unit 3 KA3 - chemical energy Flashcards
what is enthalpy
a measure of the chemical energy in a substance
when is a reaction described as exothermic
when it releases heat energy
when is a reaction described as endothermic
when it takes in heat energy
what is an issue with endothermic reactions in industry
they may incur costs in supplying heat energy in order to maintain the reaction rate
how can the enthalpy change associated with a reaction be calculated
from the quantity of heat energy released
how can the quantity of heat energy released be determined
experimentally and calculated using Eh=cmΔT
what is the enthalpy of combustion of a substance
the enthalpy change when one mole of the substance burns completely in oxygen
what does Hess’s law state
the enthalpy change for a chemical reaction is independent of the route taken. the enthalpy change for a reaction can be calculated using Hess’s law, given appropriate data.
what is the molar bond enthalpy
the energy required to break one mole of bonds in a diatomic molecule
what is a mean molar bond enthalpy
the average energy required to break one mole of bonds, for a bond that occurs in a number of compounds
how can bond enthalpies be used
to estimate the enthalpy change occuring for a gas phase reaction, by calculation the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products
what may need to be done in exothermic reactions in industry
heat may be removed in order to prevent the temperature rising
what is the difference between bond enthalpy and mean bond enthalpy
Mean bond enthalpy must refer to an average energy and to a number of compounds and bond enthalpy must relate to one compound/diatomic molecule.
