[9.3] bond enthalpies

Question 1

definition of average bond enthalpy (exam q)

Answer 1

average enthalpy change when one mole of gaseous covalent bonds is broken

Question 2

bond enthalpies

Answer 2

• energy is always required to break bonds
• bond enthalpies are always endothermic
• bond enthalpies always have a positive enthalpy value

Question 3

what happens in chemical reactions in terms of bonds?

Answer 3

bonds break and new bonds are formed

Question 4

summary of energy in exothermic

Answer 4

• energy is released when bonds form
• bond making is exothermic
• ΔH is negative

Question 5

summary of energy in endothermic

Answer 5

• energy is required to break bonds
• bond breaking is endothermic
• ΔH is positive

Question 6

define an exothermic reaction

Answer 6

when more energy is released when making the bonds than the energy needed to break the bonds

Question 7

define an endothermic reaction

Answer 7

when more energy is needed to break the bonds than the energy released when making the bonds

Question 8

how do you calculate enthalpy change for a reaction involving gaseous molecules of covalent substances? (the one with a value for C-C, for example)

Answer 8

ΔH = ∑ (bond enthalpies in reactants) - ∑ (bond enthalpies in products)

Question 9

why could the value calculated from bond enthalpies differ from the true value?

Answer 9

• bond enthalpy calculation is based on average bond enthalpies, not the actual one
• bond enthalpy calculation assumes that all reactants and products are in the gaseous state