Tuesday, 13 March 2012

Energetics

4.13 understand the use of ΔH to represent molar enthalpy change for exothermic and
endothermic reactions

4.14 represent exothermic and endothermic reactions on a simple energy level diagram

4.15 recall that the breaking of bonds is endothermic and that the making of bonds is
exothermic

Just remember this! To break smth, obviously you need energy, so you need to take it in, hence breaking of bonds is endothermic. When substances form bonds, it's usually to become more stable, and thus they release energy. E.g. reactive elements like sodium react with chlorine to form sodium chloride, your common table salt, and this is a very stable ionic compound, and the reaction is exothermic. (less energy, less violently reactive...makes sense right?)



Heat Changes in a Reaction

heat change/enthalpy change: the amount of energy involved in a reaction, measured in kilojoules (kJ) which is 1000 joules, and is represented by the symbol DH. (D is the Greek letter for 'delta', which means change. H means energy content. 

For an exothermic reaction DH is negative. This is because the chemicals have lost energy to the surroundings. 
For an endothermic reaction, DH is positive. This is because the chemicals have gained energy from the surroundings. 

Examples:
1. When one mole of carbon is burnt in excess oxygen, 349 kJ of heat is produced. This is an exothermic reaction. So DH=-349kJ. 

 carbon + oxygen à carbon dioxide
C(s) + O2 à CO2 (g)   DH=-349kJ

2. When one mole of hydrogen reacts with one mole of iodine, 52 kJ of heat is absorbed from the surroundings. This is an example of an endothermic reaction. DH=+52 kJ

 hydrogen + iodine à hydrogen iodide
H2 (g) + I2 (g) à 2HI (g)   



Energy Level Diagrams for Exothermic and Endothermic Reactions

Energy level diagrams--convenient ways to express energy changes in a reaction

Exothermic Reaction 
Consider an exothermic reaction, heat energy is lost to surroundings. This means that total energy of the products is less than that of the reactants. 



The energy level diagram of an exothermic reaction

The difference between the energy levels of the products and the reactants is equal to the amount of energy given out by the reaction. 
i.e.  DH=Hproducts –  Hreactants  (as the energy of the reactants is larger, DH becomes negative. --> taking away a larger value from a smaller one gives a negative result. e.g. 2-4=-2) 


NOTE: USE THE SAME EQUATION FOR ENDOTHERMIC REACTIONS. It is always: 
 DH= Hproducts –  Hreactants

Endothermic Reaction
Since an endothermic reaction absorbs heat from the surroundings, the products will have more energy than the reactants. The difference between the energy levels of the products and reactants is the energy absorbed during the reaction.

The energy level diagram of an endothermic reaction

Activation Energy "EaThe minimum energy required to initiate (start) a chemical reaction. Both endothermic and exothermic reactions require activation energy.


So when particles collide and react, this is called an effective collision. This only happens when the particles have the minimum activation energy, if not, they may collide but it wouldn't result in a reaction.

Exothermic Reaction
Endothermic Reaction
Gives out heat to the surroundings
Takes in heat from the surroundings
Causes an increase in temperature
Causes a decrease in temperature
Has a negative DH
Has a positive DH
Has products lower in energy than the reactants
Has products that have higher energy than the reactants


This is a great, concise video to summarise everything, watch! 

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