Bond Enthalpies

Question 1

Bond dissociation energy (E) is usually just simplified to bond energy or bond enthalpy

WHAT IS THE DEFINITION

Answer 1

The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy(bond energy or bond enthalpy
)

Question 1

In symbols, the type of bond broken is written in

Answer 1

In symbols, the type of bond broken is written in brackets after E
Eg. E (H-H) is the bond energy of a mole of single bonds between two hydrogen atoms

Question 2

Bond energies are affected by …

Therefore, an average of a number of the same type of bond but in different

Answer 2

affected by other atoms in the molecule (the environment

Therefore, an average of a number of the same type of bond but in different environments is calculated

Question 3

Bond energies are affected by other atoms in the molecule (the environment)
Therefore, an average of a number of the same type of bond but in different environments is calculated

This bond energy is known as

Answer 3

average bond energy

Question 4

Since bond energies cannot be determined directly, ;…………….. are used to calculate the average bond energy

Answer 4

enthalpy cycles

Question 5

Calculating enthalpy change from bond energies
Bond energies are used to find the ………………..of a reaction when this cannot be done experimentally

Answer 5

ΔHrꝋ

Question 6

ΔHrꝋ formula

Answer 6

enthalpy change of bonds broken + enthalpy change of bonds formed

Question 7

Step 1: Use the equation to work out the bonds broken and formed and set out the calculation as a balance sheet as shown below:

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

Note! Values for bonds broken are positive (endothermic) and values for bonds formed are negative (exothermic)

Step 2: Calculate the standard enthalpy of reaction

ΔHrꝋ = enthalpy change for bonds broken + enthalpy change for bonds formed

= (+2253 kJ mol-1) + (-2346 kJ mol-1)

= -93 kJ mol-1

Question 8

Step 1: The enthalpy of combustion is the enthalpy change when one mole of a substance reacts in excess oxygen to produce water and carbon dioxide

The chemical reaction should be therefore simplified such that only one mole of ethyne reacts in excess oxygen:

H-C≡C-H + 2 ½ O=O → H-O-H + 2O=C=O

Step 2: Set out the calculation as a balance sheet as shown below:

ΔHrꝋ = enthalpy change for bonds broken + enthalpy change for bonds formed

= (+2912 kJ mol-1) + (- 4142 kJ mol-1)

= -1230 kJ mol-1