Endo-and Exo- Energy's Comings and Goings

Experiment 14 Endo- and Exo- Energy's Comings and Goings

Overview | Module | Background | Procedure | Report
 

Introduction/Background

     Chemical energy is the energy that is stored in the bonds of all molecules. Whenever we create or break a molecular or ionic bond there is an absorption or release of energy. Outside of the laboratory, we can use this absorption or release of energy to create items for our day to day use. One simple example of this usage is the first aid hot and cold packs. Inside the packs are chemicals contained in separate sections. When the pack is "squashed" and shaken the sections are ruptured and the chemicals mix. If the reaction between the chemicals is exothermic, the liquid in the pack becomes warm. If the reaction is endothermic, the liquid is cooled or in some cases frozen.



hot pack
First-Aid Hotpacks, containing either calcium chloride or magnesium sulfate, plus water, demonstrate a spontaneous exothermic solution process.

Another very common example of how the real world uses the energy of chemical reactions is in the process of photosynthesis. Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)



photosynthesis
Photosynthesis is the process of converting light energy to chemical energy.



Key Concepts
     The key to determining the direction of energy flow into or out of a system/reaction is determining the change in temperature (DT) that can be observed. In order to determine this temperature change, we must first define the direction of temperature flow.

In any chemical process, there are two areas of interest: The system and the surroundings. The system is always defined as the reaction and the surroundings are essentially considered to be everything that is not directly involved in the reaction. For example, when an aqueous acid and base are mixed together in a flask, the reaction that takes place only occurs between the acid molecules and the base molecules, therefore, it is only these two molecules that are considered to be the "system". The water molecules surrounding the acid and base molecules don't actively participate in the reaction and therefore are considered to be part of the "surroundings". The flask in which the reaction is taking place as well as the atmosphere that surrounds the beaker are also considered part of the surroundings.

     system and surroundings
System vs. Surroundings

Now that we know how to define the system and surroundings we should set up some rules for determining the direction of heat/energy flow:



1) For heat that flows out of the system into the surroundings:
a) The reaction is exothermic.
b) The DT for the reaction/system will be negative.
c) The DT for the surroundings will be positive.

1) For heat that flows into the system from the surroundings:
a) The reaction is endothermic.
b) The DT for the reaction/system will be positive.
c) The DT for the surroundings will be negative.

Example 1: Aqueous barium hydroxide (octahydrate) and aqueous ammonium nitrate are mixed (in a 2:1 ratio) in a small beaker

The temperature in the beaker is measured by placing a thermometer in the solution. Both solutions are originally at 23.0oC and drops to 10.0oC when they are mixed. Is the reaction endothermic or exothermic?

Answer: First define your system and your surroundings:

Since both reactants are said to be aqueous, we know that the Ba(OH)2 and NH4NO3 molecules are surrounded by water. This means that the system is the reactants and the surroundings include both the water and the beaker, etc.

Next, figure out exactly what DT the thermometer was measuring.

Because the reaction occurs on a microscopic level, it is impossible to say that the thermometer is measuring the temperature change of the reaction directly. Rather, the temperature change of the water surrounding the reaction is what is actually measured by the thermometer.

So finally, if the DT of the surroundings is calculated as TFinal - TInitial and we input the values above:

DT = TFinal - TInitial

DT = 10.0oC - 23.0oC

DT = -13.0oC (Negative)

Then the DT of the surroundings is negative which means according to the rules above that the DT of the system is positive and therefore the reaction is Endothermic.


Example 2: An aqueous solution of HCl is added to an aqueous solution of NaOH in a one to one ratio. Both solutions were at 25oC before the reaction and the temperature (measured by thermometer) rose to 30oC when the two solutions were mixed.

Answer: Define the system and the surroundings:

      click to reveal

Is the reaction endothermic or exothermic?

     click to reveal


Glossary

Related Materials

Chemical Energy Changes (http://scidiv.bcc.ctc.edu/wv/7/0007-002-exo_endothermic.html)
Thermochemistry (http://www2.muw.edu/~melioff/chapter6.ppt)
Introduction to Thermodynamics (http://www.cofc.edu/~kdk/441/Powerpoint/Chapter2/CH2-1.ppt )