Calorimetry measures the amount of heat transferred from one substance to another.
The heat is exchanged with an object.
Since the previous calibration was used to establish its heat capacity, the change in temperature of the measuring part of the calorimeter is converted into heat.
Knowledge of the heat capacity of the surroundings, and careful measurement of the system and surroundings' temperatures before and after the process allow one to calculate the heat transferred as described in this section.
When an exothermic reaction occurs in solution in a calorimeter, the heat produced by the reaction is absorbed by the solution, which increases its temperature.
The thermal energy of the solution decreases its temperature when an endothermic reaction occurs.
The amount of heat involved in either case can be calculated using the temperature change and specific heat of the solution.
Scientists use well-insulated calorimeters that prevent the transfer of heat between the calorimeter and its environment.
The heat involved in chemical processes, the energy content of foods, and so on are all determined by this.
The "coffee cup" calorimeters allow more heat exchange with their surroundings, and therefore produce less accurate energy values.
A calorimeter can be made from cups.
The reaction mixture has a thermometer and a stirrer in it.
Commercial solution calorimeters are also available.
Inexpensive calorimeters often include two thinwalled cups that are nested in a way that reduces thermal contact during use, along with an insulated cover, handheld stirrer, and simple thermometer.
A well-insulated, fully enclosed reaction vessel, motorized stirring mechanism, and a more accurate temperature sensor are some of the features of more expensive calorimeters.
Simple, inexpensive models for student use and more accurate models for industry and research can be found in the commercial solution calorimeters range.
The core idea behind calorimetry is shown in a simpler example.
Suppose we have a hot piece of metal and a cool water.
When the two substances have the same temperature, the heat will flow from M to W. If this occurs in a calorimeter, all of the heat transfer occurs between the two substances, with no heat gained or lost by either the calorimeter or the calorimeter's surroundings.
A piece of steel is dropped into a large amount of water.
The piece of rebar was initially at a higher temperature.
The temperature of the copper was 338.6 degrees.
A piece of copper is dropped into water at a temperature of 22.6 degrees.
The final temperature can be calculated if all heat transfer occurs between the copper and the water.
The final temperature was reached by both copper and water.
This method can be used to determine other quantities, such as the specific heat of an unknown metal.
A piece of metal that had been submerged in boiling water was quickly transferred into 60.0 mL of water.
The final temperature was 28.5%.
The data can be used to determine the heat of the metal.
The metal can be identified using this result.
The metal we identify as copper is closest to the value in our experimental heat.
It would be hard to determine which metal this was based on the numerical values.
The observation that the metal is silver/gray in addition to the value for the specific heat indicates that the metal is lead.
The principles we have been discussing apply when we use calorimetry to determine the heat involved in a chemical reaction.
The amount of heat absorbed by the calorimeter is small enough that we can neglect it, and the calorimeter reduces energy exchange with Chapter 5.
There is no overall energy change during a chemical reaction because energy is not created nor destroyed.
This concept is at the center of all calorimetry problems.
Imagine if you could combine the two solutions so quickly that no reaction took place while they mixed; then after mixing, the reaction took place.
At the time of mixing, you have 100.0 mL of a mixture of HCl and NaOH.
The solution temperature is 28.9 degC.
The solution takes in more heat than the reaction.
We need to make a few more reasonable assumptions to proceed with this calculation.
Since the solution is water in terms of its heat and mass values, we can proceed as if it were water.
100.0 mL has a mass of about 1.0 x 102 g because of the density of water.
The specific heat of water is approximately 4.18 J/g degC, so we use that for the specific heat of the solution.
The negative sign shows that the reaction is cold.
It has a heat yield of 2.89 kJ.
A metal disc and a supersaturated solution of NaC2H3O2 are found in a common hand warmer.
A later chapter on solutions will investigate saturation and supersaturation, as well as nucleation sites around the metastable NaC2H3O2.
The NaC2H3O2 can be used again if the hand warmer is reheated.
Chemical hand warmers warm your hand on a cold day.
You can see the metal disc in this one.
The iron and water in the hand warmer can be exposed to the air if it is ripped open.
Salt in the hand warmer causes the reaction so it produces heat more rapidly.
Other types of hand warmers use lighter fluid, charcoal, or electrical units that produce heat by passing an electrical current from a battery through wires.
The precipitation reaction that occurs when the disk in a chemical hand warmer is flexed is shown in this link.
The solution becomes cold when it is dissolved in water.
This is the basis for an "instant ice pack" when NH4NO3 is dissolved in 50.0 g of water in a calorimeter.
You should state any assumptions you made.
An instant cold pack consists of a bag with a second bag of water.
The pack becomes cold when the bag of water is broken because the dissolution of ammonium nitrate removes thermal energy from the water.
The cold pack is used to remove thermal energy from your body.
The temperature decreased when a sample of KCl was added to water in a coffee cup calorimeter.
If the amount of heat absorbed by a calorimeter is too large to neglect or if we need more accurate results, then we must take into account the heat absorbed both by the solution and the calorimeter.
The calorimeters described are designed to operate at constant pressure and are convenient to measure heat flow.
This type of calorimeter has a steel container that is submerged in water and contains reactants.
The sample is placed in a bomb, which is filled with oxygen.
A small electrical spark is used to start the sample.
The steel bomb and the surrounding water hold the energy produced by the reaction.
The heat capacity of the calorimeter is used to calculate the energy produced by the reaction.
Calibration is required to determine the heat capacity of the calorimeter.
The heat capacity of the calorimeter is determined by the temperature change produced by the known reaction.
Before the calorimeter is used to gather research data, the calibration is done.
To view how a bomb calorimeter is prepared for action, click on this link.
This is a calculation using sample data.
The temperature of the calorimeter increases when 3.12 g of C6H12O6 is burned.
The bomb has a heat capacity of 893 J/degC, and the calorimeter has water in it.
The water and bomb absorb most of the heat produced by the combustion.
The reaction released 48.7 kJ of heat.
The temperature of the calorimeter increases by 8.39 degrees when 0.963 g of benzene is burned.
The bomb has a heat capacity of 784 J/degC and is submerged in water.
35 calorimeters have been built to measure the heat produced by a living person since the first one was constructed in 1899.
More recently, whole-room calorimeters allow for relatively normal activities to be performed, and these calorimeters generate data that more closely reflect the real world.
The calorimeters are used to measure the metabolism of individuals under different environmental conditions and with different health conditions.
metabolism is measured in calories per day
In your day-to-day life, you may be familiar with the amount of energy given in calories, which are used to calculate the amount of energy in food.
One cal is equal to 4.184 joules and one cal is equal to 1000 cal.
The main components of food are sugars, fats, and oils.
The calories from each of the three macronutrients are shown on the nutrition labels on food packages.
The rest of the world uses less energy than the US does.
You can use food labels if you want to calculate the total energy per portion.
Bomb calorimetry can be used to determine the calories in a food.
A sample of food is weighed, mixed in a blender, freeze-dried, ground into powder, and formed into a pellet.
The pellet is burned inside a bomb calorimeter, which converts the measured temperature change into energy per gram of food.
A method called the Atwater system is used to derive the calories on food labels.
The average amounts are derived from the various results given by bomb calorimetry of whole foods.
The fiber content is discounted for being indigestible.
To determine the energy content of a food, the quantities of fat, calories, and carbohydrates are divided by the average calories per gram for each and summed to get the total energy.
The US Department of Agriculture has a database of nutrition information on over 8000 foods.