# Energetics

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• Energetics
• Enthalpy
• ENTHALPY is a measure of the amount of heat energy contained in a substance. It is stored in the chemical bonds and intermolecular forces as potential energy.
• If enthalpy change in a reaction is negative, energy has been released to the surroundings = reaction is exothermic
• If enthalpy change is positive, energy has been absorbed to the surroundings = reaction is endothermic
• Enthalpy of combustion = reactants - products
• Enthalpy of formation = products - reactants
• Heat change    Q=mc?T
• the specific heat capacity gives heat needed to increase the temperature of unit by mass by 1K
• SPECIFIC HEAT CAPACITY     c= q/?T       heat capacity= heat change/ temperature change
• the standard enthalpy of formation of a substance is the enthalpy change that occurs when one mole of a substance is formed from its elements in their standard states under standard conditions
• Hess's Law
• Hess's law states that the enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and  final conditions, and reactants and products are the same
• it allows us to calculate enthalpy changes for the reactions that cannot be measured directly in the laboratory
• enthalpy circles
• Reversing the direction of the reaction reverses the sign of ?H
• Bond enthalpies
• MEXO: making bond is exothermic
• BENDO: breaking bond is endothermic
• Energy needed to break one mole of bonds in gaseous molecules under standard conditions
• enthalpy change of a reaction can be calculated by using bond enthalpies - Remember to take in consideration coefficients
• Ozone depletion
• the double bond in O2 is stronger than the 1.5 bond in ozone and so is broken by radiation of higher energy and shorter wavelength
• Ozone is formed from free radical substitution
• E (photon) = hv
• Ionisation energies and electron affinities
• first ionisation energy is the minimum energy required to remove one mole of electron from one mole of gaseous atoms    Na(g)--> Na?(g) + Cl? (g)
• first electron affinity is the enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms      Cl(g)+e?--> Cl?
• Gibbs free energy
• ?G must be negative for a reaction to be spontaneous
• ?G (system) = ?H(system) - T?S(system)