Lattice Enthalpy Flashcards
What is enthalpy?
The internal energy of a substance, H. Unmeasurable.
ΔH?
H products - H reactants
T standard condition?
298K
P standard condition?
100kPa
Conc standard condition?
1moldm-3
Standard Enthalpy Change of Neutralisation definition?
Δ(subscriptneut)Hθ
Enthalpy change when 1 mol of H2O is formed in the reaction of an acid and a base under standard conditions: 298K, 100kPa, 1moldm-3 - STATE THESE
Standard Enthalpy Change of combustion definition?
Δ(subscriptc)Hθ
Enthalpy change when 1 mol of a substance is completely burned in O2 under standards conditions: 100kPa, 298K (conc not relevant here)
remember when doing calorimetry calcs?
NEED TO PUT THE SIGN ON
How to do a calorimetry calc?
calc q using q=mcΔT
then, calc enthalpy change using ΔH = q/n (of water)
Enthalpy change of formation definition?
- Enthalpy change when 1 mol of a compound is formed from its elements in their normal states under standard conditions - 298k, 100kPa, 1 moldm-3
- Na(s) + 1/2Cl2(g) ➡ NaCl (s)
- can be endo or exo
first IE definiition?
- Mg (g) - e- -> Mg+ (g)
- Endothermic - need to overcome the attraction between the e-s and + nucleus
The energy needed to remove 1e- from each atom in 1 mol of gaseous atoms forming 1 mol of gaseous 1+ ions
ΔI1Hθ/ ΔIE1Hθ
2nd IE definition?
• Mg+ (g) - e- -> Mg2+ (g)
• The energy needed to remove 1e- from each ion in 1 mol of gaseous 1+ ions forming 1 mol of gaseous 2+ ion
• more endothermic than 1st IE - bc the p:e increases so the following e-s are more strongly attracted and so take more energy to remove
ΔI2Hθ
Hess’s Law definition?
the enthalpy change for a reaction is independent of the route taken as long as it proceeds between the same initial reactant & final product.
Lattice enthalpy definition?
- the energy released when 1 mol of an ionic compound forms from its gaseous ions
- Mg2+ (g) + 2Cl- (g) ➡MgCl (s)
- MAKE IT CLEAR IF FORMING OR BREAKING THE LATTICE TO THE EXAMINER
ΔLEHθ
Standard enthalpy change of atomisation definition?
- ΔatHθ
- The energy needed to form 1 mol of gaseous atoms from the element in its normal state under standard conditions - 298K , 100kPa
- Endothermic
Mg(s) ➡Mg(g)
1/2Cl2 ➡ Cl (g)
the first electron affinity definition?
- exothermic
- the energy released when each atom in 1 mol of gaseous atoms gains an e- forming 1 mol of gaseous 1- ions
- 0(g) + e- ➡ O- (g)
ΔEA1Hθ
The second electron affinity definition?
ΔEA2Hθ
- The energy required to add 1 e- to each ion in 1 mol of gaseous 1- ions forming 2- ions
- Endothermic - negatively charged ion repels the e-, so e required
O-(g) + e- ➡ O2-(g)
Enthalpy change of solution definition?
- Enthalpy change when 1 mol of a compound is completely dissolved in water under standard conditions
- Can be endo/exo
- ΔsolHθ
NaCl(s) ➡(water on arrow) Na+ (aq) + Cl- (aq)
Enthalpy change of hydration ?
• always exo - always about forming bonds between ions and H2O
• Energy released when 1 mol of gaseous ions is dissolved in water forming 1 mol of aq ions
• ΔhydHθ
Na+(g) ➡ (water on arrow) Na+ (aq)
electron affinity and IE are?
opposites
⭐ What are really important with born haber cycles?
the states
summary of definitions?
- If a number of moles is mentioned it is always 1 mole
* if a state is mentioned it is often the gaseous state or the standard state
Born haber cycles ?
Hess’ cycles for the formatio of ionic compounds
⭐ With equations, be careful because
could be 2 diff ones - e.g. atomisation and IE
How to draw a born haber cycle?
• try to get endo on the right and exo on the left
Always put LE as ? on BHC
exo
how to draw BHC for LE?
1) write equations
2) put enthalpy arrow
1) formation of the ionic compound
2) atomisation
3) atomisation (of other reactant)
4) first IE (MUST be before Ea - e- needed)
5) first Ea
(could be multiple Ea/ IE)
After drawing the BHC, use ? to calc LE?
- Hess’ Law
* change the sign when going against the arrow
Bonds mneunomic?
Breaking bonds is
Endothermic
Making bonds is
E
xothermic
forming the lattice is?
exothermic - making bonds
When explaining the differences in LE?
• look at what ion is diff
1) which is more exothermic (negative)
2) which ion attracts more strongly
3) which has the smaller IONIC radius, which has the higher charge density (same/larger charge spread over smaller ionic radius)
when answering a LE trend question?
order the substances, use comparative words
When answering a HE trend question?
- which has the most exo value?
- which forms the strongest electrostatic forces of attraction to the water molecule (not H bonds)
- which has the highest charge density
across a period?
atomic radius ⬇ bc proton number increases
Dissolving an ionic solid involves 2 processes?
- Breaking down the lattice into gaseous ions. This is the reverse of lattice enthalpy. This bond breaking. +ΔH
- Hydration enthalpy - the enthalpy change when 1 mole of gaseous ions are dissolved in water forming 1 mole of aqueous ions, under standard conditions. This is always bond forming, -ΔH
EoS equation?
endothermic LE (forming is exo) + exo HE
HE is always?>
exo
how to draw an energy level cycle for EoS?
- enthalpy of solution
- LE
- Hydration
EoS can be ?
endo/exo
2 ways of working out EoS?
- using cycle
2. using equation
how to work out EoS from experimental data?
- q = mcdeltaT
- delta H = q/n
- ⭐ data must be of dissolving the solid
the more negative the enthalpy of solution…
the more soluble a substance
Which substances would you expect to be soluble in water?
the ones with an exo ΔH
enthalpy change of neutralisation is nearly always ?
around 57
enthalpy change of neutralisation - mass is?
vols of acid + base
When the products are have less enthalpy (Exo)
more stable, less reactive, more likely to form
endo products are?
less stable, more enthalpy, less likely to form
