ChemWiki: The Dynamic Chemistry E-textbook > Wikitexts > Hope College > HOPE Chem 343: Krueger > Calorimetry


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Calorimetry is the measurement of heat associated with either a chemical reaction or physical process. Measuring the heat change means that you can tell how much heat is being absorbed and released in or from the system. Measuring the temperature before and after the process is a way to determine the heat change which can be used to find the amount of energy lost or energy gained. Constant-Volume and Constant-Pressure are the two main types of calorimetry. The tool used to make this measurement is called a calorimeter. 
The first law of Thermodynamics is useful to know since it can be used to calculate heat flowing in and out of a system which is displayed below
where ΔU is the internal energy change of the system, q is heat and w is work. Positive q is heat flowing into the system and positive w is work done on the system.
q can be calculated using the heat capacity of the calorimeter, the heat required to change the temperature by one degree, in cal/K.
q= -C∆T
where C is the heat capacity. 
Enthalpy, the total heat content, can also be calculated using calorimetry which is displayed below
where ΔH is enthalpy and W is work done on the system. Calculating the heat capacity of the calorimeter can be used to find the change of internal energy and change in enthalpy.

Constant - Volume Calorimetry 

Constant Volume calorimetry means that all variables can change except the volume. A bomb calorimeter is a popular tool of measuring the change of internal energy between the reactants and products of a chemical process or heat lost in a combustion reaction. Enthalpy can also be found using a bomb calorimeter, using ∆H=∆U+W .
The way a bomb calorimeter is used is when it is placed in a container of water. The bomb is usually made of steel to provide a constant volume system and the reactant is held in the bomb. The reaction is allowed to burn with pressurized oxygen in a sealed vessel. Since the reaction is combustion, the enthalpy of combustion can be calculated by using the equation,
The molar enthalpy of combustion was then calculated from the molar energy of combustion:
where P is the pressure of the system and V is the volume. 
A diagram of a bomb calorimeter is displayed below.

Constant - Pressure Calorimetry

Constant Pressure calorimetry means that all variables can change except the pressure of the system. A solution calorimeter is a popular tool of measuring the constant pressure release in the liquid phase. To find the change in enthalpy, ΔH can be found knowing the heat capacity of the calorimeter, C, and the change of temperature of the system. Since the pressure of a solution calorimeter is constant, the equation to find the heat q, can be rewritten as
∆H = (-C∆T)/n
where n is the number of moles. 

A system that can be used for solution calorimetry are two Styrofoam cups which keep the two liquids that are added, insulated. A diagram of the system is displayed below. 

constant cup.png


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10:06, 2 Oct 2013

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