Energetics II Flashcards
How does an ionic compound form
Metallic atoms transfer electrons to nonmetallic atoms to form positive and negative ions which attract to form a lattice
Melting point and boiling point of ionic substances?
High
Lots of energy to overcome strong electrostatic attraction between oppositely charged ions
Standard enthalpy change of formation definition
Enthalpy change when one mole of a compound is formed from its elements under standard conditions
Enthalpy of lattice formation (AKA lattice energy) definition
Enthalpy change when one mole of a solid ionic lattice is formed from its gaseous ions
Making bonds so exothermic so always NEGATIVE VALUE
Lattice energy is always…
And it is a measure of…
Negative
Ionic bond strength
General born-haber cycle to work out lattice energy if have to do calculation?
Gaseous ions -lattice enthalpy—> ionic solid (on top)
Elements (on bottom)
Arrow form elements up to gaseous ions - atomisation, ionisation, atomisation, electron affinity etc.
Arrow from elements to ionic solid - enthalpy of formation
Steps to convert elements to gaseous ions
1) Change elements in standard form to gaseous state (enthalpy of atomisation)
2) Change gaseous metal atoms to (positive) ions (ionisation energy)
3) Change gaseous non-metal atoms into (negative) ions (atomise first if diatomic then electron affinity)
Standard enthalpy of atomisation definition
Enthalpy change when one mole of gaseous atoms are produced from one mole of an element under standard conditions
Eg. 1/2Br2 (l) —-> Br- (g)
Always positive because have to supply energy to atomise
First electron affinity
XXXX
Enthalpy change for one mole of gaseous atoms
to gain one mole of electrons
to form one mole of gaseous ions with a 1- charge
Eg. X(g) + e- —-> X-(g)
——————
First electron affinity is exothermic for atoms that normally form negative ions WHY???
Second electron affinity
Enthalpy change when one mole of gaseous 1- ions
gains one electron per ion to produce
one mole of gaseous 2- ions
Eg. X-(g) + e- —-> X2- (g)
——————
Second electron affinity is ENDOTHERMIC because energy must be taken in to overcome repulsion between already negative ion and electron you are trying to add
Equation for lattice enthalpy
(energy change of elements to ionic solid aka enthalpy of formation) - (energy change the elements to gaseous ions)
Going round the cycle it’s negative gaseous ions to elements + enthalpy of formation of ionic solid from elements
How can we calculate the theoretical lattice enthalpy? And what do we assume?
Visualise perfectly spherical ions in a lattice
Calculate all forces of attraction and repulsion in the lattice
Assume perfect ionic model where ions are 100% ionic and spherical and attractions are purely electrostatic.
What kind of values are the values determined by the born-haber cycle?
Value is determined by experiment
Reflect the actual value
(We assumed perfect ionic model but if the experimental value is different we know this assumption was wrong and there is covalent character which is causes this difference)
What does a MORE -VE value of lattice enthalpy of FORMATION mean?
And LESS -VE value?
More negative value More exothermic More heat energy released Stronger bonds made Stronger lattice
Less negative value Less exothermic Less heat energy released Weaker bonds made Weaker lattice
What does a great a difference between the experimental and theoretical value of the born-haber cycle mean?
Greater degree of covalency in the lattice (and thus more polarisation)
Perfect ionic model
Ions are 100% ionic and spherical
Attractions are purely electrostatic
Polarised ionic bonding
Cation attract electrons
What does the extent of polarisation depend on?
How much
the cation pulls the electrons (polarising power)
The anion lets the electrons be pulled away (Polarisability)
Factors which increase polarisation in ionic compounds?
1) size of cation (smaller cation attracts e- more so more polarising)
2) size of anion (larger anion more polarisable)
3) increased charge of cation or anion
What type of polarisation would
K+F- have?
Low polarisation
K is a large cation
F is a small anion
low charge (both +/-1)
What type of polarisation would
Al3+ (I-)3 have?
