Thermodynamics is the study of the energy in a chemical system, and in order to understand this better a number of definitions have to be cleared up first.
Definitions
Standard temperature and pressure: The conditions used for the reaction. This has to be 1 atm and 25 0C
Enthalpy: The energy released or absorbed during the reaction
Entropy: The change in disorder in a system. Entropy change is positive when a liquid change from a solid to a liquid, or a liquid to gas because more disorder is obtained. It is therefore negative when a gas condenses to a liquid or a liquid freezes to a solid since less disorder is produced.
Standard enthalpy of combustion: The energy released when 1 mole of compound reacts with air at standard temperature and pressure.
Standard enthalpy of formation: The energy released or absorbed when 1 mole of a compound is formed from its components at standard temperature and pressure.
Standard enthalpy of atomisation: The energy absorbed when 1 mole of gaseous atoms are formed from its original state at standard temperature and pressure.
Standard enthalpy of bond dissociation: The energy required to break 1 mole of bonds at standard room temperature and pressure.
Standard enthalpy of ionisation: The energy required for 1 mole of atoms to lose 1 mole of electrons from its outer shell at standard temperature and pressure.
Standard enthalpy of 1st electron affinity: The energy released when 1 mole of atoms gain 1 mole of electrons to form a uni-negative ion at standard temperature and pressure.
Standard enthalpy of 2nd electron affinity: The energy required when 1 mole of uni-negative atoms gain 1 mole of electrons to form a di-negative ion at standard temperature and pressure.
Lattice enthalpy: The energy released when 1 mole of a compound is formed by reacting the ions at gaseous state at standard temperature and pressure.
Hess’s Law
Hess’s Law states that no matter the route taken for a reaction, the enthalpy change for the reaction will be the same.
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Bond Dissociation Energy
The energy released in any reaction is the difference in energy between the bonds of the reactants and the bonds of the products. Breaking bonds is always endothermic whilst making a bond is always exothermic.
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Reaction Profile
A reaction profile shows the energy change between the reactants and the products. This reaction change can be either positive or negative, and when the enthalpy change is negative the reaction is exothermic whereas when it is positive the reaction is endothermic.
The activation energy is the energy needed for a reaction to initiate, to break bonds in order to create new interactions. The higher the activation energy the higher the energy needed for a reaction to take place, no matter how kinetic favourable the product could be.