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1 Heat of Reactions.notebook June 13, 2017
Heat of Reactions
Exothermic reactions are reactions in which there is a net release of energy.
Ex: The thermite reaction is a highly exothermic reaction
Fe2O3 (s) + 2Al(s) → Al2O3(s)+ 2Fe(l)+ E
If you had this reaction occurring in your hand (don't try this at home!!!) your hand would feel warmer because the reaction is releasing heat.
1 Heat of Reactions.notebook June 13, 2017
Endothermic reactions are reactions that require a net input of energy.
Ex: Barium Hydroxide Octohydrate and Amonium Thyiocynate
Ba(OH)2.8H2O(s) + 2 NH4SCN(s) + E → Ba(SCN)2(s) + 10 H2O(l) + 2 NH3 (g)
If you held this reaction in your hand, your hand would feel colder because the energy required for the reaction to occur would be taken from the surroundings, in this case your hand.
Cu(s) + Cl2(g) → CuCl2(g) + 220.1 kJ
A potential energy graph ofan exothermic reaction:
H2O(g) + C(s) + 132 kJ → CO2(g) + 2H2(g)
A potential energy graph ofan endothermic reaction
1 Heat of Reactions.notebook June 13, 2017
Chemical Bond EnergyWhenever a chemical reaction occurs, there is always a change in energy. Where does this energy come from?In any chemical substance, energy is stored in:intramolecular bonds the chemical bonds that hold atoms together intermolecular bonds and in the bonds that hold individual molecules together (solid or liquid)
During a chemical change, these bonds are rearranged some bonds are broken, other bonds are formed.
Almost all chemical reactions involve changes in which some bonds are both broken and formed. Thus, some energy will be needed to break bonds, but energy will also be released as new bonds form.
When we write a chemical equation that includes the energy change, the equation shows the net difference in energy change.
1 Heat of Reactions.notebook June 13, 2017
Heat of Formation
We will look at chemical reactions in which we form one mole of a compound from its elements. We are interested in how much energy is either absorbed or released during synthesis reactions forming one mole of product.
H2(g) + O2(g) → H2O(l)
Balance for one mole of water
How much released in the formation of one mole of water?
How would we change things for 2 moles of water?