R 1.2
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What does each chemical reaction involve?
breaking and forming of bonds with an overall change in energy
What is the difference in stability of reactants/products in different reactions?
exothermic - more stable products endothermic - less stable products (higher energy are less stable)
What is bond enthalpy?
the energy required to break a bond (one mole of chemical bonds in the gaseous state)
Is bond enthalpy + or -?
always positive because they refer to bonds being broken
What is the average bond enthalpy?
enthalpy change when one mole of bonds are broken in the gaseous state, averaged for the same bond in similar compounds
Total enthalpy =
total bonds broken - total bonds formed
What is Hess’s law?
the total enthalpy change in a chemical reaction is independent of the route by which it took place, as long as the initial and final conditions are the same
What is combustion?
an exothermic chemical process where a substance is combined with oxygen gas
What is the standard enthalpy of combustion?
the enthalpy change when one mole of a substance is burned completely in oxygen under standard state conditions
What is the standard enthalpy of formation?
the energy change that occurs when one mole of a substance is formed from its constituent elements in their standard state
Breaking bonds ___ energy
absorbs
Making bonds ____ energy
releases
BENDO!
MEXO!
Why does bond enthalpy data differ from those achieved in experiments?
they are average values, not precise for specific reactions or compounds
What is the enthalpy of atomisation?
the enthalpy change when 1 mol of gaseous atoms is formed from an element in its standard state
What is bond energy?
energy required to break 1 mol of bonds in the gaseous state
What is electron affinity?
energy released when 1 mole of electrons is added to 1 mole of gaseous atoms
What do we use the Born-Haber cycle for?
to determine the lattice enthalpy
Order of enthalpies in a born haber cycle and if they are + or -:
enthalpy of atomisation: positive ionisation energy: positive electron affinity: negative enthalpy of formation: negative
What is the order that atoms go through in a born haber cycle?
• atomisation • ionised • electron affinity