Section 7: Enthalpy: Exothermic and Endothermic Reactions
Crucial Chemistry
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In the field of thermodynamics, scientists study how heat and other forms of energy are
involved in chemical reactions.
o When a reaction is endothermic overall, energy moves from the surroundings
into the system.
 To start a reaction, chemical bonds must be broken.
 The process of breaking chemical bonds is endothermic.
o When a reaction is exothermic overall, energy moves from the system into the
surroundings.
 When chemical bonds are formed, energy is released.
 The process of forming chemical bonds is exothermic.
In the field of kinetics, scientists study the rates of reactions. Many variables can increase
the rate of a reaction:
o increasing the concentration of the reactants
o decreasing the particle size/increasing the surface area of the reactants
o increasing the temperature
o adding a catalyst
 A catalyst speeds up a reaction by lowering the activation energy of a
reaction.
 The activation energy is the energy that must be gained to break the
initial bonds and start a reaction.
A reaction is considered to be spontaneous if it continues without the input of energy
once it begins:
o Two factors determine if a reaction is spontaneous:
 Enthaply (∆H) or the change in heat energy, and
 Entropy (∆S) or the change in disorder.
Scientists often draw energy diagrams to represent the change in enthalpy during a
reaction.
o If the energy of the reactants is higher than the energy of the products, then the
reaction is exothermic overall, and ∆H<0.
o If the energy of the reactants is lower than the energy of the products, then the
reaction is endothermic overall, and ∆H>0.