High polarisation
Al3+ is a small cation (more polarising power)
(I-)3 is a large anion (more polarisable)
More +ve value of lattice DISSOCIATION enthalpy means
More +ve value of dissociation enthalpy More heat energy taken in More endothermic Stronger bonds had to be broken Stronger lattice
Less +ve = weaker lattice
High polarisability (Anion) High polarisation (cation)
LARGE ANION
SMALL CATION
What causes polarisation and what does a greater polarisation mean?
Covalency causes polarisation
Greater polarisation means more covalent character
Link difference in actual and theoretical values of lattice enthalpies , covalency and polarisation
Larger difference = higher covalency degree = more polarisation
What can the actual and theoretical values of lattice enthalpies be used to find?
Degree of polarisation
Why do some ionic compounds not dissolve in water?
Strong electrostatic attraction between oppositely charged ions is too large to be overcome by water molecules
Explain how water molecules interact with an ionic compound?
Delta positive hydrogen attracted to negative ions
Delta negative oxygen attracted to positive ions
Water molecules vibrate and shake ions free from lattice
What two things occur when ionic compounds dissolve in water?
1) Lattice broken apart (endothermic)
2) New bonds form between water molecules and ions (exothermic)
Enthalpy change of solution definition
ionic equation for dissolving ionic solid NaCl
Exothermic enthalpy change when one mole of ionic compound is dissolved in excess water and ions are well separated & don’t interact with each other
Ionic solid —> hydrated ions
NaCl(s) + (aq) —> Na+ (aq) + Cl- (aq)
Enthalpy of hydration definition
Exothermic enthalpy change when
one mole of gaseous ions dissolve in excess water
to form an infinitely dilute solution (such that further dilution causes no further heat change)
M+(g) + aq —-> M+(aq)
X+(g) + aq —-> X+ (aq)
M+(g) + X-(g) + aq —-> M+ (aq) + X-(aq)
—————
Always exothermic because bonds are MADE between ions and water molecules
Enthalpy of lattice dissociation
*opposite of lattice formation enthalpy
Enthalpy change to convert 1 mole of solid ionic lattice to
1 mole of its constituent gaseous ions
Breaking bonds so always a POSITIVE VALUE
How is lattice enthalpy formation connected to lattice enthalpy dissociation?
Lattice enthalpy dissociation is the OPPOSITE of lattice enthalpy formation
Lattice enthalpy of formation is exothermic eg. -200
Lattice enthalpy of dissociation is (-ve formation) so -(-200) so +200
Characteristics of a weak ionic lattice
Small cation
Large anion
High charges on ions
What happens to lattice energy as the size of the anion increases?
Anion size increase
Charge density decreases
Electrostatic attraction between oppositely charge ions decreases
Value of lattice energy decreases (aka. strength decreases)
What is charge density?
Charge per unit volume
What happens to lattice energy as the size of the cation increases?
Size of cation increases
Charge density decreases
Less electrostatic attractions
Lattice energy decreases
What happens to lattice energy as the charge on either the cation or anion increases?
Charge on ion increases
Charge density increases
Electrostatic attraction increases
Lattice energy increases
Define enthalpy of hydration
Enthalpy change when one mole of gaseous ions
become hydrated such that further dilution causes no further heat change
(Heat energy given out when they dissolve)
How does the size of the ion affect enthalpy of hydration?
Size of cation/anion increases Charge density of ion decreases Electrostatic attraction between water molecule and ion decreases Weaker bonds made Less energy given out Hydration enthalpy decreases
Charge density decreases
Hydration enthalpy decreases
What does a higher charge on ion mean for hydration enthalpy value?
Higher charge Higher charge density Electrostatic attraction between ions and water molecules increases Stronger bonds made More heat energy given out Larger hydration enthalpy value
Enthalpy of solution equation
Enthalpy of solution =
Lattice enthalpy of dissociation + lattice enthalpy of hydration
What is lattice enthalpy of dissociation equivalent to?
