R1.2.1 – R1.2.5 Energy Cycles in Reactions

Question 1

Bond breaking is…

Answer 1

an endothermic process

Energy is required to break chemical bonds

Question 2

Bond-forming is…

Answer 2

an exothermic process

Energy is released when new bonds form

Question 3

Bond enthalpy

Answer 3

Energy required to break one mole of bonds by homolytic fission in one mole of gaseous covalent molecules under standard conditions

Question 4

Homolytic fission

Answer 4

Distributes the electrons from the bond equally between the two new species, resulting in the formation of radicals

Question 5

Radical

Answer 5

An atom, molecule, or ion that has at least one unpaired valence electron

Usually highly chemically reactive

Question 6

Bond enthalpies are affected by…

Answer 6

the bond’s chemical environment

Bond enthalpies are average values

Question 7

Enthalpy change of reaction calculated using bond enthalpy data

Formula

Answer 7

(Sum of bond enthalpies of bonds broken) - (sum of bond enthalpies of bonds formed)

Question 8

Why does the enthalpy change of a reaction calculated from bond enthalpies differ from its actual value?

Answer 8

• Bond enthalpies are average values
• Bond enthalpy data do not take intermolecular forces into account, which is particularly important when substances in a reaction are solid or liquid

Question 9

Hess’s Law

Answer 9

Regardless of the route by which a chemical reaction proceeds, the enthalpy change will always be the same as long as the initial and final states of the system are the same

Application of the law of conservation of energy

Question 10

Standard enthalpy of combustion

Answer 10

Enthalpy change that occurs when one mole of a substance in its standard state is burned completely in oxygen

Question 11

Standard state of a pure substance

Answer 11

Form it takes under standard conditions (SATP)

25.00°C (298.15K) and 1.00 x 10⁵ Pa

Question 12

Standard enthalpy of formation

Answer 12

Energy change that occurs when one mole of a substance is formed from its constituent elements in their standard states

Question 13

Most stable allotrope

Answer 13

Standard state of an element with several allotropes

Question 14

Enthalpy as a state function

Answer 14

Enthalpy change of a reaction is the difference between the initial and final values of the enthalpy of the system

Pathway between initial and final states of a reaction does not affect overall enthalpy change

Question 15

Overall standard enthalpy change for a reaction

Calculated using enthalpy of combustion data

Answer 15

(Sum of the enthalpy changes of combustion of the reactants) - (sum of the enthalpy changes of combustion of the products)

Application of Hess’s Law

Question 16

Overall standard enthalpy change for a reaction

Calculated using enthalpy of formation data

Answer 16

(Sum of the enthalpy changes of formation of the products) - (sum of the enthalpy changes of formation of the reactants)

Question 17

Ionization energy

Answer 17

Standard enthalpy change that occurs on the removal of one mole of gaseous electrons from one mole of atoms or positively charged ions in the gaseous state

Endothermic

Question 18

Enthalpy of atomization

Answer 18

Standard enthalpy change that occurs when one mole of gaseous atoms of an element is formed

Endothermic

Question 19

Enthalpy of atomization

Solid monatomic species

Answer 19

Value is equal to the enthalpy of sublimation

Question 20

Enthalpy of atomization

Gaseous diatomic species

Answer 20

Value is equal to half of the bond enthalpy

Question 21

Electron affinity

Answer 21

Standard enthalpy change on the addition of one mole of electrons to one mole of atoms in the gaseous state

Typically negative for non-metals (with exceptions)

Question 22

Lattice enthalpy

Answer 22

Standard enthalpy change that occurs when gaseous ions are formed from one mole of structural units of a solid ionic lattice

Endothermic

Solid structure to gaseous ions