9.3 Bond enthalphies

Question 1

What is average bond enthalpy

Answer 1

The energy required to break one mole of a specified type of bond in a gaseous molecule

Question 2

What is always needed to break bonds

Answer 2

Energy

Question 3

Are bond enthalpies always exothermic or endothermic

Answer 3

Endothermic

Question 4

Do bond enthalpies always have a positive or negative enthalpy value

Answer 4

Positive

Question 5

Is breaking bonds endothermic or exothermic

Answer 5

Endothermic
Energy is required
ΔH is positive

Question 6

Is making bonds endothermic or exothermic

Answer 6

Exothermic
Energy is released
ΔH is negative

Question 7

What determines whether or not a reaction is exothermic or endothermic overall

Answer 7

The difference between energy required for bond breaking and the energy released by bond making

Question 8

How can the enthalpy change of reaction ΔrH ne found

Answer 8

By calculating the bond enthalpies of the bonds in the reactants and the products

Question 9

For a reaction involving gaseous molecules of covalent substances:

Answer 9

ΔrH = Σ(bond enthalpies in reactants) - Σ(bond enthalpies in products)

Question 10

Why are there limitations in the calculated enthalpy change

Answer 10

Bonds may be different in different environments, however, the calculated enthalpy change of reaction should be in general agreement with the actual enthalpy change of reaction