Unit 1: Section 6 - Thermodynamics Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure
What is enthalpy change of formation?
ΔfH - the enthalpy change when 1 mole of a substance is formed from its constituent elements with all substances in their standard states - exothermic for most substances
2Na(s) + 1/2O2(g) -> Na2O(s)
What is enthalpy of combustion?
ΔcH - When 1 mole of a substance undergoes complete combustion in oxygen with all substances in their standard states - exothermic
H2 (g) + 1/2O2(g) -> H2O(l)
What is enthalpy of neutralisation?
ΔneutH - when 1 mole of water is formed in a reaction betweeen an acid and alkali under standard conditions - exothermic
1/2H2SO4(aq) + NaOH(aq) -> 1/2Na2SO4(aq) + H2O(l)
What is ionisation enthalpy?
ΔieH
1st ionisation enthalpy - enthalpy change whe each atom in 1 mole of gaseous atoms loses 1 electron to form 1 mole of gaseous 1+ ions - endothermic
Mg(g) -> Mg+(g) +e-
2nd ionisation enthalpy - enthalpy change when each ion in 1 mole of gasous 1+ ions loses 1 electron to form 1 mole of gaseous 2+ ions - endothermic
Mg+(g) -> Mg2+ +(g) +e-
What is electron affinity?
ΔeaH
1st electron affinity - enthalpy change when each atom in 1 mole of gaseous atoms gains 1 electron to form 1 mole of gaseous 1- ions - exothermic for many non-metals
O(g) + e- -> O-(g)
2nd electron affinity - enthalpy change when each ion in 1 mole of gasous 1- ions gains 1 electron to form 1 mole of gasoues 2- ions - endothermic
O-(g) + e- -> O2-(g)
What is enthalpy of atomisation?
ΔatH - when 1 mole of gaseous atoms is produced from an element in its standard state - endothermic
1/2I2(s) -> I(g)
What is Hydration enthalpy?
ΔhydH
When 1 mole of gaseous atoms dissolve in water (become hydrated) - exothermic
Mg2+(g) + aq -> Mg2+(aq)
What is enthalpy of solution?
ΔsolH
When 1 mole of an ionic solid disolves in an amount of water large enough so that the dissolved ions are well separated and don’t interact with each other - varies
MgCl2(s) + aq -> Mg2+(aq) + 2Cl-(aq)
What is bond dissociation enthalpy?
ΔdisH
When 1 mole of covalent bonds is broken in the gaseous state - endothermic
I2(g) -> 2I (g)
What is lattice enthalpy of formation?
ΔLEFH
When 1 mole of a solid ionic compound is formed from its consistuent ions in the gas phase - exothermic
Mg2+(g) + 2Cl-(g) -> MgCl2(s)
What is lattice enthalpy of dissociation?
When 1 mole of a solid ionic compound is broken up into its constituent ions in the gas phase - endothermic
MgCl2(s) -> Mg2+(g) + 2Cl-(g)
What is enthalpy of vaporisation?
ΔvapH
Whe 1 mole of a liquid is turned into a gas - endothermic
H2O(l) -> H2O(g)
What is enthalpy of fusion?
ΔfusH
When 1 mole of a solid is turned into a liquid - endothermic
Mg(s) -> Mg(l)
What are Born-Haber cycles used for?
Used to calculate lattice enthalpies
What is the perfect ionic model of a lattice?
Assumes that all the ions are spherical, and have their charge evenly distributed around them
Why is the perfect ionic model often not accurate?
- Ions are not perfectly spherical
- Polarisation often occurs when small positive ions or large negative ions are involved
- Some lattices are not regular and the crystal structure can differ
How do we know about covalent character?
- The lattice enthalpy found experimentally is often not the theoretical lattice enthalpy
- This is evidence that most ionic compounds have some covalent character
How does covalent character arise?
- The positive and negative ions in a lattice aren’t exactly spherical
- Positive ions polarise neighbouring negative ions to different extents
- The more polarisation there is, the more covalent the bonding will be
What characteristics does covalent character affect?
- Solubility - makes the compound less soluble
- General conductivity
What 2 things happen when a solid ionic lattice dissolves?
- The bonds between the ions break to give free ions - endothermic
- Bonds between the ions and the water are made - exothermic
The enthalpy change of solution is the overall effect on the enthalpy of these 2 things
When do substances dissolve?
If the energy released is roughly the same, or greater than the energy taken in
What is entropy?
- A measure of the number of ways that particles can be arranged and the number of ways the energy can be shared out between the particles in a system
- The more disordered the particles are, the higher the entropy is
What affects entropy?
Physical state - solids have the lowest entropy as they just vibrate about a fixed position but gases have the highest entropy as they have the most disordered arrangements
More particles - more ways they can be arranged and the more energy there is