-ve lattice enthalpy of formation
Picture born-Haber cycle for NaCl
Top: gaseous ions Na+ (g) + Cl-(g)
Midway between: aqueous ions Na+(aq) + Cl-(aq)
Very bottom: ionic lattice NaCl(s)
1) gaseous ions to ionic lattice is LATTICE ENTHALPY OF FORMATION
2) gaseous ions to enthalpy of solution is ENTHALPY OF HYDRATION
3) ionic lattice to aqueous ions is ENTHALPY OF SOLUTION
Enthalpy of solution is turning what to what
Solid ionic lattice to aqueous ions
Enthalpy of hydration is turning what to what
Gaseous ions to aqueous ions
Enthalpy of solution values can be
Positive and negative ?
A substance is soluble if…
Enthalpy of solution is exothermic or slightly endothermic
Substance is not soluble if…
Enthalpy of solution significantly exothermic (high positive value)
Exothermic reactions
Exothermic reactions give out heat energy so the products have less energy and are more stable than the reactants
Endothermic reactions
Energy is taken in an endothermic reactions so the products have more energy and are less stable than the reactants
First law of thermodynamics
Energy can’t be created or destroyed it is changed from one type to another
Hess’ Law
Enthalpy change of a chemical reaction is independent of the route taken
(Ie. Whatever steps the enthalpy will always be the same)
Entropy is a measure of
Disorder
Link disorder entropy and freedom of molecules
Greater freedom of molecules
Greater disorder
Greater entropy
What happens to molecules as entropy increases?
They become more randomly distributed
Entropy symbol
S
What happens when molecules gain energy?
They gain freedom of movement answer their entropy increases
Why do solids have lower entropies than other states?
They have more restricted movement of molecules
Perfect crystals
At 0Kelvin have 0 entropy
Why did liquids have higher entropies than solids?
Liquid molecules have greater freedom of movement
What is the entropy of complex molecules like?
Higher entropies than simple molecules because there are more ways their molecules can arrange themselves
Values of entropies trend
Gas > liquid > solid
Trend for orderedness of state
Solid > liquid > gas
What happens to entropy as gaseous alkane molecules become more complex?
Increases
Pentane is more complex than butane but it has a lower entropy value, why?
Standard state of pentane is liquid so molecules are more ordered so lower entropy value
Entropy of a system
When a chemical reaction occurs the products and reactants will have different entropy values to each other at the end
Entropy of system is the same as change in entropy
Entropy of a system equation
Entropy of system = entropy of products - entropy of reactants
For a reaction to be feasible, entropy must…
Increase
Name 4 changes that cause an increase in entropy
This 4 examples where a reaction is feasible
1) formation of gas molecules
2) dissolving solid
3) decomposition of a substance (because resulting components have greater freedom of movement)
4) gases becoming more randomly distributed (diffusion)
Spontaneous change means
Feasibility
For a spontaneous change to occur, entropy must…
Increase!
Entropy must be > 0 AKA a positive value
What has happened to entropy if the reactants go from being 5 moles of gas two products of 8 moles of gas?
Entropy has increased
more moles of gas so more ways of arranging the energy of the system over the molecules
Reactions involving a decrease in the number of gas molecules
Entropy decreases
Standard entropy is measured at
298K
Pressure of one bar
Entropy units
J K^-1 mol^-1
Special thing to always remember about entropy measurements
Measured in JOULES
Change in entropy of a reaction is the same as
Entropy of a system
Change in entropy equation
Change in entropy = sum of entropy products - sum of entropy reactants
What does the negative entropy value mean?
Entropy has decreased
Total change in entropy equation
Change in entropy of the surroundings + change in entropy of the system
Change in entropy of the surroundings equation
Change in enthalpy (joules)
- ——————————
Temperature (Kelvin)
What to remember when working out the entropy of surroundings?
Enthalpy on top of equation is often given in KJ so need to convert to JOULES by x1000
Enthalpy change units
VS
entropy change units
Enthalpy change
Kj mol^-1
Entropy change
J K^-1 mole^-1
Explain why entropy change of the surroundings might be positive even though ENTHALPY change of the reaction is negative.
