Endothermic and Exothermic Reactions

1. Endothermic and Exothermic Reactions Lyndsey Smith and Megan Purcell Mr. Therrien Chemistry 12 ADV December 15th 2009

2. Exothermic and Exothermic Reactions How do we measure them? It is hard to measure the amount of energy used and produced in a reaction, so instead we measure reactions using their enthalpy change. This is represented by ΔH. We use this equation to calculate the enthalpy change in a reaction: MCΔT= q q= heat (J) M= mass (g) C= specific heat capacity (J/g.oC) ΔT = change in temperature (oC) A device called a bomb calorimeter is used to measure the change in temperature during a chemical reaction.

3. Exothermic Reactions What does it mean? The word exothermic comes from the Greek language. ‘Exo’ comes from their prefix ‘ex-’ which means outside and ‘thermic’ comes from ‘thermein’ meaning to heat. Marcellin Berthelot, a French chemist and politician first coined the word exothermic.

4. Exothermic Reactions What are they? A reaction is classified as exothermic when it is a chemical reaction that releases energy in the form of heat, light or sound. Exothermic reactions are the opposite of endothermic reactions. In an endothermic reaction energy is taken from the surroundings into the system instead of energy being released from the system to the surroundings like in an exothermic reaction.

5. Exothermic Reactions What do they look like in a chemical equation? When making a chemical equation of an exothermic reaction heat must always be included either in the products side of the equation or outside of the equation in a negative value. This is due to the fact that exothermic reactions release energy so they use up more energy than they gain. Example: Hydrogen burning can be expressed two ways (Inside the reaction) 2H2 + O2 → 2H2O + 483.6 kJ (Outside the reaction) 2H2 + O2 → 2H2O ΔH = −483.6 kJ/mol of O2

6. Endothermic Reactions What does it mean? Endothermic means a reaction that absorbs heat. It comes from the Greek words “endo” – meaning inside and “thermic” – meaning to heat. The term was first used by the same scientist that coined exothermic.

7. Endothermic Reactions Endothermic What are they ? A reaction is considered to be endothermic when it absorbs heat light or sound. The heat light or sound leave the surroundings and enters the system. Exothermic

8. Endothermic Reactions What do they look like? When making a chemical equation with an endothermic reaction, there are two different ways to represent it. You can show the heat inside the reaction by placing the heat on the reactant side or represent the heat outside the equation in a positive value. Here is an example: (inside the reaction)2HCl+ I2 +236kJ→2HI +Cl2 (outside the reaction) 2HCl+ I2→2HI +Cl2 ΔH = 236kJ/mol

9. Exothermic and Endothermic Reactions How can you tell the difference? When an exothermic reaction occurs it releases energy; usually in the form of heat or light. These reactions can be everyday reactions like the combustion of fuel in your car that makes it work or large explosions such as mixing sodium with water. Endothermic reactions absorb the energy instead of releasing it. A simple example of an endothermic reaction would be melting an ice cube. The cube absorbs the heat on the outside warming it causing it to melt.

10. Works Cited • http://chemistry.about.com/cs/generalchemistry/a/aa051903a.htm • http://en.wikipedia.org/wiki/Exothermic_reaction • http://en.wikipedia.org/wiki/Endothermic • Mr. Therrien’s Blue Chemistry Book