enthalpy changes

Question 1

use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic

Answer 1

bond breaking is endothermic and bond forming is exothermic

less energy is released when bonds are broken than when bonds are formed

Question 2

suggest two reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretic values

Answer 2

incomplete combustion

non-standard conditions

heat lost to surroundings

Question 3

suggest why the enthalpy change of formation cannot be determined directly

Answer 3

other products may be produced

Question 4

explain the bonding in a C=C double bond

use the orbital overlap model

Answer 4

C=C bonds are formed by the sideways overlap of P orbitals from 2 carbon atoms

pi bond is formed above and below plane of molecule

Question 5

suggest why the average bond enthalpy of a C=C bond is not twice the bond enthalpy of a C-C bond

Answer 5

C=C bonds contain one sigma bond and a pi bond

pi bonds weaker than sigma bonds

Question 6

define average bond enthalpy

Answer 6

average bond enthalpy when one mole of gaseous covalent bonds are broken