. Of course, there are other elements that could be involved such as sulfur or nitrogen, but the most common types of combustion reactions are just carbon and hydrogen or carbon, hydrogen, and oxygen. Now we're gonna say associated with any combustion reaction is a heat of combustion value, which we represent as delta h not, so that little circle, combustion. So we'll abbreviate it as comb. So just remember when discussing constant value volume calorimetry, we associate it with a combustion reaction.
Heat of Combustion is the amount of heat released when 1 mole of substance is burned or combusted in a Bomb Calorimeter.
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example
Constant-Volume Calorimetry Example 1
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Here it says, which of the following statements is true about the combustion of propane? So propane is c 3h8 gas. It reacts with 5 moles of oxygen gas to produce 3 moles of carbon dioxide gas plus 4 moles of water vapor. It has an enthalpy of combustion that's equal to negative 2,222 kilojoules. Here it says it is endothermic, it is exothermic, it absorbs heat from the surroundings, or none of the above. So remember, a combustion reaction is accompanied with a negative delta h value which signifies that it is an exo thermic process. It releases heat to the surroundings. So it is exothermic, not endothermic, and here it's releasing heat to the surroundings. If you're absorbing heat from the surroundings, you are not exothermic, you are endothermic. So out of all the choices, only option b is correct.
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Constant-Volume Calorimetry
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Here we're going to talk about a bomb calorimeter. Now a bomb calorimeter is a steel container with a combustible sample submerged in a known quantity of water, and we're going to say with a bond calorimeter we have the concept of constant volume calorimetry, and this is used to determine the heat released during a combustion reaction. Now here with this whole idea we also have constant volume itself. This just means that the calorimeter has a fixed volume that doesn't expand after the sample is combusted. So just think about combustion reaction as kind of a controlled explosion. So there's a explosion that goes on within the bomb calorimeter itself, but the volume stays fixed. It does not expand outward, And we're going to say since it's a combustion reaction, that would mean that it's exothermic. This would mean that our enthalpy of standard combustion for this reaction would be equal to negative q lost. So negative q, the amount of heat that's lost by the reaction. Now if we take a look here we're gonna first look at the bomb calorimeter itself, its components. So with the bomb calorimeter we're gonna have up top these two wires here. These are actually the fuses that help to ignite the combustion reaction. We're going to say here that they go down here and inside of this orange box this represents our combustible sample. So we'll just say that this is our sample. This reaction occurs at some designated temperature and we know that through the use of a thermometer. So here our thermometer is saying it's happening at 50 degrees Celsius. Here we're going to say that this blue outline here is actually our water, and the heat that's combusted that's released by our sample. We want to make sure that heat is evenly distributed throughout the water sample, so we use this stirrer. And then finally itself we have our our, bond calorimeter. So our bond calorimeter the sample is combusted, gives off heat, the heat exits the bond calorimeter and it goes into the water. The water there, its temperature is read by the thermometer. The heat is dissipate dissipated evenly throughout the use of the stirrer. Now here with the bomb calorimeter we also have our constant volume formula. Here we're going to say this is when both the liquid and calorimeter absorb heat from the hot object. Here we're going to say that it's equal to negative q loss, so the amount of heat lost equals the amount of heat gained. So gaining heat means Q is positive plus the calorimeter itself. Remember up here we said that standard enthalpy of combustion is equal to negative q loss, so here these are the same. So negative q, loss equals our standard enthalpy of combustion. We're going to say that this amount of heat gained is related to this portion. Since the heat gain is positive, then this mcat is also positive, And then we're going to finally say that our calorimeter itself, it uses our heat capacity which is capital C times the change in temperature. So we would say that this portion when we look at it is the constant volume formula where our standard enthalpy of combustion equals positive MCAT plus our heat capacity times change in temperature.
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example
Constant-Volume Calorimetry Example 2
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Here it says, the heat capacity of a bomb calorimeter was determined by burning 12.13 grams of ethane. We're told the heat combustion equals 1,000 500 and 60 kilojoules per mole in the bomb. If the temperature changed by 15.2 degrees Celsius, we we have to determine the heat capacity at the bomb calorimeter. Alright. So remember, that is enthalpy of combustion equals positive q object, the one that absorbed the heat, plus q of the calorimeter. Here we're going to convert the heat of combustion from kilojoules to joules, which is customary with these types of questions. So remember 1 kilojoule is 10 to the 3 joules, so that is joules per mole. Remember, enthalpy of combustion is exothermic, so this is a negative sign here. Now we have 12.3 grams of ethane. Ethane's formula is c286, and here if we convert it into moles alright. So we have 2 carbons which is 24.02 grams for the 2 carbons, and then we have 6 hydrogens, each one is, 1.00 8, so their combined mass is 6.048. Alright. So this comes from the 2 carbons and the 6 hydrogens of ethane. When we add that up together, the mass is 30.068 grams per 1 mole. Take that mole, we're gonna plug it in here for the q of the object, so the moles are 0.4034 moles times its specific heat capacity, which when you look it up is approximately 52.63 joules over moles times degrees Celsius. The temperature changed by 15.2 degrees Celsius, so that's our delta t. We're looking for the heat capacity of the calorimeter times the change in temperature again, which is still 15.2 degrees Celsius. Moles cancel out. Degrees Celsius cancel out. When I multiply those together, I'm gonna get 3 22.710 joules plus my heat capacity again times the change in temperature. Subtract out what we have here. When we do that we're gonna get 1559677.29 equals heat capacity times the change in temperature divide out 15.2 degrees Celsius. And when we do that we're gonna get the heat capacity equal to 1.03 times 10 to the 5 joules over degrees Celsius. So that would represent the heat capacity of the bomb calorimeter within this given question.