Energetics II - Topic 13 Flashcards
Enthalpy of atomisation
enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
First ionisation enthalpy
The first ionisation enthalpy is the enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge
Second Ionisation enthalpy
The second ionisation enthalpy is the enthalpy
change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions
First Electron Affinity
The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge
Second Electron Affinity
The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions
Why is the first electron affinity exothermic
The first electron affinity is exothermic for atoms that normally form negative ions because the ion is more stable than the atom and there is an attraction between the nucleus and the electron
Why is the second affinity endothermic
The second electron affinity for oxygen is
endothermic because it take energy to overcome the repulsive force between the negative ion and the electron
Enthalpy of lattice formation
The Enthalpy of lattice formation is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form
What factors affect enthalpy of lattice formation
- size of ion
- charge of ion
How does the size of the ion affect strength of a enthalpy lattice formation
The larger the ions the less negative the enthalpies of lattice formation, as when the size of the ions increase the charges become further apart and so have a weaker attractive force between them
How does the charge of the ion affect strength of a enthalpy lattice formation
The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values)
Why does the first electron affinity become less exothermic down group 7
As the atoms get bigger and have more shielding so it becomes less easy to
attract electrons to the atom to form the negative ion
General trend of lattice enthalpies formation down a group
It becomes less negative down any group
How to calculate lattice enthalpy using Bohr harbour cycle
Lattice enthalpy = enthalpy of formation - ( enthalpy of atomisation + enthalpy of first/second ionisation + first/second electron affinity)
Know how to draw Bohr harbour cycle
When calculating enthalpy of solution in an enthalpy cycle and Bohr harbour how do you work out what arrow is what
Arrow pointing from solid to aqueous = enthalpy of solution
Arrow pointing from gas to solid = lattice enthalpy of dissociation
Arrow pointing from gas to aqueous = enthalpy of hydration
Why are hydration enthalpies exothermic
as water molecules have delta positive regions that naturally attract negative ion forming a bond between ion and water molecule
What affects enthalpy of hydration
- size of the ion
- charge of the ion
How does the size and charge of the ion affect hydration enthalpy
Size - as the size of the ions increase the the attraction between the gaseous ions and water molecules are weaker as they are further apart so the hydration enthalpy will become less negative
Charge - as the charge increases the forces of attraction between gaseous ions and water molecules will increase so the hydration enthalpy will increase
What do theoretical lattice enthalpies assume
They assume that ions have the perfect ionic model the ions are 100% ionic and
spherical and the attractions are purely electrostatic
Define polarised
When the negative ion becomes distorted and more covalent
Define polarising
The metal cation that causes the polarisation
What is covalent character
This is when the positive ion is so highly charge dense that it polarises the negative ion, causing the ions to become close to each other resulting in electrons to be shared between the two ions rather than being localised on the anion
What causes the polarising power of the cation to increase
- if the positive ion is small
- the positive ion has multiple charges
Why are hydration enthalpies exothermic
As energy is given out as water molecules bind to metal ions
What affects hydration enthalpies
- size of the ion
- charge of the ion
How to work out enthalpies of solution
Enthalpy if solution = lattice enthalpy + hydration enthalpy
Properties of the enthalpies of a soluble substance
- enthalpy of solution is exothermic
- hydration enthalpy = lattice enthalpy
Properties of the enthalpies of an insoluble substance
- enthalpy of solution is endothermic
- lattice enthalpy > hydration enthalpy
What happens to the entropy when a substance dissolves
the entropy increases as there is more disorder as solid changes to solution and number of particles increases
Equation for delta S total (total entropy change)
Delta S total = - delta H/T + delta S system
Or
Delta S total = delta S system + delta S surrounding
Why will salts always dissolve if the enthalpy change is exothermic at all temperatures
- This is due to the enthalpy change being negative so (minus) - delta H/T will be positive and the delta system will be positive due to the increased disorder as there are more particles
- This means that the total entropy change (delta S total) will be positive so the salt can dissolve in when delta H is exothermic
Use this equation to answer the question: Delta S total = - delta H/T + delta S system
For salts where enthalpy change is endothermic what factor depends on whether it is soluble or not
Use this equation to answer question: Delta S total = - delta H/T + delta S system
- Delta S system depends on whether the salt can dissolve
- As due to delta H being positive (due to it being endothermic) it means that - Delta H/T will be negative
- However if Delta S system is more positive than - delta H/T due to the increased disorder than delta S total will become more positive making the reaction feasible and the salt will dissolve
What is a spontaneous process
A process that will on its own without any external influence
What substances have a high entropy
Substances that have more ways of arranging their atoms and energy and therefore are more disordered
Sign for entropy
Delta S
What is in a system
- a system will consist of reactants and then products.
