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

Calorimetry

The following headers are meant as a guide to the sections that you need in your module. Here, before the table of contents, give a two-four sentence overview of the Module. Be straight to the point. No examples and no figures in this section.  Keep the following text for automatic table of contents based on the headings below (seen in edit mode only).

Introduction

 
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
 
∆U=q+w
 
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
∆H=∆U+W
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:
 
∆H=∆U+∆(PV)
 
where P is the pressure of the system and V is the volume. 
A diagram of a bomb calorimeter is displayed below.
 
Bomb.png
 

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
 

References

  1. This is meant for references used for constructing the module. They must be primary and accessible to readers at a library.        
  2. You need at least two different sources here. Websites are not allowed. DOI links to J. Chem. Ed. are ideal Do not reference class notes. Also, do not reference textbooks for maximal credit. Using the insert citation button to automatically handle references is highly suggested (bottom right button on editor toolbar).

Problems

Be careful not to copy from existing textbooks. Originality is rewarded. Make up some practice problems for the future readers. Five original with varying difficulty questions (and answers) are ideal.

Contributors

  • Name #1 here (if anonymous, you can avoid this) with university affiliation

You must to post a comment.
Last Modified
10:06, 2 Oct 2013

Page Rating

Was this article helpful?

Tags

Module Vet Level:
Module Target Level:

Creative Commons License UC Davis GeoWiki by University of California, Davis is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Terms of Use