Reactivity 1.2 Flashcards
Bond breaking… what energy?
Absorbs
Bond forming… what energy?
Releases
Energy is required to…
break a chemical bond (endothermic)
Energy is given out when..
chemical bonds are formed (exothermic)
Bond enthalpy values only refer to…
The gaseous state
Bond enthalpy units
kJmol-1
Bond breaking
Seperating atoms that are attracted by an electrostatic force
Bond making
Involves bringing together atoms that are attracted by an electrostatic force
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Strength of bonds
Are influenced to some degree by the nature of other atoms attached to one or both of the bonding atoms
Average bond enthalpy
Average energy required to break one mole of the bond in similar compounds all in the gaseous state
Average bond enthalpy strength order
Single covalent < double covalent < triple covalent
When can you calculate enthalpy changes?
When all reactants and products are in the gaseous phase
Enthalpy change for a reaction is…
Total bonds enthalpies in reactants - total bond enthalpies in products
Why is calculated change in enthalpy different to that in the data booklet?
Values used are average values in similar compounds
Hess’s law of constant heat summation
The total energy change in a chemical reaction depends only upon the initial and final states and is independent of the reaction pathway
Energy cycle - what side is worth what?
Clockwise = anti clockwise
Standard enthalpy of atomisation
Enthalpy change when one mole of gaseous atoms are formed from the element in its standard state
Enthalpy of formation
Enthalpy change when one more of product is formed from its elements in their standard state
Standard state rules (4)
- HOFBrINCl are all diatomic, everything else in monotomic
- Br2 and Hg are gases
- Noble gases are all gases
- Everything else is solid