Energetics
- Created by: camillafotii
- Created on: 12-05-18 05:40
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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 specific heat capacity gives heat needed to increase the temperature of unit by mass by 1K
- 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
- 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.
- 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
- enthalpy circles
- it allows us to calculate enthalpy changes for the reactions that cannot be measured directly in the laboratory
- 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
- 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
- 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
- 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)
- ?G must be negative for a reaction to be spontaneous
- Enthalpy
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