U1.2: Energy Cycles in Reactions Flashcards
Define bond enthalpy
- The amount of energy needed to break one mole of bonds in gaseous molecules under standard conditions
Define average bond enthalpy
- The amount of energy needed to break one mole of bonds in gaseous molecules under standard conditions
- averaged over a range of similar compounds
How to calculate bond enthalpy?
Sum of bonds broken in Endo + Sum of bonds formed in Exo
( + becomes - in actual calculations )
What is true about Ozone regarding its decomposition rate
Ozone decomposes faster than O2
State Hess’s Law
The enthalpy change for a reaction is independent of the pathway between the initial and final states
How does the Hess’ Law energy cycle look like (REP)
R –>P : Arrow is Delta Hr
E—>R: Arrow is Delta H(1) [usually positive]
E—>P: Arrow is Delta H(2) [usually negative]
Formula for Hess’s Law?
Delta H1 = Delta H2 + Delta H3
What to do if reactants and products switch in Hess’s Cycle
Sign of Delta H also changes
Define Standard Enthalpy changes of formation
- 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
Equation for Standard Enthalpy changes of formation?
ΔHr ⦵ = SUM (ΔHf ⦵ products - ΔHf ⦵ reactants)
Outline the enthalpy of formation for elements and why
- 0
- this is because no energy was required to make them in their natural state
Define Standard Enthalpy changes of combustion
- 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
What state is water in its standard conditions
Liquid
Equation for Standard Enthalpy changes of combustion?
ΔHr ⦵ = SUM (ΔHc ⦵ reactants - ΔHc ⦵ products)
For Standard enthalpy of combustion reactions, if the data is given for the enthalpy of formation, how do we approach these types of questions
- Use ΔHf formula
How else can we find the Standard Enthalpy of combustion reactions (other than formation and combustion values from the data booklet)
- Use bond enthalpy values as it is also calculated as Bonds broken in reactants - bonds formed in products
Define lattice enthalpy
Enthalpy change when 1 mole of an ionic compound’s broken apart into its constituent gaseous ions in standard states
Define enthalpy of atomisation
Enthalpy change when 1 mole is formed from an element
Define bond association energy (E)
Energy required to break 1 mole of bonds in gaseous states
Define 1st Electron A*ffinity (always exo –> favourable) and state the equation
Enthalpy change when one e- is A*dded to each atom in 1 mole of gaseous atoms
- X(g) + e- –> X(g)-
Define 2nd Electron A*ffinity (always endo–> unfavourable) and state the equation
Enthalpy change when the 2nd e- is A*dded
- X(g)- + e- –> X(g)2-
Define 1st Ionisation energy (endo) and state the equation
Enthalpy change when 1 e- is removed from each atom in 1 mole of gaseous atoms
- M(g) –> M(g)+ + e-
Define 2nd Ionisation energy (endo) and state the equation
Enthalpy change when a 2nd e- is removed from each atom in 1 mole of gaseous atoms
- M(g) –> M(g)2+ + e-
State the formula for the Born Haber Cycle
Hlat + Hf = 2(Hat - metal and nonmetal) + HIE + HEA
What is true about the ionic character of compounds when considering experimental and theoretical lattice enthalpies?
- Most compounds dont have 100% ionic character theoretically but we assume 100% ionic character experimentally
Describe the pattern for covalent character across and a period and down a group
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 %)
Describe the relationship between the discrepancy and covalent character
The greater the discrepancy in a compound, the more covalent character the element has and thus, the more purely ionic a compound (crystal) is
State what hydrogenation is
The addition of H2
Suggest why it may be useful to use Hess’s law to determine the heat of formation
- 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
