Thermodynamics Flashcards
a2 chem
Enthalpy change definiton
Heat energy change at constant pressure.
Enthalpy of formation definition
Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states. EXOTHERMIC
H2(g) + ½ O2(g) →H2O(l)
Enthalpy of combustion definition
Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states.
EXOTHERMIC
CH4(g) + 2O2(g) →CO2(g) + 2H2O(l))
Enthalpy of neutralisation definition
Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions
EXOTHERMIC
Enthalpy of atomisation definition
Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.
ENDOTHERMIC
First ionisation energy definition
Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions.
Mg(g) →Mg+(g) + e-
ENDOTHERMIC
Second ionisation energy definition
Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions.
Mg+(g) →Mg2+(g) + e-
ENDOTHERMIC
First electron affinity definition
Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1– ions.
O(g) + e- → O-(g)
EXOTHERMIC
Second electron affinity definition
Enthalpy change when each ion in one mole of gaseous 1– ions gains one electron to form one mole of gaseous 2– ions.
O-(g) + e- → O2-(g)
ENDOTHERMIC
Lattice enthalpy of formation definition
Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase.
Na+(g) + Cl- (g) → NaCl(s
EXOTHERMIC
Lattice enthalpy of dissociation definition
Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase
ENDOTHERMIC
Enthalpy of hydration definition
Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water).
Na+(g) + aq → Na+(aq
EXOTHERMIC
Enthalpy of solution definition
Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.
NaCl(s) + aq → Na+(aq) + Cl-(aq)
ENDO/EXO
Bond dissociation enthalpy definition
Enthalpy change when one mole of covalent bonds is broken in the gaseous state.
ENDOTHERMIC
Mean bond enthalpy definition
Enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds.
CH4 (g) → C(g) + 4H(g) = +1664kJ mol-1 / 4
ENDO/EXO
Enthalpy of vaporisation definition
Enthalpy change when one mole of a liquid is turned into a gas
ENDOTHERMIC
Enthalpy of fusion definition
Enthalpy change when one mole of a solid is turned into a liquid
ENDOTHERMIC
How to draw a born-haber cycle?
1) Start with solid on bottom line
2)Enthalpy of formation ( writing the elements in their standard states)
3)enthalpy of atomisation of the elements ( one mole of gaseous atoms)
4) ionisation energy (mainly metal first)
5) 2nd ionisation energy/affinity depending on the q’s
What areas of born-haber cycle are negative?
enthalpy of formation (bottom left)
(left hand side)
How is a lattice enthalpy calculated?
using a born-haber cycle
what determines the theoretical and experimental values of a lattice enthalpy?
it depends on how purely ionic the compound is.
What can data referring to a perfectly ionic model assume?
the theoretical lattice enthalpy
What makes a perfect ionic model perfect?
- Ions that are perfectly spherical
- The charge is evenly distributed in this sphere (point charges)
Why is the experimental value different from the theoretical value of lattice enthalpy?
- Suggests that the compound being experimented on doesn’t follow the perfectly ionic model and has covalent characteristics.
- The positive ion distorts the charge distribution in the negative ion ( positive ion polarises the negative ion)
What does more polarisation indicate?
The more covalent character there will be
What can lattice enthalpy values tell us?
How much a substance is ‘purely ionic’
What does bigger lattice enthalpy suggest?
more polarisation so the greater the covalent character in compound.
What process occurs when enthalpy change of solution and when can it happen?
The ionic lattice (in solid form) reacts with water to form free moving ions.
There are new bonds formed between ions and water.
The ions are hydrated.
It can only happen if the new bonds formed are the same strength or greater than those broken. If this isn’t the case then it’s very unlikely to dissolve. Soluble substances tend to have exothermic enthalpies of solution for this reason.
What is entropy?
Entropy is the measure of disorder in a system. Its the number of ways energy can be shared out between particles.
What does more disorder indicate about the level of entropy?
The more disorder the higher the level of enthalpy.
what are the levels of disorders in solids, liquids and gases?
solids have the lowest level of disorder as particles are arranged in rows.
Liquids and gases are more disordered.
What factor affects entropy change?
The number of particles also affect entropy change. If there is an increase in moles then entropy increases as there are more ways for energy to be distributed.
Increasing entropy is….
energetically favourable and some reactions that are enthalpically unfavourable (endothermic) can still spontaneously react of changes in entropy overcome changes in enthalpy.
What type of reaction is enthalpically unfavourable?
Endothermic reactions
What type of reaction is entropically favourable?
When there is an increase in moles from reactants –> products
when the state of product changes to one of increased disorder
How to calculate enthalpy of solution?
- using a hess cycle basically
down arrow from breaking up solid lattice ( enthalpy of lattice dissociation)
up arrow when gaseous ions are dissolved in water (enthalpy of hydration)
What is the Gibbs free energy equation and what does it tell us?
It tells us if a reaction is feasible or not.
ΔG=ΔH-TΔS
Gibbs free energy = Enthalpy change (mol-1) - temperature (k) x entropy change (jk-1mol-1)
What makes a reaction feasible in theory?
If ΔG (Gibbs free energy) is negative or 0
What is the rearranged equation and method to calculate temperature of which a reaction becomes feasible?
T= ΔH/ ΔS
Calculate entropy change ΔS:
S.Products- S.reactants
