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3.1.4 Energetics (Enthalpy Changes (Exothermic reaction: heat energy is…
3.1.4 Energetics
Enthalpy Changes
Exothermic reaction: heat energy is given out as reactants change to products; temperature rises; energy of products is lower than energy of reactants; e.g. combustion, neutralisation, bond making
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Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature (eg ∆H298Ɵ).
Endothermic reaction: heat energy is taken in as reactants change to products; temperature decreases; energy of products is higher than energy of reactants e.g heating copper sulfate with water, bond breaking
Standard Enthalpy of Combustion (∆cHƟ): the enthalpy change when one mole of a substance in it's is completely burned in oxygen with all reactants and products in their standard states
Standard Enthalpy of Formation (∆fHƟ): the enthalpy change when one mole of a substance is formed from its elements with all reactants and products in their standard states
temperature is related to the average kinetic energy of the particles in a system; heat is measure of total energy of all particles in a given amount of substance
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Calorimetry
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flame calorimeter: has chimney, enclosed flame, and pure oxygen, to reduce heat loss
allowing for heat loss: let reactants sit in the room for a while and measure temperatures until temperature is constant for all reactants; when mixing reactants, extrapolate line back to time of mixing and use that for enthalpy change
Bond Enthalpies
Mean bond enthalpy: the average value of the bond dissociation enthalpy for a given type of bond taken from a range of different compounds
Bond dissociation enthalpy: the enthalpy change required to break a covalent bond with all species in the gaseous state
Values from mean bond enthalpy calculations differ from those determined using Hesse's Law due to mean bond enthalpies being an average over a range of different compounds, so are less accurate then if using Hesse's Law
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