Thermodynamics

Question 1

What is bond dissociation energy (ΔEd)?

Answer 1

The energy required to break a bond

Question 2

What is bond enthalpy (ΔHB)?

Answer 2

The change in heat accompanying the dissociation of a bond (measured at constant P)

Question 3

Equation for bond enthalpy

Answer 3

Question 4

Difference in ΔHB and ΔE for gases and liquids/solids

Answer 4

• For gases, ΔHB and ΔE differ by 1-2 %.
• For liquids and solids, negligible difference.

Question 5

ΔHB is always ___

Answer 5

Positive

Question 6

Why is ΔHB always positive?

Answer 6

Because it always takes energy (heat) to break a bond

Question 7

Bond breaking is always ___

Answer 7

Endothermic

heat is added, ΔHB > 0

Question 8

For an endothermic reaction, heat is added, ΔHB __ 0

Answer 8

ΔHB > 0

Question 9

Bond formation is always ___

Answer 9

Exothermic

heat is released, ΔHB < 0

Question 10

For an exothermic reaction, heat is added, ΔHB __ 0

Answer 10

ΔHB < 0

Question 11

Symbol for standard bond enthalpy

Answer 11

ΔHB°

Question 12

What does standard bond enthalpy indicate?

Answer 12

Indicates that reactants are products are in the standard state (pure form) at 1 atm, 25°C

Question 13

What is the importance of bond enthalpies?

Answer 13

The difference between bond enthalpies in products and reactants gives an estimate of the enthalpy of reaction (∆Hr°)

Question 14

Enthalpies of reaction

Answer 14

Question 15

The difference between the bond enthalpies in products and reactants gives an estimate of the ___

Answer 15

enthalpy of reaction (∆Hr°)

Question 16

Calculation of reaction enthalpies (ΔHr°)

Answer 16