U1.2: Energy Cycles in Reactions

Question 1

Define bond enthalpy

Answer 1

• The amount of energy needed to break one mole of bonds in gaseous molecules under standard conditions

Question 2

Define average bond enthalpy

Answer 2

• The amount of energy needed to break one mole of bonds in gaseous molecules under standard conditions
• averaged over a range of similar compounds

Question 3

How to calculate bond enthalpy?

Answer 3

Sum of bonds broken in Endo + Sum of bonds formed in Exo
( + becomes - in actual calculations )

Question 4

What is true about Ozone regarding its decomposition rate

Answer 4

Ozone decomposes faster than O2

Question 5

State Hess’s Law

Answer 5

The enthalpy change for a reaction is independent of the pathway between the initial and final states

Question 6

How does the Hess’ Law energy cycle look like (REP)

Answer 6

R –>P : Arrow is Delta Hr
E—>R: Arrow is Delta H(1) [usually positive]
E—>P: Arrow is Delta H(2) [usually negative]

Question 7

Formula for Hess’s Law?

Answer 7

Delta H1 = Delta H2 + Delta H3

Question 8

What to do if reactants and products switch in Hess’s Cycle

Answer 8

Sign of Delta H also changes

Question 9

Define Standard Enthalpy changes of formation

Answer 9

• The enthalpy change when 1 mole of a compound forms from its constitute elements under standard conditions
• All reactants and products are in standard states

Question 10

Equation for Standard Enthalpy changes of formation?

Answer 10

ΔHr ⦵ = SUM (ΔHf ⦵ products - ΔHf ⦵ reactants)

Question 11

Outline the enthalpy of formation for elements and why

Answer 11

• 0
• this is because no energy was required to make them in their natural state

Question 12

Define Standard Enthalpy changes of combustion

Answer 12

• The enthalpy change when 1 mole of a SUBSTANCE is completely burned in Oxygen under standard conditions.
• All reactants and products are in their standard conditions

Question 13

What state is water in its standard conditions

Answer 13

Liquid

Question 14

Equation for Standard Enthalpy changes of combustion?

Answer 14

ΔHr ⦵ = SUM (ΔHc ⦵ reactants - ΔHc ⦵ products)

Question 15

For Standard enthalpy of combustion reactions, if the data is given for the enthalpy of formation, how do we approach these types of questions

Answer 15

• Use ΔHf formula

Question 16

How else can we find the Standard Enthalpy of combustion reactions (other than formation and combustion values from the data booklet)

Answer 16

• Use bond enthalpy values as it is also calculated as Bonds broken in reactants - bonds formed in products

Question 17

Define lattice enthalpy

Answer 17

Enthalpy change when 1 mole of an ionic compound’s broken apart into its constituent gaseous ions in standard states

Question 18

Define enthalpy of atomisation

Answer 18

Enthalpy change when 1 mole is formed from an element

Question 19

Define bond association energy (E)

Answer 19

Energy required to break 1 mole of bonds in gaseous states

Question 20

Define 1st Electron A*ffinity (always exo –> favourable) and state the equation

Answer 20

Enthalpy change when one e- is A*dded to each atom in 1 mole of gaseous atoms
- X(g) + e- –> X(g)-

Question 21

Define 2nd Electron A*ffinity (always endo–> unfavourable) and state the equation

Answer 21

Enthalpy change when the 2nd e- is A*dded
- X(g)- + e- –> X(g)2-

Question 22

Define 1st Ionisation energy (endo) and state the equation

Answer 22

Enthalpy change when 1 e- is removed from each atom in 1 mole of gaseous atoms
- M(g) –> M(g)+ + e-

Question 23

Define 2nd Ionisation energy (endo) and state the equation

Answer 23

Enthalpy change when a 2nd e- is removed from each atom in 1 mole of gaseous atoms
- M(g) –> M(g)2+ + e-

Question 24

State the formula for the Born Haber Cycle

Answer 24

Hlat + Hf = 2(Hat - metal and nonmetal) + HIE + HEA

Question 25

What is true about the ionic character of compounds when considering experimental and theoretical lattice enthalpies?

Answer 25

• Most compounds dont have 100% ionic character theoretically but we assume 100% ionic character experimentally

Question 26

Describe the pattern for covalent character across and a period and down a group

Answer 26

Covalent character increases down a group and across a period as:
- The difference between experimental and theoretical values of lattice enthalpy increases (there is a greater discrepancy %)

Question 27

Describe the relationship between the discrepancy and covalent character

Answer 27

The greater the discrepancy in a compound, the more covalent character the element has and thus, the more purely ionic a compound (crystal) is

Question 28

State what hydrogenation is

Answer 28

The addition of H2

Question 29

Suggest why it may be useful to use Hess’s law to determine the heat of formation

Answer 29

• In some cases, the heat of formation is impossible to measure directly
• Hess’s law helps us break a reaction or process into a series of small steps