The purpose of calorimetry is to use an instrument known as a calorimeter to determine the enthalpy of a substance undergoing chemical change.
In a calorimeter known as a bomb calorimeter, it is the enthalpy of combustion that is measured. This is how the caloric content of foods is determined. After opening its lid, we place a weighed sample in a cup at the bottom of the bomb.
It is sealed, and through a valve, O 2 is delivered, saturating the bomb to prepare it for ignition. The bomb is then secured within a calorimeter bucket that is filled with water the water is the environment which will absorb the heat of combustion. A stirrer keeps the temperature of the water evenly distributed. A thermometer allows us to measure the initial temperature; the ignition wire connected to a high voltage source initiates the explosion; heat is released, and we measure the maximum temperature attained.
In reality we should assume that the material part of the calorimeter also absorbs heat. For reasons that will become clear later, we'll use delta H to represent the heat change for this experiment. Specific heat capacity represented by a lowercase S is the amount of heat required to raise the temperature of one mass unit, like a gram and kilogram of a substance by one degree Celsius.
So it turns out that different amounts of heat create different temperature changes, like metals get hot really easily and cool down really easily. Others, like water, require a lot of thermal energy to raise the temperature, and therefore have to release a lot of heat to cool down. I'm always wondering, though, like what does that really mean, like physically in the molecules? Shouldn't heat raise the temperature of all substances equally and why does water in particular have such a high specific heat capacity?
Heat energy can do a lot of things besides just increase temperatures. Temperature, or the speed at which molecules bounce around is just one way that atoms or molecules can absorb energy. Heat energy can also be absorbed by the breaking and formation of bonds between molecules. And as we'll learn in another episode, the extremely high specific heat capacity of water is due to the breaking in formation of hydrogen bonds that are associated with relatively small changes in temperature.
And how do we know the specific heat capacity? Well, I am happy to report that some noble chemists have worked hard to determine the specific heat capacities of hundreds of substances so that we don't have to. We just have to look up the numbers in a table.
Okay, so a specific heat capacity times mass times the change in temperature. The mass is important because the more mass of a substance we have, the more chemical bonds that are present. And because energy is contained in chemical bonds, they have a big effect on how much energy we're able to absorb or release. And, finally, there's the change in temperature. When doing calorimetry, we calculate a change in heat by measuring a change in temperature. But, as we've said a billion times before, heat and temperature or not the same thing.
It's just that luckily, in this specific case, they are related by our handy little calorimeter formula. Now, you might not have noticed, but we are right at the interface between chemistry and physics here. Each science could claim ownership over these phenomena. But the truth is that humans made up the difference between chemistry and physics anyway. Thermodynamics, the study of heat, energy and work, doesn't care about our little rules.
Thermodynamics itself makes the rules of the universe. It is the ultimate law. So now you know, even though you might not have cared, but you should, because it's cool.
It's all wiggly wobbly bondy wondy. All right, enough talk. Let's get out there and actually do some math here. Now, remember that the formula is delta H, sm delta T. The solutions we're using here are so dilute that almost all of their mass consists of water. Therefore, we can simply use the specific heat capacity of water. If we look that up on a table, we'll see that it is 4. I used a grams of each chemical for a total mass of grams. And, finally, we need the temperature change.
If you remember, the temperature rose from kelvin to The difference between these two is 7. It's a positive value because the temperature increased. Cancel all the appropriate units and then bang on the calculator to get a final release of So we're at 6. Because this formula is based on temperature change and since the temperature increased, we end up with a positive result. But, most importantly, it tells us the magnitude of the change in heat energy.
So I wonder how that compares to the amount we would predict using Hess's law and the standard enthalpies of formation. Remember that we can look up the standard enthalpies of formation for all the products and reactants in the back of a textbook or online. The chemical reaction between hydrochloric acid and sodium hydroxide produces liquid water and sodium chloride. The standard enthalpy of formation for hydrochloric acid is negative For sodium hydroxide, it's negative For liquid water, negative Calculate the change in temperature of a substance given its heat capacity and the energy used to heat it.
There are two derived quantities that specify heat capacity as an intensive property i. They are:. The following two formulas apply:. The molar heat capacity of water, CP, is [latex] How much heat is required to raise the temperature of 36 grams of water from to K? We are given the molar heat capacity of water, so we need to convert the given mass of water to moles:. Interactive: Seeing Specific Heat and Latent Heat : Specific heat capacity is the measure of the heat energy required to raise the temperature of a given quantity of a substance by one kelvin.
When a solid is undergoing melting, the temperature basically remains constant until the entire solid is molten. The above simulation demonstrates the specific heat and the latent heat. Specific heat capacity tutorial : This lesson relates heat to a change in temperature.
It discusses how the amount of heat needed for a temperature change is dependent on mass and the substance involved, and that relationship is represented by the specific heat capacity of the substance, C. Constant-volume calorimeters, such as bomb calorimeters, are used to measure the heat of combustion of a reaction. Bomb calorimetry is used to measure the heat that a reaction absorbs or releases, and is practically used to measure the calorie content of food.
For instance, if we were interested in determining the heat content of a sushi roll, for example, we would be looking to find out the number of calories it contains. Then, we would evacuate all the air out of the bomb before pumping in pure oxygen gas O 2. After the oxygen is added, a fuse would ignite the sample causing it to combust, thereby yielding carbon dioxide, gaseous water, and heat.
As such, bomb calorimeters are built to withstand the large pressures produced from the gaseous products in these combustion reactions. Bomb calorimeter : A schematic representation of a bomb calorimeter used for the measurement of heats of combustion.
The weighed sample is placed in a crucible, which in turn is placed in the bomb. The sample is burned completely in oxygen under pressure. The sample is ignited by an iron wire ignition coil that glows when heated. The calorimeter is filled with fluid, usually water, and insulated by means of a jacket. The temperature of the water is measured with the thermometer. From the change in temperature, the heat of reaction can be calculated.
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