CHECK XX
Reaction is exothermic
Heat given out so entropy of surroundings increases
If a reaction ENTHALPY change is negative then entropy of surroundings is always
XXX
Positive
Because exothermic reaction releases heat energy so entropy of surroundings increases
What do we mean by feasibility
A reaction is possible in terms of energy (but this doesn’t necessarily mean it will happen - might be other factors stopping it)
For a reaction to be feasible…
TOTAL entropy must increase (sign must be +)
Dissolving ammonium nitrate crystals in water
Crystal breaks up into ions and mixes with water
Endothermic (takes in energy, making bonds)
Entropy change of surroundings decrease
But entropy of system increases more than this so total entropy is positive and reaction is feasible.
What can entropy of surroundings tell you?
If a reaction is exothermic or endothermic
Entropy of surroundings increases, reaction is exothermic
Entropy of surroundings decreases, reaction is endothermic
Concentrated ethanolic acid and solid ammonium carbonate equation
2CH3COOH + (NH4)CO3 —-> 2CH3COONH4 + H2O + CO2
Concentrated ethanolic acid and solid ammonium carbonate
Entropy of surroundings decreases
Entropy of system increases
‘Increase in entropy due to formation of gaseous carbon dioxide outweighs fall in entropy of surroundings’
Total entropy increases, reaction is feasible
4 typical reactions from spec where entropy changes occur
Dissolving ammonia nitrate in water
Concentrated ethanolic acid and solid ammonium carbonate
Magnesium ribbon and oxygen
Mixing solid barium hydroxide with solid ammonium chloride
Magnesium ribbon plus oxygen
Reaction is feasible
Reaction is not spontaneous (does happen instantly)
So it’s feasible but that doesn’t mean it will necessarily happen
Mixing solid barium hydroxide with solid ammonium chloride equation
Ba(OH)2.8H2O(s) + 2NH4Cl(s) —> BaCl2(s) + 10H2O(l) + 2NH3(g)
Mixing solid barium hydroxide with solid ammonium chloride
Entropy of system increases because gas and liquid formed from two solids
Using states to predict entropy we are predicting entropy of …
The system only
Entropy of system if BaCl2(s)
+591
Entropy of system if BaCl2.H2O(s)
+530
Name the four main entropy equations
🔺Ssystem = 🔺S products -🔺S reactants
🔺Stotal = 🔺Ssurroundings + 🔺Ssystem
🔺H reaction 🔺Ssurroundings = - ——————- V. T (in Kelvin)
🔺G = 🔺H - T🔺Ssystem
What to remember when working out Gibbs free energy
Gibbs answer must be in Kj mol ^-1
🔺H is in Kj mol^-1
But 🔺S system is in J K^-1 mol^-1 so you have to DIVIDE ENTROPY OF SYSTEM BY 1000 TO CONVERT FROM JOULES TO KJ
Gibbs must be what for reaction to be feasible
Vs what total entropy must be
Negative value (🔺G<0)
Total entropy has to be positive value to be feasible
Feasibility changes with …
Temperature
Eg. Decomposition of CaCO3
Some Kelvin values the 🔺G produced (when you do the calculation) is negative but others it is positive
How to calculate a temperature at which 🔺G becomes feasible?
🔺G = 🔺H - T🔺S system
Work out enthalpy and entropy of system
Rearrange so = 0
Replace = so finding value < T
Equation to find temperature at which reaction involving gases is feasible?
XXX
🔺G = -RT
What does a large + equilibrium constant mean
Reaction highly shifted right because forward reaction happening at faster pace than backwards reaction
Why might a thermodynamically feasible reaction not take place?
It is inhibited by kinetic factors such as
If activation energy is significantly high
If collisions don’t produce enough energy to meet activation energy
Strength of bonds
Do catalysts affect feasibility?
No
Do catalysts affect spontaneity (as in instantly)?
Yes
What is enthalpy
Balance of energy left once you have taken bonds broken - bonds made into account
Enthalpy changes alone do not…
Control whether reactions occur
Entropy is involved as well.
When do you increase temperature to achieve feasibility?
When entropy change of the system is POSITIVE
When do you decrease temperature to achieve feasibility?
When entropy change of the system is NEGATIVE