- It does not change temperature or pressure, and mass cannot be transferred to the
surroundings. - Energy can be transferred to the surroundings.
Trend in entropy in solids liquids and gases
- Solids have lower entropies than liquids which are lower than gases.
- When a solid increases in temperature its entropy increases as the particles vibrate more.
- There is a bigger jump in entropy with boiling than that with melting.
- Gases have large entropies as they are much more disordered
What does an increase in entropy lead to
A positive entropy change in delta S system
What factors causes an increase in entropy
- a change of state from solid or liquid to gas
- significant increase in number of molecules between products and reactants.
NH4Cl (s) —> HCl (g) + NH3(g) is delta S positive or negative and explain why
∆S˚system = +ve
•change from solid reactant to gaseous products
•increase in number of molecules
both will increase disorder
Na (s) + ½ Cl2 (g) —> NaCl (S) is delta S system positive or negative and explain why
∆S˚system = -ve
•change from gaseous and solid reactant to solid
•decrease in number of molecules
both will decrease disorder
How to work out delta S system
∆S˚system = Σ S˚products - ΣS˚reactants
Or
Delta S system = (Delta H x 1000) - Delta G/ T
Unit of entropy
JK-1 mol-1
How to work out delta S surrounding
Delta S surrounding = - (delta H x1000 to convert into mol) / T (+273 to convert into Kelvin)
What is needed for a reaction to be feasible
- Delta G to be negative
- caused by an increasing entropy (positive delta S system) and the reaction being exothermic (negative delta H)
What causes a reaction to not occur
- if delta G is positive
- if there is a high activation energy
How to calculate delta G
Delta G = delta H - T x (delta S SYSTEM/1000)
Or
(using equilibrium)
ΔG = - RT lnK
Units for Delta G
KJ mol-1
What value is delta G when a substance is changing state
0
Effect of increasing temperature on feasibility
- If the reaction involves an decrease in entropy (∆S is - ve) then increasing temperature will make it less likely that ∆G is negative and less likely for the reaction to occur
- If the reaction involves an increase in entropy (∆S is +ve) then increasing temperature will make it more likely that ∆G is negative and more likely that the reaction occurs
- if the reaction has a ∆S close to zero then
temperature will not have a large effect on the feasibility of the reaction as - T∆S will be small and ∆G won’t change much
How to work out equilibrium using delta G
K = e^(- delta Gx1000/ RT)
Values of equilibrium that is used to predict feasibility of reaction
Kc < 10^-10 - reaction is not feasible
Kc = 0.1 - reactants predominant at equilibrium
Kc = 1 - equal amounts of products and reactants
Kc = 10 - products predominates at equilbrium
Kc > 10^10 - reaction goes to completion
Hydration enthalpy
Enthalpy change when one mile of gaseous ions becomes hydrated
Enthalpy of solution
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
Bond dissociation enthalpy
Enthalpy change when one mole of covalent bonds is broken in the gasesous state
Lattice enthalpy of dissociation
Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase
Enthalpy of vapourisation
Enthalpy change when one mole of a liquid is turned into a gas
Enthalpy of fusion
Enthalpy change when one mole of a solid is turned into a liquid
How to work out temperature range a reaction is feasible at
Delta H/ (Delta S /1000)
How to determine if a reaction is thermally stable using Delta G
Delta G has to be positive for a reaction to be thermodynamically stable
