3.3 chemical energy
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What is enthalpy
a measure of the chemical energy in a substance
What is an exothermic reaction?
a reaction that releases heat energy
What may happen with exothermic reactions in industry?
may require heat to be removed to prevent the temperature rising.
What is an endothermic reaction?
a reaction that takes in heat energy.
What may happen with endothermic reactions in industry?
may incur costs in supplying heat energy in order to maintain the reaction rate.
How can the enthalpy change of a reaction be calculated?
the quantity of heat energy released
How can the quantity of heat energy released be determined
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.
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.
What can bond enthalpies be used for?
-to estimate the enthalpy change occurring for a gas phase reaction
How do you calculate the enthalpy change for a gas phase reaction?
• calculate the energy required to break bonds in the reactants • calculate the energy released when new bonds are formed in the products • find the difference from the reactants to the products