Bond Enthalpy and Mean Bond Enthalpy

Question 1

what is bond enthalpy

Answer 1

the enthalpy change when one mole of a bond in the gaseous state is broken

Question 2

what would be the equation for the reaction showing the enthalpy change for a diatomic molecule

Answer 2

XY(g) = X(g) + Y(g)

Question 3

what would be the equation for the splitting up of chlorine in which the bond enthalpy change could be highlighted

Answer 3

Cl2(g) = 2Cl(g)

Question 4

what kind of reaction is the splitting of a diatomic molecule and why

Answer 4

• an endothermic reaction
• because energy needs to be inputted in order for the two molecules to separate
• meaning the energy levels of the products is larger than the reactants
• similar to thermal decomposition

Question 5

what does the fact that they are endothermic reactions mean about their bond enthalpies

Answer 5

they are all positive

Question 6

what is the symbol for bond enthalpy changes

Answer 6

delta BH

Question 7

what is the simplest example of a polyatomic molecule and why

Answer 7

• CH4

- because there are multiple atoms composing the compound

Question 8

how many bond enthalpies would methane have and why

Answer 8

• 4 different bond enthalpies
• because each bond between the atoms has t be considered individually
• as it is CH4, there are four different bonds

Question 9

what are all the bond enthalpies of polyatomic molecules like

Answer 9

• they are all positive

- as it is an endothermic reaction

Question 10

for CH4, where you only have the bond between a carbon and hydrogen atom, would the bond enthalpies of each of these be the same

Answer 10

no

Question 11

what trend should be seen and why does it change

Answer 11

• the bond enthalpy should decrease as the number of (hydrogen) atoms bonded to the carbon atom decreases
• it differs because the bond enthalpy of C-H varies with its environment

Question 12

what do we do in order to fix this issue and make the bond enthalpy between two atoms simpler

Answer 12

we use their mean bond enthalpy instead

Question 13

the bond enthalpies between the carbon and hydrogen atoms gradually decreases as there are less atoms composing the molecule. in order they are 4x, 3x, 2x and x. what would the mean bond enthalpy for a mole of methane therefore be

Answer 13

• (4x + 3x + 2x + x) = 10x
• 10x / 4 = 2.5x
• mean bond enthalpy = 2.5x kJmol-1

Question 14

what symbol is used to represent the bond enthalpies next to the structural formula of a bond between two atoms

Answer 14

E

Question 15

how would you correctly show that the mean bond enthalpy for a single bond between carbon atoms is 347Jmol-1

Answer 15

E(C-C) = +347kJmol-1

Question 16

what is the definition of the mean bond enthalpy

Answer 16

the enthalpy change when one mole of a bond, averaged over many different molecules, is broken