Chapter 11.1 - Energy changes in reaction - by Yap Jun Xian 3Z

In Chemistry, two types of reactions can occur:
Physical
Chemical

From here, Chemical reactions can be split into two more groups:
Exothermic reactions and
Endothermic reactions.

In order to understand these reactions, we need to understand energy changes. When a chemical reaction occurs, the bonds between the atoms are broken, and new ones are formed. This requires energy. If the amount of energy required to form new bonds is higher than the amount of energy required to to break them, it releases energy. If the amount of energy required to form new bonds is lower than the amount of energy required to break them, it takes in energy. What determines the amount of energy taken in or given out is the difference in the amount of energy taken in subtracted by the amount of energy given out (Energy in-Energy out). If the answer if negative, it released energy, if the answer is positive, it takes in energy. The answer also provides us with the overall amount of energy that was used or released in the reaction.

---Exothermic reactions---
The word exothermic can be split into two. Exo, meaning 'out' in greek, and therm, meaning 'heat'. From this we can understand that exothermic reactions give out heat. Common examples of exothermic reactions include the neutralisation of an acid and alkali or combustion. In these examples, energy is taken in to break the molecular bonds, after which energy is released when new bonds are formed. Exothermic reactions occur when the amount of energy released is higher than the amount taken in, resulting in the release of heat. The graph above shows that the reactants require more energy to be broken down/formed while the product requires less to be broken down/formed.



A simple experiment on exothermic reactions can be done in a lab.

Apparatus:
Aqueous HCl
Aqueous NaOH
1 Test tube/boiling tube
2 Measuring cylinders

1) Add some aqueous HCl into a measuring cylinder, add an equal amount of aqueous NaOH into a seperate measuring cylinder.
2) Add the two solutions into the test tube.

What happens:
The reaction produced more energy forming bonds than it took in to break the bonds. Because of this, the overall amount of energy used is a negative, and the energy is released as heat.

---Endothermic reactions---
The word endothermic can be split up into two. Endo, which means 'in' in greek, and therm. From this we know that endothermic reactions take in heat, and is the opposite of an exothermic reaction. Examples of endothermic reactions are photosynthesis, the mixing of water with ammonium nitrate (NH4NO3) and decomposition of calcium carbonate into calcium oxide. In these reactions, energy is taken to break the molecular bonds, after which energy is released when new bonds form. Endothermic reactions occur when the amount of energy released is lower than the amount taken in, resulting in the loss of heat. The graph above shows that the reactants require less energy to form/break the molecular bonds than the products.

A simple experiment can be done on endothermic reactions.

Apparatus:
2g of ammonium chloride
Distilled water
1 test tube/boiling tube

1) Add 10ml of distilled water into the test tube.
2) Add the ammonium chloride into the water.

What happens:
The reaction took in more energy than it produced when forming bonds. Because of this, the overall amount of energy used was positive, and therefore, took in heat.

Thank you for reading.
*note* I couldn't find examples of experiments so I created them in hopes of them being correct.

Bibliography:
http://chemistry.about.com/b/2006/03/22/cold-packs-endothermic-reactions.htm
http://chemistry.about.com/od/lecturenotes13/a/endorxns/htm
Complete Chemistry for IGCSE page 160-163